23 replies to this topic

[coopdog]

right i have a 12g295 head and was told it had be lightly skimmed, so standard its 2.75" and mine is 2.6545 is that 100thou been taken off?
because that seams alot to me? :/

[coopdog]

right the inlet valvevs are bigger so im thinking the chamber had to be made bigger to fit them so the CR would of dropped so thats why its been skimmed?

[coopdog]

what cc of the chamber is suitable for flat top pistons?

Edited by coopdog, 24 May 2012 - 07:31 PM.

[coopdog]

You need about 26.5cc's for a suitable compression ratio, with a standard cam.......

what does the cam have anything to do with it?

[Spud_133]

YOu can run a higher comp ratio with a higher lift cam

[coopdog]

YOu can run a higher comp ratio with a higher lift cam

im going to use an mg metro cam

[jaydee]

If its for the turbo build, measure the chamber size and calculate the CR including gasket and piston dish (measure even if you have flat tops, maybe they come close to the top of the block and not exactly at top of the block) as you dont want an high CR for the turbo, you need about 8.3-8.5:1 CR.

[Foxy10]

iirc pal, you need 28cc for a turbo build.

[coopdog]

Nah its not for a turbo build :L thinks way to skimmed for that

[coopdog]

the head has about 25cc in the chambers, what kind of CR am i looking at with my flat top piston engine?

[jaydee]

Flat top N/A with kent 266 i'd go for 10:1 CR.

[thebullet]

10:1 is what I got mine skimmed to, seems nice with flat tops :)

[coopdog]

yeah its already been skimmed 80 thou but it has 25cc in the chambers, what CR will my car be if i fit it, thats what im asking :L

[jaydee]

In theory, thats what you would do, but you need to measure some elements to get it more accurate....
Engine size 998cc
Assuming flat top pistons with no dish and TDC to deck + piston rings area of about 1.5 cc
head gasket compressed volume - 2.8cc (some are 3.4)
chamber volume - 25cc
998/4 = 249.5
1.5 + 2.8 + 25 = 29.3
(249.5 + 29.3) / 29.3 = 9,51

So theoretical CR is 9.51:1

Try to work out the real piston area cc and head gasket volume (its also quoted by the producer by the way) to do more accurate maths.

(Edited wrong cc though)

Edited by jaydee, 25 May 2012 - 10:30 AM.

[thebullet]

I never went into that much detail, I should of to do a proper job. I think if I remember, I measure the gap from the piston to the top when it was at the top of its travel. I think anyway... Mini and metro then when I gave them the figures said 10:1.