What is the best pipe size for low end torque? I installed a Gibson 3" single pipe cat back on my truck(see sig)about a year ago and after reading some other posts I am not sure if the 3" diameter is the best choice for the "torque challenged" 4.6 so I was wondering if dual 2 1/2" would produce more low end grunt.
Thanks for the responses,


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*WE WANT TO RACE THE TRUCK....PEOPLE LOVE THE TRUCK*

99 F 150 4x4.ORP,S/B 4.6 4.10 gears, Gibson 3" cat back. K&N filter. Superchip

Other Toy: 1964 Falcon, 300 H.P. 306,(soon to be 347)TopLoader 4 spd. 9"Detriot Locker rear end with 3.91's

 

I doubt the Gibson lowers back-pressure a lot. Most of the constriction is in the cats and Y-pipe. If you don't mess with those, you should be OK. Besides, unless you are going with true duals, 3" isn't too big.

 

i have always been told small diameter pipes are better for low to mid. it realy has nothing to do with backpressure. i will try to explain it as best as i can so here goes. lower rpms the exhaust does not come out as fast so with small diameter pipes the exhaust will be flowing and moving but at higher rpm's you will get backpressure. now with larger pipes or dual 2.5's in the lower rpms the exhaust gets dumped in to the pipes this is when the engine bogs until the entire pipe is filled and pressureized then it will start to flow freely and of course there will be less resistance at higher rpms. if you are looking for a new cat back check out cgs motorsports.com...i hope this helps you out.

 

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[This message has been edited by max mitchell (edited 03-23-2001).]

 

Jarret,

Despite what you might read on the board here, the exhaust advice you're looking for has to do with TUNING as opposed to the backpressure argument.

The loss in low end torque that you are seeing is attributed to the geometry change in the exhaust pipe, which changes the tuning of the system. The larger pipe FLOWS better (less restriction), but consequently changes the RPMs at which the OEM pipe is tuned to perform optimally (I use this term loosely, as Ford's "optimum" may not be the exact "optimum" that you desire).

Take a look at the torque curve of the Ford Triton engines as opposed to the competition from GM. It's clear that Ford devoted a great deal of time to tuning, resulting in the very high, FLAT torque curve as compared with the GMs. If the engine is "lacking" torque in a specific rev range, the intake/exhaust geometry can be changed to compensate for the low torque values.

I've posted this several times: Just remember - reducing backpressure is ALWAYS GOOD for more POWER because the engine is trying to force the exhaust out (the lower the restriction, the lower residual exhaust trapped in the cylinder), but doing this via geometry changes may change the tuning as well. I recall posting something to explain tuning a while back, but I'm not certain of the title of the thread.

Sorry I didn't answer your question, but I wanted to make sure that this didn't turn into another "You don't have enough backpressure" thread.

Now, if there's a manufacturer that produces an exhaust system with a lower flow loss than the OEM, while NOT sacrificing the OEM tuning, you'll have an increase in torque throughout the WHOLE rev range. However, I'm not aware of anyone that produces such a product as a direct bolt-on.

Best of luck...

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VENOM 99

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[This message has been edited by VENOM 99 (edited 03-22-2001).]