You went to the Dyno after a muffler change? I suppose your vehicles came with some the most restrictive mufflers of all times. Cat-backs seem to be good for something like 10-15 horsepower (or less) on just about every stock make & model I’ve ever seen, had, or heard about. This guy gained 8 horsepower with a cut-out. If a muffler was good for any horsepower (for his car) surely running open headers more or less would’ve made more than eight. Cut-out
I too have a Titan, maybe I ought to put a muffler on that bad boy. The stock one is HUGE, I bet it weighs 40 lbs.
Here’s kind of a cool article where they test 2.5” vs 3” exhaust on a not so radical motor. 2.5” vs 3”

 

My 2012 made 296 rwhp and 388 rwtq on a Mustang Dyno with an Airaid MXP4, Magnaflow muffler and a Hypertech programmer set on the 93-octane tuning. It ran 14.60s with a muffler swap and a CAI. Ran 14.00s after the programmer. It made another ~10 rwhp with PRT 2.5" catless B-pipes with the stock manifold cats in-place and gained closer to 20 hp at ~5,500-6,000 rpm. It broke into the 13s with the B-pipes.

This was in nearly 100F heat. Ran 14.20s in the Texas heat and 13.90s in October/November when it cooled off a bit.

It was quick for a 5,350 lbs truck with an open differential on 20" street rubber.

 

What’d it run with the stock muffler? Trap speeds differences? I’ve ran a quicker or slower e/t with no changes made (I’ve also dynoed different with no changes made) but the trap speed was always about the same.

 

15.00 @ 90 stock. Gained 4 tenths and 3 mph from a Muffler and CAI. I made 100s of passes in the 1/8 and 1/4 in that truck. Sometimes 20 in a single track outing.

 

Right on target for ls1 t56 bolt on and tune with LS6 intake. Over 4:1 gears lower whp readings. My 98 ws6 has 4.10s and on a dynojet, they cost me about 10whp from I was told. Being a bolt on ls6 top end swap, made 393whp/386wtq. So had I kept stock gears for time being, would have hit 400whp. But wanted dd beater so throw on all the parts that makes a C5Z a "Z" vs base C5. Glad I did cuz LS6 cam doesn't kick in til 4500 rpms.

My 02 SLP Z t56 still has stock 3.42s. Full bolt ons minus e-w/p and has hi-flow catted y, 355whp and 371wtq. Tuned with the Diablo Predator when that was 1st released 20yrs ago. Still stock LS6 intake and factory tb (ported by Bo White). Never used a Dynocom dyno as I started with dynojets and keep with to compare with old dyno sheets... But your wtq should be more than 360wtq. Unless 4.56 gears throws off dyno's more than 4.10s ???

 

Those are good numbers for sure, I'm about 25 rwhp less with my A4, similar mods but I have less gear, 3:73 and stock stall converter. Seems like the stick cars are always a bit more then the automatics when they get on the dyno.

 

Automatics have more internal friction or drag.

 

For sure that's the case, is there a estimated percent difference between the two for losses? Just curious

 

Every trans model is different. Only the dyno tells the real tale here.

 

Yes from reading here its highly contested what different dyno's give for rwhp ratings also how the curve was derived. Mine was on a dyna jet and was SAE

 

Roughly 15% for a Manual/7.5 rear and 18% for a 700r4/4L60E with a 7.5 rear. Pretty consistent across all years from the G-bodies to a 3rd gen to 4th gen and S10s on.

 

You say you are a tuner but then put a Hypertech on your car?

 

Hey OP is your car red with ground effects and maybe chrome Y2K's? If so check this out around 5:05.