Head and piston volumes....
Turbo Mini Forums

Just finished measuring up my head and pistons, for compairion purposes :)

The head is a std 12g940 turbo head and the piston is a std size 20831.









As you can see I've used slightly less accurate tools.. but as a guide its good. The fluid was auto-trans oil, if that makes any difference.

I measured both the head and the piston 3 times each to get a decent average:

Head 24cc
Piston 12cc

Does this seam about right :s

Alex

Edited by AlexF2003 on 27th Feb, 2004.

AlexF

I didn't think the std piston was that big, but I could be wrong

Team Racing

Yes, 12cc is right for a standard Turbo piston. I've measured a few sets and thy've all cone out to that.

Phil.

WWW.TURBO-MINI.COM

12cc very right , but thought std head 22/23 ish .
on std turbo c/r =9.4 working backwards your looking at 38cc total to get that , if you have 36 already you only need 2 from gasket /ring land and block to piston height !!!

Everyone knows that instructions only have to be read if the thing doesn't work....

The 'turbo' piston is that size, but i do think the crown is a little taller than the 'standard' A+ piston (almost flush with the top of the block), which also needs accounting for

arr that makes sence Dave...

If I put the deck height as about zero then the CR comes out as nearly 9.4:1

So using the following figures....

Bore 81.4mm
Stroke 70.61mm
Gasket Volume 4.00cc
Ring land Volume 0.78cc
Piston Volume 12.00cc
Head Chamber Volume 21.4cc
Deck Volume 0cc

I get:

Compression Ratio 9.35

Alex

AlexF

so yours is not a std head then

Everyone knows that instructions only have to be read if the thing doesn't work....

I am drawing the same conclusion!

I looks std... theres no grinding marks and no porting.. the only thing thats unusual about it is there is no ware in the valve guides... so either its done very few miles or they have been re-done.

I'll have pull another head off today and have a measure...

This is of course assuming I haven't made a mistake measuring this head!

Alex

AlexF

surely you'd visually notice that much difference in chamber size?

If it is that though, it must count as being one of the worst ever!

we can see 3 chambers in the pic , and which ever way your counting from the chamber next to the end "is" not 3cc different , like dave says 3cc is very noticeable i would have thought.

Everyone knows that instructions only have to be read if the thing doesn't work....

Cylinder 1 is most left in the picture...

I'd of thought you could see 3ccs as well !!!

But I have measured each chamber 3 times... I even checked the kit I am using is accurate, and its medical quality and accurate to +or- 0.5cc

Alex

AlexF

well like dave says , quality control at its best!!

Everyone knows that instructions only have to be read if the thing doesn't work....

I'll pull another turbo head apart tonight and see if its me or my head :p

alex

AlexF

I don't know what the valve heights look like, but this makes a big difference to chamber volume.
For example, assuming 33/29 (3.3cm/2.9cm) valves;

[pi*(D^2)]/4 is the formula for working out area, hence:

[pi*(3.3^2)]/4 = 8.55cm^2
and
[pi*(2.9^2)]/4 = 6.51cm^2

hence if both valves sat just 1mm higher than those in the next combustion chamber along;
8.55*0.1 = 0.855 and 6.51*0.1 = 0.651; total = 1.5cc difference!

This could be an explanation, or at least is a good example of why valve heights must be equal between all chambers!

D

hmm at the moment I don't have a way to measure that...

I seriously need to buy some more tools :)

Alex

AlexF