High Compression Piston = easy hp?

[focusedrage]

I look at it this way:

Raise the boost one will need a better feul control and delivery system. but on the other hand a set of higher compression pistons will compress the boost more once it is in the cylander, valves closed. So i imagine one coudl get some easy hp by changign to hight compression pistons vs all sorts of boost related troubles. Is this true? one miht need a higher octance to run that compression but i would think it woudl work.

Right now i run 13 PSI at the stock compression, what kind of increses woudl i expect in hp if i swithc to a higher compression ratio?

 

[RocketBoy]

It wouldn't be cost effective. You'd be better off with a turbo-back exhaust, upper intercooler pipe, 1G blowoff valve, boost gauge, and boost controller @ 15psi.

 

[focusedrage]

hmm... yeah but consider there is a point where you cant continue raising the boost with out a feul controler and new injectors, and a pump... but i see what you saying i have most of that

 

[CBgst]

I think running higher compression may not be as reliable as running higher boost. It just sound to easy. That's my opinion.

 

[cead]

just dont go over a 9:1 ratio. if you love to tear apart engines, go for it. if not, get a bigger turbo. either way you still gotta tune the engine to reduce knock and you may need to control the fuel , or mess with the timing. depends on the job and the head, shape of the pistons, etc. boost problems stem from ignorance and haste or even bad parts. do your home work and you'll be straight. i actually went to school so i could learn all i could about cars. but you can be a skilled do it yourselfer without dropping a tuition check. unfortunately, most problems happen because all the performance stuff is so expensive, most dsmers can only buy one thing at a time, and it just kills you to not have it installed, then you find out something new came out and you want that and then you wanna smoke a stang at the light, next thing, boom!. but problems are to be expected when pushing an engine beyond it's stock intent. just remember, high compression is also a reliability issue, check out the 5.0 page some time and hear about the 13:1 + nitrous nightmares. you will see a difference. a lot of the 1g guys pop 2g slugs in and they have seen a big increase (8:1 vs 8.5:1) my buddy with an awd talon did it, kept the boost the same and it smokes firehawks worse than before. i'm not telling you not to do it, you can, just don't half ass it.

 

[nine5raptor]

errr...7.8:1 vs 8.5:1?
The thiing is knock and power go up with higher compression...however knock has a much steeper slope, so it is better to run lower compression and more boost unless you have a way of controlling knock (race fuel and water injection...et cetera)

 

[LaserRST]

Higher compression pistons do change the powerband of any car. Remember, though, that they lowered compression to handle the extra amount of volume entering the engine before compression. Lower CR ratios mean more air/feul safely forced into the engine = more power. Higher compression pistons will create better bottom end, and makes a car be more "streetable" hp wise cuz the change in the powerband. More volume, or mass when we talk about total CR ratios adding in the formula for forced induction will create more hp. So more room = more A/F= larger bang = more output. Its not really about the squish, but about whats in there when it does squish. The reason why top feulers who run turbos actually run about 7:1 with their big blocks and high boost, 55-65 psi. Its really more about volume@ compression. So to help... make a hp goal and aim for it. If it means a broader powerband to help your light to light city runs than higher CR will help, if a peaky power band and good 1/4 mile times is what your looking for than you may want to stick with stock CR or 8.5 tops.... just my 2 cents.

 

[BlueSexMachine]

The general belief is that higher compression and boost won't get along together under the same hood

 

[RocketDSM]

I'm on your side in this one rage. I'm planning a 9.3:1 (maybe 9.5:1) setup. I want to keep the small turbo so it stays spooled all the time, but want more power throughout my powerband. Hence my compression choice. BUT, I fully understand that I'm going to need a rock steady fuel system, high power ignition module and one bad-azz computer to keep knock from killing me. As long as you understand the requirements of running higher compression AND higher boost, it can be done. Especially for street engines, like LaserRST said. For those heading for the autocross/road racing routes, I haven't seen a better deal than stroking it and bumping compression. Both add to the powerband in general. Higher boost means you're working at keeping the turbo spooled, to me that interferes with the rest of the expirence. Last note though, I was born and raised a pushrodder. This is how we build the old blocks, so go figure it's what I favor as a basis.

 

[G-Ell]

Low compression + high boost is the advantage our cars have. If you just want to up the compression and have a small turbo, you might as well just get a V8 cuz all you're going to have is a small motor with half the cylenders.

My GSX had knock problems since it's first mile with the 9:1 pistons and never saw a knock sum of less than 15 the entire time I owned it. Next time, I will stick to stock compression and run 25 psi.

 

[Bohrn]

here is my thought on it.

internal combustion engines compress air/fuel. Then they ignite it and the compressed mixture explodes. We all get that.

So to simplify even further, we compress air/fuel in the cylinder. All cars compress it with the pistons forcing a volume of air into a space that is almost too small for it. It gets hot.

with forced induction cars, we also make sure that the air in the cylinder is compressed to begin with. Then the piston compresses it even more!

Now most forced induction cars have lower compression ratios to deal with this aspect. It depends on how much pressure we are going to compress it to. Most cars that have boost pressures in the 12psi range usually have 7.5-7.8:1 compression ratios (DSM, Volvo 2.1, Mustang SVO etc) Cars that have 6-8psi have compression ratios in the 8.5:1 range(Pontiac GTP, most aftermarket turbo kits)

So I see it as a balance that must be adhered to. Either high boost/low cr, or low boost/higher cr.

Also atmospheric pressure is relatively constant. Maybe those in Colorado or Mexico city could run higher compression ratios to equalise the natural loss due to altitude, whereas someone like me who lives 32' above sea level is best to keep it uner 8:1. Altho I may run 8.5:1 when I rebuild my engine, and run less boost and use a 16g turbo to keep things more efficient volumetrically speaking. More flow, less boost still lots of power. But easy on parts.

 

[iamtall77]

all of the info from this thread is very useful, but think about what is in your cars already. the 1G runs 7.8:1 with a better head, turbo, intercooler and piping. the 2G runs 8.5:1 with a little smaller turbo, intercooler and piping, but better MAS and exhuast manafold. the two engines put out about the same power, so where is the big debate. A 1G with 8.5:1 pistons should be better and not cause any real problems. A 2G with a 1G head should do wonders with no real problems. We're not talking about huge compression changes and with the right supporting mods, either setup would be good. so as long as you're not running 9:1 or higher, you should be good with simple supporting mods and not run into problems.

ps let's not forget the EVO runs on 8.7:1 pistons.

 

[Bohrn]

but most people here are not running stock cars, and here is where it becomes an issue. It is easier to run more boost than it is to raise your c/r It is also easier to do more mods with the lower c/r and higher boost for cheaply, eg no expensive aftermarket computers, etc.

 

[4pistons]

Any internal combustion engine can be simplified to an air pump. Air (and Fuel) in, compress it, ignite it, Air out. If you can get MORE air (larger volume) and fuel in and out you will make more power.

Raising the compression ratio does not increase the volume of air pumped, but increasing the boost does. Dramatically. That is why you can make far more power with boost (volume) changes than you can with compression.

Raising compression on turbo cars generally helps throttle response and off-boost power (Low RPMs).

Rick - '91 GSX

 

[focusedrage]

i think that is what i was sayign from the begining... like it would allow you to run less boost while still making power.... like run a quicker spooling turbo that pumps less volume into a higher compression area, it seems like the thing to do if one needs the low end...

 

[RipperXX]

Originally posted by CBgst
I think running higher compression may not be as reliable as running higher boost. It just sound to easy. That's my opinion.

yea probly becouse u are runing the higher compression all the time unless your giveing it some gas your not boost

 

[iamtall77]

I think Hot Rod mag has something like this debate in their "Turbo any engine" articles. it says something to the effect that you gain something like 50 HP with every point of compression on a Chevy 350 and like 10HP with every pound of boost. (i forget the #'s) but basically you get more power with boost than with compression. however, if you bump compression up a little (8.5:1 or so) and still maintain 15psi of boost you get more power and better low end. when building a motor use strong pistons and rods to allow for the increase in power. (and maybe knock) the low compression high boost engines are only good in the upper rev. range where enough A/F is compressed to make power. look at the power band of any turbo DSM and then look at the power band of an NA motor. Turbos put power at the top due to low compression.