# Exhaust: Is Bigger Better? 90mm vs 100mm (With Dyno Graph)



## Nocturnal (Nov 5, 2004)

I have been running a 90mm pipe Stainless Steel Exhaust ever since I had the car.
A few weeks ago I pick up a very nice used HKS Titanium Race Series Exhaust with a whopping 102mm pipe. The previous owner told me that his mapper wanted him to get a 3inch exhaust because the mapper likes the car to have a bit of backpressure.

So I am wondering, is bigger honestly better? Or is it just something to show off?

I have a 600bhp engine and most tuners in Japan suggest at least 90mm pipe, so will having a 102mm pipe help? Or will the reduced back pressure make the car worst off? 

Since I just got my ECU remapped for a run at this weekend Marham Trackday (going to go for some high speed run), I thought it will be a chance to check everything out along with getting an answer to this ages long question.

I took my car to IA Motorsport in Sittingbourne, who just purchase a brand new Dyno Dynamic Dyno. 
Ian who is the owner decided to help me out and give us an answer to this long time question while checking out the fueling to make sure everything is okay. 

The Test: (All tests will be done in 4th gear, with 2 wheel drive.)
1.We will run my car with the 90mm exhaust set to my max boost level (1.8Bar)
2. We will change the exhaust from the 90mm to the HKS Titanium Race 102mm Exhaust.
3. We will run the car again with the same boost controller to observe any changes to boost level and fuelling.
4. We will adjust the boost controller back to the same level as the run with the 90mm exhaust to see if power or torque changes anywhere.

I just got some picture of the HKS Titanium Race Exhaust and the base line dyno graph. I will be installing the new exhaust in the next few days and will post the new dyno graph as soon as I get it.

* some pictures of the exhaust *

BIG 102mm pipe exhaust! :bowdown1:









Join the 102mm Stainless Steel Pipe into the 4inch inlet Titanium Muffler









115mm Exhaust Tip... which is only slightly larger then the pipe itself... :chuckle:









*DYNO GRAPH*
Flywheel Power and Torque









Boost and AFR









If anyone got any opinion of what the result will be, I am interested in it.


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## SteveN (Aug 6, 2002)

Ive seen testing on 550bhp engines (Cosworth YB) going from 3in to 3.5in, and they needed to add about 10% more fuel when live mapping on road to compensate for the extra flow because the 3in was so restrictive. 
But that was with internal wastegate, i presume your running a screamer, which means less gas in the main sys.

But at same time ive seen cars get a true 900bhp on a bench dyno with a 3inch system.

Providing the true power (ie not guessed flywheel figs) is high enough to warrent it, that nothing else is restricting it MORE than the exhaust, and the mapper knows how to compensate for it, then yeah, you should get more.

But if the opposite to any of the above is true, you may well get bugger all.


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

The ti one will sound better not because of diameter but because of material type.

Power,
The way I understand it
I think all things being the same the only thing that will change will be the torque curve spool time and its unlikely you will measure it.
it makes Xhp @ xrpm with x boost on 90mm

I dont believe the 90mm will be a restriction at that power level.
It might make more boost at the same settings, but when you wind it back to 1.88 all things will be equal.

Thats my prediction


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## Cris (Sep 17, 2007)

I remember reading about this discussion with regard to evos. I think that the chaps name was the dentist and is/was a well known mapper.

My memory of it is a little misty but I think that some of his findings were odd. I think something along the lines that the smaller pipe (3.5" to 4.0" so only small relatively) produced more torque though the top-line power figure was the same. I think the conclusion was that the narrower pipe kept the gas temps higher so the gas velocity was higher.

I think there was even discussion about tapering exhausts as being a good thing.


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## moosedoog (Jul 13, 2006)

he he, my prediction for what its worth,

touch more power at the top, touch later torque curve.:nervous::nervous::nervous::nervous:


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## SteveN (Aug 6, 2002)

Cris- The trouble is, saying smaller is better, bigger is better, etc etc is pointless and totally missleading if you dont know the power of the car, the capacity, the revs, etc etc etc.

Luckily on a turbo car, post turbo exhaust stuff is less important, ie as long as its not way too small, you will be ok and any advantages or disadvantages will be negligable, and far less noticable than poor mapping etc which is way more common.

Turbine wheel diameter compared to downpipe size seems to make a suprising difference though, regardless of a cars power.


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## daytona (Jun 28, 2005)

Hi Jerrick, glad your're pleased with the pipe, looks like you've been busy with the Autosol!!:smokin: Looking forward to seeing the result of your experiment, should be interesting, as i said when we spoke this pipe was supporting approx 800BHP when i first got my car. Hope to meet up at Brands or Lydden at some point in the future.


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

> 3.5" to 4.0" so only small relatively


Ummm ....
9.62 sq in vs 12.5 sq in .....
Thats about 26% matey ...


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## V1H (Aug 30, 2001)

bit cheeky, but any chance of adding the ats to the test?


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## kismetcapitan (Nov 25, 2005)

I've been playing around with adding backpressure to my car with an ATS. I got pretty good results with it in terms of off-boost drivability. But what matters is when it's on full power  It loses some.

I clipped the spring so that the valve opens under much less pressure (the stock spring requires 1 bar of boost from 2860-5s to open up fully), but felt that I had lost fast spooling under part-throttle acceleration. And if that doesn't sound important, keep in mind that full-throttle on a 600bhp car is NOT POSSIBLE in traffic!!

So far, I had enjoyed my car the most when I had the ATS in, but had removed the valve and spring. The ATS frame, without the valve and spring, nevertheless presents some smallish degree of restriction, maybe 2 or 3 square inches. The mass (the ATS is a heavy piece of kit) added to the tip did make the exhaust note more pleasant and significantly reduced droning.

btw, my exhaust is a wide open 4" Apexi N1 with 115mm exit.


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## Nocturnal (Nov 5, 2004)

So far there is alot of mix prediction.
I should of done a pole on this. :chairshot

Should be able to see the result this weekend, but my old 90mm exhaust have a custom decat pipe, so that mean I have to modify the new Titanium Exhaust to fit the car. Lucky it have stainless steel piping so I can do some welding. 

I can get the wheel power... BUT... Since the weather station is on (a function in the dyno), this will keep the bhp on equal ground since it is not done on the same day. I am not sure if the "weather station" is calculated when displaying wheel bhp, but I'll get the graph later anyways. :thumbsup:

As for the ATS, I don't have it anymore... Sold it ages ago. :wavey:
Toby, did the clipping of the spring let the turbo open up earlier under WOT? 
I did think about medium throttle response, but then this test is only able to show full throttle response. 
If clipping of the spring in the ATS allows the turbo to spool up earlier under WOT, then it is worth while as you have less chance to melt another sensor. :chuckle:


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## driftboy (Jan 14, 2006)

Ive got the same system "HKS Drag Muffler Ti" to correct you on its title (sorry lol) to run on my own GTR, so im very interested in seeing the difference in results... 

Although as said above i really dont think there will be much eather way from previous tests ive seen on friends cars, the HKS Ti 102mm system which is used on a friends R34 GTR against numerous Trust, Blitz, Apexi 90mm systems on similar speced GTR's...

Si...


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## kismetcapitan (Nov 25, 2005)

yep, it opens up at 0.5 bars or 4000rpm, whichever comes first. I clipped two complete rings.

The melted sensor wasn't melted btw. It HADN'T BEEN TORQUED!!!! I found that out when I removed it....with my fingers :runaway:


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## Lamb (Sep 25, 2003)

Im running the same exhuast so would be curious to see the results.

Sounds the nuts!.....aslong as you like LOUD!!.......lol


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## Nocturnal (Nov 5, 2004)

We will find out more Saturday. Gain or Loose, it is just a bit of a experiment.

It will probably be more quiet then what I got right now. My looks like an Apexi or similar, with the center silencer removed... and it is quite noisy.... 
So Titanium will probably take some of the "rumble" away. :thumbsup:

Saying that, I always thought it was call HKS Titanium Race Exhaust... probably it was name in America. 
HKS USA Titanium Racing Muffler Exhaust

By the way, got this picture today; thought it looks pretty cool.


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## Cris (Sep 17, 2007)

SteveN said:


> Cris- The trouble is, saying smaller is better, bigger is better, etc etc is pointless and totally missleading if you dont know the power of the car, the capacity, the revs, etc etc etc.
> 
> Luckily on a turbo car, post turbo exhaust stuff is less important, ie as long as its not way too small, you will be ok and any advantages or disadvantages will be negligable, and far less noticable than poor mapping etc which is way more common.


I was trying to draw an example that bigger isn't necessarily better the actual numbers don't matter. The reference was one big exhaust vs a slightly smaller one. As I suggested there were little/no power gains yet more torque lower down the rev range with the smaller exhaust.

From this I would deduce that once you get beyond a certain diameter there are negligible benefits to going bigger and in fact you may find the exhaust is affecting other areas by (in effect) having having more back-pressure at low RPM.

The fact that WRC cars don't run stupidly big exhausts suggests to me that there's something in this.


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## Cris (Sep 17, 2007)

GT-R Glenn said:


> Ummm ....
> 9.62 sq in vs 12.5 sq in .....
> Thats about 26% matey ...


3.5" is still a good sized girth and by no means small in absolute terms.

If anyone tells you otherwise you need to stop hanging around with the wrong of women!


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## David (Apr 25, 2003)

IMO you cant judge this without optomising fuelling and ignition at WOT.

In my experiance larger back pressure (which the 90mm system should have) causes det.

If the engine is det limited on 90mm and not running MBT you should be able to run more ignition to get closer or to MBT with the 100mm. Hence more power, once the ignition has moved fuelling will probably need correcting to.

Interesting test


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## Nocturnal (Nov 5, 2004)

Right, I am going to modify the test a little bit due to a lot of different request.

On Saturday (we will try) to run a dyno again.
We will post up 3 graph on each dyno (Flywheel HP/Torque, Wheel HP/Torque, AFR/Boost)

1. I will set the boost to around 1.6 bar.
2. Dyno the car with 90mm exhaust at 1.6 bar boost with Blitz SBC-ID.
3. Change over to 102mm exhaust
4. Dyno the car with 102mm exhaust at 1.6 bar boost with the same setting.
5. Setup the car to 1.8bar boost.
6. Dyno the car at 1.8bar of boost to see if anymore power made.

I will also get the wheel hp for the Dyno that is posted.

I am changing it down to 1.6 bar because I am not sure if changing the exhaust will cause the car to overboost and hit the fuel cut in the ECU. Putting power down to 1.6 bar will room for the result of the exhaust to show up.

I hope this will give us more understanding on what the exhaust will do, and it is really just a bit of fun for everyone.


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## SteveN (Aug 6, 2002)

Cris said:


> The fact that WRC cars don't run stupidly big exhausts suggests to me that there's something in this.


You not noticed WRC cars run 300 horse and therefore a 2.5inch system would be more than good enough?

And they run bigger than that.


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## Cris (Sep 17, 2007)

SteveN said:


> You not noticed WRC cars run 300 horse and therefore a 2.5inch system would be more than good enough?
> 
> And they run bigger than that.


They run a little more than 300 but that was exactly my point. If there was more power to be had they'd run bigger exhausts. They also have a very odd power curve.

Are you suggesting that cold(er) exhaust gas would present less back pressure than warm gas?


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## GarethK (Aug 29, 2004)

I agree with SteveN on this one - power is airflow, and they are restricted. Their development is all about getting to the max airflow as fast as possible, and maintaining it for as long as possible.

That said, I don't expect it to make a huge difference in this test - the silencer design and bends etc will also make a difference so you won't be able to put it 100% down to pipe size!


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## moosedoog (Jul 13, 2006)

SteveN said:


> You not noticed WRC cars run 300 horse and therefore a 2.5inch system would be more than good enough?
> 
> And they run bigger than that.


iirc wrc cars also run a 34mm restrictor pre turbo and as mentioned are a maximum of 300bhp so are set up to be torque monsters.


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## WIT BLITZ (Apr 1, 2007)

Hmm interesting


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## SteveN (Aug 6, 2002)

Cris said:


> Are you suggesting that cold(er) exhaust gas would present less back pressure than warm gas?


Im suggesting your examples are pointless.
I didnt even mention exhaust gas temps.
Simple fact is WRC cars dont need massive exhausts as they dont have massive amounts of exhaust gas as they are fairly low revving 2litre engines with slightly over 300bhp.
But we are talking about a bigger engine with more revs and loads more power.


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## Nocturnal (Nov 5, 2004)

Okay, some bad news update guys.

Looks like my engine have receive some terminal damage at RAF Marham today... 
It have lost all compression it seems, so will have to strip it down and see.

Mainly seems to be the oil temp that kill it, even with an oil cooler temp shot over 115C.

So looks like this test will have to be delay for quite sometime. Sorry guys.


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## kismetcapitan (Nov 25, 2005)

that's terrible to hear - what were the symptoms? or did the car just suddenly die?


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## Nocturnal (Nov 5, 2004)

Car suddenly die as I enter the pit, very hot oil temp, and looks like zero compression across all 6 cylinder... not sure what is wrong yet...but feel terminal to me.

Anyways, always wanted a RB30.


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## Madden (Nov 14, 2004)

Shit man hope its all right.

I had oil temps above 130 going round silverstone and all was fine. 

Cambelt gone?

Good luck mate


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## freakazoid3 (Jan 19, 2008)

Hopefully it isn't anything major :nervous:


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## moosedoog (Jul 13, 2006)

thats bad news mate but oil temps of 115 isnt into terminal failure territory is it..:nervous::nervous: oilman reckons the silkolene pro race stuff is fine up to 140.. i also believed that 110 is a good oil temp cos it evaporates the water out of the oil:nervous::nervous::nervous:


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## steveyturbo (Dec 24, 2006)

I have seen one of our competition cars running 130 degrees for a full season, 115 degrees is not high at all.Any good synthetic oil can cope with temperature


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## Nocturnal (Nov 5, 2004)

Well guys,

The rebuild should be underway soon, I'll start a new thread and post some picture. Sorry I couldn't get you guys an answer to the exhaust. 

I am changing the specs, including using a new Turbo that no one have used before. Should be interesting as it will be very similar to a GT4088R, but should spool slighly earlier. Just trying out new things... but again, not sure what the result will be.


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## DarkChild (Nov 5, 2004)

So is bigger better? Why not just run a straight pipe without a muffler and no restrictions at all if that's the case? 
Can anyone with the knowledge shed some light on the matter? 

Cheers.


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## Nocturnal (Nov 5, 2004)

DarkChild said:


> So is bigger better? Why not just run a straight pipe without a muffler and no restrictions at all if that's the case?
> Can anyone with the knowledge shed some light on the matter?
> 
> Cheers.


That is done alot on drag cars... but saying that... I wonder if there is a down side of running a bigger exhaust on turbo car... such as OFF BOOST driving....


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## hpt_simon (May 20, 2006)

sorry to hear about the car, but sounds like you have a good list coming up 

was looking forward to this thread ;(, 
anyways just wanted to mention, someone posted aerlier WRC cars didnt run big exhausts, ...well y used to buil wrc cars and i can tell you, they did run big, and no where near smaller than 3", i could even say closr to 3.5" and yes only 2 liters, but they blow constantly at over 2.4 bar so that is quite alot of air to get rid of ....


simon


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## StretchGSK (Oct 24, 2006)

Lamb said:


> Im running the same exhuast so would be curious to see the results.
> 
> Sounds the nuts!.....aslong as you like LOUD!!.......lol


I am very tempted by one of these for sale here.
What sort of Db do you think it is? Is it ok cruising on the freeway?
I guess I am asking if the cops ping you every time?
Any chance of a vid?

Lastly, do people think a re-tune will be required on a stage 1 GTR going from a 90mm pipe to this?


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## kismetcapitan (Nov 25, 2005)

I don't think I have any problems with off-boost driving, and I'm 100mm from the Y-pipe and 110mm at the exit, and there's nothing in between.

I did do an interesting mod. I took my Apexi ATS, and first cut the spring so the valve opened a lot easier. The, I just took the valve out altogether, leaving only the relatively heavy valve housing. It obviously no longer does any silencing, but for some reason it doesn't drone at 3000rpm anymore either!! Very nice!


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## Totalburnout (May 15, 2005)

Just realised I have this same exhaust, sweet 
:thumbsup:


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## Philip (Jan 17, 2002)

Me too.

When I had the car in London it seriously annoyed the neighbours.

Phil


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## Nocturnal (Nov 5, 2004)

Its a good exhaust... can't wait to get the car back on the road.
The head is the issue and it is almost finish being rebuild. Will bolt everything back on to see how much a bigger exhaust and match ported head will gain on the dyno. I also took out all the squish area, so that will be interesting.

After that, I got a new turbo that should do about the same power, but will give at least 1000 RPM more response. I hope everything will be done before christmas.


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## Philip (Jan 17, 2002)

Quick question for people with this exhaust - is the tail pipe diameter 115 mm and not 120 mm?

Don't have the car here to check.

Thanks,

Phil


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## lightspeed (Jul 11, 2002)

115mm and the ATS fits just fine


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## Nocturnal (Nov 5, 2004)

115mm Apexi titanium slide finisher fit no problem also. :thumbsup:


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## eeiko321 (Oct 31, 2008)

holy S$$$$$#

thats gonna be LOUUUUUUUUUUUUUUD!!!!!!!!!!!!!!!!!!!!!!!!!!!!


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## Philip (Jan 17, 2002)

Thanks.

Phil


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## StretchGSK (Oct 24, 2006)

Any update on this Nocturnal?

Can the guys that have this exhaust comment on just how loud it is with and without the bung?
Totalburnout/Philip/Lamb any chance of a vid?

Still interested to know if people think swapping from a 90mm to 100mm cat back will require a retune?

Thanks


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## Nocturnal (Nov 5, 2004)

Engine is still apart. The test won't be accurate now as not only will I have a bigger exhaust, but I will also have different compression ratio and match ported head, so the test is out the window really.

For what I had done on my car, will need a remap for sure.

Sorry that I couldn't give a better result


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## Dandism (Dec 13, 2004)

Just my 2p, as I'm also interested how this turns out as I'm actually more of less doing the opposite and fitting a 3" exhaust (titanium Rheinhart one) instead of my 3.5" Kakimoto R. I didn't realise the Ti exhaust was smaller diameter tubing when i got it but I think it may actually flow better. 

Something that everyone has been ignoring is that the whole system inc elbows, downpipe etc isn't matched. When I ran a pressure drop calc on a 3" downpipe outlet into a 3" decat into the 3" Ti system it actually flowed better than having the 3.5" on the back as the 3 to 3.5" step created a bigger pressure drop than not having one at all. This was using a gas flow equivalent to around 500bhp.

Unless the pipe diameter is choking the flow due to very large volumes of gas (ie big HP) then the straighter and smoother the system appears to make more difference to the overall system pressure loss.

There is a pretty cool pressure drop calc tool online if you google it.


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## Nocturnal (Nov 5, 2004)

You also have to remember that exhaust gas flow slower as it cool. That is why you can have smaller elbow and downpipe than exhaust as the gas is a lot hotter as it leave the turbo. 

As far as pressure drop, I think I have a HUGE one as my downpipe goes from 75mm into 102mm... thats a big jump in size.

I noticed my old Reinhart exhaust are being move from car to car... 
Be sure to get some video and sound clip when you have it on.


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## Dandism (Dec 13, 2004)

Just come back from Ron's having had the Rheinhart exhaust fitted together with a Titanium downpipe that I picked up from Hongkong (Ron wasn't that impressed with this but the welds looked ok, it fitted and weights 2kg, the primaries also look bigger than the stock one).

Loving the sound, its actually quite a bit less noisey than the Kakimoto R but sounds awesome, haven't had chance to drive it hard yet, but the sound is a nice smooth purr ar low rpms rising to a large superbike at higher revs.

Most importantly its killed the boom that was at 2-3k rpm under cruise conditions so the car is a lot nicer to drive on motorways and dual carriageways, which i do back and forth to work. Looking forward to giving it a proper thrashing, and will try and get a vid clip sorted in the new year.


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## tyndago (Oct 24, 2002)

I have run an exhaust cutout on GT-R's before. Pick up about 10 wheel horsepower. I have to find the dyno chart from this one R34 that I did before and after on. 

Most of the real restriction is between the exhaust port and the turbine in a turbo car. After the turbine wheel. Freer flowing is better. 

YouTube - R34 GT-R Electric exhaust cutout


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## tazsgtr (Aug 10, 2006)

Quick question for anyone who might know. Would backfiring damage something like the Apexi ATS?


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