Compute compression ratio?

B

BobM

Senior Member
I've got Offy 425 heads with 70cc's of head volume on unrelieved 276 block and a installed head gasket of approx. 050" thick, compression chart I have doesn't really give me exactly what I need but I came up with 8 to 1. Anybody have an opinion as to what my compression ratio is? I once had a chart that had "total" cc volume as opposed to "head" volume and included head gasket thickness, anybody know where I can find that one? Total cc volume as I understand is an educated estimate; could I seal the valves and piston off at TDC and with head and gasket torqued down and cc through the spark plug hole and fill to where end of spark plug would be to find total volume?
 
Yes, you could fill the chamber to determine actual volume. However, it is important that the deck be level in all directions and, from my experiece, it is difficult to get all the air pockets out. To do it accurately I have the piston down about 1/4" and fill-er-up with a documented volume, screw a graduated cylinder into the plug hole then slowly rotate crank to TDC and observe the expelled quantity which is then subtracted from the original, documented volume.
 
JWL said:
Yes, you could fill the chamber to determine actual volume. However, it is important that the deck be level in all directions and, from my experiece, it is difficult to get all the air pockets out. To do it accurately I have the piston down about 1/4" and fill-er-up with a documented volume, screw a graduated cylinder into the plug hole then slowly rotate crank to TDC and observe the expelled quantity which is then subtracted from the original, documented volume.
Click to expand...
I'm with you all the way until you got to screwing in graduated cylinder into head; where do I get one of those?
 
That part you have to make. I found that a fairly accurate guestmate can be had by measuring the cylinder head volumn in cc'c then adding 15 cc'c to that and using that for the combustion chamber volume. I then have a chart in the book that gives the answer. Close, with in a few decimals.
 
Try this.
 

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The best part of this chart is the fact that it offers the cc volume of the head. Unfortunately very few heads meet this requirement. So measure, before you assume the CR.
 
Ol' Ron said:
The best part of this chart is the fact that it offers the cc volume of the head. Unfortunately very few heads meet this requirement. So measure, before you assume the CR.
Click to expand...
I measured 70cc, but haven't removed exposed spark plug threads yet.
 
There is also the chart at http://www.flatheadv8.org/flatratio.htm. They list 63 cc as the head volume for the 425. I measured 64 on mine. I traced out the gasket to get an area of 15 sq in., which gives about 12cc for the gasket. I measured the piston dome height and calculated the volume as 14cc. The one part I couldn't easily measure was the volume around the valves. According to the numbers in the chart, that would be about 10 to 13cc depending on the relief. For my 285 cid and mild relief this gives about:
1 + 583/(64 + 12 - 14 + 11) = 9.0.
 
I should have mentioned that I use a plexiglas plate over the head when measuring the volume and then fill it up through a small hole with a 60cc syringe. This is better than directly filling it, because it is hard to tell when the water level is even with the top.
 
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