I had not considered the timing advance issue with the low engine rpm that will occur with a .6 ratio, and that brings up an interesting point. The primary reason for the ultra low rpm was originally to increase mpg and factory motors were designed to work well at that low rpm. My jeep, automatic trans, lockup torque converter, cruises along nicely at 1800-2000 rpm, heck it loves ultra low rpm. But it's fuel injected, computer controlled and has a low rpm cam profile. You can also bet the timing advance curve has been optomized for this ultra low cruise rpm.
Consider the older 302 powered Mustang GT's. The stock motors (about 225 horse) were low rpm engines! They would hardly make power above 5000 rpm (if that high). They typically came with a .6 overdrive 5 speed as well.
Timing advance curve plays a HUGE roll in mpg.
We typically build for decent horse power and that requires a cam profile that will work in the high 5000 or low 6000 rpm range. Compare that to my Jeep which runs out of power in the high 4000 rpm range!
Along with the advance curve question at low cruise rpm you also have to consider fuel/air ratio, I bet the engine is running way rich under these conditions and combustion is not going to be effecient for many of our motors. Perhaps a vacuum advance unit would be of help in this scenario (low rpm cruise). Perhaps a lower compression ratio and a leaner fuel curve would also help. Then get a really 'torque monster' ultra low rpm cam profile, like a truck, Jeep or motorhome would use. Something that makes max power at 4500 rpm.
Wait,,, are we building a truck or a Cobra? I lost track somewhere along the line.
And DON'T pay attention to Ernie, I tend to ramble and could be out to lunch on any given post.
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