Re: E28 Exhaust

[Robert T Chafin <rtchafin@xxxxxxxxxxxxxx>]

At 06:20 PM 9/4/98 -0400, Kurt wrote:
>Date: Fri, 04 Sep 1998 18:11:17 -0700
>From: K F Martin <kfmartin@domain.elided>
>Subject: E28 exhaust

>Hoping that someone who has been down this road can advise me...
>I have an '87 535is.  The exhaust system is now pretty well shot 
>from head to tailpipe.  I took the car to a local guy to check 
>the damage and ask his opinion.  This guy is well-known for his 
>good exhaust work; hes honest, inexpensive, and he did a great 
>job on my vw golf last year.  He suggested that I use an exhaust
>that he will custom fabricate, using stock components...and using
>a y-pipe that will convert to a single exhaust the whole way back.
>I told him I wasnt interested in doing that, having some serious
>reservations because I imagine this car to be very sensitive to
>back-pressure changes.  I've since priced the exhaust in the BMP
>catalog...its about a thousand dollars in parts, plus installation.
>The mechanic's alternative would be about two hundred, installed.
>I guess this could offend some purists, but this is enough money
>for me to think twice about it.  so im thinking.  can anybody help?
>Id greatly appreciate any input.  (gotta decide by tuesday)  
>thank you much.
>kurt.

Kurt - This guy is on the right track; very d*** little of your exhaust
backpressure comes from the system piping itself, so there's not a
significant difference (qualifiers inserted here) for a street-spec 3.5L
engine between a single pipe and dual pipes, if you're talking about
similar reasonable pipe sizes.  For example, if your OEM pipes were dual
1.75" dia., and your local guy changes it to a single 2" or 2.12" dia.
pipe, there won't be a measurable difference in either backpressure or
powerloss. The big differences are in the mufflers, and in the transition
into the muffler. You don't want a one-size-fits-all muffler, because those
things use restrictive internals, and the resulting high backpressure, so
that whatever car they're put on will be quiet, whether it's a 4-cyl.,
6-cyl., or 8-cyl.  And you want a smooth transition between pipe and
muffler (both in and out) without radical changes in diameter.
	These points are a lot more important than whether you've got one pipe or
two.  And of course, having somebody you have confidence in do the work is
worth a lot.
Regards, Robert

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