# RB26 and higher compression



## makenski (Oct 28, 2007)

I been thinking about running higher compression ratio. Note this isn't a debate thread.

I would like to get up to 10:1 CR (I can settle with less down to 9:1 min) and eventually roll with gt2860-5s.. maybe -9s.. making around 500 hp, goal is to make a super responsive machine.

What I have is stock rb26 block and heads. I already have 86.5 pistons w/ 16 cc dome volume, manufacture states a 8.5 CR for these pistons.

I used a quick compression ratio calculator, assuming the rb26 heads are 70 cc. I calculated with a 1 mm HG that 2mm would be shaven off. However, with no idea about piston deck clearance height that 2 mm may be more or less.

The ultimate problem rests on the valve clearance.. and if I were to do a 10.25 lift, how much would I have left =/

Anyone have experience with increasing CR in a rb26 and boosting decent power?


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## regal (Oct 3, 2005)

*rb26 and higher cr*

with higher compression ratio comes the need for good fuel if you want to run decent boost. assuming that the block and head have never been skimmed before, with using a 1.0mm head gasket you will need 63 cc's in the head to achieve a cr of 9:1, that means that roughly 20 thou would have to be skimmed off the head. but you would need to cc the head etc before you start and work it out properly and not guess it.
as for valve to piston clearance, from experiance there will still be enough clearance with 20 thou skimmed off the head especially if pistons come with valve reliefs, but again you will have to assemble it up, dial in the cams and check clearance either with plasticine on top of the piston or with a dial gauge on full lift.


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## makenski (Oct 28, 2007)

Thanks for the reply. I been doing a lot of reading. 10:1 can be done with 93 AKI pump fuel all day, whereas the 11 to 12 CR have been running E85+ no problems as well boosted. From what I am reading a CR of boosted 9 or 10 is not uncommon.

There are other parameters that contribute to denotation, such as air charge temp. Which boils down to the turbo chargers.. the slightly bigger -5 run more efficient than the -9.

Then it's choice of fmic and duct work. Also ceramic coating the headers which drops the under bonnet temps quite a bit.

Even running the nismo thermostat which has a cooler opening can help.

I looked around for stock combustion chamber, you are correct that a stock is 63 cc which would mean a 1 mm skim to get 10:1... And you are correct that I will need to get my actual values before I attempt. But for now I am doing homework and collecting whatever knowledge I can.

I been inquiring since a 9 to 10 cr is not uncommon. Wonder if any GTR guys been applying it.


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## Piggaz (Sep 5, 2002)

I'm running 9.0:1 on my 26 (Tomei stroker, 86.5 mm pistons). Twin GT RS and [email protected] procams. No issues at all. Would like to see what a 10.1 ratio would be like to drive.


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## Sub Boy (Jan 28, 2008)

Piggaz said:


> I'm running 9.0:1 on my 26 (Tomei stroker, 86.5 mm pistons). Twin GT RS and [email protected] procams. No issues at all. Would like to see what a 10.1 ratio would be like to drive.


Thats about what I want to run but with a RB30, 9.0:1 :thumbsup:

10.0:1 would surely be too hight for good boost on pump gas? No?


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## Piggaz (Sep 5, 2002)

Wouldnt have a clue. It might be pushing the boundries a bit. Maybe? Also rod:stroke ratio might come into it. IE stroked 26's and motors like the OS 30 litre with their low ratios might have a better chance of surviving as the pistons doesnt dwell up at the top of the bore for as long? Food for thought maybe?


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## infamous_t (Jul 9, 2007)

Piggaz said:


> Wouldnt have a clue. It might be pushing the boundries a bit. Maybe? Also rod:stroke ratio might come into it. IE stroked 26's and motors like the OS 30 litre with their low ratios might have a better chance of surviving as the pistons doesnt dwell up at the top of the bore for as long? Food for thought maybe?


Strokers have shorter rod/stroke ratios than the rb26.
The rb26 r/s ratio isn't great to start.
Increasing compression shouldn't affect r/s ration unless your changing the pin height on the piston to achieve this increase, in which case it would make it slightly longer stroke and slighty better r/s ratio, not worse.

Dwell time for an rb30 would be much higher than any of these other RBs and they have no dramas at any comparable power level.


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## Chris Wilson (Aug 20, 2001)

Static compression ratios don't mean a great deal, you need to be considering dynamic compression ratios. Late thinking on some turbo engines is to use a high (10 to 1 'ish) CR with cams with a LOT of lift and a goodly amount of duration. The actual dynamic ratio is fairly low due to the cam timing. To run mild cams and 10 to 1 or more staic CR in an RB26 would limit you to low boost, or race fuel and higher boost. Mapping would be more critical, too. You would need to be brave...


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## Sub Boy (Jan 28, 2008)

Chris Wilson said:


> Static compression ratios don't mean a great deal, you need to be considering dynamic compression ratios. Late thinking on some turbo engines is to use a high (10 to 1 'ish) CR with cams with a LOT of lift and a goodly amount of duration. The actual dynamic ratio is fairly low due to the cam timing. To run mild cams and 10 to 1 or more staic CR in an RB26 would limit you to low boost, or race fuel and higher boost. Mapping would be more critical, too. You would need to be brave...


Sorry to thread steal....but:nervous:

Chris, What are your thoughts on 9.0:1 static with 260deg 9.15 lift PonCams on 98 gas?

Safe enough with 2860-5's?


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## kismetcapitan (Nov 25, 2005)

Sub Boy said:


> Sorry to thread steal....but:nervous:
> 
> Chris, What are your thoughts on 9.0:1 static with 260deg 9.15 lift PonCams on 98 gas?
> 
> Safe enough with 2860-5's?


that's an angle I haven't thought of before - a head skim. 9:1 comp ratio, Poncams, 2860-5s...running at 1.9 bars boost. Just how much difference would the increased CR make? Cost/benefit analysis in play here - the increase would have to justify the riskier mapping and the car always being closer to the edge.


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## infamous_t (Jul 9, 2007)

kismetcapitan said:


> that's an angle I haven't thought of before - a head skim. 9:1 comp ratio, Poncams, 2860-5s...running at 1.9 bars boost. Just how much difference would the increased CR make? Cost/benefit analysis in play here - the increase would have to justify the riskier mapping and the car always being closer to the edge.


Essentially higher CR = less boost for same output power... often with more torque, or it can be seen as extending the efficiency of your turbo.

Captain Toulene here should be able to run 10:1 without batting an eye considering newer factory turbo cars are now running +10:1 on pump rubbish


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## Sub Boy (Jan 28, 2008)

It's the off boost and boosting earlier thing that I'm thinking of.
I'm not looking to make huge HP, More like 550-600hp at the hubs (May need to hybrid the -5's yet) but having it mega responsive for curcuit work.


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## makenski (Oct 28, 2007)

Sub Boy said:


> Sorry to thread steal....but:nervous:
> 
> Chris, What are your thoughts on 9.0:1 static with 260deg 9.15 lift PonCams on 98 gas?
> 
> Safe enough with 2860-5's?


I was wondering the same, cept with 10.25 lifts and what would be the maximum boost tolerated on the -5s on pump gas.. and how are the higher rpm's effected.. ie.. 9k rpm...

From looking around, a higher CR allows for a wider power band, from track perspective that's good. 

To Kismet aka Capt Toublene:
Fortunately, now in days with more advanced computer controls the safety margin for higher compression is raised. That's why I heard that 10:1 pump gas crankers are a dime a dozen. And even greater margin of safety if you are running a stand alone.


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## kingsley (Aug 26, 2002)

I'm no expert in this but I would expect an increase in CR to give you better low down response but it may actually reduce your high boost performance because you probably won't be able to run so much ignition advance.

On a separate note, might radiusing all of the sharp edges in the combustion chamber be a good idea? Admittedly you'd be removing a small amount of material and therefore actually reducing the CR a bit, but the difference should be very small and it should reduce the chance of det (IIRC the sharp edges are where the det usually starts) thus makign it safer to run a higher CR.

Cheers,
Kingsley.


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## makenski (Oct 28, 2007)

kingsley said:


> I'm no expert in this but I would expect an increase in CR to give you better low down response but it may actually reduce your high boost performance because you probably won't be able to run so much ignition advance.
> 
> On a separate note, might radiusing all of the sharp edges in the combustion chamber be a good idea? Admittedly you'd be removing a small amount of material and therefore actually reducing the CR a bit, but the difference should be very small and it should reduce the chance of det (IIRC the sharp edges are where the det usually starts) thus makign it safer to run a higher CR.
> 
> ...


I am thinking of smoothing the edges of the pads and unshrouding the valves. For the most part both squish pads untouched. For ports...smooth out the transition in the choke area; only surface clean. Knock out the bumps in the exhaust ports. And a little port matching.

That's all I got planned for my head.


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## Mr Bizzle (Jul 17, 2008)

I just finished putting together the bottom end on a 9.0:1 compression ratio RB30/26. I have machined the quench pads out of the head and had a set of custom pistons made up that have been designed to tumble/mix the air/fuel mix. 

I'll let you know how it runs in a few weeks .

Just make sure that if your decking the head, you run a standard sized gasket and setup the squish/quench area accordingly as it makes a difference. Removing the sharp edges from the quench pads is a must (post deck/skim) 

If your running something at 500 or so hp I would highly recommend ditching the quench pads all together. I know this has been discussed at length, but I have yet to see any issues when doing this, providing you use the right pistons.


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## Pavlo (Sep 12, 2002)

I would be very wary of going for high CR if you want to run big boost. 2bar and pump fuel on a 10:1 CR engine is not a good combo, it will make less power than a more conventional CR (ie 8.5:1) because the timing has to be retarded too much. If you only want to run 1.3 bar (for instance) it might be worth a go but I would bow to the knowledge/experience of people that have already done it!


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## ChrisIsle (Sep 17, 2009)

I agree with the above. High CR means less advance at the top. So less power. A low CR engine can be mapped to be responsive at the bottom end. Lot's of advance. I'd always go for lower CR's if possible. My old car ran 8.0:1, also did 2.5bar of boost on a GT3582R. 635bhp out of a 2 litre 4 cylinder is not to be sniffed at. 

I used cheaper 9.0:1 pistons on the first build and my mapper (Dave Rowe of EPS) had to start pulling advance at only 1.5bar. made crap power too.


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## Chris Wilson (Aug 20, 2001)

ChrisIsle said:


> I agree with the above. High CR means less advance at the top. So less power. A low CR engine can be mapped to be responsive at the bottom end. Lot's of advance. I'd always go for lower CR's if possible. My old car ran 8.0:1, also did 2.5bar of boost on a GT3582R. 635bhp out of a 2 litre 4 cylinder is not to be sniffed at.
> 
> I used cheaper 9.0:1 pistons on the first build and my mapper (Dave Rowe of EPS) had to start pulling advance at only 1.5bar. made crap power too.


What engine was this? Sounds interesting


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## ChrisIsle (Sep 17, 2009)

It was my old MR2 turbo. Incredibly strong engine. Motec management.


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## Chris Wilson (Aug 20, 2001)

Blimey, that's 503E IMSA engine territory!


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## ChrisIsle (Sep 17, 2009)

Is that the Pikes Peaks engines? There have been a few impressive power 3S GTE's built recently. Mushim's was 750ish bhp, I think Nathan's was 900 but used NOS. 

I'm debating what to do with my RB. Might build it, see how the finances go. Still on a learning curve as yet. Trying to figure the strengths and weakness' etc.


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## kismetcapitan (Nov 25, 2005)

food for thought. I think the most I'll go with is a thin head gasket, 1.0mm? I haven't shopped for parts in ages, so I can't remember what the thinnest is.

Any caveats with just going with a thin head gasket?


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## infamous_t (Jul 9, 2007)

Gibson R32 GTR ran around 9.5 CR and made very good power doing it, but obviously not running pump fuel.


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## makenski (Oct 28, 2007)

kismetcapitan said:


> food for thought. I think the most I'll go with is a thin head gasket, 1.0mm? I haven't shopped for parts in ages, so I can't remember what the thinnest is.
> 
> Any caveats with just going with a thin head gasket?


Tomei sells a 1 mm gasket.

I was thinking the same thing, it would bring me to a 9.5:1 compression, if the piston is above the deck like SAU specified.

The only thing that comes in mind that would need attention:
-piston to head clearance
-valve to piston clearance
-timing belt tension


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## Pavlo (Sep 12, 2002)

9.5:1 Will be too high if you want to run on pump fuel most of the time.


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## makenski (Oct 28, 2007)

10:1 on pump fuel are a dime a dozen...and ran all day hard machines...

12:1 on E85 dime a dozen.. also...

Since 1996 with the addition of knock sensors, the safety threshold for boost has been raised. People are stuck with the 1970's lower CR mentality.

But, if you rolling with a un-tunable ecu don't bother.


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## SteveN (Aug 6, 2002)

makenski said:


> 10:1 on pump fuel are a dime a dozen...and ran all day hard machines...


At what boost level though? 10:1 is no worries if you arent running much in the way of boost. Physics and Chemistry hasnt magically changed in the last 30 years though, det is det.

Relying on knock sensors for anything but emergencies sounds like a recipe for disaster to me, but im no mapper.

People seem to think tuning is so advanced these days. People really believe technology on road cars are more advanced than 80s F1 technology?

Off boost higher comp drives nicer, but on boost higher boost drives nicer, swings and roundabouts, you cant have your cake and eat it on pump fuel.


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## Pavlo (Sep 12, 2002)

I'm not saying it can't be done, but if you want to run a reasonable amount of boost, a lower (than 9.5) will yield more power on pump fuel. The aim of the exercise is to get the best output from the engine, not to be able to claim the highest CR but run no advance, or for that matter to boast 2.5 bar but make no power because of a 6.5:1 CR.


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## makenski (Oct 28, 2007)

SteveN said:


> At what boost level though? 10:1 is no worries if you arent running much in the way of boost. Physics and Chemistry hasnt magically changed in the last 30 years though, det is det.


The magic comes in EFI.. something that wasn't common until the mid 80's... now in the present time the computer control systems are quicker and more advanced; not relying on a inefficient suction powered carburetors and spinning rotor caps....

Boost means nothing... 10 psi on a 8.5 and 10 CR motor make different amounts of power... 

They aren't rocking it with their tails between their legs either with 400 to 500+ hp

Look around, do the search... look at other platforms.. such as the EVO, the Cossie.. plenty success else where...

Also.. food for thought... The latest and greatest...cars...

2010 Mitsubishi Evolution
2010 Nissan GTR

what they both have in common... 9:1 CR..
Now as a car manufacture.. for the sake of brand reputation of reliability... why would they release turbo vehicle with a higher CR?


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## kingsley (Aug 26, 2002)

In my limited understanding of the finer points of all this, I thought that det can be minimised by things like better fuel mixing, swirl, better combustion chamber design, etc.

If I'm right (maybe someone else can comment), just because the latest engines can run higher CR with boost doesn't necessarily follow that you can do the same on the much older RB26.

Cheers,
Kingsley.


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## Corsa1 (Sep 8, 2003)

A smaller cylinder has less heat build up due to the short flame travel I.e. you should be able to run more compression in a small cylinder, that's why we can get away with 9.5 in a bike engine with a turbo


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## makenski (Oct 28, 2007)

kingsley said:


> In my limited understanding of the finer points of all this, I thought that det can be minimised by things like better fuel mixing, swirl, better combustion chamber design, etc.
> 
> If I'm right (maybe someone else can comment), just because the latest engines can run higher CR with boost doesn't necessarily follow that you can do the same on the much older RB26.
> 
> ...


A new engine and the RB are mechanically the same. What's different is the control systems.


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## David (Apr 25, 2003)

makenski said:


> Now as a car manufacture.. for the sake of brand reputation of reliability... why would they release turbo vehicle with a higher CR?


There is a very simple answer to that..... Emissions and fuel economy. Without the 1st you simply wont be able to sell the car.

They are the only reasons for such high compression, I know as I work on OEM charged engines. It has FA to do with power or responce


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## Pavlo (Sep 12, 2002)

The RB is very good for det resistance. But choosing the right CR is very much to do with the level of boost you want to run. Using the R35 as an example, it makes (for arguments sake) 600hp on pump fuel with the right turbos, but the size of the engine means it doesn't need lots of boost to do it.

High CR really only becomes a problem as you try for high boost levels. I've mapped a number of high CR 4G63 and they make peak power at about 1.8 bar, but the jump in power from 1.4 bar to 1.8 bar was very small because the advance had to be reduced to very low levels (2 degrees at peak torque, 3.5 degrees ar peak power). The power was around 560hp. Now with 109 RON Sunoco GT PLus fuel, ignition timing at 1.8 bar jumped up around 7 degrees, and we were able to run easily a 2 bar, we would have run it at 2.2 but came up against an ignition misfire. The power rose to about 650hp.

If the boost level is low, high CR is fine and will feel nice to drive, but if you go to 10:1 on pump fuel there is only so much that piston design and even the most modern EFI can do for you.


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## Cris (Sep 17, 2007)

Pavlo said:


> The RB is very good for det resistance. But choosing the right CR is very much to do with the level of boost you want to run. Using the R35 as an example, it makes (for arguments sake) 600hp on pump fuel with the right turbos, but the size of the engine means it doesn't need lots of boost to do it.
> 
> High CR really only becomes a problem as you try for high boost levels. I've mapped a number of high CR 4G63 and they make peak power at about 1.8 bar, but the jump in power from 1.4 bar to 1.8 bar was very small because the advance had to be reduced to very low levels (2 degrees at peak torque, 3.5 degrees ar peak power). The power was around 560hp. Now with 109 RON Sunoco GT PLus fuel, ignition timing at 1.8 bar jumped up around 7 degrees, and we were able to run easily a 2 bar, we would have run it at 2.2 but came up against an ignition misfire. The power rose to about 650hp.
> 
> If the boost level is low, high CR is fine and will feel nice to drive, but if you go to 10:1 on pump fuel there is only so much that piston design and even the most modern EFI can do for you.


This is very interesting.

Would I be correct in saying that if you were thinking about using smaller turbos (say Garrett -9s or -5s) with up to 1.5 Bar of boost (which is, as I understand it, as much as you'd want to on these units) then a higher CR would be possible.

The above sounds like a receipe for a lovely road car, responsive with 'enough' power.


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## Pavlo (Sep 12, 2002)

I guess so, but 1.5 bar is a reasonable amount of boost so unless someone has done it with a paticular head and piston setup and can provide concrete results, you'll be working with an educated guess at best.



Cris said:


> This is very interesting.
> 
> Would I be correct in saying that if you were thinking about using smaller turbos (say Garrett -9s or -5s) with up to 1.5 Bar of boost (which is, as I understand it, as much as you'd want to on these units) then a higher CR would be possible.
> 
> The above sounds like a receipe for a lovely road car, responsive with 'enough' power.


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## makenski (Oct 28, 2007)

Cris said:


> This is very interesting.
> 
> Would I be correct in saying that if you were thinking about using smaller turbos (say Garrett -9s or -5s) with up to 1.5 Bar of boost (which is, as I understand it, as much as you'd want to on these units) then a higher CR would be possible.
> 
> The above sounds like a receipe for a lovely road car, responsive with 'enough' power.


Exactly what I been thinking. 1.5 bar with the -5s, I am just concern what the higher rpms going to behave..


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## Mr Bizzle (Jul 17, 2008)

It’s all about correct turbo choice a lot of the time.
People run smaller turbo’s because they want them to spool early and to deliver a wider range of power. Turbo A (smaller turbo) running 1.0bar will spool faster than Turbo B (slightly larger) running the same boost pressure, but the amount of air moved by Turbo B (slightly larger) will be greater at the given boost pressure as it’s moving more air? Thus creating more power. Follow me?

The problem is that the more boost pressure you run, the more the charge is heated by the turbo. This results in the temperature of the air/fuel mix entering the combustion chamber to be significantly higher. Once the piston is on it’s up stroke, this air and fuel mix is compressed and something called adiabatic heating occurs. (*which is the increase in temperature of a fluid when under pressure) if this air fuel mix reaches the auto-ignition point of gasoline you get premature detonation or det (The air/fuel mix auto igniting before the spark plug fires – not like premature ejaculation) 

By upping the compression ratio in an engine, your increasing the heat that is generated when the gas is compressed on the upstroke (but also increasing the density of the gas/air mix and extracting more mechanical energy) this means that your inlet temperatures and fuel octane are significantly more important. Higher octane fuel has a higher auto ignition temperature and is more difficult to burn, thus making your combustion process more det resistant.

When building turbo engines static compression ratio is actually a bit of a clumsy way of measuring what’s going on because your measuring the C/R at atmospheric pressure not the desired boost level. You need to calculate the dynamic or effective ratio based on the pressure of the air your planning to put into your engine through the turbo. 



Bizzle on Driftworks said:


> It's really down to the type of Piston and shape of the combustion chamber which determines the speed of the flame front that travels across the compressed mixture.
> 
> In simple terms a higher compression ratio DOES create more power off boost which gives you extra torque down the arse end. It's really down to static vs effective compression.
> 
> ...


So with that said, a higher C/R engine will produce more power off boost and subsequently spool your turbo slightly faster. You need to aim for a power goal (whatever that is) and spec your turbo setup accordingly to produce the required air at a reasonable pressure. RB heads suffer from cooling problems around the back two cylinders which is pretty evident if you take the head off almost any 80K+ mile bolt on tuned engine. You’ll see the crackling on the quench pads – but there are ways around this.

The majority of the advice above is stuff you should take into consideration when building any RB engine – 

Notes:
* Air is a fluid


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## SteveN (Aug 6, 2002)

People should listen to Pavlo, he has been there and done it for example.
Talking about it on the net and having genuine experience of things is wildly different, and he has.


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## makenski (Oct 28, 2007)

Mr Bizzle said:


> It’s all about correct turbo choice a lot of the time.
> People run smaller turbo’s because they want them to spool early and to deliver a wider range of power. Turbo A (smaller turbo) running 1.0bar will spool faster than Turbo B (slightly larger) running the same boost pressure, but the amount of air moved by Turbo B (slightly larger) will be greater at the given boost pressure as it’s moving more air? Thus creating more power. Follow me?


Anytime air is being compressed, it heats up. However, the point of going a larger turbo is because it runs cooler, you run across the efficiency range of compressor better. So turbo B often is a cooler choice.



Mr Bizzle said:


> The problem is that the more boost pressure you run, the more the charge is heated by the turbo. This results in the temperature of the air/fuel mix entering the combustion chamber to be significantly higher. Once the piston is on it’s up stroke, this air and fuel mix is compressed and something called adiabatic heating occurs. (*which is the increase in temperature of a fluid when under pressure) if this air fuel mix reaches the auto-ignition point of gasoline you get premature detonation or det (The air/fuel mix auto igniting before the spark plug fires – not like premature ejaculation)
> 
> By upping the compression ratio in an engine, your increasing the heat that is generated when the gas is compressed on the upstroke (but also increasing the density of the gas/air mix and extracting more mechanical energy) this means that your inlet temperatures and fuel octane are significantly more important. Higher octane fuel has a higher auto ignition temperature and is more difficult to burn, thus making your combustion process more det resistant.


Knock sensors... with processing power faster that you can blink... like it checks that sensor around 25,000 times a second. Once that knock sensor reads a distinct 6400 hz of noise it retards ignition thus reducing power. Or.. it can put you in limp mode.. This is only one fraction of what tuning can do for you...



Mr Bizzle said:


> When building turbo engines static compression ratio is actually a bit of a clumsy way of measuring what’s going on because your measuring the C/R at atmospheric pressure not the desired boost level. You need to calculate the dynamic or effective ratio based on the pressure of the air your planning to put into your engine through the turbo.
> 
> So with that said, a higher C/R engine will produce more power off boost and subsequently spool your turbo slightly faster. You need to aim for a power goal (whatever that is) and spec your turbo setup accordingly to produce the required air at a reasonable pressure. RB heads suffer from cooling problems around the back two cylinders which is pretty evident if you take the head off almost any 80K+ mile bolt on tuned engine. You’ll see the crackling on the quench pads – but there are ways around this.
> 
> ...


DCR = SC*PR ... PR - pressure ratio...

10:1, with 1.5 bar of boost... pr = 1.5 +1

10 * 2.5

25... 

A true dynamic pressure is not accounted:
Air also has mass. Forcing air up against the head, it dynamically compressing itself too and that varies by rpm (and acceleration.. so rod ratio is in effect here too). Also not taken to account, dimensions. Also... depends on when the intake valve closes too. Initial Air tempature. Fuel mixture...block temp..iron heads or aluminum heads....plug location... many things changes.

If your rear cylinders are running hot, you running a bad tune.


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## Pavlo (Sep 12, 2002)

SteveN,

The internet is always right...


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## Sub Boy (Jan 28, 2008)

Pavlo said:


> SteveN,
> 
> The internet is always right...


:nervous::flame::lamer:


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## Mr Bizzle (Jul 17, 2008)

makenski said:


> If your rear cylinders are running hot, you running a bad tune.


Thanks for your input. However i disagree with the above comment. If you track or drift an RB engine then they are the two cyl's that get hottest at the rear of the engine.

To my knowledge it's one of the reasons why the HKS step 2 crate engines come with an extra port into the water jacket at the rear of the head (replace the welsh plug)


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## Chris Wilson (Aug 20, 2001)

Mr Bizzle said:


> Thanks for your input. However i disagree with the above comment. If you track or drift an RB engine then they are the two cyl's that get hottest at the rear of the engine.
> 
> To my knowledge it's one of the reasons why the HKS step 2 crate engines come with an extra port into the water jacket at the rear of the head (replace the welsh plug)


Have you a photo of this modification that HKS make? Thanks.


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## Terminator (May 26, 2002)

Chris Wilson said:


> Static compression ratios don't mean a great deal, you need to be considering dynamic compression ratios. Late thinking on some turbo engines is to use a high (10 to 1 'ish) CR with cams with a LOT of lift and a goodly amount of duration. The actual dynamic ratio is fairly low due to the cam timing. To run mild cams and 10 to 1 or more staic CR in an RB26 would limit you to low boost, or race fuel and higher boost. Mapping would be more critical, too. You would need to be brave...


Chris pointed me to this thread from MKIV.net
If it would help I would like to illustrate what Chris has said with my own experience of a HCR forced induction motor. My 2JZGTE engine was build by Virtual Works in the US. It has a compression ratio of 9.5:1, 1mm oversize valves, HKS 280 cams, Carrillo rods and forged pistons, all balanced to 9000rpm. It was recently mapped at SRR, but the car was slipping on the rollers so we did not get an accurate set of figures. Ryan of 2bar Tuning, estimated 700 -750bhp at 1.4 bar. At no point during 2 hours on the rollers or later on the roads did we get a hint of det. Ryan is confident that on a hub dyno we could map it to 1.7 bar with ease on 99Ron fuel, we hope to see over 800, my target is 850bhp on pump fuel with my GT74. The car builds power fast and I have a massive rpm range at full boost up to the current 8500 red line. Off boost performance is good due to the relatively high compression. Given my experience to date, I would not hesitate to go to a CR of 10.0:1. When it comes to rebuild time that is where I will be heading.


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## hybrid_eg (Jul 24, 2006)

quick question on this topic.. i need a rebuild sadly.. made a fatal error of not checking oil, too much slosh and spun a road bearing while drifting..

Here is I have planned, will run this on pump gas with less boost and need opinions on how reliable this will be.. R33 GTR stock motor, n1 oil pump, run 9.1 pistons, rods, clean up the block, metal headgasket, factory thickness, headstuds, mild head port, and mild cams, maybe 264's? Turbos are Nismo GT/le mans (14411-RS581) (similar to 2530's but journal type)

good idea or not? or should i just stick with stock compression and just get forged pistons of course?

Any help is greatly appreciated.. need to get my car back together asap will be pulling the motor out next week..


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## makenski (Oct 28, 2007)

Mr Bizzle said:


> Thanks for your input. However i disagree with the above comment. If you track or drift an RB engine then they are the two cyl's that get hottest at the rear of the engine.
> 
> To my knowledge it's one of the reasons why the HKS step 2 crate engines come with an extra port into the water jacket at the rear of the head (replace the welsh plug)


You can tune those cylinders to run richer and other neat tricks of the trade to try to balance the cylinders. If your engine fails because your rear two overheated.. you ran a bad tune. HKS and TOMEI got their own nitches... even because they did it doesn't mean they need to be done.



Terminator said:


> Chris pointed me to this thread from MKIV.net
> If it would help I would like to illustrate what Chris has said with my own experience of a HCR forced induction motor. My 2JZGTE engine was build by Virtual Works in the US. It has a compression ratio of 9.5:1, 1mm oversize valves, HKS 280 cams, Carrillo rods and forged pistons, all balanced to 9000rpm. It was recently mapped at SRR, but the car was slipping on the rollers so we did not get an accurate set of figures. Ryan of 2bar Tuning, estimated 700 -750bhp at 1.4 bar. At no point during 2 hours on the rollers or later on the roads did we get a hint of det. Ryan is confident that on a hub dyno we could map it to 1.7 bar with ease on 99Ron fuel, we hope to see over 800, my target is 850bhp on pump fuel with my GT74. The car builds power fast and I have a massive rpm range at full boost up to the current 8500 red line. Off boost performance is good due to the relatively high compression. Given my experience to date, I would not hesitate to go to a CR of 10.0:1. When it comes to rebuild time that is where I will be heading.


Thanks for your input! You are obviously not one who runs around with his tail between his legs. :thumbsup:

I am hoping to get 1.5 bar out of a pair of GT2860-5's, running 260 cams with 10.25 lift. 9k redline. It be nice to do it all with 10:1 CR



hybrid_eg said:


> quick question on this topic.. i need a rebuild sadly.. made a fatal error of not checking oil, too much slosh and spun a road bearing while drifting..
> 
> Here is I have planned, will run this on pump gas with less boost and need opinions on how reliable this will be.. R33 GTR stock motor, n1 oil pump, run 9.1 pistons, rods, clean up the block, metal headgasket, factory thickness, headstuds, mild head port, and mild cams, maybe 264's? Turbos are Nismo GT/le mans (14411-RS581) (similar to 2530's but journal type)
> 
> ...


Do it, you got plenty of safety margin rolling with 9.1:1 CR, just make sure you or your tuning guy is putting up with the task.


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## Terminator (May 26, 2002)

makenski said:


> I am hoping to get 1.5 bar out of a pair of GT2860-5's, running 260 cams with 10.25 lift. 9k redline. It be nice to do it all with 10:1 CR


The current fastest standing mile held by a MKIV is 198.55, this car has a static CR of 9.5:1 and is running twins. So you goals seem very realistic, however you should look at different cams or you will hurt your timing and take out what you hope to gain. I don't have a particularly high lift, but the 1mm OS valves do go some way to compensating for lack of lift, my over lap is not huge but enough to alter dynamic compression. I think the 260's overlap will be too mild for a 10.0:1 on standard valves.


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## Mr Bizzle (Jul 17, 2008)

makenski said:


> You can tune those cylinders to run richer and other neat tricks of the trade to try to balance the cylinders. If your engine fails because your rear two overheated.. you ran a bad tune. HKS and TOMEI got their own nitches... even because they did it doesn't mean they need to be done.


Doesn't that depend on the ECU you run? I didn't think the PFC's and alike did independant cylinder mapping? (im no engine mapper tho) 
I have a pic of the rear head drail water cooling line, but my photobucket doesn't work here at the office.


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## ODEngines (Jan 4, 2010)

Terminator said:


> The current fastest standing mile held by a MKIV is 198.55, this car has a static CR of 9.5:1 and is running twins. So you goals seem very realistic, however you should look at different cams or you will hurt your timing and take out what you hope to gain. I don't have a particularly high lift, but the 1mm OS valves do go some way to compensating for lack of lift, my over lap is not huge but enough to alter dynamic compression. I think the 260's overlap will be too mild for a 10.0:1 on standard valves.


Agreed with what your saying about dynamic compression. If response is the ultimate goal my thoughts are no higher than 9.2:1 with standard cams, 9.3 with 260 degree cams, 9.4 with 272 degree cams. 

Chances are if your using a cam larger than these your not after response anyway as the bigger cam duration the further up the RPM range you move the point of peak torque.

The most lift you can get at the valve combined with a low duration (260) cam and 9.2 cam would be my pick.

Using a RB25 NEO head to pick up inlet cam VCT and a smaller combustion chamber size would be worth considering as well. The down side is that you'd have to use a single throttle plenum, or adapter plate to retain the 6 throttles (which you would want to do to maximise response). The exhaust manifolds would also require some creativity as the stud positioning is differnet between the RB26 and RB25 neo heads. Of course there is also the requirement for blocking the VCT oil feed and running an external oil feed to retain VCT operation. This approach combined with high compression would produce the best low end/off boost gains.

Any which way you go you will probably end up using custom pistons of some kind. Nothing out of the box will suit your needs exactly. Cometic make a 36 thou head gasket that is worth looking at as well, to optimise the squish of the combustion chamber and help achieve the desired compression ratio.


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## jasaircraft (Feb 15, 2009)

I cant seem to find pistons with a CR higher than 9:1 .... can anyone point towards a 9.5 or 10 set please¿


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## Sub Boy (Jan 28, 2008)

jasaircraft said:


> I cant seem to find pistons with a CR higher than 9:1 .... can anyone point towards a 9.5 or 10 set please¿


What fuel are you going to run?


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## m3dim (Aug 22, 2013)

jasaircraft said:


> I cant seem to find pistons with a CR higher than 9:1 .... can anyone point towards a 9.5 or 10 set please¿


Perhaps contact your choice of piston manufacturer and see whether they could possibly custom make you a set with higher compression. 

Another option is to use a thinner head gasket...


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## Piggaz (Sep 5, 2002)

Chris Wilson said:


> Have you a photo of this modification that HKS make? Thanks.




4.5 years too late but I thought I'd chuck it up anyway. My head was done by HKS and NAPREC in Japan. They replace a plug in the back of the head and you run a -8 line around to the front and tee it in to one of the water pipes up the front. I don't have the car at home at the moment so it's a bit hard to take a photo.


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## muzzer2002 (Oct 10, 2007)

Eh isnt that the rear head drail mod for oil i could be wrong but mines is similar lol


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## Piggaz (Sep 5, 2002)

No. The "oil drain/vent" is the plug above that one.


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## jasaircraft (Feb 15, 2009)

Pure alcohol, so no det with 10:1 and 27psi on gt2830-5 (i hope)

I bought a set of supertech 8.5 but now have changed my mind and want HC ones... could buy the thinest head gasket too


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## jasaircraft (Feb 15, 2009)

Damn CP can make them but will go for 1400dollars with upgraded pins and 1300 with normal ones...


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## m3dim (Aug 22, 2013)

Actually isn't that bad. I'm going with the CP 9.0 comp pistons together with a 1.2mm head gasket. I'm guesstimating it should give a 9.2 comp maybe 9.3. I think the difference to 10 wouldn't make a huge difference to response?

I think it's good compromise for a street car...


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## TomiS (Aug 29, 2014)

Old thread, I know, but are there now more RB26's running high CR like 10:1 or even higher? I'm planning a rebuild for my RB26 and been offered CP pistons with 10:1 CR. I'm using E85 and looking for more responsive engine with no more than 600 hp. What would be the difference between 10:1 or 9:1?


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## Sub Boy (Jan 28, 2008)

If you are only going to be running E85 then go for it, more compression= faster spool, and without the downside of detenation with Ethanol it will be good. 
Pretty hard to make good power on that compression if you were only running 95/98 octane pump gas however.


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## TomiS (Aug 29, 2014)

OK, sounds great. I'm planning to use only e85 and have already build the fuelsystem accordingly. Anything else I should consider, like stronger con rods or bearings?


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