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Discussion Starter #1
I think I have discovered an important omission in the Bell formula for calculating megaphone lengths, etc.
Does anyone have a formula other than Bell?
How about just the dimensions? I can try to extrapolate backwards from the motor details (size, peak RPM) and give you back (more or less) how it was calculated or at least what the relationships look like.
Thanks.

Need help with your Mikuni VM? Click here:
http://victorylibrary.com/MIK-BT.htm
 

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I recently discoverd a flaw in the theory of relativity.

If someone will give me the formula for time travel I think I can work backwards and figure out what's wrong..... with Joe.

JohnnyB
 

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Discussion Starter #3
Since you're so interested:
The usual (Bell etc.) math suggests that the primary length is whatever is left over after calculating the diffuser specs, either based on maximum OD (3-4") or taper (14-20 degree included angle) and subtracting them from the tuned length (based on RPM, gas speed, exhaust duration, etc.). The baffle (reverse cap) appears to be less sensitive and no real details are given, it's an afterthought.
However: Jennings is very specific in his expansion chamber work that the tuned length is NOT to the major diameter, or to the end of the baffle (exit to ATM), but 1/2-way to the THEORETICAL end of the baffle (the point of average reflection for the positive wave if continued without termination at the desired outlet diameter).
If this is relevant, it means that the most common calculations (Bell) that use only length to the major diameter (primary + diffuser) are way too short, and even calculations that use total length, and I haven't seen one (primary + diffuser + baffle) are closer but STILL too short, since the theoretical length of the baffle is more than twice the actual length (viz., the baffle only reduces the major diameter by less than 1/2).
E.g., a baffle for a 4" major diameter diffuser with 16 degree (included) convergent angle to a 2.75" (ID) exit, a fairly common size for big singles, is 2.01" long, but the 50% point on the baffle based on its theoretical length, or 6.8" long, giving a 50% distance of 3.4". The old calculated length is [primary + diffuser + 2.01]"; the corrected length is [primary + diffuser + 3.4"].
The extra 1.39" length puts the predicted peak RPM above the desired input figure. For a motor with 70 degree BBDC exhaust opening and 6K peak RPM it's about 250 RPM too high.
Might this be why the tuning instructions frequently say "begin with a primary length 2-4" longer than your predicted length and shorten by test; some adjustments to the primary length may be needed".


Need help with your Mikuni VM? Click here:
http://victorylibrary.com/MIK-BT.htm
 

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I just build a few systems and test them on the dyno. Formulas get you in the ball park but I've never seen one that was right on the money. But then for the most part a few years of building exhaust systems and you can get in the ball park just knowing displacement and state of tune.

Course the Helmholtz Formula program I wrote for estimating intake track length was only about a 1/4" off on the dyno.

It's been my experience that on most vintage bikes a proper exhaust isn't worth a whole lot of hp one way or the other. Just tends to move the powerband around. Which of course can be worthwhile. Depending on the engine I design more to compensate for weak areas than to achieve a peak hp figure.

Don't know crap about two stroke exhaust.... and don't want to.
JohnnyB
 
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