exhaust back pressure. do you need it?

Diamond, thats neat, easy, and creative. But how loud is that? It looks deafaning. Oh and did you re jet cuz you removed the baffles?

Simply put yes for power band where you want it...

There is a science to it...
Some notes I found while researching for designing my own exhaust...


There are two primary considerations involved in designing/choosing an exhaust.

The first deals with the exhaust gas that travels at 200-300 feet per second. Here we are concerned with reducing the back pressure while maintaining flow velocity to get the best cylinder exhaust purging.

The second is exhaust pulses or finite-amplitude pressure waves which travel the speed of sound or at about 1700 feet per second. These pulses can be tuned to create an effect known as scavenging (sucking additional exhaust out and intake charge in, with a properly timed negative pressure pulse).

Exhaust gas - General rule of thumb, a narrower diameter pipe will improve low rpm flow hence torque but may limit top end.

A wide diameter pipe will enable top end power but have poor low end velocity (eg great for the track but lacking on the street). So choose the where you want your power band and set diameter accordingly.

Increasing or decreasing pipe diameter 1/8th will move the torque peak about 500 rpm up or down. Varying the length of the pipe will fill in the power curve around the torque peak. Shorter pipes fill above and longer pipes below.

A good starting point for exhaust pipe area is the circumference of the valves times the maximum lift plus about 15%. A good starting point for the length of the primaries is 28-32 inches.


Exhaust pulses - When a positive exhaust pulse traveling down the pipe reaches an opening or increased diameter it is reflected back as a negative pulse. This negative pulse can be timed to reach an open exhaust valve to provide scavenging.

The rpm's that this occurs is at peak torque and continues in the upper portions of the bike's power range. However, there is a trade off.

In the lower rpm's the valves are open longer and thus also allow a positive wave to hit. This usually occurs around 2/3's - 3/4's of the torque peak (between 4000-5000 rpm range on the Max) and is why you see a lot of dyno dips in the power band in this range. The positive wave pushes exhaust back into the cylinder and in some cases, if there is enough valve overlap it will travel up the intake track and cause triple loading of the intake charge (air is drawn through, pushed back and drawn through again.) Here is where the 4-1 versus 4-2-1 make a difference. The 4-1 will have a STRONG tuned pulse.

Therefore it will have a higher top end (strong negative wave) but a bigger hole in the mid range (strong positive pulse hits while valve is open). A 4-2-1 will soften and lengthen the pulses, as the 4-2 connection provides an interference wave in addition to the primary waves. A similar effect can be done with stepped pipes. The top end effect won't be as great but it will be longer. The mid range dip will also be shallower hence, the adage that a 4-2-1 improves mid-range.

Old Man Rock wrote:
The first deals with the exhaust gas that travels at 200-300 feet per second. Here we are concerned with reducing the back pressure while maintaining flow velocity to get the best cylinder exhaust purging.

That's related to the main urban myth, that "freer breathing" exhaust is always better. However, opening up the cross section area of the pipe reduces exhaust gas velocity. So, there is a sweet spot for velocity and going "more open" than that decreases performance.

cherokeekid58 wrote: Diamond, thats neat, easy, and creative. But how loud is that? It looks deafaning. Oh and did you re jet cuz you removed the baffles?

Oh, forgot to put the witty quip in there with the pictures.
That's how the pipes were on my bike when I got it and it was LOUD!!! My comment was that the exhaust pulses were the poor guy behind me's problem, not mine.
I immediately put a Kerker on it and rebuilt the carbs and tuned them for the new pipe and pods.
Those open pipes looked cool as hell and I like the low slung dual look too. They came off of a J model and I still have them. Someday, I'd like to clean them up and put baffles back in them and run them. They're pretty trick looking if I do say so myself.

Remember too, a secondary function of "back pressure" is to aid in cooling the exhaust valves.

"4 into 1 makes the best top end at the expense of low to mid "

"4 into 2 (with a crossover) makes best low to mid at the expense of top end"

Old Man Rock's very informative post reminded me of a good website on scavenging. The truth about scavenging

jga2z wrote: Old Man Rock's very informative post reminded me of a good website on scavenging. The truth about scavenging

Interesting link, but I am always skeptical when reading the sales pitch from a vendor trying to sell a product that is markedly different than all his competitors’ products that have a proven performance track record. The X-pipe pitch could be totally true, but it strikes me that the engineers at Yoshimura, Kerker, Moriwaki, Vance & Hines, etc. would be using an x-pipe configuration if it produced more power than the conventional configuration. Ed

Legit dyno test results have been repeatedly requested from the x-pipe guy for a long time, and never furnished. Just the same tired old smoke being blown and blown and . . . . .

Good Fortune!

:laugh: Those open megaphones are FAR too loud for the speed your Z goes

If you do go with an open 4 into 1 pipe, you can tune it yourself by finding the carbon ring inside the pipe which is the location the frequency and exhaust intersect. Cut the pipe exactly on that ring and you tune it for that motor. That being said all other things need to be right. It works for me on every open pipe I have run.