ok i need a little bit clearer understanding as to what this means

If you're rebuilding for stock or performance, raise the compression & overbore for an easy 15-25% HP
(depending on size).

Note: it's just the opposite with Turbos - you want lower compressions and smaller overbores!

so does that mean that if i stay at stock piston size that ill gain more or less when building a turbo'd engine?
thanx

I went bigger pistons with a lower compression ratio.

When most people build engines, they go with a larger piston size. This makes sure everything seals properly and the cylinder walls are smooth and even. If you're going for high boost with a turbo car, many lower their compression.

A lower compression allows boosted cars to run on pump gas, not race fuel.

For a supercharged car in most cases (well, at least for us), keep stock compression because it's harder to have more boost without making more heat.

N/A, you want the high compression.


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overboring actually lowers the comp. ratio. unless you plan on running tons of boost you dont need to lower compression, but you must still be able to tune for it.
rasing compression will not give you 15% more HP. if your lucky you will get 5-10 HP

do a google search on comp ratios and learn how it works

ok thanx. ya i was mainly trying to figure out if i should go with the std. bore 8.9:1 ratio or the .020 bored pistons. when im said and done on the project i plan on running like 15-18lbs of boost so is it a good idea to go for the overbore?

Yes i would personally. I have no regrets on my built thats for sure. what engine are you planning on building?

2.2 ecotec im rebuilding everything on it. i made a mistake on running nitrous and my compression on the third cylinder is off bad. but i already got the eagle forged crank and im looking into the eagle rods and wiseco 8.9:1 pistons. for starters

As stated above the reason for boring the cylinder is to you have a true cylinder so the piston rings seat well and hold the compression. In a 4 cylinder engine this usually does not yield much of a hp gain since you can usually only bore it 40 thousandths of an inch bigger. On some 8 cylinders you can bore it significantly more plus you have gained 2 times as much volume than you would with the same size overbore on a 4 cylinder.

Higher compression pistons is where a big gain normally comes from. The more you compress the air the more the power it will make. However the more you compress it the more the heat and the more likely it is to preignite. This is how diesels run. They compress the air so much that as soon as the gas in injected it ignites without a spark from a spark plug. Preignition is very damaging to non diesels. When you turbo a car is is more likely to preignite under high boost since you are forcing more air in and it is being compressed in the same ratio as a NA engine. Thus it will cause more heat. When upping the boost eventually you come to point where the only option left to prevent this is to lower compression.



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