question about pistons

[Juanka]

a friend of mine is rebuilding his motor. He found out that he need to bore a little bit the block so he want pistons with more compression. The other parts in the engine are still stock. The pistons are going to be .20 more bigger. He was thinking 10.5 to 1 compression. The question, Is this is to much or its ok for the engine?

[nando]

it'll be fine, but you'll have to run premium fuel, and it's best to retune the timing and fuel maps. if you want to be a bit more conservative, go with straight 10:1

[AdamF 88iS]

10.5:1 will require more than premium.

I've built many M20s with varying compression and found that 9.0 is the highest you can go with 91 octane gas.

92 allows you to dip into the low 9s, depending on cam choice. Anything much over 9.6-9.7 will require gas in the mid to high 90s. 10.5 will need very high ocatane fuel.

[nando]

I'm running 10:1 on 92 octane with no problems? using an aftermarket chip with more timing advance too..

[euroshark]

My euro M30 is 9.3:1 compression stock, and a bit more now. It runs great on regular or plus.

[Juanka]

he got a jim comforti chip and he always use premiun
and he is going to stick with the i cam

[Wilmer787]

How much Power you can get with 10.0=1 compression?

[digger]

The cam you use has as much influnce on knock as the static compression so what cam are you going to run? The static compression does influence whether it will knock but a bigger cam duration will allow more compression without knock because the dynamic compression is typically lower. Using a standard cam on high compression engine is asking for trouble.

[AdamF 88iS]

I'm running 10:1 on 92 octane with no problems? using an aftermarket chip with more timing advance too..

You must have WAAAYY better gas up there than we do in CA, 10:1 with a chip on pump gas would be a death wish here. Beware of carbon build up raising your compression over time...

[AdamF 88iS]

The cam you use has as much influnce on knock as the static compression so what cam are you going to run? The static compression does influence whether it will knock but a bigger cam duration will allow more compression without knock because the dynamic compression is typically lower. Using a standard cam on high compression engine is asking for trouble.

Yup. There is a good article about this on E30M3Performance.com

[nando]

You must have WAAAYY better gas up there than we do in CA, 10:1 with a chip on pump gas would be a death wish here. Beware of carbon build up raising your compression over time...

I don't know. it's possible, but it seems unlikely. maybe the MM pistons really are designed better than standard generic forged pistons..

5000 miles so far and no knocking issues. I will tune it eventually though.

[Wilmer787]

I have the stock cam, we have 93 octane here in P.R., just want to know if I can use JE 10.0=1 compression pistons without problems.

[digger]

If you aren't going to get a proper custom tune then i would not go over 10:1static CR

[swimstarguy]

Doesn't the m42 have a 10:1 CR and no knock sensors...


~Zar4

[AdamF 88iS]

Doesn't the m42 have a 10:1 CR and no knock sensors...


~Zar4

The shape of the combustion chamber has a huge effect on detonation characteristics. S14s came with 10.5, and I've built them up to 11:1 for pump gas with no problems. Hell, even M20 eta motors have 10.something compression with that little bitch ass cam. It's just the shape of the combustion chamber on the i motors that makes them pre-disposed to pinging at higher compressions.