The Stovebolt Site Forums The Shop Area Engine Shop Compression ratio question

Does anyone know what the compression ratio was on 261 engines with the original cylinder head? (not the modified 848 that most people prefer)? I've got two of those heads, and I'm wondering how far I'd need to mill them to get the same compression as the 848? If you happen to know the CC volume of both types of combustion chambers, that would be very helpful, as I have CC testing equipment available. Thanks!
Jerry

Original 261 head (casting 3836850) compression ratio: ~7.2:1 to 7.5:1 stock (varies slightly by year; 1954–1962 truck engines).
848 head (235 high-compression, casting 3836848): ~8.0:1 to 8.25:1 on 235s, but when swapped to 261 without milling: ~8.0:1+ (higher than stock 261 due to smaller chamber).
Combustion chamber volumes (approximate, from forums/inliners):

Stock 261 head (850): ~60–62 cc (larger chamber, lower compression)
848 head: ~52–55 cc (smaller chamber)

To match 848 compression on a 261: mill stock 261 head ~0.125" (or use 848 head milled ~0.030").
CC your heads exactly (with valves/seals installed) for precise mill amount. Use a compression calculator (e.g., Wallace Racing) with your bore, stroke, gasket thickness, deck height, and measured cc.
Stock 261 was low-compression truck engine; 848 swap/milling is common for better performance.

A 261 has a compression ratio of 7.17:1 for 54 thru 55.2 and 7.8:1 for 56 and later versions, which I'm assuming had the 848 head. [nope the 848 head was only used on 235's - only one PN used on 261 heads, so don't know why the difference in compression ratio] This data is from the Chevy Truck Vehicle Info Kits. With the bore and stroke, you should be able to calculate the combustion chamber volume from those numbers. Displacement + cc volume /cc volume = compression ratio. Just a little algebra needed.

[on edit] Looks like Paul weighed in while I was digging with more exact info than I could find.

OK- - - - -thanks! The rule of thumb to bring the early 235 head up to 848 compression has always been to mill .090" off the early head, and recess the intake valves the same distance to prevent burning the edges of the valves from the flame front moving across the combustion chamber. It sounds like a similar cut on the 261 head might put me in the ballpark, compression-wise. To avoid pushrod length, rocker geometry, and rocker arm adjuster problems, it's a good idea to shim the rocker shaft stands up the same distance as the head is milled, and run a lash cap on the exhaust valves.
Jerry

Kevin: Could the difference in compression ratio be because of piston design? Dished vs flat vs domed?

Bill, yes it could, but I believe both the 235 and 261 both used flat top pistons throughout their runs. I used the formula I posed above to figure out the new compression ratio of my 216 with the 235 crank. Of course, I didn't change the head. Changing from a flat piston to dished in the same engine would lower the compression ratio, and going from flat to domed (assuming no interference with the head) would raise the compression ratio. I hadn't checked every year, but just did and the compression ratio on the 261 bumped up to 8.0:1 in 1958 and later, so there were two changes in CR over the years, first in 56, and then again in 58.

The 1961 Vehicle info kit showed the 235 having an 8.25:1 compression ratio, and the 261 having an 8.0:1 compression ratio. They share the same stroke, with the only difference being bore and head volume. So swapping any 235 head to a 261 would boost the compression ratio. Not sure exactly how much though without doing some math.

Thanks for all the info- - - - -I think I'm going to go with an .090" mill on the 261 heads, and be satisfied with that modification. I'll recess the intake valves to put the protective shrouding back in, and probably increase the exhaust valve size to 1.6", using the hotrod small block V8 stainless valves. The bored and stroked 261 engines I've got on the drawing board will also have close to 300 cubic inches with Japanese diesel pistons, H beam connecting rods, and slip-fit flanged removable sleeves.
Jerry

A good many years ago, I was on the phone with Tom Langdon. He was at work at GM so you know how many years ago this was. We were talking 235 and 261 chevy head chamber volume. He looked it up on his work computer and gave me some casting numbers and cc volumes. The 848 head was 79.1 cc, the 5913 was larger. The 261 had two head castings, one was the same volume at the 5913 head, the other was much larger. Give me a few minutes. I know I have it written down.

Edit:

So with all attribution to Tom Langdon, this is what I have:

3835913 is 86.2 cc

3836848 is 79.1 cc

The above are both 235 heads

For the 261, am I am not saying that these are the only head casting numbers for a 261, just the ones that Tom gave me

3836850 is 86.2, same as the 5913 54-55 235 head.

This one is tricky as my notes are fading so I am going to have to rewrite them and resolve a discrepancy:

3703570 (or it might be 3708570) is 95.5 cc.