This an answer I posted on the "Nopistons" list.

 

At airplane RPM the primary lengths would be longer. probably 30"

I can give you some factors, and you will have to choose among them. I don't think you can have it all in one package. The Pport has a much better connection between the intake and exhaust during overlap than does the side port. So, muffling has a more powerful effect on killing HP.

The most important feature of headers is that the two primary tubes be exactly the same length.

The next most important feature is that the primaries be the right length for what the engine needs to spread its power band up or down. Where do you want the headers to help the engine the most, RPM wise)

The next most important is tube diameter. Bigger makes all other features less sharp. Smaller diameter perks things up and makes a real good feel to the engine. This is less scientific but the grin will be wider.

How big? The same ID as the port or just slightly larger. Like 2" ID max.

There is the short system as seen on one national champion. About 11" to a very short collector and a 3" 90 and pipe to a long custom steel-pack cut into the passenger door bottom and the pipe turned down right in front of the rear wheel. Those 11" were straight out of the port with a gentle radius to get into the collector.
Much better looking than it sounds, and built to help spread the torque of a Pport being shifted above 10,000 RPM. A more common system is the primaries being in the 24" to 26" to the collector. Good all around power up to 9,600 RPM. The last is the early IMSA RS system of about 102" and that puts the muffler under the rear bumper of an RX-2. But the total length is not that critical so you could do that and swap between race mufflers and street mufflers without getting under the car. In that system both primaries meet the collector just behind the rear wheel. This is described in the Racing Beat manual. If you can weld and like to fabricate stuff, they have a kit that works rather well for the street, and holds up about 2 seasons in a race car.

If the design permits it, the longer the run straight out the port centerline before the first bend. The largest radius bends you can fit into the design.

Most people leave in the liner and do some porting on the engine side of it. Other wise (If you take it out) there is a big discontinuity that makes exhaust flow unhappy. The higher the velocity of the gas, the more energy will be lost to any imperfection. So, that step must be replaced with a machined ring of aluminum to return the floor of the port to dead smooth.

Then there is the angle of the primaries entering into the collector. The longer the collector the broader the power band. Tubes at a steep angle makes it more peaky, and closer to parallel makes it less peaky.

You need the collector to blend the pulses from each primary. They are moving above super sonic and the closer you can get to a perfect blend the more power it will make.

From the end of the (6" to 8") collector (about 2 1/4" to 2 1/2" inch diameter) a long megaphone (maybe 18" to 24") up to your exhaust pipe size.

Burns stainless has some great tube bends in stainless if you like to build stuff. Use .062" minimum wall.

Exhaust pipe as close to 3" as you can fit in the car. I use 4" and 4" diameter Borla muffler to get about 100Db. at 50 feet. This is with the exhaust dumped out the passenger door rear corner and pointed at the ground. This is not a streetable system. The passenger door is on the inside of the race track and the sound equipment is on the outside of the track to protect the neighbors.


Go to Paul Yaws web site and look at his street headers. Better yet buy a set from him. All stainless and dyno tuned over years of engine building. WWW.Yawpower.com

Lynn E. Hanover