8 replies to this topic

[steve and tina]

Hi I am having a 998 engine bored out to 1098 My question is what is the difference between flat top and dish top pistons and do I have to get what ever was there before thanks steve

[jonny d]

flat top pistons will raise the compression

[Wil_h]

I don't think you have much option on piston choice at that size overbore. What overbore is that? can't be bothered working it out.

[Guess-Works.com]

no possible, that's what it is... +.120" would be close

[Wil_h]

Yes I did bother working it out in the end 1095 with +120. +120s are available in flat top only I think, you could put your own dish in though.

[Cooperman]

With flat-tops you may well need to open out the combustion chambers or the comp ratio could be much too high, especially with the increased swept volume. You need to measure and calculate accurately.
It would be good to go for about 10.2 : 1 comp ratio.
A 998 bored to almost 1100 cc is a great engine, although you do run the risk of block-scrappage when going that big as you never know quite what manufacturing tolerances your block has in terms of cylinder wall thikness. The most I've gone to is 1061 cc, that's +0.080".
I think those +0.120" pistons are made by Hastings, a highly-regarded US manufacturer.

[1977 Loud_Mini]

How do you work out what cc the engine will be after being bored then? I have a 998cc engine sitting in my garage and its +.020 oversize, i thought it was 1015cc?

[Cooperman]

The volume of a 4-cylinder engine is:
piston top area x stroke x 4

Piston top area is calulated by the following formula: piston diameter (in cm) squared x 3.142 divided by 4

So, for a +0.020" overbore, the new piston diameter 2,543" + 0.020" = 2.563"

2.563 x 2.54 = 6.51 cm

Thus piston area is (6.51 x 6.51 x 3.142)/4 = 33.28 sq. m.

Multiply by the stroke, whih is 7.62 cm and the volume of one cylinder = 253.67 cc

So, for 4 cylinders this equals 1014.67 cc, so your 1015 cc is correct.

From this you an go on and, after doing a trial-build to measure everything before final machining, work out what size the combustion chambers must be for a specific compression ratio, but that's another set of calculations.

There is quite a lot of calculations and accurate measuring when building an engine.

Edited by Cooperman, 26 February 2010 - 03:46 PM.

[1977 Loud_Mini]

Thanks Cooperman, very much appreciated.