diesel v. gasoline exhaust.

[dernst]

It is my understanding the bigger isn't always better. I even downsized my 2.9 L exhaust from 2.5 to 2.25 to increase flow and reduce back pressure.

A lot of my friends who have turbo diesel trucks feel that there isn't an upper end of size and would install an 8" exhaust if it would fit.

They tell me that bigger is better and you need to let them breathe. If my understanding is correct, these larger size pipes are creating more back pressure for the diesel motors and reducing performance.

Is this true?

 

[JohnnyU]

You're thinking backwards. A larger diameter tube creates less back pressure and increases flow.

For a forced induction engine, there is no upper limit, shortest largest exhaust the better - hence Ø6"+ hood stacks on pulling trucks/tractors.

For a N/A gasoline engine, exhaust pipe size is actually a tuned parameter based on engine size and RPM range.

 

[midget]

Im not an expert by any means but diesels are choked heavily from the factory thats why the bigger exhaust and intake help drastically but yes after a point the performance should drop down. Believe it or not I saw a truck about a week ago that had twin stacks that were made from 12"(minimum) Chimney pipe on an f250. Not only did it look and sound terrible but I bet its safe to assume its hurting the mileage as well. However I feel you would have to know the extent of engine modifications to truly know or not bigger turbos and other goodies are going to effect the diameter you should go with.

 

[Wicked_Sludge]

You're thinking backwards. A larger diameter tube creates less back pressure and increases flow.

to a point. like you said, piping needs to be matched to an engines CFM. too large of a pipe will increase backpressure and reduce flow.

 

[bigcountry]

on a diesel you DO NOT want to go over about a 4in pipe and thats with of course a turbo and performance mods like tuners and an bigger intake if you go to big in diesel exhaust you will burn the valves up before you get 5 miles down the road. so yes you can go to big on diesel exhaust. if you have a semi truck over 4in pipe would work because the motors are pushing out some much cfms of exhaust but the people that dont have have a 400 to 600 cubic inch diesel motor stuffed between the frame rails of their DD only want to go to 4in pipe and thats still pushing it now the tip of the exhaust can big as big as you want and can put on ive seen 8in tips before. now to the orgnial poster if your friends think that the bigger the pipe on a diesel the better. well that really shows they never need to work on their trucks. i have seen a diesel with a 6in exhaust and it was so funny i out ran him in my old 1990 toyota 4x4 all he did was blow balck smoke and make alot of racket im sure alot of the black smoke was partly his vavles buring up and god knows what else

 

[AllanD]

Too large a pipe cannot "increase backpressure"

what it can do is reduce flow velocity to the point where the
flow "stalls" and even "reverts" between exhaust pulses.

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[85_Ranger4x4]

5"+ tips on a diesel look about as good to me as a fart can on a fwd car, it is just too much.

 

[gw33gp]

Too large a pipe cannot "increase backpressure"

what it can do is reduce flow velocity to the point where the
flow "stalls" and even "reverts" between exhaust pulses.

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Some people may consider that increasing back-pressure. I guess it could be considered pulsed back-pressure. In any case; back-pressure, in any form, is not good for an engine's performance.

 

[RacinNdrummin]

On a turbo engine, you can run as big of an exhaust as you want and it isnt going to hurt performance. There is a point where it doesnt help, but it aint going to hurt it.

 

[Wicked_Sludge]

Too large a pipe cannot "increase backpressure"

what it can do is reduce flow velocity to the point where the
flow "stalls" and even "reverts" between exhaust pulses.

and when an outgoing exhaust pulse collides with a stalled or reverted exhaust pulse, what happens? pressure. the outgoing pulse is slowed. this is knows as "backpressure"

 

[bigcountry]

On a turbo engine, you can run as big of an exhaust as you want and it isnt going to hurt performance. There is a point where it doesnt help, but it aint going to hurt it.


not tryin to start anything but if you put a 5in+ pipe on a turboed superduty you would kill that motor in no time think about it turbos need pressure what do diesels need to run compression when the pipe is to big the turbo cant spool up to power the motor so the computer starts dumping diesel in the motor to make up for the loss of power. so there goes fuel ecomony, now not only will fuel ecomony be your only problem now it will be when you take your foot off the go pedal there will be no pressure keeping the exhaust system from sucking in air so now when you have incoming air and you put your foot on the gas the exhaust valves in the motor are going to become under even more pressure because you are not olny pusing out exhaust but also what got sucked in when you let of the go pedal so now you have to repalce exhaust valves. so now you have to hear your pos truck go down the road popping rattling and all sorts of noise. my point is your statment isnt true you see all these lil punks in their hondas,toyotas and whatever else import they have a small exhaust system to keep the turbos from losing pressure

 

[rurouni20xx]

On a turbo engine, you can run as big of an exhaust as you want and it isnt going to hurt performance. There is a point where it doesnt help, but it aint going to hurt it.

i hate to disappoint but the exhaust has to be tuned to ensure proper scavenging, even on a turbo engine. sure the compressor will force it down the line...to a point. if you put 5 inches of exhaust on a 300 hp blower you better have it dumped 3 feet after the compressor otherwise the exhaust simply collects in the pipe and creates a heat issue. we all know a turbo has problems w/ heat anyway, why stack it next to the compressor making it heat up worse? the exhaust must be tuned to the cfm leaving the pipe, even in a turbo app for the same reasons an n/a vehicle generates heat by stagnant exhaust so far down the line in the pipe. just so happens that a turbo uses larger diameter pipe cuz of more cfm exhuast being pushed out. just dont think that 6 inch pipes on a turbo motor just cuz it has a turbo wont hurt the performance and life of the compressor, especially at low rpm and idle when the turbo isnt completely spooled up if it even spools at all hardly.

 

[AllanD]

and when an outgoing exhaust pulse collides with a stalled or reverted exhaust pulse, what happens? pressure. the outgoing pulse is slowed. this is knows as "backpressure"

"Backpressure" is a term used SO OFTEN and so RARELY used
correctly that I would take it as a personal favor if everyone
would simply not use the term at all.

"back pressure" can be used to describe simply so many
distinct phenomena that the term is essentially useless.

 

[Wicked_Sludge]

then please, feel free to enlighten all us ignoramuses.

 

[rurouni20xx]

+1 i want to hear the real def then so i can use it as correctly as possible. didnt think that 2.5 yrs of schooling and 6+ yrs of experience w/ exhausts much less the rest of the systems that ive understood was all wrong. hell ive spent a lot of time self studying and reading up on tuned exhausts and tuned exhaust theory using "backpressure" "flow rates/cfm" along w/ "compression/expansion and sound wave tuning."