how low can you drop compression

[SwolenColen]

i've been planning on dropping my compression to 8.2:1 for 15 psi, but i was thinking, since im getting new pistons, why not go lower? Corky Bell, in Maximum Boost, says that anything lower that 8 is not driveable, but my friend says that he's running 7:1 on his daily driver, and doesn't have any problems with it. What are your opinions on this, and also, how much horse/torque do you lose from dropping the ratio?

[KatnipBimmer]

well...there are alot of facets to this issue that corky bell doesn't really explain in his book. (good book...but vague in some areas)

You can run 9:1 compression and still boost to 18psi if your motor is tuned correctly and you are running high octane gasoline (103ish)
If you had boost early on (very low revs) you could get away with lower compression but if you didn't have air being forced in it would be a rather un-powerful engine.

[SwolenColen]

i will only be running 93 octane, and i'm not really trying to test the limits of my engine. I'd like to drop down to 7:1, but i'm afraid i'll have problems w/ startup, idle, low rpms, etc. I'm planning on only having max boost at 4k +.

[RvolvoR]

run lower boost and a higher comp ratio, you will get better all around prformance, because all you are doing when you raise the boost at a lower comp ratio is making up for how much you dropped it

[Highboosting///M3]

Just look at the Supra guys. 8.5:1CR and they have made 1520rwhp on an auto! I would think that you aren't going for more hp than that.

With max boost at 4K rpms you are looking at maybe 650rwhp max(if you lucky). More than that and you will have more lag.

So no lower than 8.5:1CR! Why? Cause there is no reason for lower CR unless you are building a drag car....your car will just bog before boost.

[SwolenColen]

with an 8.5:1 CR I will be able to put in 15-17 psi max... bimmerjim is running 17 and getting 300 hp

[KatnipBimmer]

Originally Posted by boosting///M3

So no lower than 8.5:1CR! Why? Cause there is no reason for lower CR unless you are building a drag car....your car will just bog before boost.

Please dont rip new assholes just cause i counter this BUT; I don't think this is neccesarily true. NA cars gain power from an increase in CR and the same can be true for FI cars. We like to lower the compression in are cars because 1) it's safer and 2) we don't have to run exotic fuels just to drive around.

I'm no expert so if i am wrong, no hard feelin's. just some thoughts from another gearhead. (i'm planning on 9:1 CR for my project engine...but that is to retain driveability at low engine RPM and a turbo that doesn't kick in till high in the rev range)

[KatnipBimmer]

Also; if your only going to be running 93 octane...then you will be fine with a higher ratio (granted you aren't trying to push 2bar over the whole rev range!!)

[tjn182]

The M30 engines have 8:1 comp stock (from the e28s) -- So far we've had 22psi of boost dumped on our engine w/ stock internals (including stock headgasket). The only thing that went wrong there was the headgasket blowing. We can go much higher if we replace our headgasket with something more high performance. I've heard accounts of someone hitting 26psi of boost on stock internals as well. Some of the 745i's came with a 7.5:1 compression - and are real slugs off the line, but kick some serious ass once they build boost. I wouldn't drop the compression much past 8:1 - it's not necessary. The lower the compression, the slower it'll be off the line -- Yet the more boost you can dump into the engine. The higher the compression - the faster it'll be off the line -- Yet with less amounts of boost able to be dumped in the engine.

[SwolenColen]

the only drawback to boost is detonation. I am going to crank the boost up as high as i can w/o getting any detonation. The ways you can add more boost w/o detonation is lower CR, cooler intake charge, running rich (not an option), and higher octane gas. And I am not building a drag car, if you read the original post, I want to know how low people have dropped compression, and how that's effected the drivability.

[SwolenColen]

Originally Posted by KatnipBimmer

Also; if your only going to be running 93 octane...then you will be fine with a higher ratio (granted you aren't trying to push 2bar over the whole rev range!!)

I do not understand this at all. Why would 93 octane gas make it fine to have a higher ratio? If i have a 8:1 CR and 93 octane, lets say i can run 18 psi. If I had like 103 octane, then i could still run 18 psi on a higher compression (like 9:1 maybe), but I don't see how i will be fine w/ high compression and 93 octane.

[2jz]

you want to run a 7.0:1 on the street? I have a '95 SC300 with stock compression of 10.5:1 and used a 3mm headgasket and put it down to 8.7:1 and put down 644rwhp with a GT60 turbo. goood luck with that compression....sounds like its very streatable

[SwolenColen]

The point of this post is to find out what is "streetable", in your words, for compression. I don't know why you feel the need to mock me for considering it. If you know about this subject, please tell me why 7:1 is not "streetable".

[Dragonsi]

7:1 is very uncommon, I've actually never heard of anyone making pistons of compression ratios that low, 8:1 is the lowest. You need a decent compression ratio to spool the turbo. That's why on honda-tech there is a 11.5:1 cr on a t3/t4? and he has absolutely no lag since he has exhaust gases literally on tap. Unless you want to make a full out dyno queen, then 7:1 is useless, and/or you are gonna make some insane highway pulls

[328iTunr2]

The lower the compression, the greater the responsiveness suffers. If you lower compression more, you can add more boost. This effect is like a lancer EVO. Absolutely no power/responsiveness in lower RPMs, but a huge surge when the boost comes.