2.7 question

[Gabi_323i]

i have the 10.2:1 eta, i have both 731 head and 885 head, witch is best to use? with the 731 head i know the comp ratio will be the same but will it worth the bigger valves and bigger intake and exhaust ports from the 885 head? anyone knows what is the comp loss with the 885?
thanks

Gabi.

[Gabi_323i]

up

[chu346]

I have read that it should work out to be 9.4, but I have been told it has been worked out to be 10.2 still. I put 325 valves into a 320 head last year because at the time everybody said that it was not possible to put a 325i head on a eta bottom end without it pinking.

You could use the 325 head, but you may need some tuning because it may pink slightly, that is your only problem.

When are you doing the conversion? I might do a px for your 325 head as I want to lower my c/r for the turbo

[Gabi_323i]

we have 98 and 100 octane gas here so why get the pinging?

so the question is what combo gives more hp?

2.7 bottom end with 731 head e21 2.3 camshaft and 10.2:1 cr?

or

2.7 bottom end with 885 head e21 2.3 camshaft and slightly lower cr?

[Ant]

the 731 head has the correct chamber shape for the ETA pistons, so that would be the one to use

the ports can be opened up to match those on the 885 casting should you wish, but dont go mad,they are very thin walled in places !!

as Chu said, pop the 325 valves into the 731 head, iirc you need to machine the seats wider but otherwise all should be OK

[Turbo-Brown]

Does fitting the 325 valves reduce the compression or something?

[Simon13]

i've yet to see a 731 head 2.7 make proper power

[liam012]

and whats "proper power"?

[Ant]

I'd imagine the over 200 bhp benchmark Liam.

O/T, you got that MS on yet dude ??

[kam-325i]

Does fitting the 325 valves reduce the compression or something?

No, the 325 valves are bigger = better breathing....

[Turbo-Brown]

You only get better breathing if you enlarge the throats of the ports to acomodate the larger valves.

Otherwise you get better shrouding of the ports (especially at lower lifts) and correspondingly lower air flow.

[Gabi_323i]

i was thinking that the larger valvel, larger ports will compensate for the slightly lower cr and get instead a higher power peak?