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Old 03-05-2013, 11:01 AM
oldbogie oldbogie is offline
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Quote:
Originally Posted by BigMo View Post
Wow, I saw the same blurb with Hearthrob guy, and it mighta been on Stacy's show, but just a couple days ago.....they dynoed (chassis dyno) both ways, and the H was significantly higher HP than the X .....I was surprised, as I thought for sure the X would have a better flow....
I wouldn't take that as one being ipso-facto better than the other. There are factors other than the cross over at work so each system you use will be sensitive to something other than their configuration. Exhaust systems are still very much a cut and try proposition this is a place where the math can only get to an approximation.

You've got two major things going on with exhaust systems:

1) Is the reduction in back pressure by selection of streamlined and high flow components.

2) is wave propagation and management so if the engine is using a cam with a lot of overlap the tuning of pressure waves can be used to assist the induction by reflecting a returning exhaust pressure wave back into the exhaust system which will lower cylinder pressure which will then draw harder in the intake when both intake and exhaust valves are open. David Vizard in his writings on this subject calls this the 5th cycle of a 4 cycle engine. But this is super quirky the returning exhaust wave must be timed exactly to the valves overlap for that particular cylinder. This means it is:
A) RPM sensitive as well as;
B) Mach velocity sensitive;
C) And pipe distance sensitive.

A) Basically means that the combination effect of B and C will align at some RPMs and not at others. So the effect will be seen strong at some RPMs to little or none at others.

B) Is the Mach velocity (speed of sound) of the waves in the exhaust system.
The complicating factor here is that the speed of sound with in a gas is very dependent upon temperature and pressure. Since these two things are constantly changing within the header tubes and collector let alone the rest of the exhaust system. So calculations based upon the simple text book speed of sound at standard atmospheric pressure and temperature can at best only get you into the ball park but not necessarily into the game.

C) Is the distance that the wave has to travel from where it trips back to the valve. This in all likelihood goes beyond the effects of measured pipe lengths to where the event actually moves around inside the pipe based on temperatures of the plumbing itself as well as the exhaust gasses inside. If you will a virtual change of distance even though the physical hasn't changed at least not by much as it heats and cools.

My best guess on the paint or wax burning on the collector as indicating the optimum location for a cross over is it may be where the reflected wave is colliding with the transmitted wave with-in the collector. For typical wave reflection the H pipe is usually closer to the engine where X pipes tend to be further away. Further away means lower temperatures and pressures as well as greater distance and that probably reduces the strength of the wave's pressure pulse simply from a greater length having more surface drag. But the X pipe probably shows less overall backpressure which is a different issue.
In the end which configuration that shows the most benefit will vary by the specific installation configuration which will also be sensitive to pipe diameters as well as the entire foregoing epistle.

Bogie
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