37 replies to this topic

[minilee94]

I got asked today by bill Richards what the volume of my combustion chamber is I looked on the net on how to work it out and found a method of putting a small bit of greese round the CB chamber and putting a cd over it and filling the CB chamber up with fluid 5 ml at a time and counting how many you put in and that will calculate what CB chamber volume I have I worked mine out at 22 does this sound about right for a 12g940 head with 36ml valves thanks lee

Or have I been reading some bullcrap on the net again

[Artful Dodger]

That's how you measure it, that's a crude way of doing it but the results will be approx accurate

[minilee94]

Okay thanks I just thought id make sure before I tell bill Richards 22 lol so it its at 22 and I have 7 cc dished pistons what will the Cr be

[james962]

What bore size, how far down the bore are the pistons at TDC?

Edited by james962, 06 March 2013 - 10:05 PM.

[Cooperman]

I use a piece of 1/8" thick perspex with 2 holes in, one to put the fluid in and the other for the air to get out. Put a thin smear of grease around the edge, as stated and use, typically, paraffin, meths or even water and a 5 ml shrynge from Boots the Chemist.
It's fairly acdurate and easy to see when the chamber is full. The surface of the head must be level, so use a spirit level to achieve this.

[Cooperman]

Okay thanks I just thought id make sure before I tell bill Richards 22 lol so it its at 22 and I have 7 cc dished pistons what will the Cr be

Bring a piston to TDC and measure how far down the bore the piston top is at TDC. If it's a 1275 cc engine there will be 1 cc of volume for each 0.010" the piston top is down the bore.
Yes, and what is the engine size? Is it standard 1275, +0.020", +0.040" or something else. What is the part number of the piston (this will give the piston dish volume which may be 7 cc or not). With that info the CR can be calculated.

[minilee94]

The pistons are omega pistons and says on box 7cc dish and I don't know as far as how far down bore etc as I have not have it rebored yet etc and the engine is a 1275 being rebored to 1380

[Dan]

And what's the ring land and what gasket are you going to be using?

[Cooperman]

The pistons are omega pistons and says on box 7cc dish and I don't know as far as how far down bore etc as I have not have it rebored yet etc and the engine is a 1275 being rebored to 1380

If it's being re-bored you will be doing a 'trial-build' at which time the initial distance down the bore can be measured. The engine is then stripped and the block deck is normally machined to bring the piston tops to be level with the block deck at TDC or within about 0.003" of the top. Then that volume can be ignored.
However, with a 1380 and 7 cc piston dish, 22 cc chambers in the head and 4 cc gasket volume that would be an 11.24:1 compression ratio. That's a bit high, so you need the pistons to sit a bit down the bores. For 10:1 C.R. your piston tops need to sit 0.040" down the bores after the block deck is skimmed. If you can't get this you can have either a higher C.R. you can open up the combustion chambers a bit by further grinding out or machine a bit off the top of the pistons.
I've allowed 0.7 cc piston ring land, but that makes very little difference.
I built a turbo engine and had to increase the combustion chamber volumes by 3.4 cc and that was with 11 cc dished pistons.

Edited by Cooperman, 06 March 2013 - 10:30 PM.

[MRA]

That's how you measure it, that's a crude way of doing it but the results will be approx accurate

Approx accurate ? is that similar to almost nearly ??

[MRA]

Don't lets get carried away folk, the OP did ask for the combustion chamber size not the cc's of the engine.. Bill if he is building your engine will work all that out from the figure that you supply.

[phil hill]

That's how you measure it, that's a crude way of doing it but the results will be approx accurate

Approx accurate ? is that similar to almost nearly ??

Or a little bit pregnant ??

Back on topic, I did as Cooperman said: Perspex, smear of grease, 40ml syringe bought from a Vets (yes honestly !!) and some light oil. Syringe was graduated in 1cc increments, which was fine for me. I did both the Combustion chamber and built-up bottom end so the only "guess" was Head-gasket volume.

I seem to remember reading that a "standard" 12G940 chamber volume as 21.4cc, but I may be mistaken ??

Phil.

[minilee94]

It can't be 21 as mine is 22 and Cooperman bill is going to trial build it for me I am waiting for a engine build on paper to come through with al calculations on it Cr everything I need

[minilee94]

Dan what you mean ring land and im using a bk450

[Cooperman]

12G940 castings had different chamber volumes on different engines (Turbo for example), but the original standard volume was indeed 21.4 cc.
Bill R. will sort this out if he is building the engine and machine it all to suit.