14 replies to this topic

[jb93]

Hi all,

I am in the stage of purchasing a new exhaust for my 998cc mini, and if possible would like something cleared up.

I am unsure on what exhaust diameter would be best. I have read various different opinions which have just confused me more. I have decided Playmini with a maniflow manifold.

Which diameter???
Would a 2 inch tailpipe have a 2 inch bore (http://www.minisport...it_Exhaust.html)
or should I go for a 3inch tailpipe, or would this strangle the power of the car?

Cheers guys

toaster93

[andymini12]

1.75" is the best exhaust diameter and is the best for power increase , so something like maiflow all the way would be better. and 2" tailpipe will be a 2" link pipe which will zap the power as i have a 2" one on mine at the mini which does zap the power. and 3" will defently zap the power.

[Cooperman]

Let me post again on this.
For a 998 you need a 1.625" internal diameter pipe from the end of the exhaust manifold right to the final single outlet pipe. The Maniflow system is about the best there is in terms of quality and performance. A larger final exit pipe will cause a measureable loss of power as when the pipe internal diameter increases the gas velocity decreases, the pressure in the larger pipe increases and this causes a distinct loss of efficiency. It is necessary for the gas velocity to stay the same right to the final exit point.
This is all fairly basic exhaust technology and it is correct that for a 998 pulling less than about 7000 rpm at peak power the 1.625" int. dia. is the best solution.
For a 1275 the optimum int. dia. is 1.75" or 1.875 for a highly tuned unit pulling 7000 rpm.
I hope this helps.

[Turbo Phil]

The size of the tailpipe will have minimal effect on overall power. But as already said the main pipe diameter is critical. To large a diameter & the gas speeds will be slow, keeping the diameter smaller will keep gas speeds up & help to "extract" the gases.
This is why a 2" system will lose power on a 998, one of the smaller bore Maniflow systems as mentioned above will be ideal, or the classic RC40. The RC40 is available with a 2" tailpipe if you must have the "bling" factor ! Lol

[Cooperman]

The size of the tailpipe will have minimal effect on overall power. But as already said the main pipe diameter is critical. To large a diameter & the gas speeds will be slow, keeping the diameter smaller will keep gas speeds up & help to "extract" the gases.
This is why a 2" system will lose power on a 998, one of the smaller bore Maniflow systems as mentioned above will be ideal, or the classic RC40. The RC40 is available with a 2" tailpipe if you must have the "bling" factor ! Lol

Sorry to contradict, but the diameter of the end pipe will have a lot of effect. Look at "Bernoulli's Theorem" for the actual physics. It's also known as the 'venturi effect' and states that when a fluid flows along a pipe, if the pipe daimeter increases then the fluid velocity decreases and the pressure increases. So you get a back pressure which screws up the exit flow with consequent loss of exhaust extraction efficiency and loss of BHP. For optimum power and engine efficiency it is vital that the pipe is the same diameter right to the final exit point.
Now imagine a hose pipe of a certain diameter with water under pressure flowing down it. When it gets to the end it is ejected efficiently at the same speed as it has flowed down the pipe. Now imagine that the final 6" is at double the cross sectional area, i.e. the diameter is larger than the mainm hose pipe. The water will not be ejected so efficiently.
It's called 'fluid dynamics'.

[jb93]

Thanks all, help much appreciated

[Turbo Phil]

Theorys are all well & good, but in practice it showed no measurable difference on the rolling road when we changed a friends backbox from a standard RC40 to 2" tailpipe box.
If science it o be followed then surely as the exhaust gases cool & slow down as they pass along the exhaust, then the pipe diameter, if anything should decrease in size to keep the gas speeds up ?
Not having an argument at all, just enjoy an interesting debate.

[Carlzilla]

If youre referring to the tip, its that short it makes no difference whatsoever, in my theory anyway, the gases are seen as escaped as soon as they leave the rear silencer, the gases are unrestricted in the final piece of tailpipe if you have one of a larger bore.

I have a 1.75" single box rc40 on mine, including 1.75" fletcher lcb manifold in mild steel, with a stainless rc40 side exit backbox with 2.25" exit.
if im correct, you want a freeflow manifold on the 998 if your car is lowered, the lcb catches on speedbumps on a lowered 998, as its designed for the taller 1275 engine.

Edited by Carlzilla, 28 December 2010 - 11:09 AM.

[Cooperman]

If youre referring to the tip, its that short it makes no difference whatsoever, in my theory anyway, the gases are seen as escaped as soon as they leave the rear silencer, the gases are unrestricted in the final piece of tailpipe if you have one of a larger bore.

I have a 1.75" single box rc40 on mine, including 1.75" fletcher lcb manifold in mild steel, with a stainless rc40 side exit backbox with 2.25" exit.
if im correct, you want a freeflow manifold on the 998 if your car is lowered, the lcb catches on speedbumps on a lowered 998, as its designed for the taller 1275 engine.

The tip is absolutely critical. The gases must not be allowed to slow and increase in pressure locally in any way.
Take a look at the cars prepared for maximum power, such as the works competition Minis. They all used constant diameter pipes right to the very end for the reasons I've given. The straight-through silencer is designed so that the gas velocity is maintained through the silencer and thus to the final outlet, all at optimum speed. To go back to the hose-pipe analogy, if you put a large tip on the very end of the hose, imagine what would happen to the water jet out of the final exit - it's velocity would be lost and lesswater would actually flow out. If the tailpipe is larger, then the gases will slow, the local pressure will increase and any advantage from the rest of the exhaust system will be lost.
Fluid dynamics is quite complex, but note that F1 cars have constant diameter pipes right to the tip to keep the power/exhaust efficiency as high as possible.

[Turbo Phil]

As I said, we saw no measurable difference, as that's real world testing it's good enough for me.

[Wright&Wright]

However back pressure can be useful in terms of the physics in an exhaust.
If you view an exhaust manifold in terms of it being a harmonic tube, closed at one end and open at the other, back pressure rebounding from the exhaust valve just before the exhaust valve opens can create a vacuum effect in the manifold causing more gases to be extracted from the cylinder. This is why manifold pipes are all the same length to where they join the Y piece. The effect is also taken into account on inlet manifolds, if back pressure is created in the inlet then it can be disastrous in terms of power. Hence why the correct length inlet can always increase power.

[Carlzilla]

you have less pressure at the tip, so the exhaust gases there are as good as gone.
Anyhow, if the difference isnt measurable, as turbo phil explained, then you sure as hell aint gonna feel it in your right foot, which is where the difference really counts.

Edited by Carlzilla, 28 December 2010 - 04:58 PM.

[sedmann]

Sorry to bring up a very old topic but, i was interested as i have 3" straight through bore all the way to the end and i decided to get this because i liked the look of it. Then after purchasing, i though, 'do i have as much power?' In my right foot i feel exactly the same except even better because of the beautiful exhaust note haha. So, my inexperienced opinion, for a road mini, go for what you think sounds and looks the best, forget stupid figures and statistics.

[Verno]

Hi I'm after a new exhaust for my mini 998 and thinking of getting a twin dtm. I already have a sportex single centre exit on, will the new twin dtm and 2" link pipe just fit straight on?
Jake

[Dusky]

Someone should make an exhaust system wich changes diameter based on revs and load. That would optimise power, the only reason you don't see it in F1 is because it was forbidden, so it must be good