I have been looking at replacing my Dynomax Ultraflo muffler with a Dynomax ST 17739 with the stock dual outlets but am worried about losing power [slightly modded L98]. The Dynomax rating for the 17739 muffler is 175 Hp loss free and the Ultraflo is something like 515 Hp.

Looking at this comparison test http://www.pontiacstreetperformance....p/exhaust.html
this guy is losing less than 0.1 of a second - out of a high 12 second run - going from an open pipe to a Dynomax 17749.

Now the rating on the 17749 is 193 Hp and the 455 in the LeMans wagon has to be making a whole lot more than 193 Hp to run 12s???? What gives - are Dynomax being ultra conservative rating their mufflers this way or is this all you can expect to lose? What am I missing here? Seems like a pretty damn good result given that the muffler is only rated at 193 Hp and is a 2.5" inlet and outlet...

 

If you have stock L98 any muffler you go with will be an improvement. You may want to wait though and just get a whole cat-back.

 

386 HP.

The LeMans had duals.

The recommendations on the Pontiac site are pretty good, but not completely universal. For instance, if you're running Warlock mufflers, you'd want the outlet to be located at the "torque tube" length for uncorked operation, not as far back as possible.

There's a lot to be said about being able to drive to the track w/o looking over you shoulder for cops, and running at the track without having to crawl under the car to unbolt things. Of course, most of what they did wouldn't apply to a single pipe 3rd gen exhaust.

 

The muffler part of the testing was done using a single system...

"...To allow muffler variations to be more accurately determined on the LeMans wagon, a "single muffler" exhaust system was fabricated. By forcing all of the exhaust through a single muffler, its effect on engine performance and sound was magnified. The charts showing backpressure, sound levels and performance provided by different muffler's are shown in charts 1 through 4. After analyzing the results, the large bodied Walker #17749 Dynomax muffler was chosen. It’s quiet, flows well and is relatively inexpensive. The muffler body is 20 inches long and it has an oval cross section that’s roughly 9.75 inches wide by 4.25 inches thick. This muffler has offset 2.5 inch diameter inlet and outlet connections...."

[I]

 

I guess I didn't read that carefully enough. I thought that was only for the sound tests. All of their pictures show duals, as well as their recommendations talk about duals. Oh, well.

The HP ratings are probably conservative to establish the "loss free" part. So, it doesn't mean you can't make more HP using the muffler, just that there will be some loss above those values.

 

No worries, I was just worried that the muffler I was looking at was rated at only 175 Hp when mostly stock L98's will make at least 220-230 Hp at the flywheel.
I guess if a rating of 193 Hp barely annoys an engine that must be making at least 400 Hp to get a LeMans wagon down the quarter in 12's, then they must be super conservative about their Hp ratings I reckon!