cams and compression ratio

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Mar 3, 2004 | 09:25 AM

76 F150 Ranger XLT

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From: Crownsville, MD

cams and compression ratio

I notice that on the cam spec card posted on the Crane website they include a recommended compression ratio under each cam description. According to Crane, the 901 would be OK with 8-9.5 compression while the 941 would be OK with 8.75-10.75 compression. How critical is this? Will the 941 work poorly with stock 8.6 compression pistons or will you just not get the full benefit of that particular cam? And why?

Mar 3, 2004 | 10:32 AM

Scouder

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From: Evanston, WY

The reason for increased compression with increased duration is that the effective compression, called dynamic compression, is reduced with long duration. The intake valve does not close until the piston is already on it's way up the compression stroke. Longer duration typically causes it to close even later, reducing the amount of stroke that is "left over" to create compression. So, to compensate for the "lost" compression the static compression ratio is raised by a commensurate amount.

So you can see from this that if you don't raise compression with a longer duration cam the net result is lowered compression and the HP loss associated with it.

There are other factors to be considered when bumping compression like this, but the cam manufacturers are usually pretty close.

-Scouder

Mar 3, 2004 | 10:38 AM

76 F150 Ranger XLT

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From: Crownsville, MD

Thanks. That makes more sense than I expected. I am pretty certain my 1976 390 has stock 8.6 compression so I would probably be wasting money, gas, and horsepower if I went with the Crane 941.

Mar 3, 2004 | 03:47 PM

stephenw

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From: Central Oregon

You can also compensate for less than desireable CR by using their High-Intensity lifters. They bleed down a little at lower RPM's (oil pressure) and that has the effect of delaying the valve timing a bit, but after the engine gets a few more revs the cam timing comes back to spec and you get the high-end too! I did that with a 390 at Crane tech's suggestion, and it worked like a charm!
JFWIW. Steve

Mar 4, 2004 | 02:34 PM

76 F150 Ranger XLT

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I hate to demonstrate my ignorance, but what exactly is "bleed down" and do I need to be concerned about it?

Mar 4, 2004 | 06:46 PM

stephenw

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From: Central Oregon

There is what is called "bleed off" which reduces some of the dynamic compression with a more radical cam that Scouder is talking about, which helps to enable a higher CR.
The "bleed down" that I am referring to essentially shortens the High-Intensity lifters at lower RPMs because there is less oil pressure to pump them up to their max. This basically has the opposite effect of "bleed off", because it leaves the valves closed longer at low RPMs and allows use of a radical cam in a low(er) CR engine.
Camshaft design is a real art in many respects, so the cam mfr will have the best shot at matching up to your expressed needs/desires/application, rather than your buddies in the back yard! And I have found the Crane techs very helpful.
Good Luck, Steve

Mar 5, 2004 | 06:36 AM

76 F150 Ranger XLT

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From: Crownsville, MD

Sorry to keep dragging this link back to the top of the board but Your response leads to several other questions. When you installed the 941 and the high-intensity lifters did you do any oiling mods to the heads and/or install a higher volume oil pump?

Also, what CR were you running on the engine with the 941?