Thread: 1937 Plymouth 5 Window Coupe

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Thanks Roger, I probably should have explained it better. Any cam you screw into the motor will have a certain cruise rpm range, where the cam will be effective. RPM's under that, due to using an overdrive transmission, will be disappointing in both acceleration and steady-speed fuel mileage. You don't have to take my word for it, here it is in black and white from the Crane Cams engineers.....read the left column to see what cam has what recommended cruise rpm range and also the static compression ratio that is recommended for that cam to work correctly. You cannot choose a cam for a motor until you have nailed down the exact static compression ratio. That's why I always recommend buying the cam next to last and the torque converter last. Been there, done that, many, many times. I guess it's just human nature though, fellows will run right out as soon as they unload the newest hulk off the trailer and buy a cam for it, without knowing any of the parameters that need to be in place to make an intelligent choice.
http://www.cranecams.com/56-67.pdf

Here's a for-instance.....
700R4 4th gear ratio 0.7:1
Ring + pinion ratio 4.11:1
Tire diameter 27"
RPM at 60 mph 2148
Looking at the Crane application list, a cam with something around 204 intake degrees @0.050" tappet lift would be a nice match, while moving up to 206 intake degrees would be outside the window. Of course, if you wanted to cruise at a higher speed, then you could use a cam with more intake duration. For instance, moving the cruise speed up to 70 mph (2506 engine rpm) would allow you to use a cam with 210 degrees intake duration @0.050" tappet lift. The exhaust doesn't matter so much, it's the intake closing point that matters.

I know this cam timing/gear relationship is totally shocking news to some fellows and I've been told on other forums that I don't know what I'm talking about (that's what most fellows say to me when I catch them on something that won't work and they're embarrassed over it, particularly if they've already bought the parts), but all it is is simple math. Do yourself a favor an read down through the rest of the Crane offerings, noting the cruise rpm's and suggested static compression ratios.

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