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mrl 03-05-2007 12:59 PM

8500 rpm vs 6500 rpm
I suppose this may sound like a vague question, but what makes one engine capable of 8500 rpm vs one that is limited to 6500 rpm? Is it strictly a matter of better internal moving parts being used? What about shorter stroke vs longer stroke? Is there a limit for a bore/stroke ratio? The following is fairly obvious: proper carb size and fuel supply, single plane intake, higher flowing heads, high perf valvetrain(proper cam, pushrods, retainers, locks, springs, valves, etc...), forged pistons, great rods. Does it come down to "what is the application, race or cruise" and build accordingly? If both are built properly, will an engine designed and built to peak at a higher rpm make more power than the same engine (type, displacement) which peaks at a lower rpm?

killerformula 03-05-2007 01:12 PM

To me it seems like you're thinking of it somewhat backwards. You'd build a motor to make power, not to run RPM. If people could get the same horse at 6500 RPM as they could at 8500 nobody would run 8500 because the stresses and costs are much greater. If a motor is set up to run 8500 with proper cam, compression, valving etc and has proper lowers, then yes, it will make more power than a similar motor which is capped for whatever reason at 6500.

Motors become less efficient as they spin faster. Cylinders tend not to fill as completely and frictional losses multiply. Power is defined as work over time, so even if your motor makes slightly less power per stroke at 8500 RPM (assuming it can run 85), you're running 25% more strokes per time unit. Horsepower drops off when your motor's power per stroke drops off faster than what is gained by running more strokes per unit of time.

Hope that helps, maybe somebody can explain it better-


johnsongrass1 03-05-2007 03:36 PM

peices that can withstand the punishment of rpm's dictate the max rpm. For most engine's, the valve train is the biggest problem.

You need to burn a specific amount of air and fuel to make enough work energy to get the job done. in this case, it's to move a car. You can adjust either the engine's part's parameters like or the RPM, both burn more A/F in the same amount of time.

So smaller engine like fours and sixes need more rpm to do the same amount of work of a bigger engine with less rpm.

A 350 can injest enough air at 9600 with the correct part's to make 650 hp.
A 454 can injest enough air at 5000 with the correct part's to make 650 also.

It take roughly ten times the money to make the 350 live at 9600.

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