The Stovebolt Site Forums The Shop Area Engine Shop 235 vs 261 Head on a 261

I have a 261 I'm building and I was wondering what the HP and torque difference would be between the stock 261 head and a 848 head, neither head milled. I'm using a Bull dog cam and the block is bored +.060. I'm going to use Langdon's holley 2 barels, finton headers. Just want a comparison because I have a couple of NOS 261 heads that I can use and I do not have a 848 head.In the end is there that big of HP difference in the heads. I know the compression ratio will be higher but nobody ever says you will get this HP with a 261 head and this HP with a 848 head.
37Chevy

Just by dumb luck I have a 261 with a 848 head on it. I thought I was purchasing a 235 from a neighbour, off of his barn floor. It needed a rebuild and then I discovered it was a 261, widely used in Pontiacs here from 1954 til 1962. A year after the rebuild, I heard that the 848 head was a desired head so I checked my numbers on the head and it was an 848. I have no clew how much horsepower it has.

I've got a "848" head on the 56 - 261 I'm runing in my truck. I also have an "848" head on a 62 - 261 on an engine stand that I plan to build for the truck. I think every little bit helps and that's the way I look at using an "848" head, or anything helpful, especially if you can obtain one easily.
"Every little bit helps!"

The Canadian Pontiac 261 left Oshawa rated at 148 H.P. with the 848 head and included steam holes. For comparison the triple carbed, dual exhaust, high lift cam (same as 261) 235 in the Corvette was rated at 150. Joe would you know if the 913 head (originally on the vette) had the same size combustion chamber as the 848 and if the early temp sender hole is the only difference? To the first question custom pistons (which are available and not too outrageous) address those compression issues without carving up the old iron and look like a good way to go to me.

Curt B

IIRC the 848's nominal compression is 8.25:1 on the 235, with a combustion chamber volume of 5.414 cubic inches (88.7cc).
A 261 with the 848 (assuming nominal chamber volume, which is seldom accurate) may be as high as 9.0:1.
I assume a volume error for most heads at 3-5cc, depending on the shape. A large area cast chamber like the stovebolt will favor 5cc, so the 88.7 may be closer to 94cc and yield 8.6:1 - but still a substantial increase.

One thing that is seldom explained sufficiently is why more compression is a benefit. Too often, the "more is better" approach is taken without any idea of what's actually being accomplished. Early engines (1920's) had compression ratios in the 4:1 range, or a bit higher, and they developed ridiculously low power for the size of the engine. As gasoline refining technology improved, and anti-knock additives such as tetraethyl lead became available, it was possible to compress fuel mixtures more without getting into the explosive "detonation" range of combustion chamber pressures. This process peaked with the mid-1960's factory hotrods, some of which had stock compression ratios of 10:1 or more. Those engines needed gasoline with 100+ octane ratings.

Enter the unleaded gas era- - - - -not a hotrodder's dream by any means! Compression ratios with unleaded gas need to be below 9:1, and 8.5:1 is better, even with a well-shaped combustion chamber and proper mixture and ignition timing adjustments. The camshaft profile has a lot to do with how high our calculated compression ratio can be, because valve timing has a definite influence on actual combustion chamber pressure.

In order to build cylinder pressure, both valves must be closed. Unless the intake valve duration is limited to 180 degrees of crankshaft rotation or less, part of the bottom of the compression stroke is used up before the intake closes. The average intake duration is at least 250 degrees, part of it happening before the piston reaches Top dead Center on the exhaust stroke, and the rest accomplished by keeping the valve open past Bottom Dead Center on the intake stroke, effectively shortening the compression stroke. As cam grinds get more radical, (longer intake duration) there's less of the compression stroke left to build pressure. It's necessary to squeeze the mixture tighter at the top of the stroke to achieve the same pressure that was there with a milder cam. Combining a high compression ratio head with a stock-duration cam is a recipie for lots of detonation- - - - -making it necessary to retard the ignition timing.

I can recall one situation where a friend insisted on using 12:1 pistons in a Triumph TR-4 engine, and staying with his stock-duration cam. That engine detonated so badly we had to set the ignition timing to 10 degrees AFTER top dead center, and the car had less power and performance than before we raised the compression! Too much cam or compression is a very bad situation. If you go with a longer-duration cam, it's absolutely necessary to bump the compression ratio up. The combustion chamber pressure is going to stay the same as original, or increase slightly, because you're compressing less of the total cylinder volume with the radical cam.

Bottom line- - - -with the Bulldog cam, depending on intake duration, the 848 head on the 261 is probably the best way to go.
Jerry

Does anyone have the specs on the bulldog cam????

Thanks to everybody for there opinion. The specs on the Bull dog cam are as follows.
Duration 254 Intake, 264 Exhaust
Duration @.50 212 Intake, 218 Exhaust .435 Valve lift.
I got these from Delta when I ordered my camshaft on Thursday and it shipped on Friday. Now that's service!
I have been told by several well know inliners that there is less than 4 HP difference between a 261 (6850) head and a 848 head built the same on the same short block.
37Chevy