Compression ratio on (premium) pump gas

[Altaran]

Hi,

Since I am planning to rebuild my motor, it will be getting some goodies including forged pistons raising the compression ratio to 12,5 : 1.

So far so good. However I also have a P54 cylinder head on stock that I acquired some time ago (mainly because it's BMW motorsport ported). This one has been ground off by 0,6mm compared to new OEM.

By my calculation that will further raise compression to 13,44 : 1, which does seem quite high.

Has anyone ever ran something like this or higher on premium pump gas? For reference the late GT3s have been at 13,3 : 1.

I can still counteract this by ordering a thicker head gasket...

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[9kracing]

I've run my Hondas on 91 octane at around 13:1, but you need a really good tuner, and I'm not sure I'd run MAF at that point.

That seems awfully high for a street car, I'd probably also be looking at headgaskets.

It looks like SCE used to make a copper one but it seems to be disco

[Altaran]

Oh, I did not mention that I run an Alpha-N mapping.

TZR (German retailer) sells gaskets from "Cometic" in various thicknesses. I still have to do some research on them...

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[Tbonem3]

Cometic was the brand to use in my motorcycle racing days.

Since our cars are a bit known for HG issues, esp cars in Europe for whatever reason, maybe a slightly thicker HG isn't a bad idea.

[WestBankM4]

Can vouch for Cometic, used it in my Japanese swapped SR20DET Silvia, upgraded turbo, was dyno'd at ~480whp.

[BBRTuning]

Hi,

Since I am planning to rebuild my motor, it will be getting some goodies including forged pistons raising the compression ratio to 12,5 : 1.

So far so good. However I also have a P54 cylinder head on stock that I acquired some time ago (mainly because it's BMW motorsport ported). This one has been ground off by 0,6mm compared to new OEM.

By my calculation that will further raise compression to 13,44 : 1, which does seem quite high.

Has anyone ever ran something like this or higher on premium pump gas? For reference the late GT3s have been at 13,3 : 1.

I can still counteract this by ordering a thicker head gasket...

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12.5:1 is fine on pump gas, even 91 octane (95 RON), with larger camshafts. If your compression will be as high as 13.4:1, you will absolutely want larger camshafts, at minimum 288/280, ideally 296/288. Not only will this help lower the effective compression ratio, it will really take advantage of the higher compression. With 288+ cams and at least 98 RON and a good tuning you should be able to make it work. Keep in mind you will have to be extra careful about checking piston-valve and valve-valve clearances at these specs, when assembling the engine.

[Altaran]

There will be a set of 296/288s going in with the rebuild.

Usual Premium gas here is at least 98 ROZ.
Shell Vpower has 100 ROZ. Aral Ultimate supposedly has 102 ROZ, although some tuners recently discovered it performs worse on the dyno than regular 98 ROZ.

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[digger]

Run e85 as-is, or lower it for with thicker gasket for I presume 98ron. people way overestimate the benefits of compression ratio and if you have to cut back ignition timing too much you’re loosing more than you gain from compression so net effect is less power.

you should always build an engine based on its intended operation requirements/ application and not partly hamstring it to suit incorrect part choices

[Obioban]

I would not want to go past 12:1 on 93 octane.

[bmw m3 s50]

I am guessing it's all in the tune on the octane that you need to run. The tune would determine the values to retard or advance timing. Higher octane would definitely allow the higher compression motor to run more advanced timing. I think the new 911 GT3 runs 13:3:1 compression ratio stock, and would need to be able to run 91 octane.

[EthanolTurbo]

I am guessing it's all in the tune on the octane that you need to run. The tune would determine the values to retard or advance timing. Higher octane would definitely allow the higher compression motor to run more advanced timing. I think the new 911 GT3 runs 13:3:1 compression ratio stock, and would need to be able to run 91 octane.

Also one thing to consider, I saw in your profile your from Germany. I believe European premium gas runs much higher in octane than the USA, unless that changed. Last time I was over there, recall seeing 97 octane. That could play as a difference, since here in the USA we only see 93-94 Octane as the highest and in some parts of the country only 91.

European octane ratings are different, they're about equivalent to 91/93 octane here.

[Altaran]

Correct, 98 RON is equivalent to 93 octane in the US.

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[Obioban]

The new, very high compression ratio NA engines are using the spark plugs as ionic knock sensors to nearly instantly pull/push timing.

Without that ability (which the MSS54(HP) does not have), I wouldn't want to exceed 11.5:1 on 91 octane or 12:1 on 93 octane.


... and, yes-- 98 RON (euro) is the ~same as US 93 octane.

[bmw m3 s50]

European octane ratings are different, they're about equivalent to 91/93 octane here.

Actually, I think your right. I didn't think of that. Sorry.

[digger]

The new, very high compression ratio NA engines are using the spark plugs as ionic knock sensors to nearly instantly pull/push timing.

Without that ability (which the MSS54(HP) does not have), I wouldn't want to exceed 11.5:1 on 91 octane or 12:1 on 93 octane.


... and, yes-- 98 RON (euro) is the ~same as US 93 octane.

The biggest difference is direct injection that allows a CR increase of about 1 full point straight off the bat. Also combustion modelling and understanding has come along way since late 90s. People can’t look at what oems have done more recently and think they can simply copy them