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
