21 replies to this topic

[Daniel_moody25]

Going to be boring out a 1275 to 1293 and obviously I need new Pistons so my question what is better flat top or dished? I theory a flat top would have a higher compression ratio so more power if I'm right so would that make flat tops better

[Spitz]

I'm not an engine builder.....but

I think you need to work out what your compression ratio is going to be with either one.
Too high, and I doubt you'll run well with regular pump fuel...or even high octane, without an additive

[Craig89]

Dished everytime on a large bore engine. Flat top pistons would result in a compression ratio of about 12.5:1 with standard head chambers, which, with these engines, is much too high for 95% of applications.

But as said above you need to do the CR calculations to determine what you need

[Daniel_moody25]

I don't know any of the measurements to even start working out the compression ratio, also at what would be a decent compression ratio to stick to?

[Craig89]

That depends entirely on what sort of engine you want. Standard then about 8.5 - 9.5:1, modified 9.5 - 10.5:1 race >10.5:1.

But it all depends on the spec, primarily the camshaft profile.

Take a look here for calculations:

http://www.theminifo...75-60-cooper-s/

[jaydee]

Use dished to get a better combustion chamber, then skim the cylinder head to achieve the correct CR.

[Daniel_moody25]

The cylinder head is getting a skim anyway, So these would be fine? http://www.minispare...px|Back to shop

[Craig89]

I'm sure they would be fine, these will be good also

http://www.minispare...|Back to search

But you haven't said what your plans for the engine are, is it just a standard rebuild?

[Daniel_moody25]

Oh yeah sorry, well I plan on doing well a normal rebuild but adding a cam ( thinking 266 at the moment) was going to do roller rockers but looking at a recent topic on here they don't seam worth it, so standard rockers, double valve springs, Vernier timing gear. Maybe dry deck depending on price of everything. Going to be a carbed engine with an aldon ignition. Standard box and diff. I want to be fast road that I might do the occasional track day with

[Craig89]

Pretty much exactly the same spec as the engine I am building. Wouldn't bother with the dry deck, that is also a race only thing really. Spend your money on a top spec cylinder head, that's where the real gains are unleashed.

by the way you want around a 10:1 CR with that spec

[Mini Manannán]

It's been said many, many times: you really need to work it all out before you start buying stuff willy-nilly and plonking it in.  Here is Coopermen's CR calculator: http://www.theminifo...o-calculations/

 Also his rebuild measurements: http://www.theminifo...-2#entry3092350

[Spider]

With the same CR in a flat top vs dished, you'll usually get more power, particularly at lower revs when running flat tops, the engine will also tend to run slightly 'cleaner'. Flat tops offer a better squish at TDC than dished.

[Daniel_moody25]

Cheers guys, I don't have the engine just yet but was wonder what would be better. I'll make sure I do all the measurements once I've got it and the rebore has been done, but basically i what I should be aiming for is a cr of 10:1?

[kezzkitkat]

to be honest you will be defeating the object with flat top pistons as said cr will be to high
Then the only way to lower the cr is the chamber size
Quite a few people seem to think dish pistons are for forced induction but this is not true
Dish in the pistons do vary in cc but manufacturers will give a rough cr ratio with there pistons

[kezzkitkat]

http://m.ebay.co.uk/...880?_mwBanner=1