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Discussion Starter #1
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

Pictures:
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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08 convertible
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...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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Discussion Starter #4
After operating the car for a couple of months with this modification, I wanted to update the thread and mention any untoward effects encountered, if any.

A month ago or so I inspected the car and found nothing loose or any signs of distress. After a bit of checking, I removed the ½ “ spacers I’d placed at the front mounting points of the brace and substituted much thinner shock absorber washers which moved the brace up to a near stock position. The clearance between the pipe and brace are now nominally 1/8” to 3/16” which is much less than stock, but I found no evidence of any interference between the pipe and brace in the now two months of operation. I’ll review it again after considerably more mileage and see if any changes are warranted.

I should have clarified the “working hard” comment I made in the original post. I agree an engine works harder the more mechanically stressed it is and understand that intake restrictors used in racing aren’t detrimental to engine longevity as they then operate under less mechanical stress. And true, choking an engine on the intake side won’t make it ”work harder”. But, exhaust restriction is a different animal as all engines expend some effort (horsepower) to expel the burnt gasses and the power used isn’t available to move the car. More restriction takes more effort (especially at high RPM) which is where my “working hard” comment came from. The more restrictive convertible pipe apparently was enough for GM to rate the engines (as installed) at less power. And, I've found no published references that the convertible's engines are mechanically or programatically different from the sedan/coupe engines. Sorry, I didn’t mean to mislead.

Mark
 

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Discussion Starter #6
I didn't do before and after testing so I can't prove it. My goal was to take the 13 horsepower at least theoretically and hypothetically even the playing field a bit between the convertible and the coupes/sedans. (Life is tough for the convertibles anyway as there's no losing the extra 500 lbs.)

Mark
 

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Mark, no data needed, owner to owner, what does the dyno in your butt say?
Sure, some of it may be emotional, but does it feel snappier, a little more ummph at mid-range, anything, or just plain nothing.

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Discussion Starter #8
Okay. Low end is still strong. Mid range up seems to come on with a bit more urgency. I'm satisfied and feel it was worth it.

Mark
 

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Cool, that's pretty much what I suspected would happen.
I doubt I will be able to build any data, but there is a section of pipe and a lucky car that will soon meet.
I am realistic that it is not going to be day/night, but am excited.
Thank you.




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Discussion Starter #10
Your car is a beauty. Good luck with your project. Here are a couple of tips from my experience:
  • Looks like you found a good donor pipe. The convertible appears to have the same cavity and heat shields for the pipe and muffler as the coupe/sedans. Be careful to mount the pipe as far up in the cavity as possible. Some might remove the resonator, but I left mine in. The installer I used was an artist. Wish I’d pursued welding when I was young. Maybe in my next life.
  • The two bolts that secure the front of the brace (near the center of the car in the rocker panel area on each side) might be rusty and difficult. (May even be damaged if the car was ever high centered.) Use penetrating oil. Breaking these will ruin your day. The backside of them isn’t accessible. Though I tried a couple different lengths in my experimenting, I ended up using the originals in this location. I used a dab of “anti-seize” when I reinstalled them hoping to fend off problems in the future.
  • You’ll need (4) longer bolts for the rear mounting of the brace. I believe they are M12’s. Use Property Class 12.9 (hardest available). The local Ace Hardware actually had all I needed in stock in several lengths. I don’t remember the lengths I settled on for the rear, but the length isn't limited and you’ll easily see once you have the brace re-positioned and held up with the floor jack.
Good luck and take care. - Mark
 
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