Convertible exhaust pipe change... - Pontiac G6 Forum
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post #1 of 3 (permalink) Old 08-23-2019, 12:47 PM Thread Starter
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Convertible exhaust pipe change...

I know there’s nothing likely new or original in what I’m describing here as these cars have been around a long time and have been worked on by many before me. I just thought I’d offer this in case anyone else might be interested.

It’s widely known that the convertibles are rated at less horsepower than their coupe/sedan counterparts. Pontiac advertised as much in their brochures for the G6. (7 horsepower less for the 3.5 and 13 less for the 3.9 for ‘07.) The difference is attributed to the convertible having a more restrictive exhaust system which I hoped from the beginning I could remedy. I didn’t want to increase noise, remove the cat or resonator or lose the stock muffler. I just wanted to equal the coupe/sedan system and perhaps give the engine a break as its job is arguably hard enough.

The difference between the two systems is the proprietary intermediate exhaust pipe used on the convertible. This pipe is routed on all models from the resonator rearward, around the fuel tank and on to the muffler. On coupes/sedans, it runs the path without obstruction. Convertibles however have two reinforcing “X” Braces mounted under the body and the pipe is sandwiched between the rear brace and the body. It’s easy to see where the restriction is when you compare the two pipes. The coupe/sedan pipe is a nominal 2” ID and has no surprises. The convertible pipe is a nominal 1 ¾” (though I found places where mine measured closer to 1 ½”). The most dramatic visual image is where the smaller diameter pipe is spliced into a 2” piece just before the muffler. (If your car has the two outlet muffler, this exhaust pipe is smaller than either outlet.) There are also two places where the convertible pipe has “reliefs” crushed into it where the “X” brace runs under it to provide clearance and prevent rattles. The “X” braces themselves are bolted on allowing them to be removed for servicing the componentry above them (ie: power train, fuel tank, exhaust system, etc.).

The front brace is obviously structural, made with sturdy square tube and secured in multiple places. I’d recommend you don’t mess with this brace. The rear brace is basically a steel strap running from the body center on each side (sharing a mounting point with the front brace) to the opposite side rear suspension support member. It works on tension (compression would simply “bow” it), is secured only at its four corners and doesn’t contact anything in between. It appears intended to provide more stability to the rear suspension bracket by tying it to the opposite side rocker panel area. An earlier thread by another poster described how he spaced the rear “X” brace (using lug nuts per his picture) to give clearance for his modified exhaust system. I didn’t plan to do exactly as he did, but if I could get enough clearance to allow a normal coupe/sedan intermediate pipe to fit with minimal impact, I thought I’d try it.

I started with a good coupe/sedan pipe I sourced from the local Pull-N-Save wrecking yard. Of course the muffler shop could have made one, but the factory pipe fits as close as possible in the confined space and had good bends and consistent diameter. I then removed the rear “X” brace myself and took the car to the muffler shop to have the pipe put in. Once the pipe was in, I took the car home and put the brace back on loosely to see where the interference was and what would be needed to correct it. I had already decided on a limit of how much spacing I would allow on the brace at as close to ½” or less as possible. When you relocate or space things that are or will be stressed, you change the dynamics they operate under. Since the brace seemed to have a singular function and considering the hardware and materials involved, I was comfortable that it would sustain a change within this parameter. Much more would be outside of my comfort zone. As it turns out, ½” spacing at the front and 5/8” at the rear gave sufficient clearance between the pipe and brace. I decided to run it this way for a time, checking it frequently. My part of the work was simple with the R&R using common tools and a floor jack. I can provide details if requested. The costs to do this was low and I could revert it back if need be.

Is it worth it? Psychologically, yes. Can I prove my car should now be rated at 13 more horsepower? No. But, I feel comfortable the engine may not be working quite so hard even if the convertible does weigh 500 lbs more than the coupe/sedan (and I can’t help that.)

Good luck everyone. – Mark

Coupe/Sedan pipe I used.
Convertible pipe from my car. Note diameter, crushed depressions and 2” splice to muffler (at bottom).
Rear brace removed. Floor jack helps position it under car but its’ not heavy.
Pic under car showing spacer hardware at rear (foreground) and front (distant).
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post #2 of 3 (permalink) Old 08-23-2019, 08:08 PM
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Nice write up - thanks.
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post #3 of 3 (permalink) Old 08-24-2019, 10:51 AM
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Originally Posted by 1of4885 View Post
...But, I feel comfortable the engine may not be working quite so hard even if the convertible does weigh 500 lbs more than the coupe/sedan (and I can’t help that.)
I'm on board with everything you said except for this part. What exactly constitutes "working hard"? Well, if you do a max acceleration run, like a drag race, you will be developing 13hp more. So the engine will be 13hp more stressed mechanically. Definitely working harder, not less.

If you're cruising at 70mph, you will be using the same 40hp or so (just a guess) regardless of the max power capability. So the engine is working equally hard if "working hard" is measured by how mechanically stressed the engine is. With the free flowing exhaust you will likely operate a tiny bit more efficiently at 40hp, so you may save a bit of fuel.

It's a bit counter-intuitive, but choking an engine doesn't make it "work harder". There are classes of racing that require intake restrictors. That reduces max power and, since race cars operate near max power most of the time, actually helps the egines last longer. Because they operate at lower hp levels than they would without the restrictors.
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