Sooooooooo - my 3" duals hasn't made any noticable drop in torque at the off idle range. C'mon now, it's a 400, plenty of room to play! Another benefit of free flow is a cooler running engine. Again, - to each his own. (don't sweat the petty things, pet the sweaty things)

 

Ken pretty much said it. Backpressure is not required. However, overly large pipe can create backpressure under certain conditions. An exhaust is typically subjected to such a wide array of conditions, that really, the system has to be built for a purpose. A large exhaust that may be well suited to the track, may not be good for a daily driver. What may be good at 3500 rpm might not do so well at 2000 rpm.

My rule of thumb is to use the SMALLEST size that is adequate to get the job done. This in my book is usually either the same as the largest section of pipe in the factory exhaust system, or one size larger than the factory tailpipe.

 

Uh O, bring out the popcorn everybody, this is going to get fun!

 

Big pipes flow more, so is bigger better? Answer: absolutely not. Primary pipes that are too big defeat our quest for the all-important velocity-enhanced scavenging effect. Without knowledge to the contrary, the biggest fear is that the selected tube diameters could be too small, thereby constricting flow and dropping power. Sure, if they are way under what is needed, lack of flow will cause power to suffer. In practice though it is better, especially for a street-driven machine, to have pipes a little too small rather than a little too big. If the pipes are too large a fair chunk of torque can be lost without actually gaining much in the way of top-end power.



Got this from superchevy.com

 

heres a backpressure chart!!!

 

Whats that Velocity? The thing I was talking about? Thanks for making me right. I never said to go as big as possible. I was merely correcting the comment of NEEDING backpressure. You dont.

 

Heres a picture of a cookie!!!



Whats your point? That chart doesnt say much of anything RELATED to this topic.

 

I know super chevy is lame but it is just a place where i could get some facts from..But will you agree with me that you do need backpressure to make torque..

 

Brad: Can you or do you have any videos of your exhaust? I notice you and me practically have the same set up..Doug Thorleys with true duals...And your running no cats, and thats what i have been wondeirng about...Thanks

 

I dont agree at all. Thats the whole point Im trying to make. Its not backpressure that makes power or torque. Its velocity, and the design of the entire system, and of course the engine that makes torque.

 

I have speed videos but none of the exhaust..Yeh no cats for my exhaust...When i get back from spring break i'll make a couple for you if the roads are good..Which wil be the week of april 7th..I hope you can wait that long .Because it has to be warmed up for it to sound good...It is actually quite quiet for some reason..

Theres a video of it inside accelerating..



http://www.youtube.com/watch?v=XUc7BQ_cNgI



And i finally got that performance throttle body 2 months ago and had only problems with it...I got tear able gas mileage so i took it off last week ...For some reason my computer was not adjusting right for the air/fuel mixture and so it always ran rich..But once the weather gets nice here i'm going to monkey with it..I'm thinking i might have to get a special chip burnt for all the mods..I think it just pushes my computer to the max-o..

 

I think you and I have some of the fastest close to stock 460's out there man, thats impressive, nice truck, and 180,000 miles, thats real good.

 

Well the motor has only around 120,000 because the idiot whoever put tje truck together but the wrong dip stick in it and it always ran a quart low so after running a quart low for 60,000 it caught up with its self..I'm Leaving today so i'll make the video's when i get back....

Have a good one everyone

 

Sweet man, cant wait to see them man.

 

Air, including exhaust gas, like all other matter has mass. When it is in motion, it is called inertia, This means that it matter in motion will resist changed to its motion. The exhaust will move as pulses which have momentum. If you have an exhaust that is too large, the larger volume or air already in the pipe will resist new air coming in from the engine. This is also true of excessively long pipe, such as that found on most motorhomes. The pulses expand to fill the diameter of larger pipe and the pulse will slow down rapidly, releasing heat in the process. Even when the exhaust stream starts moving, it will move more slowly. Because it is moving slowly, it will have less inertia and will not have the power behind it to resist the many sudden changes that happen in the exhaust. Every time the valve opens, the exhaust has to expand into the large pipe and cools in the process, and also creates a lot of turbulence. Every time the valve closes, the entire exhaust stream will pull against the void that is created, and will quickly slow down.

Too small a pipe creates excessive friction that robs power as well, but generally, it is much easier for the heated exhaust pulses to force the entire exhaust stream into motion. The gases will move much more quickly and with much less turbulence. When the valves open, the gases will flow into the rest of the exhaust at high speeds and push the rest of the column of exhaust along at similar speeds. When the valve closes, the exhaust stream will be moving quickly enough that the it cannot slow down or destabilize and will instead create a vacuum wake that will for in the entire exhaust manifold, helping to pull hot exhaust from the next valve that opens. This push-pull scavenging effect is what builds torque and allows for rapid throttle response.

The correct size really depends on a lot of factors. Engine displacement is a huge factor, as is the stroke to bore ratio, rpm range, valve configuration, the length of the system. Longer systems should have large pipe towards the end to help overcome frictional losses and to project the exhaust in a one way direction. Staged exhausts also allow you to use larger tailpipes and that means larger exhaust tips.

My general rule of thumb, is that the factory pipe size is usually fairly close to the correct setup. A 4.0L V6 does well with a 2.25" exhaust, a 4.6L or 5.0L V8 does well with either 2.25" or 2.50", and the 5.4L, 5.8, 6.2L and 7.5L all do well with 3.00", thought he 7.5L won't complain with 3.50" either.

In dual setups, the pipe size of the factory y-pipe is usually what you want. If the engine has other modifications, it can benefit more from larger pipes, since its ability to efficiently displace air also improves.

Backpressure does not create torque, rather, the high velocities of the exhaust gases create torque. By the same token, larger pipe does not necessarily create less backpressure, in fact as mentioned above, it can actually create backpressure, and create it inside the exhaust manifold, which further resists the exhaust flow as it first exits the cylinders.