Registered: Sep 2003
Location: Buckeye State
Posts: 128

dan
do you have any exprerience with increasing fuel air mixture velocity by reducing the port size and allowing the extra velocity of the mixture to "supercharge" the cylinder AfterBDC if the valve timing is correct. is the supercharge effect not large enough to give a substantial increase in HP. in theory larger ports would produce a slower air velocity and would not have as much inertia as the faster moving air produced by smaller ports therefore not providing as much cylinder fill AfterBDC. i guess this theory only works if the piston is pulling the same amount of air no matter the size of the ports. is the amount of fuel entering the cylinder BeforeBDC when using larger ports that much more than when using small ports that the supercharge effect cannot compensate for the lack of fuel/air lost BeforeBDC? i realize cam timing has a great effect on how all this works but i was just curious what you thought
i thank you for the time you spend on here answering our questions and also thanks for whatever you are doing to Joe's motor cause i am a big fan of his and you two got something good going. keep it up!

thanks
scottie

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Registered: Oct 2002
Location: Nevada
Posts: 8284

ATVriders Patron

I know that you asked the question for Dan, but I'm going to jump in here anyways.

Many camshafts and ports already take advantage of the "supercharging" effect you mention. However, it's not universal. It doesn't work perfectly at all rpms and engine sizes. Smaller ports may increase air velocity, but they also present more restriction. Eventually, you reach a point at which those restrictions cause more problems then they solve.

Take it to an extreme example. You can get very high air velocity if your ports were only a quarter inch in diameter. However, with such a small port size, your engine would be starving for air in the higher RPM range, and you actually couldn't rev very far beyond about 5000 rpm, because the engine simply can't suck enough air in.

Now, when do cross this fine line? Where is the magic compromise between velocity and too much restriction? Believe it or not, it's right about the stock port size. The ports flow very well for the stock engine and RPM range, and they already take advantage of supercharging effect you mention. This usually corresponds fairly close to the peak HP. You can still rev past peak, and make decent power, but the power drops off, largely because the engine can't pull in enough air at those higher RPMs.

When you increase the size (big-bore), or have a chamshaft specifically designed for higher RPMs, you want to open those ports just a little more. You want the engine to breath better at higher RPMs to shift that magic compromise a little higher. Now, instead of your supercharging being most effective at 7000rpm, you've shifted it up to 8500 rpm.

This is the exact reason that porting is best left to a professional. You don't want to just hog out your ports and open everything up. It's important that portwork be designed on a flowbench, to get the best compromise between airflow and air velocity.

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Registered: Jul 2005
Location:
Posts: 355

Wow where do I start? there is alot to be gained by having a corectly sized port having the right port with the correct camshaft can yeild huge power increases. Big ports can be made to make good power if done correctly it ussualy involes correct valve angles and the proper camshaft. Like mentioned the RPM that you want to use the motor will play a big part in what the port crossectional area will be. Where you want the power is the million dollar question. Having the right cam so it does notopen to fast to reduce velocity in the port makes a differance also.

Having 100% ve is when you fill the engine with 450cc of air per intake stroke. When you fill it with more you increase power and your CR goes up. So that is why a engine with 13:1 can run on pump gas with a poor hear and when you put a good head on it you CR goes up with extra cylinder filling. and will require race gas. So when poerting you must consider what the use is and RPM of the engine, fuel used, Cam shaft, carb size and valve size it actually gets quite complex.

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Registered: Sep 2003
Location: Buckeye State
Posts: 128

thanks for answering the question.
now if you do have a static CR how do you determine what the dynamic CR is when you have a well ported head and the correct camshaft for the porting?

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Registered: Jul 2005
Location:
Posts: 355

quote:

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Originally posted by mscottiel
thanks for answering the question.
now if you do have a static CR how do you determine what the dynamic CR is when you have a well ported head and the correct camshaft for the porting?

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Registered: Apr 2003
Location: MD
Posts: 4069

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quote:

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effective VE plus static Cylinder volume gets increased from 450cc x 1.05 (ve) gives you effective or maybe 95% ve depending on the head x.95 It get into some extensive math that I admit I am not the best at but I do my best to struggle through

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