My friend has a Camaro with a carbureted 305 that we're mildly building up (new heads, carb, intake, and headers). I'm looking for a cat-back exhaust kit for him. He already has a Flowmaster muffler, and I believe the inlet is 2.5". He wants mandel-bent pipe to replace his stock pipe. On a slightly-modded 305, would the extra 0.5" of a 3" intermediate pipe really matter? Would it be a hinderance since the 305 is on the smallish-side?

 

3" inch for sure. no question, especially on a modded 305. using anything smaller would be stupid.

 

if youre already spending the $$$ on mandrel-bent pipes, might as well get the right size of muffler as well. 3" for sure.

 

Too many people concentrate on the diameter of the pipe.

Trust me, that's not as important as how many CFM the system flows. If you're pushing some serious RPMs, then a larger diameter MIGHT effect the peak HP, as long as nothing else is causing a restriction, but that's the only real benefit.

One problem a lot of people overlook is the muffler. A good flowing 2.5" system with a high flow muffler will flow more than a 3" system with a restrictive muffler.

AAMOF, a lot of people say that going to a true dual system made a huge difference over their single system. It should.

Think about it. That's also TWO mufflers that now share "blank" amount of exhaust flow, so the amount they're required to flow has now been halved. Pretty simple really.

Flowmaster ain't bad. They aren't the best flowing mufflers, but they have that scavenging design which helps in RPM ranges other than peak HP.

How much HP are you expecting to make?

Here's a tip. Take the HP, multiply it by 2.2, and that's how many CFM @ 20.3"/H2O you need for the muffler to flow. (Divide that amount by two if using two mufflers.)

Then call Flowmaster and ask them how many CFM that muffler flows. See if you get an answer.

If you do, tell your friend to compare it to a Dynomax muffler, since they proudly display the CFM numbers on their website, as well as how much "loss free" HP the muffler is good for either in a dual system, or single.
www.dynomax.com

 

Go with the 3", that leaves room for more mods....

 

Can't argue with that.

If you wanted to do that, and save your friend some money, just get the 3" pipes, then use a reducer to stick it in the 2.5" muffler.

 

I went from a bone stock exhaust to 3" and headers, big difference in power Couldn't believe the factory exhaust was still on the car. Well, maybe I can after now remembering who the previous owner was

 

Thanks for the help, everyone. My friend bought the Flowmaster muffler because he was tired of the sound his car made -- it was practically silent. He'd like to keep the Flowmaster muffler if possible, but wouldn't mind spending some extra money for a 3"-inlet muffler.

I figure that his 305 will be pushing 200-220 hp... nothing incredible but better than what he has now.