mperformance

What happens if the exhaust ports are ported extremely large compared to the intake ports on an N/A engine? Let's say keeping a small streetport (intake) and doing a radical exhaust port? Can this work?

The_7

im not a porting expert but i know it all depends on which way you take the port. i think the common practice is to make it a little wider and extend it to close later. this will hurt your low end alot but you will get some nice top end. even if the intake ports are small the large exhaust port will scavange alot out in the high rpms which increases your volumetric effeciency and gives a power boost

mperformance

Thanks The 7

I also found this...from rotarygod, very helpful:

The more port overlap you have, the less your volumetric efficiency will be at lower rpm's. As stated already, some exhaust will dilute the intake charge which not only leaves less room for fresh air but also heats up the intake air in the chamber. At higher rpms inertia helps keep the gasses flowing in the appropriate directions. In other words the velocity of the exhaust gasses leaving the chamber at higher rpms will help pull all the remaining gasses out of the chamber regardless of overlap. Another good side effect of this is that at high rpms because the suction through the exhaust is so good during overlap, it will create a low pressure zone in the chamber which actually helps to pull in more air through the intake. This is why it is so important to have a properly scavenging exhaust system. If it is restricive, less gasses will want to leave the chamber and will subsequently go back through the process again.

The Renesis has no overlap what so ever. It actually has 64 degrees of dwell. This is solely based on the port location. Since there is no overlap it can not get the higher amounts of dilution into the intake side. This keeps heat out of the intake as well as leaving more room for fresh oxygen rich air. Volumetric efficiency goes up. This in turn leads to better fuel economy, better emissions, and more power down low. The downside to no overlap is that at higher rpms there is not as much benefit from a good scavenging exhaust. It it still important though. It just can't help suck in more air to the intake side. It is actually a very simply concept to undestand. It is also one that should probably be seen to comprehend. When you see it you'll wonder how you didn't know it the whole time. Its pretty cool.

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so actually I asked because I made a 12a with huge exhaust ports that was impossible to start unless pushstarted...it had a really small street port but extremely huge exhaust ports, kind of like the size of the exhaust opening without the sleeves!!!. It reved very freely and made some decent power...it just never fired up on it's on... hehehehe

karism

M Performance : You right. I couldnt have explained it better myself.If i can refer to Boingers for a moment.
Aftermarket cams normally have more intake timing,and lift than the exhaust cam.This forces more air into the combustion chamber ,which gives a "supercharging" effect seeing that it almost forces the cylinders to use the extra air and fuel.

Take the 20 Vw engines as an example .3 inlet,and 2 exhaust valves.

This is the same with Rotaries.A rule i have allways use,is to make the exhaust +- 80% of the size of the intake,for na engines.NOS engines normally have large exhausts,to improve the flow,seeing that there is addiction air+ fuel added.

I`am also still learning,but it has helped me alot to make the exhaust port smaller

peejay

The exhaust ports are the main choke point on these motors.

Effects of huge exhaust - Inability to wipe ****-eating grin from face due to lumpy idle, increased power both above and below current powerband due to increased exhaust flow (and therefore more useful intake flow) and increased overlap, slightly *better* cruise MPG, slightly worse idle/low-speed MPG, and a bit more noise.

I wouldn't change the opening side (bottom of port) any, though.

Tmann26

The only good use for a street port would be for turbo/supercharged engines then, since more air would be forced into the motor?

The_7

no, porting is the best way to get more power out of an n/a motor. the larger intake ports are less restrictive so at high rpms the volumetric effeciency rises, boosting your power.

karism

On my previous 12A ,my exhaust port was too large.It wouldnt start easilly.But when it was going,i went like you wouldnt believe.So,on my next attempt, i kept to the 80% rule,and now its much better.

Something interesting : The weekend,i restricted my exhaust on my Brideport.(Placed a 1.88 " pipe inside the existing 2.25",in a attempt to silince it a bit) The idling was very low,it was shaking violently as it tried to idle at 1100.I also wouldnt start without depressing the accelerator,and revved up very slowly.

The outcome : Exhaust porting is very-very important.The flow,and also the velocity of the port,and last but not least,the shape.

Your intake and exhaust should be matched,in order to make the most power of a wide rpm band.Your are just taking away alot of torque,by enlarging the port to much,and also creating alot of overlap.This in turn makes the motor a pig,and the results are allways dissapointing