Contour Enthusiasts Group Archives Forums Modifications Duratec Performance Torque Vs Backpressure...

I can say that via my plugged cat test (ie less restrictive exhaust) that the torque went way down,,,,,,,,,, HEHE.

Obviously, there has to be a balance between flow and back pressure. If this was no the case, then explain why the power and torque dropped across the board when we installed a very high flowing Hermoff cat on our SVT.

As another example, we have a local SVT owner who decided to install a SHO shop off road y-pipe in place of his stock y-pipe and cat. This resulted in a drop of 6lbsft of peak torque and a loss of HP across the board.

His previous peak numbers before the Y-pipe install were 171hp/157lbsft. His runs after the install were 168.2hp/150.8lbsft. The numbers were lower across the board after the main cat was removed.

We also have a Focus owner that installed a 2 1/2" cat back exhaust on his relatively stock Focus (at the time). His before runs netted an average of 107hp/116lbsft. His runs after netted an average of 103hp/111lbsft.

Does lower backpressure causes lower torque? I would have to say that it is definitely related, and not just peak torque values but values across the entire power band, when the backpressure is reduced too much. I don't claim to be an engineer, but I speak of what I have experienced. I am sure there are others on the board that can give a more scientific explanation.

Bottom line is that in our recent experiences, it appears, on duratec equipped vehicles, it is critical what mods are performed on the exhuast side up to the main cat. After that, the sky's the limit (pun) to what you can do with a cat back exhaust. It would appear that the average gains for any good quality cat back nets 9 to 10hp at the wheels.

We'll assume for the moment that dyno numbers are accurate. There are two aspects to consider regarding flow. The first is the simple restriction of flow due to the diameter of the exhaust, or construction of the muffler/cat. This would be the electrical equivalent of DC resistance. The other factor would be resistance as a function of RPM (frequency), and is the electrical equivalant of AC impedance.
The two exhaust systems would have to be characterized for both aspects. Simple resistance would be measured on a flow bench. For the impedance an acoustic tranducer, impedance analyzer and a tone generator would be needed, and measurements would have to be made from 0 - 1125 hz (3rd harmonic of 7500 RPM).
For an engine that's making over 75 hp/l, you have to figure that the factory engineers had a good idea of what they were doing.

Guys Cawong11 said that he has a local SVT guy that instaleld a SHO shop y-pipe and cit the cat and lost power?? how can this be. The Sho shop y-pipe is supposed to be much better than the stock. And cuting the cat should be good for at least 2 hp....It shouldnt go down at all.. Why did it?
Joe

I agree with Terry on this subject, but I also think it is possible for a car to lose power with parts that help to lower backpressure. For example if a car was making x amount of horsepower and you were to put an offroad y-pipe that caused it to run lean power will probably drop. In my case my car increased power with the offroad y-pipe but then lost power when I put the Pro-M on. If I had put the Pro-M on first I guess I could say it was the lower back pressure of the y-pipe that caused a loss of power. I think it is a good idea to dyno your car as you make changes but you still have to look at the big picture and realize how one part relates to another.

Dang it Terry, you know backpressure is a really poor way to describe it

It's all about velocity and flow. Backpressure is such a poor way of describing the situation its pitiful.

Ideally, you want the exhaust to be able to flow all gases the engine can throw at it, but at the same time, if you use too large an exhaust pipe, the velocity of the exhaust gases goes in the toilet. This is a problem because of somehting called scavenging; you see, if exhaust velocity is high, it creates a scavenging (or suction) effect on the cylinder when the exhaust valve is still open, allowing more complete cylinder filling (ie less exhaust gas is left in the cylinder, so more oxygen rich air can enter, and make a bigger explosion )

Anyway, the end result is that, just like anyhting else, the exhaust is part of a larger system, and all the parts must be sized appropriately to work together well. Too large is bad, and too small is bad. Do your homework, and make your exhaust the right size for your application; for optimum results anyway .

Backpressure, for the sake of backpressure is dumb, and ill-informed. If properly sizing the system for your engine's flow, and to maintain adequate gas velocity results in some "backpressure" so be it, but don't go hunting for backpressure, all that does is choke things off.

What about Forced induction apps? isn't bigger always better then?

c'mon Terry... just spill the beans. What do you consider an ideal exhaust (for power) on a 2.5L Duratec? 2/2.25/2.5" pipe? Single or dual?

Come on guys, you still wont bite the bullet,where are all the 'must have backpressure(sorry Rara!)supporters???, who has put a 0-10 psi pressure gage on the Y pipe to prove back pressure gives more torque....I'm still confused .Cant reduce the back pressure but fit the biggest 'cat back' system to reduce the flow????, where are some of these ides comming from...prod the grey matter guys....

OhSigma,
It depends on your type of forced induction...
If you are running a supercharger...These same rules apply, BUT if you are running a turbocharger, you have to relate the Crosses sectional area of your Turbine outlet to the size of your wastegate, and also the style and size of the Exhaust turbine... You see on a tubo application, the turbo itself acts as the restriction. What rara spoke of as far as scavneging does not really apply to a NON-N/A motor(but on a N/A motor scavenging is key to power gains). Ideally on a forced induction motor you want as little overlap in your cams as possible(intake vs. Exhaust) but I digress from the whole topic at hand...

Terry,
I agree with what you are saying and also what rara is saying, but I beleive that what alot of people are missing here is that the "restriction"that you want to impose should be at the "front" of the exhuast, IE the size of the Header tubing, not jsut some catback exhaust diameter(once you run into CAT backs as your imposed restriction for this test you'd also have to deal with cooling of the exhaust gases slowing velocities etc.. where as at the header these temps will remain VERY high compared to , lets say at the resonator... this would eleimate some of the variables that people will try to claim caused the drop etc..)(also,i know its cost restrictive to try differrent sizes, so we would have to go witht he fact that the ford guys got it right for most of our apps)In this instnace, larger diameter header tubing would definately slow the exhaust velocities, thus killing your torque, and power in general accross the power band. But too small of a header tubing and you would be horrendously starved for breathing. So as far as headers go, yes there is a fine line for restriciton vs torque.
Just another aside, If any of the supercharged guys had a crappola amount of money lying around, Id like to see a set of larger diameter headers with Velocity stacks at the exhaust ports on the heads to increase flow velocities into them.. i bet these would yield the biggest gains overall.
I think I strayed way off course on the topic here, forgive me.. I'm rambling