when ive read posts bout the differnt exhaust set ups (no cats, straight pipe, etc) i always hear talk bout losing torque. from what i under stand opening up the exhuast by gettin bigger pipes, better flowing cats and muffler (or go no cat and straight pipe) gives better sound and reduces backpressure which gives better hp because theres less resistance on the exhuast stroke in the engine. but ive also heard that a loss in back pressure means a loss in torque.

how does backpressure convert into torque? or am i confused in my understanding? im a newb still, so be gentle

 

it's actually pretty easy. if you have a motor that has a really free flowing exhaust and not enough air coming into the engine to match what the exhaust wants to put out then you have too little backpressure. so just run a 2 1/2 inch pipe all the way back and you will be fine. of course have some mufflers because straight pipes IMO sound pretty raunchy and stupid.

 

yea, ive heard that on some trucks straight pipe sounds good until you get up to a higher rpm and then it sounds kinda 'tinny'

right, now im actually waiting for my kandn fipk to come in the mail so ill be getting more air intake soon.

what my most ideal exhuast set up would be (definatly not in the near future, but still this is what i would like to do) is:

new headers, and get true duals, 2 cats, 2 mufflers (maybe flo-pro, or flowmaster)

whats a good ball park figure on this. this is sumthing i would have to save up for for a while, id like to get it all done at once so i could go into a muffler shop and try to haggle a lil since id be gettin it all done in one place. what kind of backpressure issues would i run into with that.

how big of a differnce would 3 in be apposed to 2 1/2 inch?

what involved in going from single exhuast outlet to true duals? cuz i remember this one guy in highschool had duals on his truck, but it was actually 1 pipe that split into two, im guessing it was a 1 in 2 out muffler but never actually saw it so i dont know. i never really liked taht, if imma get duals then i want real ones.

 

3" is about right for a single, but for true duals, use about 2-1/4 or 2-1/2 max. Free flow is great for top end, but you want flow velocity for low rpm torque. Realistically, most of your driving is in the lower RPMs, so that's where you want your power unless it's on a speedway flat out or doing 1/4 mile blasts. Torque is what gets a vehicle moving and accelerates it. Horsepower is basically a rating of torque over a period of time. In a heavy truck the torque rating is even more important than in a car. Do your modifications with this in mind and you'll be much happier with the results. BTW, there was a guy at my work with a SWEET sounding Chevy. I finally asked him what exhaust he had. Answer: 3" to converter, 1-in-2-out converter, 2-1/2" duals from there with no mufflers. It sounded great, but not really loud. Funniest part is I also found out it was a V6 instead of a V8!

 

Higher exhaust velocity scavenges the cylinders and pulls more air fuel mix into the cylinders. It is a matter of tuning the exhaust for the rpms you want to run at. Smaller pipe is better for low rpms.

 

This is for BMW's but applies to all EFI engines. DME refers to the computer.

http://www.uucmotorwerks.com/html_pr...torquemyth.htm

From Magnaflow's Site:

BACKPRESSURE = TORQUE?
An old hot-rodder's tall tale: Engines need some backpressure to work properly and make torque. That is not true. What engines need is low backpressure, but high exhaust stream velocity. A fast-moving but free-flowing gas column in the exhaust helps create a rarefaction or a negative pressure wave behind the exhaust valve as it opens. This vacuum helps scavenge the cylinder of exhaust gas faster and more thoroughly with less pumping losses. An exhaust pipe that is too big in diameter has low backpressure but lower velocity. The low velocity reduces the effectiveness of this scavenging effect, which has the greatest impact on low-end torque.

Low backpressure and high exhaust stream velocity can be achieved by running straight-through free-flowing mufflers and small pipe diameters. The only two exceptions to this are turbocharged engines and engines optimized for large amounts of nitrous oxide. Both of these devices vastly increase the exhaust gas volume and simply need larger pipes to get rid of it all.

 

amal, think about that explanation. Exhaust should be high velocity with low backpressure. Backpressure helps create velocity. What you really need in a good exhaust system is a compromise that gives enough backpressure for lower RPM and that is free flowing enough to flow enough volume at higher RPM.

 

oh yea and forgot to mention in that set up i want to get. id like to have the dual pipes comming out infront of the back tires istead of behind. i think that'd look kool. what that cause a major difference in backpressure?

how woul di ensure low back pressure and high exhaust velocity?

 

The trick is to use the right size pipe, free flowing mufflers, and keep bends, and kinks to a minimum. Crush bends are cheaper, but they hurt flow, and the actual pipe diameter is smaller at the crush bend. Get mandrel bends if you can. Also, putting the mufflers as far foward as possible will help deepen and mellow the exhaust tone, IMO, making it sound better.

 

Yeah, the good old boy that did mine said he would try and do the bends as best he could. I wasn't that concerned since I went with dual 2.5s, but he did a great job. Smooth bends. I didn't know putting the mufflers forward helped sound, but mine are as far up as possible, with the pipes coming out in front of the pass. tire and they sound nice. I also don't have a cat, which helps with sound. Ted

 

Here are other principles that have influence on power ranges.
Stock manifolds the cats and the stock muffler are plenty restrictive and impede the flow. Remove or lessen these restrictions and velocity is increased.
Exhaust is elastic which means it will compress and expand at different points in the system. Each point that it does this takes energy.
Removing or lessening the compression/expansion relieves the load on the engines input into this system.
At certain RPM there are resonant nodes based purely on the volume and length of the exhaust system. When this resonant frequency (rpm) is achieved the 'drone' happens in the cab, actually the whole truck. This indicates a system balance of pressures at this rpm and also at its harmonics to a lesser degree.
This is when the scavenging is at its greatest.
Adjusting the tube size, length, muffler, etc changes this resonant frequency because the cubic volume is changeing.
This is the principle of expansion chambers on dirtbikes for more low end torque.
Hence a tuned exhaust, specifically tuned for particular rpm ranges.

Wavelength = 1100 X 60 X 1/RPM X 1/4 X 1/2

or

Wavelength = 8250 / RPM

Wavelength = standing wavelength
1100 = speed of sound in air in feet per second
60 = convert RPM to Revs per second
RPM = RPM
1/4 = four cylender firings per revolution (make this 1/2 for "true
dual")
1/2 = standing wavelength is half the wavelength of a "normal" wave

 

thanks alot yall this is some good info.

one more thing. the other day i was crawling around on the ground looking under my truck, and i noticed that since i have 2 gas tanks it takes up alot of space under there, im not sure where they would run the other exhuast pipe. anyone know how they would run dual pipes on a truck with dual tanks? cuz the 2nd pipe wouldnt be able to take a similar path as the 1 pipe i have.

 

with the dual tanks, i dought that the pipes will be able to exit in front of the rear tires. my '87 has dual tanks the exhaust was both run down the passenger side then split at the axle. exits behind the rear tires.