Doug Roberts

ok I have a 99 5,3 and going to cut cats off what and how do I make back pressure for the values and how much do I need I want it loud and mean I have no muffler on it now

Mille Racer 69

The whole backpressure thing is more of a myth from people that do not take into account several variables when it comes to modifications. What you want is velocity to get the exhaust out as quickly as possible with as little resistance as possible.

Think of it like the hose at your house.
If you just turn tge water on it comes out.
If you partially plug up the end it comes out faster.

lets say you have a Monte Carlo or whatever that you wanna sup up.
So you put on a monster 3.5" exhaust on, you notice you now have less power.
Not necessarily because of backpressure, but due to a loss of velocity I.e. the garden hose.

If you have longtube headers, that's probably all you'll need.
If you want, put on a couple extra feet of straight pipe after the collector.

Doug Roberts

well I have 3in stock tube and was going to have pipe welded on and make it stock looking I don't want to lose that much hp

Mille Racer 69

Do you have longtube or shorttube headers?
Or stock manifold?

Doug Roberts

stock
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Mille Racer 69

before welding anything, I'd just give it a try as is.
do a 0-60 test 3 times to get an average time with the truck stock, exhaust and all.
then try it again with the exhaust system removed and see how the time compares.

another test to compare is torque. Start burning out in place for a sec,
then let off the brake and measure the distance of your skidmarks.
do that test again with the exhaust removed and see how it compares.

The stock manifold may be resistive enough that you don't have to do anything.
If your tests are suffering noticeably after the exhaust system was removed thus only running on the manifold,
then it'd be time to think about welding a pipe on.

zkl1995

No back pressure will kill your valves and fuel economy. Trust me, I have a lot of friends that regret doing it and you cant fix it by putting the cats back on. I have an 03 4.8 with no mufflers just true duals with 4 inch tips and stock cats. It is plenty loud and i like loud. If you are going to cut your cats off, then put something like 2" pipes on it with true duals. I changed mine to 2" and cackles a little less but it got an insane amount more power.

Mille Racer 69

Was your friend running stock exhaust manifold, or headers or nothing?

Probably not the stock manifold because there is enough resistance that the valves won't
Get sucked back.

Your exhaust pulses out, 8 pulses to be exact, and in the short ammount of time in between those pulses, you want a continuous flow of exhaust outward.
Exhaust has very little oxygen in it, and oxygen is what burns valves.
If you have a manifold or headers on your engine, they should be long enough to prevent oxygen from getting to your valves. Since the tubes should be long enough that it won't suck fresh air back to the valves. If you run with no headers or manifold, your sucking in fresh air which is what will kill valves.

Though I do agree, in theory you should get better MPG, not to mention a much nicer ride, and you wouldn't have to worry about exhaust fumes or resonance with a muffler.

Doug Roberts

so if i cut right at cat for now i will be ok till i put a ext pipe there and bring to the back will be stock 3in pipe no cat no muffler not dual yet no funds
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zkl1995

They had stock manifolds