Here is the exhaust port I did on FRANKENTENTH

 

Outside looking in

 

Onje more

 

Compared to a bone-stock port

 

different angle

 

exhaust side

 

The later you close the exhaust port, the more overlap you induce. On a street-ported motor, it's a complete waste.



The earlier you open the exhaust port, the less you give the power stroke to produce torque.



The wider you make the port, the more stress the apex is given as it tends to 'bend' at the centre w/ less metal to ride on at the sides.



Enjoy your humongous exhaust port.



B

 

"n/a's LOVE overlap" - Sort of. If you're increasing overlap without the effourt of enlargening the intake port, then you're wasting mass air charge.



"And Big exhaust ports spool turbos faster too." - Not necessarily. If the port is too large, then there's a chance that the exhaust velocity will be substantially slower, therefore making the turbochargers' boost threshhold *later* in the RPM band.



Like I said, bigger isn't always better. Size isn't what you need to focus on; it's something else entirely different.



B

 

Just remember that there is a very small range of rotor position where torque is generated on the eshaft. Opening the exhaust earlier than bottom dead center was taboo a generation ago on piston engines for thought of reducing the expanding exhaust gasses' ability to do work moving the crankshaft. However, once engineers began experimenting with earlier opening of exhaust valves they realized that exhaust gasses really only have a small timeframe of crankshaft rotation to effectively produce work, and it is far before BDC . . . the rotary is the same way. For example, many people find a performance loss with using 91-93 octane fuel in NA rotaries because it burns slower than 87, causing the exhaust expansion to happen later and slightly "missing" the window of greatest mechanical advantage on the Eshaft that 87 octane is timed for on stock ignition timing.



In short, it has been proven by more people than Mazdaspeed7 that early exhaust opening is not as detrimental as the rule of thumb about overlap or wasting the exhaust pulse out the port would tell. After the sweet spot of maximum leverage on the eshaft, there is little to gain by retaining exhaust gasses any longer but much to gain by letting them flee the chamber with as little resistance possible.



The good thing about this forum is that you guys will probably understand what I am talking about. The members of the other forum are not into this kind of stuff.

 

I agree that there is a lot more to making power than just making the port as big as possible. BUT, on my engine without the exhaust sleeves, I LOVE the powerband, and it still idles at 600 rpm. It does have a little less torque below 3K, but not enough to affect drivability.