Porting overlap is when the intake and the exhaust port are open at the same time. How much is acceptable for turbocharged? How much is acceptable for N/A?



When we left off the other thread we were on the point where the overlap may be counterproductive for cars with larger turbos (exhaust backpressure). What do you think?

 

actually, I don't think we specified large vs small, just poorly selected turbos.



If I'm not mistaken a smaller turbo will have more backpressure, not larger like stated above.

 

Thats one of the points i was gonna make. But it seems what i gathered from that was different from what was actualy meant.

 

i don't think the big boys will give out their secrets because then everyone would be starting their own porting biz...

 

I think im gonna aim for a reasonable overlap on my turbo engine. My goal is to keep a cleaner charge of air ready for combustion. I think the N/A requires little or no overlap because of relying more on the supercharging affect of the intake plenum and in attempts to retain torque.

 

for a turbo motor it works like this. say you're boosting 15psi, that means you have 15psi in the intake manifold. and on the turbo side you have the exhaust system back pressure plus the backpressure the turbo has. which can be like 25psi. now if you have over lap where the exhaust is open and the intake is open at the same time, which way would you go?



http://www.maxcooper.com/rx7/how-to/...ystem/why.html



play with the chart on the bottom to see what i mean

 

I see, do you think the velocity of the incoming air will combat this to some extent?

 

Obviously yes, or else all the bridge/peripheral port turbo engines around the world wouldn't be making loads of power like they do.

 

yes, but high overlap motors are the ones that are really sensitive to exhaust restrictions



mike

 

I believe overlap is sort of a bandaid to poor flow. 2 examples here. First, pushrod V8's. The heads dont flow very well compared to a DOHC motor. But they still make crazy power by using lots of overlap. Indy cars, F1 cars, and many high revving DOHC motors run zero overlap. The simple fact that an engine at the highest form of racing, in possibly one of the highest form of motor design, technologically speaking, uses zero overlap speaks pretty loudly to me.



Now, relating that to rotaries, the renesis has zero overlap, and revs to 9K in stock form. Overlap isnt a necessity for a high redline, or power. BUT, overlap can increase the flow within its powerband, but reversion out of its powerband would reduce the power somewhat. But like I said, its more of a bandaid for an imperfect engine than a perfect solution. More overlap will make more power, but the powerband will be more peaky.



And like J9 said, high overlap motors are the ones that are really sensative to the exhaust especially, but also the intake.