Lower Compression with Headgasket

[Team STILLEN]

So I was thinking. I know on the old 3sgte in the MR2 you could get thicker head gaskets to reduce the compression ratio. I know HKS has a head gasket set that lowers the compression ratio to 12.4 from 12.5. Would it be possible to lower the compression ratio down closer to 12.0 with a head gasket without other issues?

[FrsDuke]

Might start running into issues with header fitment if you raise the effective deck height too much. Also if you start getting too thick you could potentially have a serious reliability issue with the gasket.

[diss7]

I don't see why it wouldn't be possible. At least that way, bumping the compression down would allow for a more knock tolerant 350-400whp.

However, it's doing nothing to strengthen the rods, obviously. The rods will are always the weak point in a high revving high compression JDM motor. They make them as light as they can to make the engine rev.

I'm not knocking the idea at all. I think its a good way to build in a bit of fat with regards to knock threshold. However it's not the same as a built motor, is the point I'm making.

[mid_life_crisis]

Now that is old school,

[diss7]

Originally Posted by mid_life_crisis

Now that is old school,

Just because it's old school, doesn't make it false. Most JDM 2000cc motors can quite happily be boosted to 400whp unopened, on pump gas. This motor, because of knock, because of its high compression, it limited to 300whp or thereabouts.

[diss7]

Originally Posted by FrsDuke

Might start running into issues with header fitment if you raise the effective deck height too much. Also if you start getting too thick you could potentially have a serious reliability issue with the gasket.

Valid point. However, if you're going to the trouble of pull the motor and removing the heads to install a new HG, cutting and spacing/modifying the headers is far from difficult.

It's the intake manifold which will be more difficult.

[FrsDuke]

Originally Posted by diss7

Just because it's old school, doesn't make it false. Most JDM 2000cc motors can quite happily be boosted to 400whp unopened, on pump gas. This motor, because of knock, because of its high compression, it limited to 300whp or thereabouts.

I've seen more than one shop claim 400+ with forced induction on stock internals on this very forum. The original post was asking about using only a thicker head gasket. Which is going to have limits as I said in the first reply. Head gaskets are weaker than heads or blocks so if you go too thick it will fail. Also as you get thicker the intake and exhaust ports are getting further apart because you are raising the head further from the block meaning stuff like the intake manifold and header no longer fit.

[FrsDuke]

It would be a hell of a lot easier to do rods and pistons than to reinvent a header and intake manifold. So the idea of cheaply lowering the compression with a head gasket is just straight limited at best

[diss7]

I was talking about pump gas being the limiting factor to 300whp, because of the compression.

Where as if it were lower compression, you could push it further to 400whp, just like any sti motor, sr20, 3sg, 4g63, rb20, 1g, fj, I could go on.

Regarding the rods, yes there maybe people pushing more than that, but for how long? One thing in some of these builds favour is the lag of the turbo. Running an early spooling turbo that can make 400whp or more, like a gtx2867r or a efr7163 and you're putting even more strain on the rods.

[diss7]

Originally Posted by FrsDuke

It would be a hell of a lot easier to do rods and pistons than to reinvent a header and intake manifold. So the idea of cheaply lowering the compression with a head gasket is just straight limited at best

I agree with you. Well it depends on the persons situation really. If they have a lot of time, and welding/fabricating ability, modding the manifolds isn't hard. Certainly easier than paying for a built block.

But for most of us, that would be paying either way, you defintely have a point.

Still going to be much cheaper to modify manifolds than to buy a built short block.

[Calum]

I'd be curious about the length of the timing belts (chains I think, but either way) and how far off the cam timing would be when adding that much to the gasket thickness.

Anyone know the combustion chamber volume so we can do some calculations to find out how much deck height we'd need to add, otherwise this is just pure speculation.

[Team STILLEN]

I did not have the time at work to calculate the size head gasket to decrease the compression ratio by X amount. I didn't even think about the manifolds lining up right as an issue because I am used to inline 4's and 6's. Maybe that is why the HKS only drop the CR by 0.1?


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[FrsDuke]

That would be my guess. Boxer engine is kinda tied to one spot

[humfrz]

Originally Posted by mid_life_crisis

Now that is old school,

So true ....... but back in the day, we were going the other way .....

Personally, I think increasing the head gasket thickness on a 12:1 compression ratio engine is just asking for trouble.

humfrz