How does back preasure make torque?
 

I drove a guys carbed 3.0 yesterday. Certainly more power on the top end, but he was running a bypass pipe, and it did not seem to have nearly as much low end torque.

Back pressure does not make torque.

An exhaust system needs to have the right velocity of gasses through the system for proper scavenging.

Too big of an exhaust will slow down the velocity too far, and reduce performance.

It's a compromise. Smaller exhaust will have the proper flow at lower rpm's, while a larger exhaust system is appropriate at higher rpm's.

High school physic's PV=NRT at work.

Also- different cams work best in different rpm ranges. Make sure your cam profile is compatable with your exhaust.

Too many people bolt "go fast" parts on their car, only to reduce performance because the new parts do not play well with other parts.

PV = nRT is a good start, but you need more...
2nd year college physics - wave superposition -- "scavenging"

search on those words -- if it doesn't make sense then you'll need diagrams.

I know I oversimplified, but that's because I'm a simple-minded type of guy.

Think of exhaust gas pulses in the system as balls in a pipe. A ball travelling through a pipe will have a high pressure area in front of it, and a low pressure area behind.

To keep with the simple-minded theme, we basically want to time a low pressure area next time the exhaust valve opens to help suck out the next ball.

Now if the diameter of the pipe is twice as big as the balls, we loose the low pressure area. Or, if the balls are travelling slowly, the effect disappears.

Ow. I'm making my own head hurt. Boil it all down, and we see that with exhaust systems, bigger is not always better.

Exhaust also works in conjunction with the intake system. If you open one up and don't match the other one you are wasting time and money. I can't speek for the carbed engine but I know some opening on a 3.0 injected engine is a good thing. I opted for what some folks here look down on (more than likely because it isn't high dollar) The Bursch 1in 1out muffler and the simple "test pipe" The proper back pressure is maintained because of the length of pipe. I lost no bottom end but the gasses are kicked out very well on the upper end, and I love the sound, The police don't even look at me when driving like a sensible person and everybody looks at me when I am driving like...........well..............I'm driving a Porsche.
I would imagine if your buddy chucked the carbs and bolted an MFI on his car it would be lacking nothing

Too little backpressure can allow fresh mixture to escape out the exhaust port at low revs reducing the amount left for the next combustion.

A bypass pipe won't change the torque.
something else must have been wrong with the engine.

think of it as a big aggressive cam verse a stock cam...one will work well for a entire range, the other will only be good at the top..the stock cam will not be near as good at the top...its all a compromise

good analogy, Dantilla

the other thing to remember is with a big cam is there is a lot more valve over lap ...hard to get power at low rpm with both valves open..but now you have to start the math with- volesty-..just imagine air being a more "fluid "like, and all that goes on as the rpm heads for 4000 to 6000 Gs-....... things start to look much better as the engine sort of starts a scavingins /self induced action....air is still coming in even as the piston is headed north !! (pretty good deal)