backpresure

[Josh350]

I heard that 88-98 fullsize chevy/GMC work off back presure and if you take off your cats you lose power? I was wondering because im going to put true duals on my 93 GMC from magnaflow and i dont want to lose any power. Plus im puting hooker headers on too.

[NEUMANNZZ]

you want some back pressure to save your engine from backfiring which is dangerous. just dont go too large on diameter of pipe( ie 3 inch single pipe or 2.25 to 2.5 inch dual pipe with x or H pipe installed).

[lmadore]

All engines need some backpressure. The back pressure helps scavenge exhaust and unburned fuel away from the cylinders. Too large of an exhaust allows the gases to cool off too quickly as well. Cooler gases move slower

[HemiHunter]

U guys think a stock 6.0 with stock cats, stock manifolds, an X-pipe and true 3" duals will have enough backpressure?

[BigNutsZ71]

If I were you, I wouldn't go bigger than about 2.5 on your pipes. Just my two pennies...

[HemiHunter]

What would be the difference in doing that or just running a dual 3" i/o muffler if the stock pipes are already 3" all the way to behind the muffler?
Thinking about some sort of forced induction down the road, I know dual 2.5" would be probably be fine,but would dual 3" hurt performance?

[mikeoxlong]

My old man does exhausts. Believe me, you don't want 3" duals, your performance will suffer. I know you're thinking S/C, and in that case 3" duals would be okay. I personally have 2.5" duals with my KB, and it flows plenty well.

[Quyonmob]

forced induction requires less backpressure.

diesels require none.

[denmason]

Originally Posted by Josh350

I heard that 88-98 fullsize chevy/GMC work off back presure and if you take off your cats you lose power? I was wondering because im going to put true duals on my 93 GMC from magnaflow and i dont want to lose any power. Plus im puting hooker headers on too.

What does backpressure have to do scavenging?? Backpressure in your exhaust system does not aid in scavenging the pipes..... it constipaites the pipes.
A stock engine that cannot adjust its fuel delivery needs backpressure to work correctly.... somewhat untrue. Some people think that too little backpressure will lead to valve burning, but it is only part of the problem. The reason the valves burn is because the engine leans out. Not enough fuel being delivered to the combustion chamber will cause burnt valves. Easy cure, add an AFPR and tune your air-fuel mixture to the 14.7:1. If you add headers to your motor you will start running lean because more air will be flowing through your system. You can use a multimeter connected to your O2 sensor and an adjustable fuel pressure regulator to up you fuel flow to get to the optimum 14.7.1
I'm not saying that bigger pipes are gonna help or hurt performance. Your headers will be the right size to "scavenge" your system. A good header not only relieves the backpressure, but goes one step further and creates a vacuum in the system. When the next cylinder's exhaust valve opens, the vacuum in the system pulls the exhaust out of the cylinder. This is what the term "scavenging" means.
There has been a lot of misconception about exhaust systems and how they work, but it really makes more sense when you know how they work and why. I hope this makes sense, wish I was better at explaining things. There are some very good tech articles out on the net, find them, read them, they can really make things easier to understand.
I'm burnt and need to sleep now, so I'll stop my babbling and say "goodnite"

[HemiHunter]

O.K. I understand that 3" dual is a bit extreme, and I know what your sayin' about backpressure, But if I'm running stock manofolds, and stock cats, wouldn't that provide enough, considering the system only flows as good as its weakest point? I have intentions on a custom tune and eventually buying a S/C as soon as Kenne Bell releases thyre intercooled kit, what would it hurt untill then? Don't want to sound stupid. On my old 94 w/ a 350 I had hooker full length headers, no cats, and a Gibson split rear catback. The truck picked up sound, power, and mileage. And that had to have little if any backpressure.
Thanks Guys

[brehnborg]

here is a trick that my uncle does when he build mod race cars. put your straight pipe in. paint the pipes 6in for the headers white all the way back. start your truck up and get it hot. turn the truck off and get under and look at how far back the pipe burned the paint right at that point where the paint isn't burned anymore cut off the remaining pipe. that is all the pipe you need for backpresure and will give you max amount of torq.

[HemiHunter]

Souds like a good idea. Anyone on here try 3" true duals? Did a search and didn't come up with anything.

[Josh350]

Well I put on Magnaflow dual-in/dual-out with no cats and it sounds great I love the way it sounds. I actually gained power without cats. I got duals all the way back in the muffler there is an x-pipe which evens out the flow. and its set up for headers when i get the money to buy them.

[cedjunior]

Originally Posted by denmason

What does backpressure have to do scavenging?? Backpressure in your exhaust system does not aid in scavenging the pipes..... it constipaites the pipes.
A stock engine that cannot adjust its fuel delivery needs backpressure to work correctly.... somewhat untrue. Some people think that too little backpressure will lead to valve burning, but it is only part of the problem. The reason the valves burn is because the engine leans out. Not enough fuel being delivered to the combustion chamber will cause burnt valves. Easy cure, add an AFPR and tune your air-fuel mixture to the 14.7:1. If you add headers to your motor you will start running lean because more air will be flowing through your system. You can use a multimeter connected to your O2 sensor and an adjustable fuel pressure regulator to up you fuel flow to get to the optimum 14.7.1
I'm not saying that bigger pipes are gonna help or hurt performance. Your headers will be the right size to "scavenge" your system. A good header not only relieves the backpressure, but goes one step further and creates a vacuum in the system. When the next cylinder's exhaust valve opens, the vacuum in the system pulls the exhaust out of the cylinder. This is what the term "scavenging" means.
There has been a lot of misconception about exhaust systems and how they work, but it really makes more sense when you know how they work and why. I hope this makes sense, wish I was better at explaining things. There are some very good tech articles out on the net, find them, read them, they can really make things easier to understand.
I'm burnt and need to sleep now, so I'll stop my babbling and say "goodnite"

Old thread..

I love how everyone just kind of ignored this post and kept going on and on about the backpressure. Backpressure = bad. Velocity = good. When you use pipes that are too big in diameter then you lose exhaust gas velocity, not backpressure. Try blowing air out of a straw as hard as you can, then the same out of a cardboard tube from a roll of paper towels. Even though you move more air through the cardboard tube, the velocity of that air is much less than the air coming out of the end of the straw. This velocity is also needed for proper scavenging. The more velocity the exhaust gas has as it enters the manifold or header primaries, the more powerful the vacuum force will be on the rest of the cylinders.

[bigolblackie]

Originally Posted by cedjunior

Old thread..

I love how everyone just kind of ignored this post and kept going on and on about the backpressure. Backpressure = bad. Velocity = good. When you use pipes that are too big in diameter then you lose exhaust gas velocity, not backpressure. Try blowing air out of a straw as hard as you can, then the same out of a cardboard tube from a roll of paper towels. Even though you move more air through the cardboard tube, the velocity of that air is much less than the air coming out of the end of the straw. This velocity is also needed for proper scavenging. The more velocity the exhaust gas has as it enters the manifold or header primaries, the more powerful the vacuum force will be on the rest of the cylinders.

Thats about what they told me when I went to get my exhaust done. They are experts,and know what theyre doing. I wanted 2 1/2",but they recomended I go with the 2 1/4". I think they were right,my truck has good torque,and throttle,which is want I wanted,but didnt know.