6 replies to this topic

[jackson-minis]

need abit of help if anyone can spare a minute
these are my calculations so far:
1310cc = 327.5cc per bore
8.4cc 21253 +40 pistons
ringland 0.8cc
headgasket 3.4cc
standard big valve head 21.4cc
piston to block height ( havent got the pistons as of yet but ive been told to use an average of 3cc because the block hasnt been decked before? )
with these calculations i get 9.8:1 cr, im aiming for 10.5:1 (is this right for my cam?) (piper 285). To achieve this i calculate i need 2cc off the head to increase it to 10.35:1 cr or 2.5cc off the head for 10.49:1
assuming that the head is standard (which it is has been checked) how much in thou would need taking off roughly to reduce 2.5cc volume?
thanks alot
Gazz

[pikey7]

RLV of 0.8cc? naaah. more like 0.08cc. piston to bore clearance as a rough rule of thumb... but it differs for different manufacturers... should be about 1000th of the piston size, so in your case about 0.0072mm. it's tiny.

you also need to KNOW the deck height. 0.15mm difference makes 0.1:1 difference in your CR, so guessing does you no good.

I calculate your current CR as 10.02:1, needing to get to 19.6cc head chamber volume to get 10.5

How much that is that need to be skimmed is anyones guess as it depends on the chamber shape. you must know the csa of the chamber to calculate it. The best way anyway is to speak to you machinist/engine builder and tell them the target head volume and let them skim to suit. You also need to ensure that each volume is the same. Kinda makes no sense if they aren't.

Edited by pikey7, 13 October 2010 - 02:54 PM.

[jackson-minis]

Brilliant mate, thanks for that!

will have another check over, and do some more accurate calcs i.e deck height, will do a dry build first and go from there.

Gazz

[Dan]

Also you must measure and calculate your cylinder swept volume, not just divide the (rounded off) total engine size by 4.

[Ethel]

land volume will be more than .08

Piston clearance may be 1/1000th of the bore to get the volume you have to multiply it by the piston height above the top ring then the circumference of the piston - Length x breadth x height is a volume.

You might find the David Vizard videos on youtube that we were looking at a few nights back of interest. Apparently 8.5 to 1 is the figure to aim for, but we should be calculating an entirely different ratio.

[pikey7]

yeah, but the volume is then minus the piston diameter. it's only that small 2-and-a-bit thou sliver...

Edited by pikey7, 14 October 2010 - 11:25 PM.

[Ethel]

erm ok, still seems like a very small figure - I didn't do the sum, the Vizard book quotes 0.78cc in its "how to calculate CR" example, could that be a mistake too?


Actually your deleted sum looks right. Piston diameter is already taken in to account

Height = ring to crown, width = bore clearance, length - the circumference of the bore, straightened out.

If you haven't yet seen the Vizard vid, it's worth a look - makes a very good point that static compression doesn't matter so much: it's the effective compression ratio after the valves close.