nismo3183

iTrader: (1)

I'm having a shortblock built and I am undecided on what compression ratio I should go with. I have an E85 station 4 miles from my house but E85 stations are few and far between here. I'm going to tune for both E85 and 93 on stock ECU but primarily I will be using E85 as this car is not my daily driver. Basically I'll be using 93 when I can't make it to the gas station. My setup will be as follows:

4g63 with 153mm rods and 94mm stroke
EFR 8374 1.05 t4
head is unported and 99% sure I'm using S3's with supporting valvetrain, but I haven't ordered the cams yet.

Does anyone have any suggestions for compression ratio for my setup? I currently do not have a tuner so I can't just go and ask my tuner this question.

nismo3183

iTrader: (1)

Anybody have any input? Any regrets with a particular setup you went with?

lostsoulsevo

iTrader: (10)

10:1 if your using E85

nismo3183

iTrader: (1)

Has anyone had any issues with 10:1 if they had to run 93 octane? engine is primarily on e85 but if I had to run pump and I had a pump tune as well is 10:1 still high?

browningderek

iTrader: (7)

With a big turbo 10:1 is fine on 93 for sure. Reshaping and smoothing the combustion dome helps a lot too as far as knock goes. I have ran 26psi on mine with no knock showing 560hp in virtual Dyno.

JohnBradley

iTrader: (30)

9:1 and here is why.

10:1 feels snappy on the street, should (and I use this loosely) make an addition 4% power for a given boost level as long as timing doesnt change, help keep bigger cams a little more civil.

BUT

On 93 (and you can imagine 92 or less out west) you still get limited to the amount of boost you can run before the timing has to drop to make the best use of the boost you want to run. Let's take Derek's boost level as a hypothetical example.

Derek runs a large turbo and 26psi on a 10:1 2.3 on 93 octane. At this boost level we are both convinced his car is knock free and as he indicated it made 560whp in VDR. We up the boost but now we have to start pulling timing at the same level to keep knock at bay so we get to the point where we make more torque but timing is so low it doesnt really make that much more power.

I have seen logs (not Derek btw) of 10:1 motors trying to run 29psi of boost on 93. Timing is 7-8* at 8,000. I know my car at 29psi is going to be 12-13* on 92 at the same boost level, same turbo but a 9:1 motor. I also happen to know that 1* of timing is normally 10whp. A few pieces of math based on my car at that level-

650whp at 29psi and 12*

vs the same car but 10:1 and 7-8*

650 - 45whp (4 or 5* less so let's split and say 45) = 605
605 (if it was 9:1) * 1.04 (+4% for a 10:1) = 629.2 at the same boost level with lower timing

So you look at it and say its only 21whp...why loose even 21whp though right? In addition lower timing elevates EGT and makes the risk of a misfires even greater. If its on E85 this is a non issue but what if E85 goes away and you get stuck running 93? Then you need racegas or meth injection to get the timing back, drop EGT, get reliability, etc.

Our cars have run just as fast as 9:1 and 55psi as they do at 10.5:1 or 11.5:1 and 55psi.

browningderek

iTrader: (7)

I agree the only reason to run 10:1 is for off boost power. My car didn't make any more power and spooled the same as arrons Evo though I have almost 2 points more compression. At 26psi my timing was 8 degrees I think which is low for sure. But with that said its AWESOME to have that extra off boost power!

I guess the op needs to decide what hes going to do most of the time. 9:1 is more pump gas friendly for sure but 10:1 is doable. Maybe a slight loss in max power but has a low rpm torque boost... I figure 4 bangers need all the help they can get.

browningderek

iTrader: (7)

Lower compression engines idle smoother also, just saying.....

JohnBradley

iTrader: (30)

Least on 92, not as lucky out here, I have done back to back testing with what I can get away with on a 10:1 motor and a 9:1 motor with everything else being the same.

Luke's car really doesnt like more than 20psi and I can squeeze 24 if I start dropping timing and getting less return. His car as a 9:1, 26psi all day and no complaints, 28-29 and I have to start pulling the same amount of timing as I do at 24. I dont like giving up boost since its airflow that makes power not compression.

I use a formula like this to illustrate it-

Compression + boost * timing = whp

Timing is the multiplier, compression is a constant, so boost is the only variable we can control. In order to get more power we need more boost, compression can sit on the corner for all I care. Then again I run a 2.4 with MIVEC so response is pretty good off boost. Add a good twin scroll setup to the motor and things get even better (and allow for more boost...win-win)

Aaron

nismo3183

iTrader: (1)

Good info, thanks for the posts guys ER is actually building the motor as soon as I decide on a compression ratio. Thanks for chiming in Aaron

Mauricio Evo IV

iTrader: (1)

What about 9.7:1 CR...

I´m building a 2.1L (4g64+87mm pistons+156mm rods+88mm crank) and pistons are 9.7:1 CR.
I use my only for drag with VP Q16 but sometimes I use normally on weekends and we only have a crap 89oct gas (+meth). Can I use it for DD with that CR and 89oct gas?

I think this can help to OP =)

nismo3183

iTrader: (1)

Ended up going 9.5:1CR 153mm rod 94mm stroke 4g63. Myles took care of me at ER. Will update this in 8 weeks when the block gets here