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Honda Cub (Passport) Forums
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Latest updates on progress of the C70 commuter
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<blockquote data-quote="69ST" data-source="post: 174435" data-attributes="member: 5"><p>That's a <em>BMF</em> variable. Minimum is one thing, what about the maximum & mean average? Add a headwind and the power requirement can go off the proverbial scale. Then, there's the gearing, especially on long grades. On a 10% grade, I'd expect to see ~70-80% of top speed potential, <em>sans headwind</em>. That assumes a completely healthy engine that is tuned & geared optimally. As long as the engine can make a certain hp number, the only effect of cam profile would be the rpm at which that occurs...you'd match the gearing to allow this. That said, something here isn't right. If you end up downshifting to first gear and the motor can't pull that, let alone without overheating (and that's the sense you're giving), horsepower is down...way down. 20mph should only require ~1/3 the power needed to reach 40mph. In stock trim, it should be damned near impossible to overheat one of these motors.</p><p></p><p>You've introduced some interesting variables: head, carburetion & gearing. Having a 4-speed tranny would be helpful, in that crucial midrange. But that isn't going to solve the overheating/pre-igntion & power loss. First thing I'd want to know is why you're getting pre-igntion. In stock form, 87-octane should suffice...at sea level. Does running pump "premium" (91-93 octane) make any difference? What's the static compression ratio of this engine, as fitted with the aftermarket head? Is the air:fuel ratio near the rich limit? What kind of oil temps are you seeing? That would speak volumes. I could see adding a high volume oil pump and, possibly an auxiliary cooler to bring down both oil & head temps.</p><p></p><p>If you're considering going back into the motor, a stroker crank will add more oomph than any other mod...and the longer the better.</p></blockquote><p></p>
[QUOTE="69ST, post: 174435, member: 5"] That's a [I]BMF[/I] variable. Minimum is one thing, what about the maximum & mean average? Add a headwind and the power requirement can go off the proverbial scale. Then, there's the gearing, especially on long grades. On a 10% grade, I'd expect to see ~70-80% of top speed potential, [I]sans headwind[/I]. That assumes a completely healthy engine that is tuned & geared optimally. As long as the engine can make a certain hp number, the only effect of cam profile would be the rpm at which that occurs...you'd match the gearing to allow this. That said, something here isn't right. If you end up downshifting to first gear and the motor can't pull that, let alone without overheating (and that's the sense you're giving), horsepower is down...way down. 20mph should only require ~1/3 the power needed to reach 40mph. In stock trim, it should be damned near impossible to overheat one of these motors. You've introduced some interesting variables: head, carburetion & gearing. Having a 4-speed tranny would be helpful, in that crucial midrange. But that isn't going to solve the overheating/pre-igntion & power loss. First thing I'd want to know is why you're getting pre-igntion. In stock form, 87-octane should suffice...at sea level. Does running pump "premium" (91-93 octane) make any difference? What's the static compression ratio of this engine, as fitted with the aftermarket head? Is the air:fuel ratio near the rich limit? What kind of oil temps are you seeing? That would speak volumes. I could see adding a high volume oil pump and, possibly an auxiliary cooler to bring down both oil & head temps. If you're considering going back into the motor, a stroker crank will add more oomph than any other mod...and the longer the better. [/QUOTE]
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