The Venturi increases the pressure from that point all the way back in the exhaust. What this means this now denser exhaust gas is going to move at the same speed as it was accelerated to passing through the venturi.

The big thing in this example is that we have exhaust manifolds and a y pipe designed for a 2.5" system. The collector has been changed to 3" with a 3" can and exhaust attached. A venturi is the only way I am aware of to increase exhaust velocity and patch this system together outside of replacing the manifold and y-pipe to make the full system 3" compatible.

 

Ahhh it all makes sense now..so would any random exhaust shop carry these or should I order online and bring it somewhere tp have welded in

 

Is your exhaust a dual exit?

 

This is not really correct. The only thing that will change the velocity is to reduce cross sectional area (up to where turbulence reduces flow)l, or increase volume of fluid (exhaust gas) moving through the system.

Generally speaking, the amount of exhaust will stay the same regardless of the size of the pipe it is running through, because the engine is determining the volume.

The venturi will neck down the system, producing a higher velocity at that point. The same volume of gas moving through the 3 inch pipe will flow at the same velocity it would have otherwise. It will be at a higher density though, since the venturi will create pressure.

It is like a slow moving lake picking up speed when it flows through a narrow river, or water through a hose moves slower than the water through the nozzle.

 

Don't think diesel with this. Turbocharged engines can't get rid of exhaust fast enough....and yes, pipe diameter and heat/flow play a big part in the equation.

For a 3.8L minivan engine, slowing the exhaust velocity by going to a 3" from a 2.5" diameter will not adversely affect performance. If nything, it'll deepen the tone of the exhaust note.

 

no its not..that was really my main concern is if it was just meant for dual exit setups...but mine not being dual i still feel i have lost some low end power either way...do i have any options?

 

Thanks guys I am dropping a section of pipe under the floor boards and moving the muffler there (Flow Master) I am running 2.1/2 exhaust pipe back so I should OK.

 

The exhaust gases are compressible leaving denser gas as it exits, and perhaps my verbiage wasn't correct. The denser gas downstream of the venturi will not allow it to flow back and the exit nozzle will not create any upstream exhaust flow as may happen as a river hits a narrow section of rapids, then widens suddenly creating back flow along the banks.

As the gases further downstream expand to their normal state, flow is maintained. Certainly there is still the slow down of contraction but there isn't a point of back flow after the nozzle enters the broader pipe.

At issue with this particular exhaust system is there is a system designed around a 2.5" collector from the exhaust manifold to the end of the y pipe, followed by a 3" muffler and exhaust pipe. The venturi and effectively the nozzle as the gas exits will not allow upstream flow of the exhaust gases. That in turn should return some of the lost low end performance without crippling the high end.

It's been a long time since I took flow dynamics in college and obviously my ability to understand it exceeds my ability to explain it. Perhaps this may help,

h t t p://en.wikipedia.org/wiki/De_Laval_nozzle

 

A 2.5" single exit system should be ideal. A loss of low end power is a bit baffling in comparison to the stock can. Perhaps greater loss of heat, chambered muffler design (not sure on the magnaflow kit) or a long fat exhaust tip (doubtful). If it has truly changed where your power band exists, I don't have any solutions. A flow enhancing venturi might help but I would be hesitant to install one as your exhaust system is very much in line with what the engine produces in terms of power.

Did you make other changes at the same time you did your exhaust?

 

I got my venturi installed today. I showed the guy a picture of what it looked like and he made one on the spot and welded it in, all for $13

Results: Some butt dynoing I did says I got a some of the lost low end edge back, but it's definitely not back to the way it was when the pipe was 2.5". It's a midpoint, or maybe a 1/3 point.

I'm thinking about whether I should leave it as it is, switch back to the 2.5", or perhaps take the middle road and replace the current free flowing muffler with a "normal", more restrictive muffler (and more quiet, since the sound right now is a a little bit annoying and I'm a stereo guy... I would enjoy the quiet), and hope that the middle road gives me an acceptable balance between what I lost and what I gained.

Opinions?