I started this thread to show Banks' agreement with DK Customs - for everyday riders - NOT for drag racers.

Everyday riders aren't interested in max horsepower at a jillion rpm. Horsepower is a function of torque times rpm. It is a derived number. Torque and rpm are directly measured, and everyday riders are interested in the shape and peak of the torque curve IN THE NORMAL RIDING RANGE OF RPM.

Take the race talk elsewhere. Start your own thread.

 

Not sure how the 2 are in agreement. I thought Banks is saying lower back pressure and DK is saying raise it.. ???

IMO the applications have nothing to do with each other. For one, Banks is talking about a exhaust system that is 12 - 20 feet long with pipes that need to route gases away from the operator with 8 cylinders and DK is talking about a 2 cylinder motor where there is still a strong pressure wave reaching the end of the pipe.

 

Unless I missed it (quite possible), I don't think DK is claiming back pressure is a positive or one should try to increase it. I noted in reply #10 that DK stated that back pressure is a detriment, should ideally be minimized, and has a tradeoff relationship with reducing reversion.

That said... I do feel they seem to see everything as a nail for their very specific hammer (same with many of their products). I tend to depart ways with the notion that performance enhancements (worthwhile or otherwise) can always be (and should be sought) achieved on any/all appropriately spec'ed, properly designed, decently performing exhaust components/systems by mutilating them and installing lollipops.

 

My impression is Bank's is agreeing that less backpressure = good, and DK Customs method of doing so in short pipes is good. Right?

 

I'm not saying that DK is claiming back pressure is a positive. In fact they are claiming that their insert has little effect or since it increases velocity at the insert that scavenging is better. I don't buy the theory. I do buy that it's better than the old lollipop since it allows for more coherent flow and their use of works..

IMO, their theory is all wrong.. They are simply changing the tuning of the exhaust system.

Here is the original post from #10 over on the sportster forum..

https://www.hdforums.com/forum/sport...l#post17075742

Tell me how does increasing the velocity at the exhaust end help reduce reversion at the exhaust port?

IMO the problem is not the buzzword "reversion" but 80% of the time what I call over-exhausting. What happens is the exhaust flows so freely that at TDC overlap, the velocity in the exhaust port simply stops and no scavenging occurs.

 

My friend I offered my opinion for better performance at all rpm except below 2500 because I think that’s Unrealistic to concern one’s self with. In fact, I think grabbing a lot of throttle below 2500 is actually hard on your oiling system, to keep up and protect your bottom end. (Lugging) When I mention drag pipes, I’m only saying that with proper tuning they make similar power even torque curve as the best 2 into 1 offerings but above that 3500-4000 mark they make more power.. so it’s a win win with many combinations. Step tuned drags at proper length and careful consideration can make higher averages than many perhaps most 2 into 1’s. THEY ARE VERY STREETABLE and well mannered. Most are not saying that.. either they don’t know any better or they have no experience.

 

Yes, they are tuning the exhaust system, using test data.

Isn't that the point?

 

The above image that is masquerading as a DK image, or as an image that purportedly shows what happens with the TTI's is inaccurate. Below is the image from our website that shows what happens when TTI's are installed.



Two Important Points regarding the TTI's-

1.
A. They were street developed
B. They were street tested
Then once we could feel a significant improvement in torque, we began fine tuning the design with the help of a dyno
C. They are dyno proven
There are many dyno charts on the site, and many videos showing improvement, as evidenced by dyno runs. Here is one of them:




2.
How they work.
A. When the exhaust valve opens two thing happen. A pulse (wave) of energy enters the exhaust pipe, generally at 1300-1700 feet per second. This can be understood as a shock wave from an explosion.At the same time the spent combustion gases enter the exhaust pipe at 150-300 feet per second. (Generally the faster the exhaust gases travel, the better the inertial scavenging and the less engine pumping loss)

B. The energy wave that is heading toward the end of the pipe will get to a low pressure area faster (low pressure being a bigger area of the pipe OR the end of the pipe)WHEN that happens some of the wave will reverse direction and collide with the slower moving exhaust gases that are still traveling toward the end of the pipe. This will slow down the speed of the exhaust gases. This will produce the result of less felt torque at the rear wheel.

C. Reduced reversion will increase exhaust gas velocity, which will result in a better intake charge, which results in more power. Because of the unique shape of the TTI's the little bit of restriction (backpressure) that is there is far overcome by the reduction in reversion AND the venturi like effect from the shape of the engine side of the TTI.

It's pretty simple. And if one does not agree with how we say it works, forget about how it works....just look at the dyno charts. They work.

 

You are correct, I modified the picture but IMO it was so it was more correct. One thing to note is that an pressure wave hitting open exhaust hitting an open exhaust will cause a negative reflection. A pressure wave hitting a restriction will cause a positive reflection. Ref Smith's book.. When you have that device in the way of the exhaust flow, as your diagram, the device creates a positive pressure pulse to reflect off of it. You believe that all pressure energy passes the around the device and none is reflected back?

You section 1 is right on.. I don't argue with any of the results..
I'll disagree with your section 2.

A.
I don't know what you mean by shock of the explosion.. I assume that you mean a sound wave. This has very little energy and from the papers I've read, it has no effect on tining. The slower wave is the pressure wave.. That one from the simulators vary with throttle opening and at WOT can easily close to the speed of sound. Say 1000 feet per second. Here is the result of a higher compression motor at 5500 rpm. In this case overlap is working real well. Green is exhaust Vel. Blue is intake.



You are correct the better the exhaust is flowing the better as the valve closes, the better the scavenging.

B.

To me if you are talking wave energy of sound, which seems to be what you were talking about in A, then I'll have to disagree. Sound energy is really low and has no effect.

C.

By adding the device, you have not only removed / reduced the negative wave but you've created a positive pressure wave to be reflected back up the pipe.

I can't argue with the results but strongly disagree with the theory.

 

This is a bit funny, you have designed a piece of equipment that actually works, but it seems you dont understand why

The reason reversion reduces engine power is not that it slows down exhaust velocity.

The reason is that at some rpms the reflected sound wave will cause a pressure maxima at the exhaust valve, at the moment the valve opens, this pressure will work against the combustion gasses trying to leave the cylinder.

And if cams with long overlap are used the pressure will blow the intake charge back out through the carb, and then it will get sucked in again. This means the air passes through the carb three times, and actually gets fuel all three times. This leads to an overly rich mixture resulting in poor engine power at that rpm.

If you run without air cleaner there will be a little cloud of gasoline mist outside the carb, usually referred to as a standoff, and your pants will smell of gas.

This is why bikes with hot camshafts are more sensitive to reversion than stock bikes, the stock cams have very little overlap.

And the reason lollipops reduce reversion I believe is that the sound waves reflected at the open pipe is 180 degrees out of phase to the reflection against the lollipop, and cancellation occurs.

There are a lot of other things going on in the pipes, with gas velocity, scavenging and such, but the main impact of reversion is the rich condition. This can be mapped away on an efi bike, but is very difficult to fix with a carb.

And action, let the tomatoes keep flying