Me and my dad were discussing this in the car. I asked him what it was and he told me is was how much P.S.I. the piston compresses in the cylinder on its upstroke. I went on to say that when boosting the horsepower on your vehicle alot, you would want to have low compression pistons so the added compression wont damage the engine. He disagreed, saying you don’t want lower compression pistons. Again being the know it all kid I told him if you add a turbo, supercharger or a nitrous oxide booster you need to have low compression piston, like a 8.0:1 as opposed to a 9.8:1(this application is for a toyota 22R/E engines). Well now that I lost people who ran out of interest, my questions are. What exactly is engine compression, and what is the compression ratio and how is it figured?
The compression ratio is the volume of the cylinder with the piston at bottom dead center divided by the volume at top dead center. For example:
The volume of the cylinder at BDC is 140 cubic inches, and the volume at TDC is 20 cubic inches, the compression ratio would be 140/20, or 7:1.
Engine compression probably refers to the cylinder’s mean effective pressure. Forced induction such as turbocharging does not change the engines static compression ratio, but does increase the intake manifold pressure and the cylinder’s effective mean pressure. Too much pressure can lead to detonation, so an engine with forced induction will typically have a lower compression ratio.
forced induction motors typically have LOWER compression ratios than aspriated engines. This is due to the fact that when the forced induction is working the cylinder pressure syrockets. If lower compression pistons were not fitted, damge would / could occur.
The engine compression ratio is the difference between the initial volume of the cylinder (or max volume with the piston down) and the final volume of the cylinder at the top of the compression stroke. This measurement is especially important in diesel engines as they are compression fired (and thus generally have higher compression ratios).
On the face of it it would seem higher compression will get you greater power. However, while I’m no expert, I strongly suspect the real answer is a good deal more complicated and the overall engine design will determine the real answer. Power vs. efficiency vs. engine tolerance and so on. They build diesel engines to get 15:1 compresion ratios so clearly an engine can be designed to easily handle it yet they don’t push ‘regular’ internal combustion engines anywhere near that usually (at least for street cars). Not being a mechanic or engine designer I can only deduce that there are good reasons for not wanting to increase compression at a whim.
Actually thinking more I can see why you wouldn’t want to push the compression of a non-diesel engine. At some point if the compression gets too high you might very well cause ignition without the spark plug firing. I can only assume this would not be considered a good thing.
[sub]Isn’t this what causes engine knock so you have to go to a higher octane gas to stop this (higher octane gas being less explosive)? [/sub]
My uderstanding is that compression ratios compare the volume of the combustion chamber when the piston is at BDC (bottom dead centre) with the volume of the chamber when the piston is at TDC (top dead centre ). Example: volume at BDC=400cc & volume at TDC=50cc. In this case a volume of 400cc gets compressed into a 50cc space providing a 400 to 50 ( or 8 to 1 ) compression ratio.
Everyone else posted while I was posting and I totally missed this which seems to answer my question/assumption.
Thanks Joey, that makes alot more sence to me, now I have another question. If I added high compression pistons to my 2.4 liter L4 what affect would it have to it? Which would be better to have, high or low compression? I am trying to figure out what I am going to do when I rebuild my engine, if it will be to costly then I am going to swap it with a 2JZ and a 6 speed tranny.
It would be better in terms of making power to have a higher compression ratio. But you may be limited to using gasoline with a higher knock resistance (octane rating). Your engine already has a 9.3:1 CR, so you probably won’t be able to go too much higher on pump gas.
Go for the six, if you can. It’s cooler anyway.
Yea, I am planning on the 6 since it would cost less for the 310hp and would take minimal work to get more(headers, bigger bore exhaust, cold air intake, and a modified EFI system to adjust the system), plus for what I have heard from some people, the mounting should be easier with stock engine mounts, but the 22R does have some powerful potential.LC Engineering makes parts for the 22R/E, 20R, tacoma and similar toyota engines(Find out on the site). It has the right stuff to get alot of hp our of the little L4 I got, but it depends on the cost.
Anyone knowing about these type of toyota engines might help, how does puttin a 20R head on the 22R up the power?
And the engine I have now is no less than 9.8:1 because that is the low CR. although it has about 190,000 miles and has been driven like a street racer/rally truck since my dad first got it in 93. If anyone says toyota trucks suck has never driven those little things.
nitpick- all engines are aspirated (or else combustion could not occur).
You mean NORMALLY aspirated.
thanks dutchboy for showing the difference, I didn’t notice that until you pointed it out.
I’m an engine neophyte, so forgive me, but by forced induction, you mean something like a turbocharger or supercharger bolted on?
I thought it might be a couple of reasons, but what I did find out from searching was there are a lot of finicky variations on the 20/22 series heads, and many combinations don’t seem to work well together.
In general, swapping heads on similar engines can give you a performance increase if the combustion volume of the new head is smaller than the old head (thus, increasing compression with the stock pistons), or the new head has a better flow path/design to it, or possibly larger valves. However, the stock 20R has smaller valves on it than the 22R, and is “reputed” to not be ported nearly as well as the 22R. Thus, likely people are trying to increase the compression ratio by head swapping, where it is possible.
I put a 1.3l FIAT head on a 1.5l engine once, and ended up increasing the compression ratio by nearly a full point, with the stock pistons. It also was only a 4-hour job to do, as opposed to swapping pistons. So it can be done with some engines with success.
Una
Yes, that is typically what people mean by that.
Lower compression is intended for blown or turboed engines while nitrous is high-compression friendly. I have a friend with a 10.5:1 LS1 (camaro) with a 150 shot of nitrous and no problems.
I heard on the radio that NOS made a new system that is supposedly a 500hp boost which can be put on “Stock” engines without damage, I am skeptical of this unless I actually see people doing this, but for now I will be adding a NOS octane booster.
Smeckel, remember that a camaro has a lot stronger engine than little rice burners and euro imports. Also I am sure your friend is smarter with his car than alot of other would be racers, who bolt on a turbo, add a stonger cam then think NO2 would make them win races when they end up blowing their engine.
A friend of mine did mud drags here locally. He had a chevy blazer with a 350CI engine. He had open headers and a K&N air filter for a fuel injected car strapped to the 4bbl. He told me when he raced he took I believe it was 3 bottles of NOS’ octane boosters and poured it into the tank which was about 1/2 full and raced. I wish I could have see it.
I know his block is going to be stronger but i was just showing that nitrous is generally intended for higher compression engines. If he took out the juice and strapped on a decent sized blower, he’d almost certainly be having detonation problems unless he was running a very high octane.
Assuming boost is maximized to just prevent detonation:
A lower compression piston will allow more air/fuel mixture to enter the chamber and give you higher horsepower at boost. It will also lower horsepower land acceleration before the boost kicks in (likely above 2000 rpm).
A higher compression piston will give you a better “street start” (engine starting at idle), but will limit the total horsepower.