Yes. Better "scavenging." Aerodynamics (Not the aircraft type.) Similar to fluid dynamics but different. From 2 into 2, to 2 into 1? Depends on the volume of the "can." It can be as much as 12-15%. Well, the benefits ARE there. The larger the "can", the more volume it can muffle without losing as much power.

Actually, "2 into 2" systems that offer small diameter pipes that are also short in length offer the worst of performance. Just noise. Slow noise.

 

2 into 2 has the problem of "reversion". Cylinder fires, exh gas pushed out, but, then air comes back up the pipe to the head running into the next firing of exh gas. This is detrimental to bottom in power...

2 into 1 pipes stops reversion by scavenging. Each exh firing is drawn out by the other exh so both exhaust are continuously pulled out by the collector.
most 2 into 1 usually quiet the exhaust a bit and make a deeper rumble sound. Should also help with exh pop noise because of no reversion.

 

If you have done (or plan to do) any mods to release your EVO from the EPA restrictions, a 2/1 pipe is a necessary part of the equation.
I love my Thunderheaders.

 

I haven't had a stock system dyno tested, only true duals and my 2-into-1, on my S&S 107" engine, done by the same shop and the same operator. The headline numbers with both systems were very close, not different enough to argue over! However that is only the start of the story. The TDs had a dip in the mid-range of the torque curve, which is typical of separate pipes. The 2-into-1 lifted the torque curve over that dip and gives much improved torque over the low/mid range of engine speed that most of us are likely to ride in. The unexpected bonus is that with the 2-into-1 the engine runs much more smoothly and is far nicer to ride. No contest!

A bit of history. Back in the 1960s IIRC BSA produced a model with separate pipes, but with a balance pipe between them. My understanding is that they wanted to use two small mufflers for styling purposes, so used both mufflers for both cylinders. Unexpectedly they found they got a useful improvement in torque, so adopted a balance pipe across their range of twins. Within just a few years every twin cylinder bike manufacturer on the planet adopted a balance pipe, which can still be seen today. Early Harley dressers had twin duals with a balance pipe between the two pipes, which must have been a b1tch to make, so they simplified their solution to the slightly odd set-up we are all familiar with. It gives similar benefits to the BSA approach.

What's the science? Every vehicle on the planet with two or more cylinders comes stock with a siamesed exhaust system of some sort, in other words the exhaust output from each cylinder is joined up with the others, no matter how many cylinders there are. My V-12 car had two 3-into-1 manifolds on each bank, which are joined together. My V-8s have a 4-into-1 on each bank, and so on. The reason for all this is that the cylinders can be tuned to interact with each other in a beneficial way, giving better results than having a separate pipe for each cylinder, especially for increased torque.

Does it guarantee increased power, without other changes? No. Some systems may give a little better HP, but IMHO the main benefit will be increased low/mid range torque. There are some questionable things said above, which I don't hold with, however is a 2-into-1 worth the expense? Most certainly YES! Scott of Hillside Cycle loves the SuperTrapp Supermeg, which I'm pleased to report I bought long before I discovered that!

 

No, that is not true. Because of atmospheric pressure and any cam that closes ATDC there will be reversion (during overlap). There will always be reversion. Is it possible that the amount of reversion is limited by a good 2-1 pipe? I would think so, this is why we can run the motor out to a higher rpm...

Yes a a good 2-1 will help to clear the combustion chamber when the overlap occurs BTDC.

 

Sounds to me your agreeing with what I said. Maybe I should have not said stops reversion but greatly improves the problem, but Im not trying to split hairs with the details of the improvement.

 

That difference on the Dynosheet looks like a big improvement on torque.

What about stepped headers, for both styles of exhaust ?

 

My bike has the original cross over pipe. The bike came with hd slip on’s on the bike I also received the original stock muffler’s, they came with the bike, the slip on’s sound good ( I don’t know if they would be considered sceamin eagle we are talking 1992) but would I have anything to gain by getting some cycle shack slip on’s.?? The bike is all stock except for an Andrews 23 cam. I am gonna do piston’s , different cam and head work at some point but I want to stay in stock appearance. Thanks

 

I suspect they only work well with a highly tuned engine, when chasing big numbers. They also feature on systems that come with an angry sounding trade name, which suggests they are intended to waken the dead! A Supertrapp is tunable, by using a closed or open end cap, also has stackable baffles, which is a great solution to getting the right combination of sound and performance.

 

No, stepped is not good for an 80"er. It doesn't move enough air to take advantage of stepped pipes. You want to keep your exhaust velocity up, a stepped pipe will reduce it...