View Full Version : Effect of exhaust on Torque

01-10-2006, 12:02 PM
I made an interesting discovery this weekend, found that the cats on my 1995 SC had been hollowed and welded back together and were leaking. So I put on a set of 1989's I had at the house (after modifying the passenger side to fit). Low and behold the bottom end power jumped a bunch (seat of the pants Dyno 2000). The engine is as follows standard crank, ..030 pistons, total seal rings, comp cam 210/220-.498, LT-1 pushrods, new plugs and Ford wires, Felpro all the way, ported late model supercharger, heads and intake, 1993 blower pulley. The car is a five speed and has the motor I just built this fall swapped in. I still hate the bottled up exhaust and want to improve the flow, but not a the expense of the low end grunt. What success have any of you had with maybe a 2.25" catted exhaust to keep the velocity high, VS the 2.5" exhaust that seems to be so popular? Or will the addition of the cats on a 2.5" system give back the low end. Previously it was Ho-Hum to drive til it got to about 2,800 rpm then it was a bat out of....well you get the idea. The way it is now the car will break the tires (Goodyear 235/55's) loose and fishtail through first from a rolling start. Second gear is also very exciting from about 2,500 to 5,000 rpm. Boost did not seem to change from previous, it is 12 psi then slips to 9 or 10. (Maybe the belt). Thanks

01-10-2006, 12:17 PM
Why did you put the stock cats back on? The driver side on the stock y pipe is restrictive because of the way it is designed. If you dont have to echeck I say ditchem and just run a set of downtubes, if you do have echeck still ditch the stock y pipe, make some downtubes with high flow cats. you might have picked up a little low end torque from restricting the exhasut but your hurting you high end horsepower because of the flow

RBH Racing
01-10-2006, 01:04 PM
This caused some ruffled feathers in another thread and I'll tell you what I said in there(more or less).

The effect of a metered restriction on your exhaust will depend totally on the design of your heads and cam. Especially on an engine that is pressure aspirated. Many times what can happen is with stock or greater cam duration and valve overlap(when both valves are open at once, to help flush the cylinder), an open exhaust system will allow an exhaust rate and vacuum that will actually draw a large portion of the air/fuel mixture right out of the cylinder. It's an effect refered to as "scavenging", It is reflected greatest in the low end torque curve. The amount of money that would have to go into the bottom end of one of these engines to get it to handle the RPM that would justify open exhaust is large and better spent on a nice set of pistons, headgaskets and an N2O system.

Leave them there, you have proven to yourself that they help your performance. The key to a fast street car is torque, NOT horsepower.
(Torque is how hard you hit the wall, Horsepower is just how fast)

If you can accelerate 0-60 in 3 seconds you only need to do 75mph to win a 1/4 mile race and stay with a damn low ET. 60 foot will win you a race everytime.

I can imagine that a set of aftermarket cats may be your best bet if your on a 5 speed since ford has admitted the failure of these cats with these transmissions. They were revised in the open market versions, and with Ford by 1992. If nothing else the cleaner our cars are the less evil we look. The EPA world is really starting to frown on performance and race cars. We should do what we can to stay competetive and not give them a reason to scream.


01-10-2006, 10:17 PM
My '90 S/C has the stock cats hollowed out & the Flowmaster set up from the "cats" back - resonator & mufflers. Are you saying that by adding cats the low end torque will increase? Will it also increase with the high flow aftermarket cats? Lastly, how much difference is there in the tone of the exhaust with aftermarket high flow cats?