3 replies to this topic

[Miki Leyland]

Sorry if this sounds a bit of nonsense, what I'm wondering if it would be possible to fit an old 850/998 cylinder head on a 998 A+ with flat pistons.

 

Standard 998 A+ head is the CAM4180 (25.5 cc) while the 850 is 24.5 cc. Would it raise too much CR?

 

Thanks

[Cooperman]

You need to measure how far down the bores the tops of the pistons are at TDC then do the calculations.

That will give you an answer.

With regard to compression ratio it should be no higher than 10:1 for a road engine.

The 850 head has quite small inlet valves so performance may be even more limited.

[Miki Leyland]

You need to measure how far down the bores the tops of the pistons are at TDC then do the calculations.

That will give you an answer.

With regard to compression ratio it should be no higher than 10:1 for a road engine.

The 850 head has quite small inlet valves so performance may be even more limited.

 Thanks Cooperman, but don't both 850 and 4810 heads have the same inlet diameter? 1.0625" according to some websites

[Spider]

Yes, that will be fine. There will be some loss of power, though, likely to be almost academic amounts.

 

It would be very wise to either CC the total combustion space to be 100% sure the CR won't be too high, or another, but less certain way is to be 100% sure the head you are fitting hasn't been skimmed previously.