6 replies to this topic

[087dave]

Hi all, is there much difference between flat top and doomed pistons as i have
the two and dont know which set to fit.
There are going into a 998 with a stage3 head (not being skimmed std size), sw5 cam and lightened flywheel

Thanks in advance!!!!!!!!!!!!!!!!

[Sprocket]

We're Doomed! we're all doomed! aye

Lol, sorry.

Domed pistons are not normaly used on the A series, There were the old 997 raised crown pistons which could be considered Domed, but they have not been used in YEARS.

Flat top pistons are used in the High compression 998 engines and the dished pistons in the low compression engines. if you still want a higher compression ratio, skim the head. Make sure you calculate the compression ratio acurately by measuring all the parts you intend to use on the engine build

[087dave]

Thanks for that how do you calculate the CR i know the head has 26cc

[Ethel]

swept volume + chamber volume divided by chamber volume.

swept volume is bore (area) x stroke

chamber volume is head volume + gasket volume + piston volume (if any) + a bit for ring land....

[087dave]

Thank you so much but,
where do i get the gasket volume+piston volume from

I was thicking to go with dished or flat top piston
Dished because it gives a swirling affect with the fuel air mixture
Flat top because of better compression
Please advise
Thanks

[Ethel]

Go with whatever gives you the correct CR - that's what the BL/Austin/Rover did.

The economy version of the A+ used flat tops giving a CR of 10.3:1 - ever so slightly high for a road engine one today's petrol.

I'm sure someone will know the volume of a smallbore head gasket, somewhere around 4cc's I'd think. There's also any volume between the top of the piston and the top of the cylinder.

[Cooperman]

Small-bore copper-faced gasket is 2.7 cc.
Allow about 0.6 cc for ring land volume and don't forget to allow whatever is between piston crown and block deck. Allow 0.85 cc for every 0.010" on a small-bore engine.
I hope this helps.