Peak RPM/HP

General engine tech -- Drag Racing to Circle Track

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11secAvanti
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Peak RPM/HP

Post by 11secAvanti » Mon Nov 26, 2018 10:45 am

What determines and controls the Peak Rpm Hp Point of 4 cycle engines? By point I mean the highest arc or crossover point as seen on dyno graphs and printouts. Thanks

Rowdy Yates
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Re: Peak RPM/HP

Post by Rowdy Yates » Mon Nov 26, 2018 11:13 am

Cam duration and Lsa I would think.

Rick360
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Re: Peak RPM/HP

Post by Rick360 » Mon Nov 26, 2018 11:18 am

Nearly every component plays some role in setting peak hp rpm in every combination.

The size and shape of the intake tract is by far the most influential factor in determining peak hp rpm.

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Re: Peak RPM/HP

Post by hoffman900 » Mon Nov 26, 2018 11:18 am

Choke flow causing the plateauing of air mass, assuming the engine isn’t artificially running out of fuel.

Whether that be at the valve, pushrod pinch, carburetor venturi, restrictor plate, etc.

Your lawnmower, NASCAR, jet engines, Formula One, etc all work that way.

https://en.m.wikipedia.org/wiki/Choked_flow
-Bob

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Re: Peak RPM/HP

Post by twl » Mon Nov 26, 2018 11:21 pm

Technically speaking, peak hp is reached at the rpm point where torque begins falling off faster than rpm can increase.

Varous factors can influence it, but usually it amounts to running out of breathing for some of the reasons already mentioned above.

digger
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Re: Peak RPM/HP

Post by digger » Tue Nov 27, 2018 5:31 am

hoffman900 wrote:
Mon Nov 26, 2018 11:18 am
Choke flow causing the plateauing of air mass, assuming the engine isn’t artificially running out of fuel.

Whether that be at the valve, pushrod pinch, carburetor venturi, restrictor plate, etc.

Your lawnmower, NASCAR, jet engines, Formula One, etc all work that way.

https://en.m.wikipedia.org/wiki/Choked_flow
Most engines won't actually have have choked flow in inlet. A restrictor engine yes but but otherwise generally not as the engine looses filling efficiency before that point

mag2555
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Re: Peak RPM/HP

Post by mag2555 » Tue Nov 27, 2018 7:44 am

In a NA motor in a magical world if you had unlimited Cam, port area/ airflow and unlimited rpm potential then at point the motors internal fiction load would match the HP being made and the HP curve would then hit its peak and nose over.

pdq67
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Re: Peak RPM/HP

Post by pdq67 » Tue Nov 27, 2018 10:59 am

I like engines made so that both Hp and T keep going up past where they cross at 5252 rpm!

pdq67

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Re: Peak RPM/HP

Post by LSP » Tue Nov 27, 2018 11:40 am

11secAvanti wrote:
Mon Nov 26, 2018 10:45 am
What determines and controls the Peak Rpm Hp Point of 4 cycle engines? By point I mean the highest arc or crossover point as seen on dyno graphs and printouts. Thanks
Intake runner length, cam can rock the curve some, but not move the peak hp much.
Wish I had time to be a Guru

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Re: Peak RPM/HP

Post by gruntguru » Tue Nov 27, 2018 10:08 pm

pdq67 wrote:
Tue Nov 27, 2018 10:59 am
I like engines made so that both Hp and T keep going up past where they cross at 5252 rpm!pdq67
So you like little engines?Image?

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Re: Peak RPM/HP

Post by David Redszus » Wed Nov 28, 2018 12:24 am

The maximum torque or power that can be developed is determined by the amount of fuel that can be burned.

The amount of fuel that can be burned is determined by the amount of air mass trapped in the engine.

Trapped air mass per stroke determines torque.
Trapped air mass per unit time determines power.

The key is "the amount of fuel that can be burned", not merely the amount of fuel sent to the engine.

Torque, at any given rpm determines power.
If torque is fixed and rpm increases so will power.
If rpm is fixed and torque is increased so will power.

If both are increasing, peak values have not been reached.

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Re: Peak RPM/HP

Post by Olds455 » Wed Nov 28, 2018 2:59 pm

gruntguru wrote:
Tue Nov 27, 2018 10:08 pm
pdq67 wrote:
Tue Nov 27, 2018 10:59 am
I like engines made so that both Hp and T keep going up past where they cross at 5252 rpm!pdq67
So you like little engines?Image?
Personally, I don't give a shit if it's 600cc or 600ci. As long as it makes good power for the application, that's all that matters to me. Ford, Honda, Pontiac, Yamaha, whatever...

And it doesn't have to be small.


So you like coyote engines? See? I can do that too.


Image


What exactly was the purpose in saying that anyway except to start shit?

gruntguru
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Re: Peak RPM/HP

Post by gruntguru » Wed Nov 28, 2018 7:35 pm

Its actually a valid technical point. The rpm where torque and hp "stop going up" is primarily determined by the displacement (and particularly the stroke) of each cylinder.

Valid technical point #2. 5252 rpm is only a meaningless number that varies depending on the unit system you happen to be using.

I am sure pdq67 understands both of the above but there are many visitors who could mis-interpret what he said.

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Re: Peak RPM/HP

Post by randy331 » Wed Nov 28, 2018 10:16 pm

Rowdy Yates wrote:
Mon Nov 26, 2018 11:13 am
Cam duration and Lsa I would think.
Many think that,.. but that hasn't been the case in my experience.

Induction tract seems to be top of the list.

Randy

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Re: Peak RPM/HP

Post by digger » Wed Nov 28, 2018 10:51 pm

randy331 wrote:
Wed Nov 28, 2018 10:16 pm
Rowdy Yates wrote:
Mon Nov 26, 2018 11:13 am
Cam duration and Lsa I would think.
Many think that,.. but that hasn't been the case in my experience.

Induction tract seems to be top of the list.

Randy
I agree with this, take an engine with long runners let's say 20" total and shorten it to 14-15" total then peak hp rpm may increase 1000rpm

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