7 replies to this topic

[johnv]

What’s the difference between pistons described as “low compression “ vs ones described as “high compression”? Is it just the dish size? If so, surely it’s up to the head etc to decide? Thanks

[Ethel]

Yes, just the dish. Obviously there can be other differences between makes etc.

[Cooperman]

If comparing the 21253 high-comp pistons with the 21251 low-comp you will find that the oil drain slots/holes are different with the low comp thus not being as strong in terms of overall robustness.

[Spider]

As well as dish size, some have a lower Pin to Crown height.

[sonikk4]

It does depend on the chamber size as well if the heads been modified, skimmed, block decked and so on.

 

6.5cc dished pistons, 21.5cc chambers and the block machined equated to a compression ratio of over 11.1 (cannot remember the exact figure that Oz said) now going for 10cc dished pistons to bring the compression ratio down to a more manageable level.

 

Not a standard engine by the way.

[Ethel]

That's true, I suppose it depends on why Johnv is asking.

 

The factory produced high and low compression models by just changing the pistons, that was to cater for different grades of fuel in different markets mostly. Production cylinder heads only vary in chamber volume where they are also different castings, usually with different volume pistons too e.g. 998 Coppers using 12g295 heads with raised crown pistons.

 

If you're putting your own modified engine together then you have to take everything in to account - You could open up a 12g295 for better flow & use dished pistons, but still get the compression higher than a 998 Cooper by skimming the head, though flat tops would be preferable.

[johnv]

Cooperman answered it really .. I can calculate the dish I need, I just wondered if I opted for a low compression piston would I be compromising on the performance of the piston .. it seems I would

[Cooperman]

The 21253 is an excellent piston. I have used them in rally engines with a CR of up to 10.5 and then revved them to 7000 rpm with no problems.

 

In my full-on rally Cooper 'S' I have usually used Karl Schmidt pistons which are fantastic, but they are very expensive and I believe I payed about £500 for the last set I bought which were +0.040" (i.e. 1310 cc).

 

I don't think that there are any 'bad' pistons on the market, just that some are a better design and finish than others.