first off
I have searched and searched on this topic.

I am building a 2.3 daily driver that will see some autocross maybe 3-4times a year if that. 91 octane is the highest and my hp goals are about 400plus. I do have supporting mods.

gsc s2 cams and springs
turbo back exhaust
02 housing
ams intercooler
walbro pump
bbk full turbo
soon to have borla exhaust manifold
injen intake
70mm throttle body

I have 9:1 pistons but been on the fence of goin lower. 8:1 or 8.5:1
any suggestions.

 

under boost 8/1 will work waaay better. off boost the higher compression is very noticable and more pleasant to drive. if you are moving to 2.3 from 2.0 I would go with 8/1. woosener makes great pistons at 8/1. you wont notice any looses going from 2.0 to 2.3. it will be gains everywhere. the original 4g63 6bolt was 7.8 compression. I built my first 2.4 motor at 7.7 compression. motor was fantastic for 15000 hard miles before a failed meth pump took it out. but it was loving 34psi all day long on 93/meth

 

This advice is contrary to what most persons say but I agree with it. My experience has shown me that for low octane fuel, low compression works better. You can still make good power and it is much less prone to knock and less sensitive to changes that cause knock, like higher temperatures etc. Engines like richer fuel mixtures on lower octane also.

I tuned a car on straight 90 octane that ran 30 psi. The compression was below 7:1 because it had stroker pistons with a regular crank It was pretty quick and pretty much couldn't knock.

 

in general, more compression: more response and quicker spool. lower compression = more power. it just depends on what your goals are, really.
i agree that going from 2.0-2.3L allows for lower comp/more power without noticing much loss off boost. however, if you are like me and like power and response down low, you could keep the compression up and just run less boost and be okay with a bit less peak power.
totally up to you.
personally, i'd love to try a 10:1 2.3L with a small turbo and lower boost. but i think our 91 is just so bad i couldn't really get away with it. but could you imagine an e85 tune??
good luck with whichever you choose.

 

Lower compression = more power??? Really?

Please elaborate on that one.

I wouldn't run anything less than 10.1

The motor runs more efficiently, less boost needed to meet the same power goals

 

lower compression = more boost = more power. or at least that's been my experience with 91oct. i like compression, though. i ran an 11:1 engine with 13psi of boost (Honda) and it felt amazing.

 

so basically 8:1 would be the ideal situation I guess

 

Completely depends on your goal. For max, peak whp, most likely.

 

I think there is a misconception that increased or higher compression ratio will improve spool characteristics, I think the opposite is correct! However, due to the overall increase in trq across the power band, one may perceive it as an improved spool.
Whether you increase your cylinder pressure mechanically by the compression ratio or thru tuning with spark and valve timing (if you have control over it), I think they are both the same. A tuner should be able accommodating for higher CR when low octane fuels are used. Consider the Mazda Sky active engines for instance, they are claiming 14:1 compression ratio.
I think someone mentioned above that he had gone with a different engine configuration and enjoyed the results, I would be curious of the difference in spark used for the same displacement but with a changed compression ratio.

 

High compression on low octane fuel is no fun. I did that experiment already (10.5:1 compression on 90 Octane). It was tuned not to knock but timing had to be very low, boost had to be lower and the end result was not satisfying - i.e. it was less powerful/slower than the stock piston setup.

Not sure if you were referring to me, but I don't remember the details of the low compression engine I spoke of. However, it definitely allowed more ignition timing and more boost than with higher compression setups. I wouldn't really recommend such low compression though. The owner made a mistake and bought stroker pistons for a 2.0 and fit it up anyway.

 

9:1 will be fine, but i wouldn't go any higher, especially with a smallish turbo.

The onset of knock becomes so sensitive with elevated exhaust back pressures associated with a smaller turbo..If you had a larger turbo, fp-blk for instance it wouldnt be as bad, but far from ideal.

i have run 91 on my 2.4L hta-grn, it sucks! Im currently @ 11:1, as i knew it was primarily a e85 motor....but sometimes things happen & your forced to run 91.

I am planning to build a 8/5/9:1 pump gas motor in the near future.

 

I think it really depends on what your set up is going to be and what it is used for. sure you can run high compression on crap gas but how long do you want it to last. unless you go with e85 or a meth kit, i would stay away from anything higher than 9.5:1. being that this is your dd, there is no need for a crazy set up, especially shooting for the numbers you are. you won't notice any power gains because your turbo is going to make the numbers you want and you aren't going kill mode 40+ psi on a huge turbo. like someone else said, they used a higher compression and pump gas and had to run less boost and timing because of it. I would personally just use the 9:1 pistons you have and call it a day. my 2.3 build will be done in the next couple weeks and my engine builder/tuner recommended 8.5:1 for my dd. good luck with your build.

 

Wreckleford, your results seem to be in line to what I was expecting. Do-able, but no fun in some cases!

 

well the machinist said he can take some off the piston to lower the compression but it wont lower it that much unless I put a thicker gasket to lower some. But I prefer not to go that route. I think it would be best to bite the bullet and buy 8:1 pistons and keep the 9:1 for the extra block I have. I have been thinking about this all weekend because I need to make a decision shortly. They plan on slapping the motor together next week.


You think it is ideal to run 8:1 or a 8.5:1 ? I will only use race gas or e85 when I autocross out of town. There are no stations here where I live that have e85.

 

Thank you everyone for your input on this major decision for me.