Length of Tail Pipe Efficiency

[jeff88]

Just wondering, all else equal, is there any efficiency difference between a long tail pipe (say from exhaust manifold to rear bumper) and a short tail pipe (say from exhaust manifold to right behind the front wheel)? I know the circumference of the pipe will make a difference, but will the length?

[oil pan 4]

Gas or turbodiesel?

In theory it takes power to push gas throught the pipe even if there are no bends. N/A engines seem to like a certian length of pipe.
The front bumper exhaust will not be legal in most places, would cause you to breath exhaust and I dont think it will help fuel economy.

[mcrews]

no.
the law requires that the pipe exist the fumes AFTER the passenger compartment.
certainly would NOT pass inspection in CA with the shorter pipe.

[jeff88]

Originally Posted by oil pan 4

Gas or turbodiesel?

My car is gas, but I was wondering from a general perspective as well.

I get the concept of the engine working to push the exhaust out, but legality aside, is there a length that is necessary for best efficiency to keep some back pressure (which I believe is necessary to have some), but not be so long that the engine has to work more than necessary to exhaust the gases. I assume it is based on engine size/power, but is there a generic rule of thumb?

Also, no inspection in California, just smog check.

[mcrews]

lived there 24 yrs.
Guarantee that if the pipe to hang the 'sniffer' on isnt at the rear bumper.....your not going to pass!!!

you may not realize it but it is a visual also.
If you dont have the right eqipment for the car.....you dont pass.
Had a 66 6 cyl mustang, CA car. Missing the air injectors on the rebuild engine. Was at 8% of allowable smog and flunked.
Had to go to the Air board to appeal.

Why do you think alll the 'bolt-on' ricky racer stuff says "for off road only in CA"

[oil pan 4]

For a turbo diesel go with the shortest pipe you can get away with.

[Ryland]

On 4 stroke motorcycles I've added more pipe on and increased both power and fuel mileage, it's all about having the right size and length for the engine size and RPM, get the math right and you will see an improvement.
Back Pressure is a myth, you can get exhaust back pressure gauges for checking for blockages and even with an over sized or shortened exhaust you will often see the pressure rise because you are loosing exhaust momentum! that momentum is helping a great deal.

[jeff88]

Originally Posted by mcrews

lived there 24 yrs.
Guarantee that if the pipe to hang the 'sniffer' on isnt at the rear bumper.....your not going to pass!!!

you may not realize it but it is a visual also.
If you dont have the right eqipment for the car.....you dont pass.
Had a 66 6 cyl mustang, CA car. Missing the air injectors on the rebuild engine. Was at 8% of allowable smog and flunked.
Had to go to the Air board to appeal.

Why do you think alll the 'bolt-on' ricky racer stuff says "for off road only in CA"

Yeah, I get the visual inspection part. My mom's truck failed because the EGR system was there (visual), but wasn't working properly. I guess it really depends on what Test-Only you go to. Some guy might just miss it. But for purposes of this argument, I just wanted to know what is most efficient, laws/safety aside. I wouldn't want to breathe my exhaust either, but I do want to know what would be best for the engine.

That sucks about the Mustang, but isn't it old enough to not need a smog now? Or was that in the past? What ended up happening with CARB?

Originally Posted by Ryland

On 4 stroke motorcycles I've added more pipe on and increased both power and fuel mileage, it's all about having the right size and length for the engine size and RPM, get the math right and you will see an improvement.

That's what I'm looking for! What is that math? How can I determine what is best based on the variables of a particular vehicle? I know different people have different rules of thumb, but is there an "equation" of some sort to help determine it? I know I can go to a muffler shop and ask an expert, but I wanted to be able to figure it out on my own.

Originally Posted by Ryland

Back Pressure is a myth, you can get exhaust back pressure gauges for checking for blockages and even with an over sized or shortened exhaust you will often see the pressure rise because you are loosing exhaust momentum! that momentum is helping a great deal.

I'm not sure I get you here. So back pressure vs. momentum. Do I want it as free flowing as possible, minimizing the back pressure as close to 0 as possible? Does that mean a shorter pipe would be better, because it allows the most momentum straight out the pipe, instead of the exhaust gas ping-ponging its way down the pipe? What exactly is that momentum helping?

[Ryland]

Originally Posted by jeff88

That's what I'm looking for! What is that math? How can I determine what is best based on the variables of a particular vehicle? I know different people have different rules of thumb, but is there an "equation" of some sort to help determine it? I know I can go to a muffler shop and ask an expert, but I wanted to be able to figure it out on my own.

Exhaust shops have experts in bending pipe and welding, they also have experts in selling fancy fart cans, math does not tend to be their strong point.
There are entire books on exhaust design, but for the most part if you find an online exhaust length calculator that will work, I've found a number in the past but the links die after a few years for one reason or another, so it will take some searching on your part, exhaust diameter is the critical part but length is important too most people don't have a choice in the length tho.

I'm not sure I get you here. So back pressure vs. momentum. Do I want it as free flowing as possible, minimizing the back pressure as close to 0 as possible?

You are never going to get -0- back pressure... well you could in theory without any exhaust pipe at all, but at that point you get an engine that will hardly run either because the exhaust exits and has no momentum!
If, like I said, you put a pressure gauge on where the o2 sensor screws in, and you look at the pressure, you will have a small amount of pressure from the moving exhaust gases, if you want that pressure to be -0- you'd need to hook a strong vacuum up to the exhaust port at the head, the next best thing you could do is have a proper sized exhaust pipe that carries the pulsed slug of exhaust gas out and creates a slight negative pressure wave as the next pulse exits, because they are a moving wave they will show up on a pressure gauge as a slight "back pressure", but exhaust is moving in pulses not a steady flow like water in a hose.

[jere]

Ryland exhaust pulses only matter until the collector merge section, of the header/s, or to the turbo. Each cylinder fires at a different time and pushes out exhaust at a different time, "pulses". Different pairing of the exhaust from each cylinder can make a big difference the efficiency of the exhaust as you suggest.

The length of the exhaust runners matters if the car has optimized header/s. If the car is non turbo a good amount of gains power wise can be found in exhaust runner length and size. That said there is generally only a certain rpm band that can benefit from this tuning. Most factory manifolds don't care about this in there design. Especially those with a pre-cat. After the collector, benefits could be had from little or no exhaust. Things like proper pipe diameter and good collector design (RPM where you want the power) will be important otherwise you might find negative results with a short exhaust.

If we are talking about exhaust length after the collector in an N/A car you want no bends or interference, straight pipes. If you can't get that then you want the smoothest bends (mandrel) and no reductions or snaking mufflers (yeah ricer like straight thru mufflers). Side exiting exhausts with resonator style mufflers tend to do this well.

And for a turbo car no exhaust after the turbo is ideal, and backpressure has more of an effect. On my turbo Nissan I put an electric exhaust cut out right after the turbo (0 backpressure) for a while. Fuel economy and power improved through the entire RPM range as did noise . While this was a lot of fun it might not be for everyone. The car you might see with exhausts through the front bumper are most likely all turbo cars.


So the short version is if you design everything right, short is generally better.