Cubic inch horsepower at given RPM

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rapidride2
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Cubic inch horsepower at given RPM

Post by rapidride2 »

Is there a calculation or calculator or rule that would dictate an RPM difference where peak horsepower/torque would happen in the RPM range based off of a difference in cubic inches?

Example: 496 chevy with matching parts has peak power
at 6300.
468 chevy with matching parts has peak power
at 6800 to make the same hp as 496
rapidride2
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Re: Cubic inch horsepower at given RPM

Post by rapidride2 »

I think i found it on Wallace.

Estimate RPM That Peak Horsepower Will Occur????
Enter Head Air Flow in CFM (at 28 in. of water):???
Enter Engine CID: ???


Assuming I enter same head flow for each cubic inch calculation I "think" this will work.
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Re: Cubic inch horsepower at given RPM

Post by Tom68 »

rapidride2 wrote: Tue Nov 29, 2022 4:03 pm Is there a calculation or calculator or rule that would dictate an RPM difference where peak horsepower/torque would happen in the RPM range based off of a difference in cubic inches?

Example: 496 chevy with matching parts has peak power
at 6300.
468 chevy with matching parts has peak power
at 6800 to make the same hp as 496
That's not always the result depending on what else is limiging the engine and what is creating its peak.
An 80s style long runner manifold is gunna limit the revs of peak horsepower on any size motor.
Ignorance leads to confidence more often than knowledge does.
Nah, I'm not leaving myself out of the ignorant brigade....at times.
rapidride2
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Re: Cubic inch horsepower at given RPM

Post by rapidride2 »

I see. But if all parts are the same except cubic inches shouldn't the rpm difference be accurate.

Im basically trying to see how much more or less two different cubic engines make the same power at.

Such as how much more rpm i would have to turn a smaller displacement to achieve same power of a larger displacement.
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Re: Cubic inch horsepower at given RPM

Post by rapidride2 »

I see. But if all parts are the same except cubic inches shouldn't the rpm difference be accurate.

Im basically trying to see how much more or less two different cubic engines make the same power at.

Such as how much more rpm i would have to turn a smaller displacement to achieve same power of a larger displacement.
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Re: Cubic inch horsepower at given RPM

Post by Stan Weiss »

Maybe Mike Jones will way in. But I don't see the 2 engines wanting the exact same cam.

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Re: Cubic inch horsepower at given RPM

Post by panic »

No.
The larger engine will pull more vacuum from the same heads, so the CFM @ 28" input isn't correct.
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Re: Cubic inch horsepower at given RPM

Post by F120 »

you are assuming that torque increases linearly with displacement
Horsepower = Torque x RPM / 5,252
496 500ft-lb at 6300rpm is 600hp
468 463ft-lb at 6800rpm is 600hp
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Re: Cubic inch horsepower at given RPM

Post by PackardV8 »

Studebaker V8 heads are intake CFM limited to an approx max of 220. I ran a DynoSim of the OEM displacements and made the same max horsepower, but at different RPM. IIRC,
224" - 8300 RPM
259" - 7300
289" - 6800
305" - 6300

And yes, of course, each required a specific cam profile to make that horsepower.
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Re: Cubic inch horsepower at given RPM

Post by F120 »

displacement x rpm / new displacement = target rpm

have fun :lol:
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Re: Cubic inch horsepower at given RPM

Post by rapidride2 »

F120 wrote: Thu Dec 01, 2022 6:49 pm displacement x rpm / new displacement = target rpm

have fun :lol:

Thank you!!!
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Re: Cubic inch horsepower at given RPM

Post by panic »

displacement x rpm / new displacement = target rpm

or not
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Re: Cubic inch horsepower at given RPM

Post by panic »

If the heads, cam, etc. are too small, the bigger engine will make very little more power.
If the heads are too big, the increase will be closer to the displacement ratio.

NEVER linear as described.
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Re: Cubic inch horsepower at given RPM

Post by rapidride2 »

panic wrote: Fri Dec 02, 2022 9:39 pm If the heads, cam, etc. are too small, the bigger engine will make very little more power.
If the heads are too big, the increase will be closer to the displacement ratio.

NEVER linear as described.
Okay, I was taking into account that each engine was an optimized combination.
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