Inliners International Forums General Discussion Engines What is the compression ratio?

I'm rebuilding a 1954 261 engine with a 1959 848 head and I've been told I should wind up with an engine of "about" a 9:1 compression ratio. I understand the concept (I think) in general terms but not much more. The changes to the engine are

.030 over Egge flathead pistons
head surface milled .003
block surface milled .003
crank bearings .010 / .010
valve sizes remain the same without porting done inside the cylinder head

My intent is to run higher than standard octane fuel, as needed to prevent knocking

Curiosity is driving me crazy, any help appreciated.

Ted

You'll need to be a little more precise with some actual measurements to be able to get a true number. First, you need to cc the combustion chamber of the head, you need to know the head gasket thickness, how much deck height the pistons have, and the bore and stroke. So to recap:

Bore size
Stroke
Combustion chamber cc's
Head Gasket thickness(actual volume will be more accurate to know on these gaskets)
Piston Deck height
Dome or valve relief volume(if true flattop with no dome or relief, then it will be 0cc)

All of the above values are required to calculate the compression ratio, so be as accurate as you can.

Ted,
You really do have to measure all the stuff CNC said.
However, as a first guess, IF only the stuff you mentioned changed, I'd say your CR went up by about 0.2, from say 8.25 to 8.48 or so.
But, while its apart is the time to measure it.
I always measure the OLD CR as its coming apart (deck, piston cc, head cc, etc) so I know the old CR and then the new CR

For dinner table discussions like this I find it helpful to use one of these Moroso Calculators (about $10) to reverse engineer the question.

Assume the 261 is bored .030" over. Now you are working with about 264 ci.
Assume DeuceCoupe is right saying you have 8.25CR with that 848 head. I dunno - if that is correct then we can proceed.

My calculator says your total chamber cc's are at 99 at 8.25:1. To achieve 9.0:1 CR you are going to need a chamber volume of 90 cc. Cleaning up the deck and head with a .003 cut is going to remove negligible cc's (I don't know the bore/stroke of this engine - if you do then you can calculate this amount more precisely.). I am going to guess it is about 1 cc, putting your final CR at 8.35:1. Today's fuels are of higher octane than when that engine was designed so I'm thinking that ol' bird will do just fine on regular grade pump gas.

The combustion chamber of the 59 848 head is about 79.1 cc. Not sure of the cc volumn of the compressed head gasket

This is getting more and more fun as we go along, thanks for all the information so far!

I spent time at my friends machine shop last night and we milled the deck to have the deck parallel to the centerline of the crank. The front of the block, next to cylinder 1, was exactly 0.047 taller than the rear of the deck, as it related to the centerline of the crankshaft. So we removed the front 0.047 and an additional 0.003 to ensure all of the pistons travel the same distance on the compression stroke, resulting in consistent performance. (we shimmed the block and removed 0.050 from the front and 0.003 from the rear of the block)

I also found out my machine shop has a 100 cc column for alcohol that I may borrow to measure cubic displacement in the head and later in the block as I assemble it. Stand by for more information!

One of the guys above asked for bore and stroke dimensions. According to the 1954 Truck Shop Manual

the bore is 3 - 3/4" (we took it .030 over, = 3.780")
the stroke is 3 - 15/16"

Thanks,

Ted

I found this compression calculator. It may help.

http://www.angelfire.com/fl/procrastination/motor.html

I built a 261 a few years ago,848 head milled only .010, Ross .080 flat top pistons with intake valve relief at zero deck,cam was 212 degrees duration at .050.. and so on.
I measured the 848 chamber at 79 cc's, compressed Felpro head gasket at .052
All added up, this gave an actual static compression ratio of 8.9. With are recurved stock distributor running 12 degrees initial advance ,36 total,vacuum advance limited to about 12 degrees,180 F thermostat the engine ran without detonation on 89-91 octane gas.
This was in a 50 Chevy truck ,3.42 gears and the original truck four speed.
You results may differ....
All the stock replacement pistons I've seen for 261's have the pistons sitting about .070 down in the hole at TDC.

Tony,

Terrific information, I brought the block home last night so will continue to take measurements and compare them to yours. Thanks!

Your comments generate another thought, on a different issue.

I also have a rebuilt 1959 235 (with 848 head)in my '51 stock truck with a RV cam (212 @ .050, etc.)and the total available vacuum from the engine is about 13. You are running 36 degrees advanced on the stock distributor, what rpm does your engine run at?

Thanks,

Ted

I never reved it beyond 4200 rpm. 36 degrees total advance also seemed to work the best on another 261 I built years ago. Maybe spending time on a dyno might find more information on the best timing...or not