Educate me: N/A Exhaust with zero overlap.

I have an engine with zero overlap (where both the intake and exhaust are open at the same time).  In fact, I think there is something like four degrees of duration where both are closed before the intake opens again.  I've heard an argument that said "exhaust scavenging through methods like equal length header runners (etc) do not apply since the exhaust pulse would basically be pulling on a vacuum"

(Does that make sense?  I was trying to sum up a much longer statement)

To me that sounds too fishy.  Mostly because those SAME people are telling me that a 70mm exhaust is probably the largest I'd want because of "backpressure" (which I always change to "the effect of exhaust scavanging").

So which do we think it is?  "Who cares about runner length and diameter because you're not going to be able to scavenge anything anyways"?

Or

"The same rules apply.  Keep the runner length equal, and keep your exhaust at the recommended diameter, etc"

Opine the, if you will.  Thanks!

(Renesis Rotary engine.  Overlap will not be possible.  Shouldnt change the physics of the exhaust though)

Hungary Bill said:

 

To me that sounds too fishy.  Mostly because those SAME people are telling me that a 70mm exhaust is probably the largest I'd want because of "backpressure" (which I always change to "the effect of exhaust scavanging").

The rest of it made my head hurt but this part didn't. I think the exhaust scavanging you get here is from one exhaust pulse pulling the next one. Say one pulse is half way down your exhaust pipe. It will pull a vacuum behind it to help along the next pulse. I'm assuming the larger the pipe the less exhaust velocity you get and therefore less help for the next pulse. Or I could be full of E36 M3. 

I had a response... and then you sad rotary. 

I assume you are talking about a full bridge motor. To be honest, I have no experiance with rotaries and I would assume they are a completely different animal than a piston engine.  

Looking forward to the thoughts of others. Free bump. 

In reply to NordicSaab :

Nope.  Just stock and shopping for bolt ons.  If what they say about not needing equal length runners is true, I can chop half the price off a set of headers.

Stampie said:

Hungary Bill said:

 

To me that sounds too fishy.  Mostly because those SAME people are telling me that a 70mm exhaust is probably the largest I'd want because of "backpressure" (which I always change to "the effect of exhaust scavanging").

The rest of it made my head hurt but this part didn't. I think the exhaust scavanging you get here is from one exhaust pulse pulling the next one. Say one pulse is half way down your exhaust pipe. It will pull a vacuum behind it to help along the next pulse. I'm assuming the larger the pipe the less exhaust velocity you get and therefore less help for the next pulse. Or I could be full of E36 M3. 

 

You've summed up my thoughts exactly.  My brain has been chewing on this for a week.  I tried hitting up the model specific forums but the line between fact and armchair expert was blurred at best.  

When in doubt, ask grassroots 

Zero overlap = perfect for boost, and lost of it.  You have my blessing!

Equal length headers are needed so the VE of the cylinders does not differ.

When tuning you will find they differ differently at different RPMs. Creating a tuning nightmare.