the P port is weird! its more tame then a stock engine, but its a lot more pyrotechnic too. if the exhaust is pointed inside the building, it'll blow all the rags around... its also axle foley proof, it'll shoot the banana right out of the tail pipe!

 

Reminds me of "the bitch" Grand National... 560whp on a Mustang dyno, through 3" pipes, it blew a bench across the shop.

 

On my Semi pp build i plan on having a cutout valve before the rear muffler, that way i can run a fairly low noise level on the street, and with the flip of a switch have a exhaust wich suits a semi pp.

 

That won't help at all. You need a zero-restriction exhaust so that the engine has good street manners.

I ran something similar on a street port I had over ten years ago, what I found was the engine needed to be retuned with the cutout open vs. closed. If you were running EFI that used a mass airflow sensor, or if you had a map that altered fueling based on exhaust pressure (custom code), then it would make more sense... but it still wouldn't get around the problem of the engine.

 

So my plan sucks then?,, need to rethink my muffler strategy. Not like i'm in a hurry, still gathering parts for the engine build.

 

i was thinking about making a post about streeting a P port, but here is the muffler/exhaust bit.

exhaust. the challenge is to make it quiet enough to use on the street (or track!), yet flow enough to allow the engine to run.

for me the next exhaust system is going to get built on the dyno, the crucial bit being header length. once i've found a header length that the engine likes, then i get to pick mufflers.

the two rules of thumb for mufflers are.

1. the larger the body of the muffler, the more it should quiet. (shape matters too, but round will stand up to the rotary better)

2. pipe size has an influence over volume and tone. to wit. i have an extra 3" muffler i can cram in the tailpipe of my P port, and it makes it quieter, but not by a lot. however a reducer from 3" to 2.5" seems like a larger difference, it takes away the low frequency boominess.

going even further i have a pair of resonated tips, that are maybe 1" ID each, and with that and nothing else, it sounds remarkably like am RB turbo back system. its quiet enough to street, it would pass the police noise test.

however putting on the tips, drops the idle 100rpm, AND it lowers the "it stops running because there is no airflow" point from 6500 (stock air cleaner) to about 4k.....

the flap actually may work, but really you'd probably need to drill a couple big holes in it

 

You could always put the cut-out on a low-pressure switch plumbed into a copper line off the collector. As long as it registers and opens by 1 psi or something it shouldn't affect running that badly, right?

And I'm not sure exactly how he does it, but Logan at Defined seems to have come up with a home-made solution for his 3-rotor semi-PP that is non-restrictive, yet quite quiet. Something to do with steel wool...

 

1psi is too much for an idling/cruising high overlap engine. My car runs at 0psi until WOT over about 4000rpm, and it's "rotary quiet".

I haven't talked to him about any details, so all I'm gonna say is based strictly on inference and pictures I've seen online:

The collector style is key, a choke style collector helps to decouple the exhaust system from the header. This is part of why new V8s make more power with stock exhausts than with traditional style long-tube headers unless you put freeflowing (loud) exhausts on them. I've only ever seen two other headers use that kind of collector.

And the rest is just simple muffler mechanics - the more muffler shell size you have, the quieter it's going to be. Ultimately you end up with an exhaust system that is all ate up with muffler, but if you do them right they won't have any restriction and will still flow well.

I realize now that I'm measuring exhaust backpressure in the wrong spot. What I need is a high speed transducer mounted up at the engine flange, and read it with a scope that has another channel reading crank angle. All that matters is the backpressure at the port during the overlap period, and really it should be a vacuum. Good setups will have 10" of vacuum or more at the exhaust port during the overlap period.