average cross section vs minimum cross section
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average cross section vs minimum cross section
Which one is most important? Which one determines the rpm limit of the engine and what engine size the head will work best on?
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bill jones
- Guru
- Posts: 2651
- Joined: Sun Sep 05, 2004 6:38 pm
- Location: salt lake city, ut
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-The way I see it, assuming the entire port length is considered and has been built for racing, both areas work to limit the rpm.
-But the cubic inches of the cylinder works directly hand in hand with the rpm to set the limits.
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-If you have the ports minimum square inch area as large as you can conceivably make it and that is at the bowl throat and the rest of the port tapers out larger clear out to the intake conduits entrance, and the port is standing up real straight and tall like on a nice racing heads raised runner head, then I'd say the minimum square inch area is likely the ultimate limiting factor.
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-If you have some low port line of sight stock production head then I feel the average square inch area and the curves within the intake tract end up setting the limits.
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-You can use less cubic inches and run a higher rpm or if you use a lot of cubic inches the rpm limit drops.
-Other factors are type of and amount of fuel within the air stream, like a methanol engine should lower rpm limits because of the huge volume of fuel in the airstream compared to a lean running gas engine.
-Altitude should also affect this as when you get to a high elevation the air is thin and therefore the engine should be able to rev higher than at a low altitude.
-The point within the intake tract where the fuel is introduced also has to be considered.
-If you run carburetor/s you have an entire different situation than if you run a down nozzle injected sprint car engine where no fuel is in the intake port.
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-Another determining factor is how badly the intake valve is shrouded.
-If you have the intake valve right up against the shroud wall it can't flow near as good as if it was a hemi with virtually no shrouding.
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-It'd be nice if we had a clue as to what the engine application is you are interested in.
-I don't believe you can say that for every possible engine the minimum cross section area is always the absolute main concern.
-But the cubic inches of the cylinder works directly hand in hand with the rpm to set the limits.
----------------------------------------
-If you have the ports minimum square inch area as large as you can conceivably make it and that is at the bowl throat and the rest of the port tapers out larger clear out to the intake conduits entrance, and the port is standing up real straight and tall like on a nice racing heads raised runner head, then I'd say the minimum square inch area is likely the ultimate limiting factor.
----------------------------------------------
-If you have some low port line of sight stock production head then I feel the average square inch area and the curves within the intake tract end up setting the limits.
-------------------------------------------
-You can use less cubic inches and run a higher rpm or if you use a lot of cubic inches the rpm limit drops.
-Other factors are type of and amount of fuel within the air stream, like a methanol engine should lower rpm limits because of the huge volume of fuel in the airstream compared to a lean running gas engine.
-Altitude should also affect this as when you get to a high elevation the air is thin and therefore the engine should be able to rev higher than at a low altitude.
-The point within the intake tract where the fuel is introduced also has to be considered.
-If you run carburetor/s you have an entire different situation than if you run a down nozzle injected sprint car engine where no fuel is in the intake port.
-------------------------------------
-Another determining factor is how badly the intake valve is shrouded.
-If you have the intake valve right up against the shroud wall it can't flow near as good as if it was a hemi with virtually no shrouding.
------------------------------------
-It'd be nice if we had a clue as to what the engine application is you are interested in.
-I don't believe you can say that for every possible engine the minimum cross section area is always the absolute main concern.