2 replies to this topic

[runkthepunk]

Hello

I am not too great on engine power upgrades so just wanted to ask.

if you have exactly the same 1275 block, head, rods cam etc would it be more 'powerful' with flat top pistons or with standard dished pistons?

basically just swapping the pistons over same size just dished or flat.

Or is there more to it than this?

Thanks

[Cooperman]

It rather depends on what the compression ratio is to begin with.
The dished pistons will have a dish volume of around 8.3 cc, so you need to measure and calculate what you have. If it is already aroiund, say, 9.5:1, removing over 8 cc from the combustion volume by fitting pistons without any dish might give you such a high compression ratio that you mkight well need a new distributor with a different advance curve and also need to run 98 RON petrol.
Building engines is not as simple as just bolting bits on and 'hoping for the best'.
In fact, it's generally believed that dished pistons are best in terms of combustiuon efficiency and the best thing is to set the compression ratio by both making sure the pistons come right to the top of the block, then skimming the head to achjieve the correct combustion chamber size for a comp ratio around 10.3:1.

[runkthepunk]

Just replacing the 6cc (iirc) dished pistons in a 1275, will result in a compression ratio of about 12:1, which is not suitable for this reason.

Thanks for the replies guys

Thats what I wanted to try and understand and this makes it easy for me to get my head around. So if I were to use flat top pistons the head would have to be modified to lessen the compression rate.

So I guess the result of running large CC chambers on a standard 1275cc (10cc dished for example) would be the opposite. The comp would drop below standard rate so no good for normally aspirated engines more suited to turbos etc.

Thanks for the replies guys

Edited by runkthepunk, 02 February 2012 - 09:48 PM.