33 replies to this topic

[Cooperman]

 

To add. I tested all the systems I assembled on a mini hill climb between the A505 bypass and cadington (before it got ruined)for those of you that know it.

With a full RC40 kit fitted I could hold third quite comfortably, it would still pull but I would lose speed if I selected 4th while still on the steep section of the gradient and have to shift back to third.

With the system I run now with the 4 1/4" oval exit not only can I shift into 4th but it allows me to continue to accelerate.

Same hill same engine. Proof of the pie as they say.

At the time I spoke to an engine test bed engineer, told him how I had gone about my research for want of a better word and he said your approach in engineering terms was not pretty but you seem to have fluke'd it. Your combination of bends, pipe and back box have worked out well against the odds, I'm surprised at how well it seems to have worked.

You can't change the laws of physics or Bernoulli's Theorem, but what I think has happened is that your exhaust end pipe is so big that the actual final exhaust exit is the end of the smaller diameter pipe and that is probably about right. The oversize box is not a part of the exhaust as such at all as it does nothing.

Imagine a hose pipe with a nominal 1/2" bore and a good water pressure. At the end the water will come out strongly in a long arc. If you increased it to, say 1" the water would lose it's velocity and not exit efficiently. However, if you put a 3" diameter end pipe, the water would just come out strongly without touching the large diameter end.

Think about it. Top performance cars have constant diameter exhaust pipes. My wife's 2.5 litre BMW has twin 1.75" final exit pipes, so a 1275 Mini engine would, by logic, need a single 1.75" pipe, which is what all the experts advise. With the resources of a major manufacturer, especially one which wants to compete with Mercedes and Audi, if a much larger exhaust could give more power then they would use it.

Edited by Cooperman, 07 February 2014 - 07:03 PM.

[Mr Joshua]

If you know the box you will know that there is a sizeable reducer inside and where as the pipe you see May be 4 1/4" the actual exit is smaller. The shroud caused by the outer forms a kind of baffle where it rolls back in on itself this helps reduce the noise that would be generated if it were a true parallel section of pipe. I studied fluid flow and thermodynamics so I'm not trying to reinvent the wheel. Like the engineer said you put the right bend in the right place and it can transform how an exhaust works. Again a fluke as he put it.

Edit: missed a word oops!

Edited by Mr Joshua, 09 February 2014 - 07:18 PM.

[samt88]

 

If you want twin exit pipes on a 1275, the diameter of the pipes should be about 1.25" each. The reason, as has been explained on here umpteen times, is because the most efficient way for the exhaust gas to exit is at the same speed as it moves down the pipe. If the cross sectional area changes near the end the gas velocity decreases and the gas pressure increases at the point where the cross section increases.

The twin parallel pipe exhausts are, quite simply, 'power-sapping noise boosters' and are not performance up-grades in any way.

 

Ha, thats funny tho because when my mini was on the road and I fitted a stage one kit, with one of these  'power-sapping noise boosters' (for context it was a maniflow twin tail pipe DTM style zaust, linky ) and still got a 15 to 20% increase in power when set up on Slarks rolling road... soo bit puzzling eh lol

 

I have printouts if anyone wants proof.

 

I can appreciate that ridiculous twin 3" pipes would prlly not do any good, but maniflow's offering seem to net me some more horses before AND after a rolling road sesh.

 

I'm assuming you fitted this instead of a standard pea shooter exhaust? Of course its going to add some power over the standard but whats been said here is that you would get even more power if it didn't have the "power sapping noise booster"

[Cooperman]

Most cars are not set accurately in the first place, so taking a 998 with a nominal 38 bhp, it might only have 34 bhp before modification. If simply put on the rollers it might go back to 38 bhp, which is an increase of 11%. The 'Stage 1' kit could add another 5+ bhp with the right exhaust, so with the wrong exhaust it might add another 1 or 2 bhp, making the figures seen and the %-age improvement.

 

One thing is true, the laws of physics cannot be altered.

 

To explain a little further, the velocity of the molecules of exhaust down the pipe give the exhaust gas an inertia. Inertia is mass x velocity. This inertia has a scavenging effect which 'sucks' the exhaust gases out of the cylinder thus helping to fill the cylinder with fresh inlet charge when the inlet valve opens. So anything which alters the overall speed of the gas down the pipe from the optimum will be detrimental to the production of power. To increase the cross sectional area of the pipe towards the end will alter the gas velocity and thus alter the power output. Also, as the gas travels down the pipe it cools which reduces the volume so, really, it might help slightly if the entire pipe had a gradual reduction in cross-sectional area from the end of the manifold right to the end so that as the volume reduced the velocity remained unchanged. It would only need to be a small reduction, but it would need to be a continual reduction and this would make the exhaust pipe very expensive to make in production. 

 

My son works in F1 design and they are working on how to harness the energy from the exhaust noise. To make noise takes power, fractional maybe, but a finite figure nevertheless.