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Hi all. My 347 is being built for a road car (65 Mustang). My question is, can I run hi compression and keep the cam timing mild to make it nice and drivable? Was think the hi compression would help make hp, mild cam timing would keep it drivable.

Thoughts please.

Thanks, Dave
 

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Cam timing

Hi all. My 347 is being built for a road car (65 Mustang). My question is, can I run hi compression and keep the cam timing mild to make it nice and drivable? Was think the hi compression would help make hp, mild cam timing would keep it drivable.

Thoughts please.

Thanks, Dave

Yes you can but just because you can do something DOES NOT mean that you should. If a high static compression ratio is used with a mild cam (i.e. and early intake valve closing point) then the mixture may end up being "over-compressed". This will lead to excessive compression losses, detonation and could even lead to head gasket or piston failure.
On the other hand, an aggressive cam with a late intake valve closing point will work well at high RPM. But at low RPM the intake valve will close too late for sufficient compression of the intake charge to occur. This will result in torque and performance loss. If a low static compression ratio is used with an aggressive cam then the mixture may end up being "under-compressed". The correct way to run high compression is to use a high performance cam with long duration. That way the engine can benefit at high RPM from the maximized amount of intake charge afforded by the late intake valve closing, and still achieve sufficient compression of the mixture as a by-product of the dynamic compression ratio.
 

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As long as no detonation is occurring, raising compression 1 point will increase hp by roughly 3-4%, up to approximately 13:1, where the increase in hp falls off to more of a 2% gain, since the energy required to compress the mixture also increases. Fuel mileage will also rise with higher compression. Again, assuming there's adequate octane in the fuel to prevent detonation.

With 93 octane gasoline and a mild camshaft up to a range of 210-230 duration @ .050", you can get away with about 9.5:1 compression with iron heads. With aluminum heads, you can go 1 full point higher than you can with iron heads. Raising higher compression than that will probably cause detonation, which you can alleviate by retarding the ignition timing, but that reduces power and efficiency. In that situation you could also retard the camshaft timing with an adjustable timing chain set to slightly lower the cylinder pressures at low rpm, but that's a lot of work once the motor's in the car, and might require experimentation to get it right.

With a more aggressive cam grind, you can sneak in higher compression, but with all the variables, it's somewhat chancy. I'm running 10.95:1 in my street motor, but the heads are an alum canted valve design, and the camshaft is pretty aggressive, with 260 duration @.050". Even so, 93 is barely enough for it, and I need to shut it off in gear when it's hot, or it will run on.
 
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