Low comp or high comp pistons with a turbo

[Lazarus]

I said you should use lose compression pistons for turbo. Some guy said you should use high compression pistons:

"thats pretty low for an allmotor honda.

this guy wants around 250whp with a turbo. now if you werent living with 1982 turbo knowledge you would agree that higher compression will net him more power for every pound of boost he is running, and lead to much better response, better off-boost performance, and less lag.
our cars are already low compression, if he runs a gt28rs with 10 or 10.5:1 compression he will have a much better running car than 8.5:1.

if he needs the safety net of lower compression if he wants a ton of useless fwd hp then go with 8.5:1. but if he's building his motor for lower power applications and a nice fast street setup, then going with an allmotor style build will net him better results.

turning car into fireball at 10:1 ? what a joke. i guess all of those turbo celicas and itr's on stock motors and turning into fireballs left right and centre...."

[hjr]

all motor means naturally asperated, in which case high compression is desirable.

turbo motors generally like lower compression which you seem to know already. maybe this is a simple mistake in communication.

[Lazarus]

Originally posted by hjr
all motor means naturally asperated, in which case high compression is desirable.

turbo motors generally like lower compression which you seem to know already. maybe this is a simple mistake in communication.

No this guy just doesnt know what hes talking about. Scientifically you want lower compression pistons because they cause less heat.

[tsi_neal]

scientifically you want less compression because its less likely to detonate due to cyl pressure, nothing to do with heat.(heat causes detonation but not in this case) The heat of the charged air is the exact same weather its 15psi going into 8:1 or 11:1 however the dynamic compression of the 11:1 and 15psi will likely be much higher than any pump gas can handle...

It has been proven many times that when dealing with a specific grade of gas that lower compression will create better power on boost. but if you run the gas to handle the boost and higher compression than the higher compression will make better power. Example you run a turbo that spins up to 20psi before is looses all efficency and want to make the most of this and have vp116 gas to run all the time. then id consider 11:1. but if your stuck with 91 or 94 pump then id probably consider 8.5 or 9 to be the max, but it will be faily volitile at that. where 8:1 youll run that 20psi on pump pretty easily.
OR
take a turbo that will spin 25psi and you want to make the most of that youll have to drop the compression

Remember this 1 point in static compression ratio is worth ~5% gains and this is regardless of boost or not. but the extra psi you may be able to run with lower compression SHOULD make more power than compression. Its all a trade off, depends on specifics of the cam profiles, head design, turbo choice, intercooler choice, etc. The safe bet is drop compression tho

[streetarab]

i thought it was basic knowledge that high compression and lots of boost break things...
also that its not very practicle

[legendboy]

250whp doesn't requrie scientific examination of engine compression. Run the motor stock. Any twin cam honda motor will do 250who no problem.

On a side note. You can run more boost, make more power, safer on pump gas with low compression. You don't see any high compression oem turbo setups do ya.

[blizare]

couldn't you say, for the sake of argument;

a 250 whp HI comp motor would need 5 psi of boost compared to a low comp motor running say, 10 psi to equal the 250 whp.


there, that was easy.

Maybe not %100 correct but thats the idea.

[ninspeed]

Originally posted by legendboy
250whp doesn't requrie scientific examination of engine compression. Run the motor stock. Any twin cam honda motor will do 250who no problem.

On a side note. You can run more boost, make more power, safer on pump gas with low compression. You don't see any high compression oem turbo setups do ya.

not on pump gas anyways....

[Hollywood]

Originally posted by legendboy
250whp doesn't requrie scientific examination of engine compression. Run the motor stock. Any twin cam honda motor will do 250who no problem.

On a side note. You can run more boost, make more power, safer on pump gas with low compression. You don't see any high compression oem turbo setups do ya.

8.5 for the win!

Major automobile manufactorers around the world can't be wrong here. Seriously.

[tsi_neal]

Originally posted by Hollywood


8.5 for the win!

Major automobile manufactorers around the world can't be wrong here. Seriously.

Im gonna try 9.0:1 and 20psi on pump gas on the motor im building for the talon. Should be interesting.

[Daxin]

Neal, your car will not like it very much. You'll get too much knock!! This is from 14psi.

[awdterror]

Higher compression DOES NOT spool the turbo quicker, period, amen. Mr. Evans tested a GSR, dropping from 10:1 to 9:1 compression resulted in 2-4 WHP off boost (not even noticeable) and about 12 peak WHP under boost. He posted the boost curve, the spool up is EXACTLY the same.

I'll take 9:1 compression and a buttload more boost for a street motor, thank you kindly. The benefits of lower compression outweight the downfalls.

http://www.boosted-hybrid.com/viewtopic.php?t=1612

There's the 10:1 vs. 9:1 post.

And finally, for what it's worth, if I were running some mondo huge turbo on my honda like a GT35R or a T3/T67 I would most certainly run 10:1 or 10.5:1 compression. 10.5:1 84MM LS/VTEC + T3/T67 will clear 400 WHP at 10, maybe 11 psi. Then when you add race gas into the mix you'll make a bit more peak horsepower than what the low comp setup would make.

[Teach]

All right, if you are really curious about what to run for a static and dynamic compression ratio, go to one of the manufacturer's websites. They will give recommendations for static compression usually with a chart for varying levels of boost and the resulting dynamic compression ratios.

As well they will have their recommendations for fuel octane requirements for the various ratios. If you go too high of a dynamic ratio, you will detonate severely and rattle the bottom end losing horsepower and ultimately destroying the main bearings.

If you are having trouble finding charts, BDS and Weiand offer this information for sure on their websites.

FYI anything above an 10:1 compression with a turbo or supercharger is normally reserved for alcohol injected engines.

And finally, Your performance will be much better with lower compression and higher boost.

[2.0turbo]

Originally posted by tsi_neal


Im gonna try 9.0:1 and 20psi on pump gas on the motor im building for the talon. Should be interesting.

What turbo are you going to run? I ran my evo316g on my 9:1's at 19 pounds, a big fat 0 on the knock count (well most of the time, highest was 7 which is still 100% safe) and rediculously fast, at traction levels anyhow. As far as I know, your talon won't knock unless it starts to get too hot. I run a supra side mount with hard piping and it does the trick.

[tsi_neal]

Originally posted by 2.0turbo


What turbo are you going to run? I ran my evo316g on my 9:1's at 19 pounds, a big fat 0 on the knock count (well most of the time, highest was 7 which is still 100% safe) and rediculously fast, at traction levels anyhow. As far as I know, your talon won't knock unless it starts to get too hot. I run a supra side mount with hard piping and it does the trick.

Evo316g, FMIC (overall similar size to the supra unit), MAFT, etc
Its looking more and more like ill end up with 2g pistons tho, for the extra 5hp off boost i dont think its worth the extra cost to me at this point.

Stock Evo3's run 8.8:1 and 17 or 19psi (not surewhich) to make a conservative 275 or so. I have no doubts that anyone that knows what they are doing could push much higher psi #'s (with the right turbo) and be totally safe

[2.0turbo]

Originally posted by tsi_neal


Evo316g, FMIC (overall similar size to the supra unit), MAFT, etc
Its looking more and more like ill end up with 2g pistons tho, for the extra 5hp off boost i dont think its worth the extra cost to me at this point.

Stock Evo3's run 8.8:1 and 17 or 19psi (not surewhich) to make a conservative 275 or so. I have no doubts that anyone that knows what they are doing could push much higher psi #'s (with the right turbo) and be totally safe

I was watching some tuner show where they do up imports and they had an evo 8 on their. They put in hks cams, a fuel cell for race fuel, a boost contoller hooked up with the switch to change gas and some suspension mods. Stock the car did 19 psi, on race gas they ran 26. Everything else stock. The car gained over 70 hp. The evo316g is in my opinion, the best all around turbo for our motor. I love how fast it spools and how hard it pulls all the way through the rpm.

Anyhow, my answer to high or low compression is neither, meet in the middle at 9 to 1.

[Blunt_osoquik]

i always thought that when using a turbo it was better to have low compression
so wut some of u guys r saying that when running a turbo set-up and race gas its better to have high compression????

[boostless]

race gas just wont detonate as much in comparison to pump...so u can get higher compression that way with race gas than with regualr 94 or 91 octance, for calgary i wouldnt think contantly driving with race gas would be practical tho...

[legendboy]

Originally posted by boostless
i wouldnt think contantly driving with race gas would be practical tho...

unless your name is rage2