Can someone give me an idea of the negative
effects of baffles/mufflers on an engine
with a crossover exhaust system? I have CC
inserts in order to be road compliant. I am
fouling plugs rather quickly, and am wondering if this extra backpresure could be part of the problem.

Any comments, experiences?

I don't have an answer to the question you posed, but I would venture a guess that your spark plug fouling problem is more likely to be due to running too rich or simply using too cold a spark plug for the driving you're doing...
At low speeds and light throttle, backpressure is minimal..and any that exists has to do with exhaust restriction rather than the collector branches.
It souknds like wrong plugs, or the engine is running way off tune: too much fuel, or very poor timing

I have 1 3/4" header tubes, wide open resonators, and 3" outlets. Without the
exhaust inserts, the car sounds like the space shuttle lifting off. The db limit
here is somewhere around 95-100. My exhaust system is a lot louder than that.
I had a plug fouling problem, due to a
leaky needle valve seat, and uneven floats.
I rebuilt the carb, which should take care of the plug fouling.
My original question was, due to the inherent
scavenging design of a crossover exhaust
system, would restricting it cause a backpressure problem? Perhaps you guys in the
UK have some better type of baffle that is
less restrictive, but accomplishes the noise reduction?
I would greatly appreciate any suggestions.


In my experience you would need to introduce large increases in back pressure to end up with a rich enough mixture to have plug fouling problems. At high RPM (if you could achieve it under load) the exhaust would be coming out at such velocity that it would create a whooshing noise (like a vacuum cleaner in reverse). If you are convinced it is your mufflers you could try running with out them with the rear clip removed to avoid fires, but I think it probably more likely you have a jetting/plug heat range/carb fault as others have indicated before me.


John, Frank, et al,

The fouled plugs was due to a carb problem, which I cured by rebuilding the carburetor.
The backpressure issue is something else
entirely. The exhaust is far too loud to
be street legal without the exhaust inserts.
The inserts are more of a restrictor, than a typical baffle.
Will this increased backpressure cause a loss
of horsepower or any other problems?

On my MK-IV I'm using Borla oval mufflers set verticaly to fit and Borla tips covered by a MKI type heat shield. This works very well. Quiet and powerfull. My car also has crossover headers.
Bill, I have just written an article for the GTD40Club magazine on silencers, Join the club and read it? it may be of some interest to see what we are doing in UK! fRANK
Thanks for the input guys!

If Borla mufflers work on J6, they will certainly be more than adequate for my car. I had intended to call Borla after picking some brains on the Forum. Thanks

I will be joining your club, for the silencer
article, as well as to spy on what kind of
tricks you UK guys have up your sleeves!


Ron Earp


I don't think the backpressure is going to seriously change your setup, as you've already found out.

Frank, I thought you good folks in the GTD Club were going to send some mags over to me? I'd love to take a look, that is for sure.

Bill, get some photos up of your DRB, I'd love to take a look.



Hi Ron,
We are sending you the Spring magazine with the Summer edition to follow at the end of June.
As well as Frank's article on silencers there are articles on our hillclimbs in the Isle of Man, fitting telemetry to a GTD, lowering the engine and gearbox in a GTD chassis, a report from Brian McCarthy (a regular forum contributor) on his build, an article on racing a GTD in Greece, details of our Club section on the Forum and a whole lot more.
Watch out for the Mail Man.