26 replies to this topic

[icklemini]

Fit the simpliest exhaust you can...
maniflow on the mighties and sevens... single box, centre exit.
Its quiet enough, centre exit means its easier to get behind the hubs should you need to do any work in a hurry at the track single box as its lighter than a two box system!

[m1key]

thanks for the replys pepole

[Cooperman]

Fit the simpliest exhaust you can...
maniflow on the mighties and sevens... single box, centre exit.
Its quiet enough, centre exit means its easier to get behind the hubs should you need to do any work in a hurry at the track single box as its lighter than a two box system!

Absolutely right there.
However, the thread is titled 'Rally Exhaust' and a single-box maniflow won't pass a rally noise check. Even a twin-box Maniflow fails noise test after a few events which is why I run a twin-box 1.75" bore RC40 system. It's not as well-made as Maniflow, but I don't have any scrutineering issues and I've welded extra skid plates on for strength on rough roads.

[Rich.]

You will lose a few bhp with that final exit for the reasons given above. To optimise power the final exit pipe MUST be the same cross-sectional area as the main exhaust pipe in order to keep the flow of exhaust gas molecules as constant as possible and at the right velocity.
Exhaust noise has little to do with what is best in exhaust design and efficiency.

This confuses me.

That box posted above appears to be two seperate smaller bore pipes which go into the back of the leterbox tail pipe.

Due to the smaller diameter of these two pipes, i bet the combined bore of the exit after silencer is the same, maybe marginly larger than the bore of the input pipe.

Are you suggesting that removing the letterbox tail finisher and adding an extra 2" of pipe out the end of the box will give a power increase?

If this is the case, why wouldnt all supercar manufacturers be sticking the longest exhausts on they could? For example cobras with side exit exhausts, probably a good 2 feet shorter than running to the back of the bodywork.

I fully understand what your saying about gas velocity, and agree that the most consistant the velocity the better, but i dont understand how two inches of pipe will effect this, especially after its just exiting a silencer, which will have screwed it up abit anyway as i doubt the pipe run would have been long enough for the gases to stabalise?

Cheers
Rich

Edited by Rich., 29 August 2010 - 03:52 PM.

[liirge]

You will lose a few bhp with that final exit for the reasons given above. To optimise power the final exit pipe MUST be the same cross-sectional area as the main exhaust pipe in order to keep the flow of exhaust gas molecules as constant as possible and at the right velocity.
Exhaust noise has little to do with what is best in exhaust design and efficiency.

This confuses me.

That box posted above appears to be two seperate smaller bore pipes which go into the back of the leterbox tail pipe.

Due to the smaller diameter of these two pipes, i bet the combined bore of the exit after silencer is the same, maybe marginly larger than the bore of the input pipe.

Are you suggesting that removing the letterbox tail finisher and adding an extra 2" of pipe out the end of the box will give a power increase?

If this is the case, why wouldnt all supercar manufacturers be sticking the longest exhausts on they could? For example cobras with side exit exhausts, probably a good 2 feet shorter than running to the back of the bodywork.

I fully understand what your saying about gas velocity, and agree that the most consistant the velocity the better, but i dont understand how two inches of pipe will effect this, especially after its just exiting a silencer, which will have screwed it up abit anyway as i doubt the pipe run would have been long enough for the gases to stabalise?

Cheers
Rich

He never suggests adding the 2" of pipe, i suggest you re-read it, cross sectional area is not length.

[mini93]

Absolutely right there.
However, the thread is titled 'Rally Exhaust' and a single-box maniflow won't pass a rally noise check. Even a twin-box Maniflow fails noise test after a few events which is why I run a twin-box 1.75" bore RC40 system. It's not as well-made as Maniflow, but I don't have any scrutineering issues and I've welded extra skid plates on for strength on rough roads.

mine never failed any noise tests, tested at 85Db's (unless your castle combe who feck with there meter to try get people to buy those stupid mufflers from merlin motorsport)

[Cooperman]

With an efficient exhaust what is being looked for is the optimum diameter to give the optimum flow at optimum rpm. It's a bit of a compromise, but in general terms you want to get the exhaust gases flowing at a good velocity from the end of the exhaust manifold where the separate pipes merge right to the final exit. A good silencer will maintain the gas velocity by virtue of its 'straight-through' design. Going into two separate, albeit slightly smaller, final outlet pipes will compromise the gas velocity as it is no longer a straight run at constant and ideal velocity. Adding a 'letterbox' end will further screw-up the gas velocity and reduce exit efficiency and cause a measurable power loss (a friend of mine calls such systems 'noise-boosting power sappers).
In physics there is a thing called 'Bernoilli's Theorem'. It states that when a fluid passes along a pipe, if the pipe diameter increases, then fluid (i.e. gas) velocity decreases and pressure increases. So, suddenly going from a straight-through silencer (typically Maniflow of RC40) to a larger bore end pipe will reduce the velocity and thus increase the pressure at the end of the exhaust. That ain't good for performance or fuel consumption.
It's a bit strange that some people will pay for a better head and/or better manifold to gain, maybe, 8 bhp on a 1275 engine, then lose 5 bhp of that with a twin exit final exhaust pipe.
Now, that's not a question of pipe length, its to do with having the correct and constant internal diameter along the entire pipe length. The ideal length is another calculation which I won't go into here as 'tuned-length' exhausts are not readily possible on an improved standard production car like the Mini.

[Rich.]

You will lose a few bhp with that final exit for the reasons given above. To optimise power the final exit pipe MUST be the same cross-sectional area as the main exhaust pipe in order to keep the flow of exhaust gas molecules as constant as possible and at the right velocity.
Exhaust noise has little to do with what is best in exhaust design and efficiency.

This confuses me.

That box posted above appears to be two seperate smaller bore pipes which go into the back of the leterbox tail pipe.

Due to the smaller diameter of these two pipes, i bet the combined bore of the exit after silencer is the same, maybe marginly larger than the bore of the input pipe.

Are you suggesting that removing the letterbox tail finisher and adding an extra 2" of pipe out the end of the box will give a power increase?

If this is the case, why wouldnt all supercar manufacturers be sticking the longest exhausts on they could? For example cobras with side exit exhausts, probably a good 2 feet shorter than running to the back of the bodywork.

I fully understand what your saying about gas velocity, and agree that the most consistant the velocity the better, but i dont understand how two inches of pipe will effect this, especially after its just exiting a silencer, which will have screwed it up abit anyway as i doubt the pipe run would have been long enough for the gases to stabalise?

Cheers
Rich

He never suggests adding the 2" of pipe, i suggest you re-read it, cross sectional area is not length.

When did i mention this? I dont think you understand what im saying.

Im saying i think the cross sectional area of the twin pipes are going to be similar to the larger single inlet pipe.

Which is why i assumed when he said 'You will loose a few extra bhp with the final exit' he ment due to the letterbox on the end, which is why im saying that the 2" of extra pipe wouldnt make much difference in my eyes?

Lets just say, for this puprous (Obviosuly i dont know), that those exits are 1.25" or so, and the inlet looks to be 1.75 or near enough.

So cross sectional area is Pi*r^2

Intake = 3.142*0.766 = 2.41 Sq Inches

Exit = 3.142*0.39 = 1.23 * 2 = 2.45 Sq Inches

Almost Identical cross sectional areas, so the only thing left that could 'loose extra Hp from the final exit' would be its length?

Rich

[Rich.]

Going into two separate, albeit slightly smaller, final outlet pipes will compromise the gas velocity as it is no longer a straight run at constant and ideal velocity.

Now this clears it up a little. I never even though about the fact there will be some resistance in the splitting of the gas flow.

Thanks
Rich

[liirge]

You will lose a few bhp with that final exit for the reasons given above. To optimise power the final exit pipe MUST be the same cross-sectional area as the main exhaust pipe in order to keep the flow of exhaust gas molecules as constant as possible and at the right velocity.
Exhaust noise has little to do with what is best in exhaust design and efficiency.

This confuses me.

That box posted above appears to be two seperate smaller bore pipes which go into the back of the leterbox tail pipe.

Due to the smaller diameter of these two pipes, i bet the combined bore of the exit after silencer is the same, maybe marginly larger than the bore of the input pipe.

Are you suggesting that removing the letterbox tail finisher and adding an extra 2" of pipe out the end of the box will give a power increase?

If this is the case, why wouldnt all supercar manufacturers be sticking the longest exhausts on they could? For example cobras with side exit exhausts, probably a good 2 feet shorter than running to the back of the bodywork.

I fully understand what your saying about gas velocity, and agree that the most consistant the velocity the better, but i dont understand how two inches of pipe will effect this, especially after its just exiting a silencer, which will have screwed it up abit anyway as i doubt the pipe run would have been long enough for the gases to stabalise?

Cheers
Rich

He never suggests adding the 2" of pipe, i suggest you re-read it, cross sectional area is not length.

When did i mention this? I dont think you understand what im saying.

Im saying i think the cross sectional area of the twin pipes are going to be similar to the larger single inlet pipe.

Which is why i assumed when he said 'You will loose a few extra bhp with the final exit' he ment due to the letterbox on the end, which is why im saying that the 2" of extra pipe wouldnt make much difference in my eyes?

Lets just say, for this puprous (Obviosuly i dont know), that those exits are 1.25" or so, and the inlet looks to be 1.75 or near enough.

So cross sectional area is Pi*r^2

Intake = 3.142*0.766 = 2.41 Sq Inches

Exit = 3.142*0.39 = 1.23 * 2 = 2.45 Sq Inches

Almost Identical cross sectional areas, so the only thing left that could 'loose extra Hp from the final exit' would be its length?

Rich

Aye, either I'm getting older or your last post was a tad on the ambiguous side! but now i see what you were driving at.

Edited by liirge, 29 August 2010 - 06:12 PM.

[Rich.]

Aye, either I'm getting older or your last post was a tad on the ambiguous side! but now i see what you were driving at.

I do tend to over complicate things rather!

[m1key]

right, so basicly the twin box rc-40 is the best one to get for sound and power