Exhaust Diameter Theory

[Baz]

Just something I have been curious about recently and decided it might be a good idea to pop the question.....

We all know that "generally" the larger the diameter of the piping in an exhaust system the more power it will generally produce and the better that it would respond to modifications as well. But unfortunately the larger you go, the more impractical it becomes due to the excessive noise that it would make.

Now here is my question. Btw cat convertors are disregarded for this experiment.

Now lets say if I hypothetically had an full turbo back 3" exhaust like most people generally buy for there turbo cars which was straight through until it reached the rear cannon, where it had only one to help baffle the sound of the exhaust. Lets say that this exhaust produced 90db.

In my second example lets imagine I had a full 3.5" turbo back system which was helped to keep it down to regulation noise by use of a couple of mufflers (same design as our cannon in example one, same restriction/flow) to keep it at our regulation noise of 90db.

Now between these both setups. Which one would produce more power? Would one produce more than the other? Would one respond better modifications than the other? Or would they be actually the same?

Just wondering.

Cheers

[Scooby]

Hi Baz;

Good question, you could work that out in rough terms by using the figures for back pressure imposed by the second muffler vs increase in flow through the larger diam pipe, they should be available somewhere. Intuitively I reckon you'd need to go up to a 100mm pipe to overcome the pressure created by the second muffler but there would have to be a break even point somewhere. It also depends on the quality of the exhaust design, as opposed to just the diam.

Then again some of the better quality mufflers are expensive becuase they can do both very well ie cancel noise and produce horsepower; it's always going to be a tradeoff and I don't think the 'perfect solution' exists.

Cheers

[R31Nismoid]

if your making upwards of 300rwkw then there is a difference over 3" to 3.5" without a doubt.

Stock turbo i doubt you'll see anything

Adding in extra mufflers wont hurt the power if they are straight flowing ones.

You'll be hard pressed to get its to 90db tho

[Cubes]

2 x 2.5" from a nice 3.5" or 4" dump works very well.

Its amazing how quiet a twin system is compared to a big 3.5".

[discopotato03]

Interesting idea Cubes , where does it go from single to double and does it go back to single down the back .

I had a bad experience with noise on an FJ20ET with a GT28RS and 3" tube . The dump pipe was formed to match the turbo outlet flange ie not a saparate waste gate pipe . There were no soft spots and the noise at cruise was LOUD . I think turbos that vent into a large volume and large pipe promote the dreaded drone . The split dump seems to have performance benefits and less noise as well . I've often wondered if the pipe venting the turbine is similar or a smidge larger that the outlet promotes flow and forms an anti reversion restriction when the pipe opens out to the main pipe diametre . I think large pipes can act as an auxilary engine tie bar if the exhaust has no give ie those braid covered stainless steel bellows gadgets , and can transmit engine noise directly to the body making it a boom box .

Corky Bell says that gas velocity is directly related to pipe size and quotes pipe diametres in relation to power output . I think the golden rule is to try to keep the exhaust manifold pressure as close to inlet manifold pressure (on boost) as possible . To do this exhaust pressure needs to be measured pre turbo , post turbo and before and after every cat/silencer in the system . Remember the turbo can be a sizeable restriction in the exhaust system which is why I keep harping about the importance of efficient (free flowing ) turbines and housings .

My 2 cents only A .

[Bl4cK32]

mine goes to twin 2.5" after the cat, and stays that way all the way to the rear, where it has 2 x 3" mufflers. It doesnt drone at all and is nice and quiet.

[30et]

im assuming zorst flow is like current. 2 restrictions in series = double the restriction. whereas 2 restrictions in parallel (seperate pipes) = same restriction... but now im thinking.. .would that cancel out the effectiveness of both of them in a row as far as noise goes? i still think it would be quieter with an overall larger flow area (2x2.5 vs 3)

[Bl4cK32]

ive had a 3" system, then changed to a 2x 2.5" without changing anything else...Im telling you its quiter, and doesnt drone anywhere near what it used to....

[Cubes]

I assume APS decided to go a 2 x 2.5" system rather than a big 3.5" single. 2 x 2.5" keeps drone to a minimum on the big ford six.

Head over to the LS1 forums and you will see the 2 x 2.5" systems are quieter than the 3.5" singles on the v8's also.

[GTRgeoff]

Fluid flow is significantly different to electricity in practice despite the claims otherwise. Flow potential increases with a radius^2 relationship, and the conservation of mass/volume has to be maintained.

So essentially if you stick to the simple A1.V1=A2.V2=volumetric flow rate. A and V being Area and Velocity then you can't go wrong.

consider if you push 300rwkw through a 75mm (4415mm^2) pipe, and you increase to a 100mm pipe (7850mm^2) and the gas velocity remains the same the potential flow for power is around 530rwkw (which is far more than linear relationship attributed to just the diameter increase)

Moral, 100mm pipe is just too big when 75mm (3") can deliver well over 300rwkw.

[Cubes]

Thats with regards to peak power but how does it affect mid range etc? Can the theory be applied so to speak?

I have know ppls fit up 3.5" exhaust and picked up quite a bit of mid range and turbo spool but still made the same peak power.

[R31Nismoid]

Moral, 100mm pipe is just too big when 75mm (3") can deliver well over 300rwkw.

3" cant deliver much over 300... from what testing i have seen and been told about... changing to a 3.5" can net upto 20rwkw when you get into the mid 300 + range

[GTRgeoff]

I'll dig out some laminar and turbulent pipe flow data later and get some theoretical flow values. It'll take a while as I've got a pretty busy day and I'll calculate completely the airflow needed for certain power figures, the fuel required, the expansion due to the heat present and finally the pipe size needed. Then I'll do some acoustic calculations for SPL drops. Time to exercise the brain.