Inliners International Forums General Discussion Hi Performance 848 head,one more time

I searched...but... Awhile back the combustion chamber volumes of 235-261 heads were listed.....Anyone have them to see ? Thanks

That may have been me. A few years back tom Langdon of Stovebolt gave me some casting numbers and combustion chamber volumes while he was still working for GM. Tom is a friend so I have to give him his due. I posted them a while back for a fellow who had a bad machine job done on his 848 head. I don't have them handy right now but if you drop me an email mdonohue05@aol.com I will send them too you as soon as I can dig them out. I seem to remember 848 heads at 79.1 cc, some other 235 heads at 82 or 83 cc and 261 heads at like 95 cc.

Depends on whether they're giving the complete chamber volume (which includes the deck clearance, gasket volume, and head interior, and can be calculated by: (displacement ÷ 6) ÷ (CR-1). If a 235 with 848 is 8.25:1 (as some are, IIRC), the complete chamber is 5.41 in3, or 88.7cc.
Obviously, the head itself is always smaller, but how much...?
-.030" deck is another 4.9cc, and a .030" head gasket is 6.9cc, so it could be 77cc or near.
Remember that all engines with cast chambers are a bit bigger than nominal, so expect an extra 3-5cc just for casting irregularities.

Mdonohue05,

Thank you for posting those when you did. I copied them at that time and have them handy:


3836848 79.1 cc's
3835913 86.2 cc's
3836850 86.2 cc's
3703570 95.5 cc's

Thanks,I'll assume this is just the combustion chamber and not the total cc's.As always,you have to cc the chambers yourself to be sure.Generally speaking head cc's specs are just the combustion chamber,at least for popular engines.If this is the whole volume ,it's a whole different deal.
Using 80 cc for an 848 head on a .060 overbore on a 261 with flat top pistons,.010 below deck with a .060 head gasket is about 8.7 compression ratio.About because the head gasket is odd shaped.
If 80 cc is the total volume then the compression ratio is about 9.25

There is a great chart/formula for compression on the KB/Silvolite website..If you use your cam timing the ratio will definately disappoint you.............JD

Wow, what a difference! My static was 9.97 and my dynamic was 8.25. I guess that's good since I'm turbocharged?

IMHO the use of DCR is over-rated. Yes, it tells you if you cam is too big for your static CR and vice-versa, and some indication of low speed knock resistance, but that's about it.

Yes,I also think the DCR is often just numbers to talk about.It certainly might matter for a highly modified engine making a lot of power.But for vintage engines lucky to make .7 HP per cubic inch,I doubt it matters much.Just go with what's known to work well.

I have played with these 235's for a lot of years and I can tell you that even though the 848 head theoretically should get you 8.5 CR, it is rarely the case. the machine work, particularly the block deck heights rarely come up to spec. In addition, the production chamber volume spec rarely is accurate. Careful machine work will get you a clean 8.5 but more than that, in my experience is sometimes difficult to obtain with flat top pistons and/or without really decking the block and head (which gets into all manner of valve train geometry). I am glad I could help you with the chamber volume.

Machining the head and or block does not change the relative positions of the rocker, valve or stand.
The geometry is only going to be off if:
1. you run out of pushrod adjustment (and you should replace them when this happens)
2. the intake seat has been sunk to prevent P/V contact.
What makes the geometry irritating to adjust on the engines is that the valves aren't at the same depth, and not parallel.
The intake is close enough that you can just the stands by the same amount as the new seat depth (sink the seat .1000", raise the stands .0998" due to the small angle).
Unfortunately the exhaust valve hasn't moved at all. If the stand is raised by the same .100", it's too high on the exhaust stems, which need to be longer by about .104" (due to the angle). An .080" lash cap is probably close enough to work.
The geo is much more likely to be off due to lift changes, since the original is very low.
The "mid-lift" (50% lift @ 90° rocker angle) adjustment method doesn't work in these engines, the rockers were designed for another purpose.