9 replies to this topic

[IainNeon91]

Hi everyone,

 

I am rebuilding an A series engine which has dished pistons (see photo). They seem like they're the standard pistons for this engine...

 

Anyway, I'm just wondering if my 12g295, which i had skimmed 88thou from it's standard thickness, would provide a decent compression ratio for this engine??

 

I'm also going to be putting in a MD266 cam. 

 

Have any of you ran dished pistons on a 'fast road' 998??

 

Many thanks,

 

Iain.

 

Photo:

 

[BritishRacingGreen]

Only way to know for sure is to calculate what compression ratio you have.

 

You'll be aiming for 10-10.5 with that spec.

[IainNeon91]

I'm currently running 10.1 with the 12g295 on my A+ with flat tops.

 

I just didn't know how much the dish will bring down the CR, i worked out it to be around 9.5-6...

[BritishRacingGreen]

what dish are they?

[IainNeon91]

I think they're 5.8cc standard size... That was from a quick search, not sure if that's right

[BritishRacingGreen]

Using 5.8cc dished pistons instead of flat tops in an engine that had 10.1 : 1 CR will give you a CR of 8.5 : 1.

 

If I've done the maths right.. But it's not really ideal for a fast road engine. If you wanted to use them you would need to take 6cc out of the head to get back to a sensible CR.

Edited by BritishRacingGreen, 07 March 2014 - 08:04 PM.

[carbon]

Yes, I ran a 998 using dished pistons and a skimmed 206 head (similar to 295) for several years. This went very well using a std 1300 cam in a light car.

 

I have no idea what the compression was, suspect somewhere about 8.5 or 9 to 1

[IainNeon91]

Using 5.8cc dished pistons instead of flat tops in an engine that had 10.1 : 1 CR will give you a CR of 8.5 : 1.

 

If I've done the maths right.. But it's not really ideal for a fast road engine. If you wanted to use them you would need to take 6cc out of the head to get back to a sensible CR.

Thanks for that.

 

I'm pretty sure a 1mm skim is about 1.9cc, is this right? 

[IainNeon91]

That would mean taking another 3mm off the head... It's already had a 2.25mm skim and apparently 4mm is about the max you can skim on a 295... Damn.

 

Flat tops it is then... haha

[miniman24]

Im running a modded 12a1456 head (pic below) on a 998 with dished pistons (5.8cc dish) and chose it because skimming a 295 to a 10.1 compression ratio on dished pistons would likely break the oil gallery or at least, make the engine un-reliable. My head has a smaller combustion chamber than a 295 so less needs to be skimmed to reach the 10.1 ratio, whilst still being a good head. I reckon flat tops are the way to go for you :)