From: charlie@elektro.cmhnet.org (Charlie Smith)
Subject: Re: Engine theory 102 section 002
Keywords: pipes, jets, carbs, bleagh.
Organization: Why do you suspect that?
Date: Fri, 22 Jan 93 01:19:31 GMT

In article <FRAHM.24.0@UCSU.COLORADO.EDU> FRAHM@UCSU.COLORADO.EDU (Joel A. Frahm) writes:
>	I'm wondering about a few things.  One is, how come some mufflers
>require rejetting and some don't?  The F1 series mufflers 'require no
>rejetting' or so they say.  Perhaps that means they don't REQUIRE a 
>rejet, but it sure helps.  I'm not really interested in replacing the 
>stock 4-1 on my FZ700.  What I want to do is take the bike to Bonneville
>when we bring the cars out there this spring, and run it too.  I thought
>I'd cut the stock muffler off and put a slip-on adapter on it, so 
>when I go to Utah I can pull off the can and put a straight extention on.
>But I am concerned about running too lean as a result.  I'm hoping to 
>get 160 mph out of the bike with no major modifications, other than 
>gears and exaust, but if the bike as a lean miss or something like that, 
>who knows?  What is the difference between a pipe that requires a rejet, 
>and one that doesn't?  Hype?  Back pressure?  I know I've had to rejet 
>my car after modifications like cam, headers, and the like but not 
>neccisarily for a muffler change.  Anyone buy an F1 series muffler and 
>get it to run right with no jet change?  Anyone run open exaust without
>over lean condition?
>
>Joel.
>     

The primary reason some exhaust system changes (not just mufflers) require
intake mixture adjustment (carb jets, FI adjustment, chip change) is that
the new exhaust system scavenges the cylinders differently thatn did the 
original one.  For example:

If the new system, or new muffler, is either less restrictive or it creates
a different cylinder scavenging condition because of (usually) more 
optimal pipe tuning for the rev range where you want to run it, then this
means that there is less exhaust back pressure in the cylinder when the intake
valve opens, or even better, there is a higher partial vaccuum (sp?) in the 
cylinder when the intake valve opens.  Either condition means you are going 
to pull more air through the intake system; the greater this effect, the
greater the need to change jets, etc.

If you don't require a rejet, your muffler change probably isn't making
any difference on that end.

You aren't as likely to see the effect in a car, because you don't normally
have tuned headers on a car.  Take the Porsche 911 before 1975 - it's a
flat 6, 3 on each side.  The exhaust is 3 equal length pipes joining at the
same point about 12" before the muffler.  This engine is VERY sensitive to 
muffler changes, and also to intake system changes.  The usual effect that
you create is to create huge flat spots - in a car like this or in a bike.
Bikes are more likely to have a tuned pipe exhaust system, so will be 
more sensitive to muffler changes.

End of Soapbox.  (tm)



Charlie Smith   charlie@elektro.cmhnet.org  csmith@dsac.dla.mil  whois ces10
  DoD #0709   doh #0000000004  &  AMA, MOA, RA, Buckey Beemers, BK Ohio V
                BMW K1100-LT, R80-GS/PD, R27, Triumph TR6
                          Columbus, Ohio USA

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