Question RE: Mufflers and Horsepower Loss

If a exhaust system gives you a extra 50 hp at peak rpm and full throttle it will give you some proportion of that 50 hp at lesser rpm and throttle settings.

so, If I was producing about 200 HP (50% of max)at 3/4 throttle with open exhaust, it would drop to about 175 (less 50% of the muffler max loss) with restrictive?

I would need slightly more throttle to get the same pull away feeling ?

Yes.

The challenge is to engineer a muffler setup that cuts down the noise but does not reduce the power.

I was hoping towards the ideal the muffler flow becomes "choked" nearing maximun output and the back pressure effect on the loss of power only be noticable near max and not be a propertional restiction over the entire range

Looking at various dyno print outs mucflered vs unmuffled it appears there is a hp gain immediately off of idle.

Looks like the gain is virtually equally proportional across the useable rpm range.

There seems to be a noise gain to me but not measured and therefore not known if it is directly proportional to the hp gain?

Fiberglass filled mufflers (Hall & Mind Train) perform well when they are new, meaning they rob less power from the engine with the trade-off of only a small increase in sound level. But as the fiberglass filler packs-in with carbon & oil the mufflers become very loud and power output usually suffers as well. Magnaflow mufflers have a good reputation for low back pressure and low sound level (when they are new) but they also have a reputation for performance dropping-off quickly, and sound level increasing quickly, as they pack with oil and carbon. Since Magnaflow doesn't make a muffler in the compact size needed for the Pantera, owners using Magnaflow mufflers are also left with a pair of mufflers hanging out the back of their car, all in the name of a few extra horsepower. There are compact mufflers using baffles instead of fiberglass filler, this seems like the ideal solution, right? Unfortunately the two I am aware of are quite loud. Pantera mufflers appear to be too small to achieve free flowing (low back pressure) operation, longevity and low sound level operation all at the same time.

Factory appearance, longevity and quiet operation are the greater priorities for me. In my estimation the GTS tailpipes from Hall Pantera are therefore best choice for me and my priorities, until something better comes along (if ever). To use them my strategy is to build the engine knowing there's going to be a 10% loss at the tail pipes and accept that as a reasonable trade-off for having a quiet exhaust, the factory look, and good longevity. Frankly, unless I am racing the car competitively I'm not going to miss that approximately 40 bhp. If I were of a mind-set where a specific number was important, like 400 horsepower, I would compensate by building the engine for 10% extra horsepower (445 bhp) with open exhaust. Seems simple enough to me ... but I am 1/2 Okie after all.

Lets be realistic. 400 bhp is enough horsepower to melt the rubber off any set of the widest possible street tires. It will accelerate the Pantera from a standstill to 60 mph in less than 5 seconds, limited by the traction of the tires and the skill of the driver. Top speed will be in the 170 mph range, if the car has enough down force to remain in contact with the road (note: front and rear spoilers needed) and enough open road, sans law enforcement and slower cars, to maintain such a high speed.

Whenever gas flows across a restriction, the pressure drop across that restriction does not increase linearly with increases in gas flow, but logarithmically. A muffler is a complex restriction, one having both mechanical and acoustical properties that will influence the flow of exhaust gases. Its much easier to quote a certain number, or a certain percentage, and we all do that; but in reality the loss of horsepower shall depend upon where we are in that logarithmic flow verses pressure drop curve. At the "low end" the curve is only gradually sloped, losses shall appear rather linear although they really aren't. As we go further out along the curve the slope becomes more pronounced, horsepower losses shall increase more dramatically (logarithmically). I'd like to point out two results of this property: (1) the GTS exhaust system is much better suited to a 351 cubic inch engine than it is for a 400 cubic in engine, because for any given rpm the 400 cubic inch engine is stuffing about 14% more exhaust gases in to the exhaust system. Increased gas flow moves the exhaust systems performance further out along "the curve". (2) Losses will also be greater at 7000 rpm than they shall be at 6000 rpm, for the same reason.

One last aspect I'd like to point out: exhaust valve timing also comes into play. An engine equipped with a cam that opens the exhaust valves early (wide lobe centers) is less impacted by the mufflers than an engine equipped with a cam that opens the exhaust valves later (narrow lobe centers). Its an old hot rodders axiom that a narrow LSA cam "demands" a better exhaust system.

In summary, a 351 equipped with a wide LSA cam (112° to 115° LSA) shall extract better performance from the GTS exhaust system than a 408 equipped with a narrow LSA cam (108° to 111° LSA).

Food for thought, respectfully submitted.