47 replies to this topic

[Spitz]

Can "d top" pistons be used with a 12g940 head?

[jonny95]

Can "d top" pistons be used with a 12g940 head?

i would imagine that the d-tops with the 12g940 woould make your compression ratio too high as the 940 has a combustion chamber that is much smaller than on the 295

also with raised pistons you may well get valves hitting the pistons with the 940 head

[Cooperman]

The valves won't hit the pistons, even if they are D-tops as the valves are never fully open when pistons are at TDC. The compression ratio will need to be measured and checked anyway with a 940 and, if necessary, the chambers will need to be enlarged by a bit of grinding out to get the right figure. Personally I would go for a 12G295 head as fitted to the 998 Cooper, but with slightly larger inlet valves and full gas-flowing.

[Spitz]

OK....I was actually asking for the OP

[minimk1]

I'll get am12g295 then

[MiniElliott]

Even if the bores are too worn to hone you could get the block liner ed back to original size. The you could use the original pistons with new rings and a un-skimmed 295

[minimk1]

What's the advantage of an in skimmed 12g295

[jaydee]

If you bore it out to 1060 with a 940 and a 286 you could hit the 80 bhp mark if built accordingily
Power band will sit pretty high in the rev range though

[Cooperman]

On a 998 I built with a 286, 12G295 head, flat-top pistons, twin HS2's, etc, it gave 72 bhp at 6600 rpm. It really had nothing below about 4000 rpm. With SCCR gears and a 4.1:1 FDR it went well, up to its effective max speed of about 90 mph at 7000 rpm.

Edited by Cooperman, 24 April 2013 - 02:44 PM.

[Spitz]

I wish I had a "rolling road" within 1000km of me.......I'd love to know what my 1098 puts out. Someone give me a guess:

1130cc ( 1098 +40 )
12g295 ( shaved to 24cc chambers......080" )
SW5
twin 1.125" ( sometimes I run an HIF38 with AAU )
LCB
rc40 equiv.

Now...as it's a long stroke engine I'm not going to be running at 6k + very often so may be hard to compare with a 998 running at 6600.
( I did install a better piston that doesn't have the ring below the pin )

[Cooperman]

My BHP guess would be about 68, but who knows!

[minimk1]

See I'd quite like to have low down power but still be able to have about 65 bhp

[Cooperman]

Sounds like you need a standard 1275 engine then as that's what they give. To get 65 bhp from a small-bore engine will require the use of lots of revs which means there will not be much low-end torque.
A 1098 will give good low-end torque, but to get a lot of power is difficult because of the rev limitation on the crank.
The problem with any modified engine is that the power/torque band becomes narrower and higher in terms of revs. Unless, of course, you go to forced induction.

[minimk1]

I want usable power is what I'm saying so I don't have to wait for the cam to come on and I want it to rev high I just dont
Want to have to rev to get my power if that makes any sense I want it to be a revy little engine and there's no point with a 1275 as that would bore me I'd rather have a racy 998 that goes as fast as a 1275 than a standered 1275

[minimk1]

By higher in revs do you mean that if you drove round revving the nuts off of it you would still get your power