# Compression ratio after combustion chamber squish pads have been removed?



## SteveN (Aug 6, 2002)

As title, with the standard compression 8.6:1 pistons and stock thickness gasket.

Loads of tuned RB26s come over from Japan with whats listed as "Combustion Chamber Processing" which is basically the 2 squish pads in each combustion chamber removed to give a nice smooth bowl, therefore de-shrouding the valves for better flow, and of course lowering the comp.

But what does it make it to? Ive never actually found out.

Rumour had it drops it to around a full point, and my experiences with other cars would tend to say thats not too far off the mark, but I honestly dont know.

Cheers


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## T.F.S. (Feb 5, 2004)

full point seems a bit much i think, i rounded the edges off on mine but was too shy to remove totally..


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## SteveN (Aug 6, 2002)

Depends, I know cars that drop by a full point or more (certain small block V8s go down by about 1.5points) just by enlarging it by about 25cc, which isnt much or any less than what fully removing the squish pads (making a proper bowl) would give idve thought.

Pics from the JUN site
www.junauto.co.jp/processing/explain/images/chamber-mc1.jpg
www.junauto.co.jp/processing/explain/images/chamber-mc1.jpg


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## professor matt (Nov 1, 2005)

dont know about the rb26,but when i had my bmw engine built by AVA they completly removed them,allowing them to run more boost

they recommend run 7:1 comp with more boost on my rb26


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## T.F.S. (Feb 5, 2004)

sen the pics before stav but i really dont like the look of it, they were there for a reason surely?


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## SteveN (Aug 6, 2002)

professor matt said:


> dont know about the rb26,but when i had my bmw engine built by AVA they completly removed them,allowing them to run more boost
> 
> they recommend run 7:1 comp with more boost on my rb26


Yeah, youd unlikely find any serious power turbo car (built by someone who understands what they for) leaving them in. Tho on big power n/a cars its far more important.

The compression thing you mention is interesting, as tho im a fan (ie I understand how to make I work well, lol) of low-comp, AVA go mentally low, lower than id ever consider for the spec, BUT always produce cars with fantastic low down power and response still, as well as the mega torque you expect with a well specced low comp big boost car.
550bhp+ 2litre engines with great power from 3500rpm up isnt shy at all, esp as they got proven performance times to match, on pump fuel too, so cant really say they doing it wrong, whatever they doing!
They must have cam specs etc totally nailed, something I know they spend a lot of time on.



T.F.S. said:


> sen the pics before stav but i really dont like the look of it, they were there for a reason surely?


Well, look at the standard piston shape.
The standard piston and combustion chamber shape makes them fit together like a jigsaw puzzle almost.

This is to direct the fuel/air mix towards the spark plug, and also give the optimal squish/quench area for best efficiency in standard spec (which mostly means fuel economy and emissions), same as all cars with similar setups.

When going for pure performance on a turbo car though it matters very little, compared to the advantages of getting rid of them (mostly de-shrouding the valves and removing and sharp edges) hence why its done. Esp as most racing pistons etc wont be made with that standard shape with the range centre.


Still no answer to what I wondered tho!
Does make me smile tho, instead of saying the REAL compression ratio, most adverts say 8.6:1 pistons, but in reality the compression is much lower as its had "combustion chamber processing" :chuckle: 
And then loads of people slate anything lower than stock comp, without realising most of the top cars are, not to mention they need to run race gas as the comp is still too high for full power on pump fuel...


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## [email protected] M/S (Nov 30, 2003)

*Squish area*

!.4cc for inlet side and .95cc for exhaust side.

Tony


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## kismetcapitan (Nov 25, 2005)

motherf*cker!!! a year ago I was told my head was unusable because I had pitting in the squish pads. I wanted to have them removed, make a polished bowl, completely rebuild the head, but I was told that it was impossible, that if the squish pads were pitted, the head was trashed. So I had to buy another one, which was a pain, and cost me plenty of cash - cash that would have covered a full machining, port, polish, flowed, valves, the whole nine yards, with plenty left over.

Grrr! 

At the very least, it makes sense to me to round them off so there isn't a 90 degree edge in the combustion chamber. And it's good to hear that it is indeed not only possible, but something that's done. good!!!! 

My old head's still at the garage. I'm getting it and gonna use it to learn how to build a head properly. Dremel's at the ready....


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## roadie (Feb 6, 2006)

I started a thread a while back about lowered compression ratios. Though I found some of the comments very informative I never felt quite satisfied by the response. There are many turbos available which perform incredibly above 2.0 bar. To me this is free horsepower. Yet the bulk of RB owners run 1.5 and under. I have to assume that high cr's force them to do so. My compression is 122lbs-126lbs accross the board. As I slowly inch my way up to 2.5 bar (pump gas), the only issue I currently have is keeping the boost pipes together.
Looking forward to some more responses.......opcorn:


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## R32 Combat (Jan 29, 2004)

Don't remove inlet squish. Speak to uncle Tony @ Abbey. With a skimmed head and no squish, I had 65cc


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## Corsa1 (Sep 8, 2003)

Take a look on how its been burning when you have removed them.


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## kismetcapitan (Nov 25, 2005)

why? how does it burn with a spherical combustion chamber?


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## kismetcapitan (Nov 25, 2005)

R32 Combat said:


> Don't remove inlet squish. Speak to uncle Tony @ Abbey. With a skimmed head and no squish, I had 65cc


So when you took out the squish, are you saying that 65cc was far too much for the combustion chamber?


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## SteveN (Aug 6, 2002)

just a thought, but i dont think the standard shape piston on a GTR would work well with the squish pads removed as the raised centre would move most the fuel/air mix away from the central spark plug, which wouldnt be great for combustion really would it.

Better off with your usual flat top or dished piston id say, as the raised section would turn in to a hinderance.

But any good engine builder wouldve worked that out im sure, rather than thinking it was voodoo that the burn is suddenly rather poor.



tonysoprano said:


> !.4cc for inlet side and .95cc for exhaust side.
> 
> Tony


just over and just under 1 cc per side tony?

are you sure? a cc is basically a ml, ie barely a drip, lol.


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## R.I.P.S NZ (May 7, 2004)

RB30, Flat tops, no squish, C/R just in the 7's (depending on gasket), comb cc aprox 69cc. 

Rob


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## Corsa1 (Sep 8, 2003)

SteveN said:


> just a thought, but i dont think the standard shape piston on a GTR would work well with the squish pads removed as the raised centre would move most the fuel/air mix away from the central spark plug, which wouldnt be great for combustion really would it.
> 
> Better off with your usual flat top or dished piston id say, as the raised section would turn in to a hinderance.
> 
> ...


Steve n you got in one with the shape of the gtr piston.


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## R32 Combat (Jan 29, 2004)

1cc = 1ml = 1cm X 1cm = 10mm X 10mm

Barely a drip??? A drip with what viscosity??


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## SteveN (Aug 6, 2002)

[email protected] said:


> Steve n you got in one with the shape of the gtr piston.


Indeed.

Its like all engine components, need to be specced as an entire unit to work well together, as whats a help with one spec is a massive hinderance with another spec.


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## R.I.P.S NZ (May 7, 2004)

You'd be amased the response I got from some tuners when they heard I used flat tops.
Some said it was stupid, couldn't possibly make decent power etc.
As Steve says, each part is chosen to complement another and we have proven pretty big power on flat tops in street engines.

Rob


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## Corsa1 (Sep 8, 2003)

Rob you would be surprised how many tuners in the uk follow a trend. we have tried some vauxhall xe flat tops.
[email protected]


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## paul cawson (Jul 3, 2003)

When I ported the head on my black GTR I removed the inlet quench pad and not the exhaust side. If you search the japanese tuner sites this is what they recommend for up to 700bhp, over this and for drag use remove them both.

The lump in the exhaust port is also removed


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## Corsa1 (Sep 8, 2003)

paul cawson said:


> When I ported the head on my black GTR I removed the inlet quench pad and not the exhaust side. If you search the japanese tuner sites this is what they recommend for up to 700bhp, over this and for drag use remove them both.
> 
> The lump in the exhaust port is also removed


WHY


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## paul cawson (Jul 3, 2003)

[email protected] said:


> WHY


Cut/copy of
From - Mr . GT-R . Mostly relating to R32 GT-R's.
ENGINE

Only minor head porting is required , cleanup and bowl work
Push intake splitter back and knife edge
Unshroud intake valves and remove all sharp edged to combat detonation , leave exhaust quench pad alone for under 800ps.
For 800 +ps remove both quench pads like Porsche 996 turbo combustion chamber.



Its to reduce hot spots as well as better flow,as you know the standard head and camshafts are designed for around 350bhp
700bhp and you have twice the air/fuel to pump through the head


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## SteveN (Aug 6, 2002)

I think you missed his point mate, I think he was saying what is YOUR reasons for it, and that seems to be "because the japanese men told me so", which is prob why he said it.

In my mind at least, leaving the exhaust one in place dont really make much sense to me, as that combined with the usual piston shape (even most/all jap forged ones are stock shape) will push the majority of the fuel/air mix away from the 2 places you want it for decent combustion, close to the sparkplug and exhaust valves, and in to the open space around the inlet valves. 



paul cawson said:


> Its to reduce hot spots as well as better flow,as you know the standard head and camshafts are designed for around 350bhp
> 700bhp and you have twice the air/fuel to pump through the head


Is that something you said, or they said? As its a bit of a strange statement, depending how you take it, as engines, esp turbo ones, dont work quite how that seems to read.


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## paul cawson (Jul 3, 2003)

SteveN said:


> I think you missed his point mate, I think he was saying what is YOUR reasons for it, and that seems to be "because the japanese men told me so", which is prob why he said it.
> 
> In my mind at least, leaving the exhaust one in place dont really make much sense to me, as that combined with the usual piston shape (even most/all jap forged ones are stock shape) will push the majority of the fuel/air mix away from the 2 places you want it for decent combustion, close to the sparkplug and exhaust valves, and in to the open space around the inlet valves.
> 
> ...


Its something I said, the basics do not change for a turbo charged engine. 

The quench area is there to cool hot spots and average the chamber temperature, it will not move the fuel away from the spark plug.

I remove the inlet quench area to improve flow and reduce hot spots, if someone has data to show it does not work I will reconsider, however for now I am happy to benifit from the R&D work carried in Japan by the top tuners and proven on race cars (GT500) and not try to reinvent the wheel.


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## paul cawson (Jul 3, 2003)

Worth a read Nissan Skyline Gtr Z Tune Head Photo


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## kismetcapitan (Nov 25, 2005)

well shit, if Nismo removed it on the Z-tune, then surely there must be something to it.

skimming the head, use of flat aftermarket pistons, and a 0.9mm head gasket. It should work. I just wonder what lower compression, say 7.5:1, will do to off-boost performance, such as puttering around town, etc.


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## SteveN (Aug 6, 2002)

kismetcapitan-

providing the engine has been specced (and mapped) for lower compression as a whole, rather than the usual spec engine with some low comp pistons in, i doubt you would notice a difference. in the various engines ive had like it, it wasnt really noticable, its not as if they got storming power pre-boost anyhow, and specced right you will have mega midrange torque for once instead of all top end like most.
if you are running lairy cams it might even drive a bit nicer once its done (if, like said, engine is specced as a whole).
only one way to find out really, its just a 2.6litre straight 6, nothing voodoo about the RB26.

7.5:1 is proper 2bar+ and well over 200bhp per litre on normal pump fuel and mild cams style compression tho IMO, bit of a waste unless thats the plan.
you want to of course run as high compression as you can get away with, but its that speccing engine as a whole thing again.

Might be interested to know in the good old unlimited boost days, the 2.6litre 4cyl turbo indycar engines that were nigh on 1000bhp only ran 260deg cams, and they only had 16valves and 4ports per side, not 24 and 6.


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## kismetcapitan (Nov 25, 2005)

well, I've got your typical 2860-5 (2530) setup - Tomei Poncams, HKS pistons at 87mm. I do push the boost though, so I'm thinking this would be a good move, plus allow me to use my old head and revive it into something better, instead of just binning it.


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## Rain (Apr 8, 2006)

Mine be gone too  funny, looks like the ones Sugino san did look cleaner than the Nismo ones in that pic :chuckle:


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## kismetcapitan (Nov 25, 2005)

unfortunately on my old head, I will have to remove both squish pads as they had suffered predetonation damage. I wonder if that will still work out with my particular engine spec....or let me comfortably run a little higher boost


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## EndlessR (Nov 6, 2007)

kismetcapitan said:


> unfortunately on my old head, I will have to remove both squish pads as they had suffered predetonation damage. I wonder if that will still work out with my particular engine spec....or let me comfortably run a little higher boost


How bad is the head? Photo?
I dont know your spec, but tuning is not guess work, its calculated.
We would recommend you consult your tuner before you decide......so many factors are involved...its not as easy as just removing the squish.


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## gibson (Feb 21, 2005)

old thread but why.... As det inflects most of us when the squish areas are in position so why not just say yep its a good idea to remove. Once boost exceeds high levels (take that as you will) det becomes very present ( yes i know fuel , mix, type and octane play a part ) but the squish zones are for a more effective burn to combat government regulations on emmision not for BHP/torque improvement... correct/ not correct.. I would like to know for sure also are japanese forged pistons flat or domed


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## GT-R Glenn (Nov 9, 2002)

This site has some pretty cool pictures

Nissan RB26 CNC Cylinder Head


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## R.I.P.S NZ (May 7, 2004)

Very nice pix indeed.

The only thing that would concern me is that although they can get every port and chamber identicle, its still only a copy of the port job they initially did and what results/power/flow numbers have they got to show that it is real good?

I'm not for 1 second saying what they done is no good, but there's no point cnc'ing something that is not as good as it can be,

Do they have any cars racing, or dyno numbers V boost?

Rob


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## freakazoid3 (Jan 19, 2008)

R.I.P.S NZ said:


> The only thing that would concern me is that although they can get every port and chamber identicle, its still only a copy of the port job they initially did and what results/power/flow numbers have they got to show that it is real good?
> 
> Rob


I was thinking the exect same thing, it might look nice, but thats not what it is about now is it? Maybe it IS a good job, maybe it isn't who will tell opcorn:


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## GT-R Glenn (Nov 9, 2002)

Im sorry , I cant answer that question.
If you click on ther link, then put the curser in the address bar and use the back key untill you get passed /components/ you can probably answer those questions yourself.
They appear to know what they are doing, who knows ....


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## gibson (Feb 21, 2005)

I like the look of those pics..

I can't answer either BUT then again why would anyone go to such lengths (CNC)if there was not the advantage.Squish to my knowledge is only for better burn in standard form to combat GOV regs on emmisions 

Do Those ports look a little to shinny ??

Cheers for the pics Glenn


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## nozza1 (Jan 13, 2007)

Yes l know old thread, but would like to know what would be the effects of removing both the squish pads.

Is it quite evident when you start the car and drive, or is it only felt when pelting it through the gears.

Or have l missed the point altogether.

Need some info on real life/real time differences between the two.



naz.


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## roadie (Feb 6, 2006)

Well, 1 1/2 years later and now a bigger turbo (gt4094r) my car running 125lbs/cylinder is still going strong. The boost gauge is burried on a regular basis past 2.5 bar.
Off boost performance is acceptable, on boost is amazing....Knock values max around 30 using pump gas.


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## bigmikespec (Sep 5, 2008)

R.I.P.S NZ said:


> RB30, Flat tops, no squish, C/R just in the 7's (depending on gasket), comb cc aprox 69cc.
> 
> Rob


Digging an old thread.

Rob, what head gasket are you using and deck height do you have to acheive a 7:1 compression ratio? 

Cheers, Mike


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## bigmikespec (Sep 5, 2008)

Come on Rob! Honest discussion here... no bible bashing.


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## R.I.P.S NZ (May 7, 2004)

I've never done a 7:1 motor


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## wildboy (May 2, 2010)

I assume it was closer to 7.99:1 lol


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## bigmikespec (Sep 5, 2008)

_RB30, Flat tops, no squish, C/R just in the 7's (depending on gasket), comb cc aprox 69cc. 

Rob_

Ok, going by a post I had assumed you had done it. Have you done any heads with the combustion chamber squish removed and custom piston to bring compression back up?

Reason I ask is that I want to do exactly that above... cheers.


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## R.I.P.S NZ (May 7, 2004)

bigmikespec said:


> Ok, going by a post I had assumed you had done it.


"just" into the 7s, as in 7.9:1 or higher depending on gasket thickness.



bigmikespec said:


> Have you done any heads with the combustion chamber squish removed and custom piston to bring compression back up?


No.


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## bigmikespec (Sep 5, 2008)

Ok thanks mate.


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## jasaircraft (Feb 15, 2009)

Interesting thread, nismo must have removed them for a proven reason, not sure which though.


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## bigmikespec (Sep 5, 2008)

jasaircraft said:


> Interesting thread, nismo must have removed them for a proven reason, not sure which though.


High (too high) squish velocity!


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## jasaircraft (Feb 15, 2009)

bigmikespec said:


> High (too high) squish velocity!


Hi, could you elaborate on that please?


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## bigmikespec (Sep 5, 2008)

jasaircraft said:


> Hi, could you elaborate on that please?


As the mixture is compressed, it is "squished" into the combustion chamber by the piston travelling upwards. The squish pads act to confine the mixture to the middle of the combustion chamber, the velocity of the mixture as is passes the squish pads is the squish velocity. 

If you want to make lots of power reliably then get rid of the squish pads and no quench (i.e. piston further away from head at TDC).


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## jasaircraft (Feb 15, 2009)

I wonder though why the nismo guys remove the intake only.. Having in mind its a 500~hp engine... Unless the picture shows the beggining of the process an they removed both...


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## kismetcapitan (Nov 25, 2005)

jasaircraft said:


> I wonder though why the nismo guys remove the intake only.. Having in mind its a 500~hp engine... Unless the picture shows the beggining of the process an they removed both...


IIRC, on the Z-Tune only the intake pads were removed. I could be wrong though; I'm getting old and senile 

seeing this thread come up brings up some memories that really bends me out of shape. As I had a propensity to really push my engine, the squish pads were a point of worry due to the sharp edge on them - any edges can be a source for pre-ignition. I had blown an engine, melting all six pistons, and in the rebuild, there was a bit of shallow pitting in the squish pads. I suggested that we machine out the squish pads to create a completely smooth combustion chamber. The engine builder looked at me as if I were on crack and suggesting the most ridiculous idea ever. So I had to buy a new head, and my "useless" head somehow managed to disappear.

Either I was cheated, or they were completely stupid and threw out the old head. We have all seen how some precise dremel work and measuring combustion chamber volume with hot wax and a syringe can produce excellent results. If only if only if only!! if only I knew then what I know now about GT-Rs....


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## AleX-34 (Mar 6, 2013)

Does anyone have the patience to tell me more about squish??

I do now about how heads work but not about squish.

Thank you. Alex.


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## Rain (Apr 8, 2006)

Alex there are lots of articles about the different thoughts behind the squish/quench pad areas. not all in relation to RB engines, but the theory is basically the same. 

just do a google search on the topic, and you will get the basic idea of why they are there and why some people remove them.


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## AleX-34 (Mar 6, 2013)

I did and have red about it in the last hour, as you say I was thinking that it was something special about the RB... Thank you for your answer


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