Brad man look this one up on google if you don't belive me but the last friggin thing you should ever and I mean EVER want on any exhaust weither it is for a 4 banger a lawnmower or a truck is an exhaust with a restriction of any amount. Back-pressure is a myth spread around by folks who don't understand things like flow dynamics and exhaust scavenging.
To put it in simple terms you want a system that is properly sized for the engines application in the case of a 302 lets say a single 2.5 inch system on a stock engine with no mods other than a header. Now some might say it is to small but with a single pipe setup like this the exhaust can in effect pull itself out at a decent velocity in the low end while still being free enough in terms of pipe diameter to make the most of the engines power even towards the top of the power band (In this case about 4500 if your lucky). Go to big with pipe size on that stock engine and it might make great HP right at the top but torque is gonna suffer in the lower rpm band as well as HP. Likely wise to small a pipe and it will do great putting around and in the low end but will not be making near what it should near the top end of the power band.
That's about as simple as I can explain it to you Brad and this issue has been beat to death at least 200 times on this forum and many more time on other forums time and time again. Not trying to be a butthead just trying to help us all out in the long haul.

 

Don't need to look it up. I have installed enough exhaust and put them on dynos to see the loss of power at lower RPMs with straight piped exhaust.

But carry on

 

You boys are arguing over semantics, but what Brad is saying is correct. If the exhaust is too open, ie, too large of a pipe, then it hurts scavenging, which is needed at low RPM.

 

You can buy aftermarket heads with 180/210/220cc etc. runners to size the head to your HP needs. The same with header primarys. Big isn't always better...

 

I'm not trying to be a smarty pants here but he said it needed a restriction in the exhaust for it to run better which is in itself a false statement, that is what I was originally referring to with my earlier reply. I'm just trying to help out the OP in the long run, I'm not trying to bash Brad or you my good man just trying to end a myth once and for all.

 

Well thanks guys. I think ill just run 2.5" straights. I like it loud.. Id rather hear the engine screaming than the radio anyway haha. Thanks again

 

And like I said. It hurts the BOTTOM end of the RPM band. And lets face it, that is where trucks need the most advantage. NOT at 6,000rpm just before the governor kicks in

 

I can go out right now pop the mufflers off my truck and make the exact same amount of power in both the low and high end with no issues. The guy has the right size pipe so all he needs to do now is run it past the cab and it will be perfectly fine with no muffler so long as the law is not involved in terms of the noise violation.

 

No you will not. I dare you to dyno your truck, then remove the mufflers and dyno it again. I GUARANTEE it will loose hp and torque on the bottom end

 

I have never done it but my buddy who graduated from a motor performance tech school in AZ told me by adding mufflers that some of the engines they tested like a 350 small block gained 25 low end hp. He has no reason to lie about it so I believed him. My 0.02. Those engines did not have cats either

 

Years ago we put a 426 wedge on the dyno, ran it with factory exhaust setup, uncapped the headers, gained 75HP. Ran a Zundapp 250 mc without muffler and ran so much faster it was scary! So depends on application.
But I never heard anyone around the drags or circle tracks say "Needs some restriction!"

 

You are also talking MAX RPM. I am talking Low RPM. BIG difference

 

And if you do have too large of a pipe, then go down to your local Super Advanced Discount Zone Of Auto Parts store and buy a choked up "performance" muffler, then you've reduced the flow thru your too-big pipe and now things get better. This is where the "backpressure" idea comes from. But whether you use too big of a pipe and "restrict" it with a muffler, or use the right size pipe with no muffs, the end result is about the same.

 

I'm going to try and explain this as much as I can without being able to use my hands for demonstration....I know the word backpressure is the way it is described but in reality it is not backpressure.


OK, imagine you are going to put a pipe on just one exhaust port. Say you put a 3" pipe on that one port, when the exhaust valve opens and the piston pushes out that exhaust then closes the exhaust gas just stops moving cause it has plenty of space. The next time the valve opens same thing happens and it just kinda moves the exhaust along. Now put say a 1 1/2" pipe on the same port, when the valve opens the exhaust is going to move faster just cause the space is less. When the valve closes the exhaust is still moving cause of the speed it is traveling, thus it halfway creates a vacuum. Now the next time the valve opens the vacuum helps pull the exhaust out along with the piston pushing it. So what you try to do is get the right size pipe so that all the cylinders work together to help pull the exhaust along with pushing it(scavenging). This tremendously helps the lower end torque as Brad said. It doesn't help as much at high RPM but it still helps. Hope this helps instead of confusing even more.

 

That makes perfect sense.