FI & lower compression pistons

[PaulMP3]

I plan on ordering my piston/rods shortly, but i wanted some opinion on what compression to go with... especially from those with protege running lower compression. I noticed a handful of guys are staying with stock compression and some have went lower. Whats side effects have you noticed running lower compression.. power/mpg/other??? I dont want my new pistons to hold me back on power goals... as of my current plan i just want to be able to max out my current turbo set up for maxium power, without swapping turbo's, or IC's. so probally between 15-20psi out of my t3 super60.

 

[Kooldino]

I'm basically staying w/ stock compression so the car runs just like stock when I'm off boost, and I get good off boost driveability and fuel economy.

 

[YP5 Toronto]

paul..I am running 8.5:1 compression. I have not seen a drop in my fuel economy and the off boost performance "feels" the same as stock compression.

Remember...I was running stock compression with the Hiboost kit and I am running 8.5:1 now with the hiboost kit...so I do have a reference to work from.

 

[InsidiousMSP]

I'd definitely go with 8.5:1

 

[bruce95fmla]

I was reading the corky bell book, and it seems on par with what a lot of people are saying ... Lower compression may allow you to run somewhat higher boost, but higher compression will give you better off turbo response (acceleration). I would say that I would stick with the stock compression (forged internals) or even raise it to 9.5:1 for even better low end and the forged internals should give you the strength to withstand the boost.
Just don't forget your tuning. That will make or break you.(from what I am reading)
------------
I am staying at my stock internals 9.0:1 in my protege.
Bruce

 

[jred321]

8.5:1, enough off boost that you won't notice any difference, but it will let you run more boost safely. you don't buy a turbo for the low end performance, you want to be able to push it up top. 8.5:1 will let you do that and won't kill drivability. raising it is ludicrous

 

[PaulMP3]

thanks for the input everyone...

anyone running 8:1????

blah.. i dont care much about low end

 

[seatbackfurther]

srt-4's are running 8.1:1 ... don't think anyone one here has gone lower than 8.5

 

[jred321]

thanks for the input everyone...

anyone running 8:1????

blah.. i dont care much about low end

1g dsm's run 7.8:1. if people upgrade their internals they usually go with 8.5:1, which is the 2g dsm stock compression ratio

 

[igdrasil]

I would do higher compression, but thats just me.
Once you have engine management, you dont need to run lower compression....it sucks to spool the turbo too much to get some decent power.

My friends 323 FSDE engine has 11:1 pistons on the engine running very high boosts, no problems.

 

[jred321]

i was looking on dsmtalk and found this to be pretty informative explaining the whole situation. it is a bit more complicated, but this gives some basics

Compression vs Boost
By MattC on the I-Club

Higher static compression creates more power throughout the rpm band, but it'll lower your maximum allowed boost before the onset of detonation. Boost is worth way more power than compression, because boost raises your compression and your total air flow at the same time. With the down side of, when you're not on the boost, you have slightly less power.

Effective Compression Ratio = static compression ratio x (1 + boost/14.7)^1/2

For a car running 8.5:1 pistons and 18psi(~max on pump gas)
8.5 x (1 + 18/14.7)^1/2 = 12.67 ECR

If you run 9.0:1 pistons and want to maintain the same 12.67 ECR (~max on pump gas), you'll have to lower your boost to: 14.4psi
[(12.67/ 9.0)^2 - 1] x 14.7 = 14.4 psi

So you have to run 3.6 psi less boost to maybe pick up a tinny bit of bottom end. Or to take it even further for 9.5:1 you can only run 11.4psi. I'll tell you right now that the difference between 11.4psi and 18psi is huge. And at some point (12.67:1 in this case) you can run no boost and be maxed out on ECR for pump gas. And how fast is a N/A car with 12.67:1 compression, ask the Honda boys running 15's.

Of course you get to a point where you don't want to go the other way too much ether. A 6.0:1 car isn't too much fun on the street, but it can run 50.8 psi boost on pump gas. But the kind of turbo that could support that much boost would never spool up. You have to find what's right for you, but 8.5:1 is what most of the DSM guys like. My friend just built a motor for his DSM with 9.0:1 JE's and he is quite upset to find that he's getting lots of knock running 16psi. While lately in the cold weather, I've been running 20psi (8.5:1) on pump gas w/o any knock. And I drove his car and the difference in bottom end grunt isn't even noticeable. And his top end performance is sad compared to mine.

I'd be real interested to hear what you guys think of this stuff.

Kevin 90GSX


That basically covers all of it.

http://dsmtalk.com/forums/showthread.php?s=&threadid=56810&highlight=compression+ratio
the whole thread

 

[YP5 Toronto]

good find... i read something extremely similar.

 

[jred321]

a more complicated, but also more in depth, version

I don't even know where to start...

OK, any combustion engine converts chemical energy into thermal energy and the thermal energy into kinetic energy. How well the engine is able to convert the heat (thermal energy) into power (kinetic energy) is quantified by the engine's thermal efficiency (which is between 20-30% on most modern engine0. The higher the compression ratio of the engine, the higher the thermal efficiency of the engine is going to be. The maximum amount of compression ratio is dictated by the detonation threshold of the engine - which is affected by the quality and octane rating of the fuel, temperature of the intake charge, ignition timing, etc. Here's something shocking, though: Forced induction does not affect the engine's thermal efficiency. Forced induction increases only the volumetric efficiency of the engine. Of course, the easiest way to reduce chances of preignition and detonation is lowering the compression ratio of the engine, but with lower compression also comes lower thermal efficiency, and although the power gained by increasing the volumetric efficiency of the engine outweights the power loss as result of lowering the compression ratio, it's just like robbing Peter to pay Paul - which leads to conclusion that boost versus compression is never ending compromise between volumetric and thermal efficiency of the engine.
The detonation threshold of the engine is also affected by the design of the combustion chamber and the pistons and two different 9:1 compression engines may have completely different detonation thresholds. As general rule, the combustion chamber should have very small volume which can be achieved by converting the squared circle design of our 4G63 heads into cloverleaf by adding more material and reshaping the combustion chambers.
One also has to realize that it's not boost what makes HP, but the mass of air and fuel mixture in optimal ratio (= chemical energy) one is able to stuff into the engine, although boost increases the volumetric efficiency of the engine - which would consequently bring us to turbo selection.
In simple terms, the compression ratio should be chosen more or less on individual basis since higher compression reduces margin for error. In general, any higher compression than 9:1 in case of our 4G63 engines in stock form is not recommended for street use on pump gas. 8.5:1 will leave considerably large size of the tuning window with enough range to compensate for low quality gas one may occassionally take, reduced intercooler efficiency in hot weather or increased intake charge temperature with turbo ran out of its peak efficiency range. The 9:1 engine will make approximately 2% HP more than the 8.5:1 engine due to increase in thermal efficiency, while the 8.5:1 engine is going to be able to run approximately 2 psi higher boost levels than the 9:1 engine (based on effective compression ratio) - and that's where the turbo selection arises since 2 more psi on maxed out T25 or 14B won't do much good (that's why boost does not equal HP).
Personally, I've gone with Wiseco 8.8:1 pistons (for a bit more safety), but 9:1 Arias or Ross pistons are perfectly streetable on pump gas as well. 9.2:1 would require some very careful tuning and 9.5:1 would be playing with fire... JMHO.

 

[Kooldino]

Interesting stuff. Woner how far I can push it with my new 9:1 pistons.

There's also other ways to lower the compression...thicker head gaskets will drop the compression a bit.

 

[KzA]

I was thinking of going like 8.1:1 and using nitrous until my boost hits like 5 or 6 psi...I just saw a BOOST activated swtich made by NOS that'll cut the power to the solenoid at a certain amount of boost pressure..Nice to use for low compression and a big turbo to get that fatty to spool up quickly..just a thought

 

[PaulMP3]

I am most likely gonna get 8:1 but i need to read up why some dsm guys went from 7.8-8.5, maybe that will change my mind..

20psi will be fun

 

[acidbug]

Wow there was some good reading in this thread...

Personally i think that the compression in our protege's is right on the money..forged internals..are the best thing..lower compression causes less power off boost..and how many people really stay on boost all day long?

But that's only my opinion..i am not plannin on forged internals..but am happy to see that people are taking the road's less traveled!

G/Luck
Chas

 

[jred321]

I am most likely gonna get 8:1 but i need to read up why some dsm guys went from 7.8-8.5, maybe that will change my mind..

20psi will be fun

when you go that low, you have basically no off boost power. on my laser it was either boost or nothing at all. so for daily driving, it takes away from some of the fun and you don't really gain anything from it, just more leeway on the tuning side. i believe the lower cr also makes spool time take longer but i'm not sure, it wasn't terrible in the laser, 15psi by 3.5k on a 16g with stock exhaust, not terrible but could be better. my vote is still for 8.5:1, plus you need some bigger injectors