4 replies to this topic

[Brams96]

I tried the search engine but without much luck so my Q is:

If I use a 1275 +.020 piston with an 11cc dish, what compression ratio would that give me and would it be any good? Would an 8.4cc dish be better or should I just go all out for a 1275 +.060?

Any thoughts from those in the know??

[Cooperman]

You need to do the calculation properly. An 11 cc dish is very large for a 1275 engine and will give a low compression ratio unless the head is skimmed the correct amount.
The correct amount will also depend on how far down the bore the top of the piston is from the deck of the block.
Then again, there is the queston as to whether your head is absolutely standard or has been previously modified and/or skimmed.
All these things need to be measured before final assembly. That's why a proper engine build is never cheap and cheap ones are never right.
Decide what engine capacity you want and why. As a very rough guide, going from 1275 to 1330 cc will give you more torque and approximately 4 bhp without doing anything else, so long as the comp. ratio is correct.
After getting the engine bored you need some linished down gudgeon pins to temporarily fit the pistons to the rods so that you can do what is known as a trial or dummy build before having the pistons press fitted to the rods. This ensures that the crankshaft will rotate properly and enables you to measure the piston crown to deck height. Only then can you work out what volume you require in the head to achieve the comp ratio you have selected. A good ratio would be about 10:1 or slightly higher.
I would recommend the Hepolite 21253 piston as the best value there is. Also, use the BK450 (TAM1521) head gasket which has a compressed volume of 4 cc (well, that's what I recently measured it as).
If you need more information with regard to measuring and calculating, either personal email me or come back on here.

[Brams96]

Cheers Cooperman. PM'd.

[mini7boy]

I tried the search engine but without much luck so my Q is:

If I use a 1275 +.020 piston with an 11cc dish, what compression ratio would that give me and would it be any good? Would an 8.4cc dish be better or should I just go all out for a 1275 +.060?

Any thoughts from those in the know??

In order to compute the compression ratio, you need to know the volume of the combustion chamber. These can vary quite a bit depending upon whether or not the chamber has modified by grinding or by skimming the head. Since the combustion chamber volume is the largest component of the unswept volume, it has the most effect of any unswept component on the Compression ratio.

Regardless. An 11cc dish is just about the biggest that is available in a piston as it comes from a manufacturer. Such large dishes are usually used only for turboed or supercharged engines because it is necessary to run lower compression ratios with them than for a normally aspirated engine. Any piston can have a dish added or increased/decreased by machining the crown of a piston of there is sufficient thickness to the crown to allow such machining. Custom piston manufacturers, like JE, Ross and Wiseco will normally allow the customer to specifiy dish volume along with crown thickness and pin height, thus allowing just about any compression ratio desired.

Without knowing your combustion chamber volume, thickness of compressed head gasket and ring land volume, it's impossible to know what you compression ratio is and if an 8.4cc dish would be better. Realize that increasing the bore size also increases the compression ratio, all else being equal.

If you are building a normally aspirated road engine, it is most likely that you will want a compression ratio between 9.5:1 and 10:1. But it really depends upon the cam you are running. Cams with more duration can generally tolerate/require higher compression ratios than low duration cams. Ask the manufacturer of your cam, or an A-series expert like AC Dodd which compression ratio to run with your cam.

[Paul Wiginton]

You would be better off with a 6cc which is an off the shelf piston.

Paul