12 replies to this topic

[-scott-]

I currently have a 1.75 inch RC40 exhaust on my car. one of my mates has offered me a Twin DTM exhaust but it is 2". Would something like this work to make it fit?

http://www.minisport...o_MSLPM43X.html

thanks

scott

[h4wx1e]

i got one of these to do pretty much the same job and i had no luck with it so decided to weld it instead, but that was my experience and i had mine from p&l minis not from mini sport.

[Dave33]

personally i would stick to the rc40,if you change the back box you will lose the performace that the rc40 gives.
If you want the look i think minispares sell a range of rc40 backboxes in different styles.
hth
dave

[Cooperman]

If you do that you'll lose power and increase fuel consumption. A guy I know calls twin final exit pipes "noise boosting power sappers".
The diameter of the exhaust pipe should be the same all the way through. If the cross-sectional area increases at the final exit, the gas exit velocity is suddenly reduced and back-pressure increases (it's technically known as 'Bernoulli's Theorem). The ideal size is 1.75" internal from end of manifold to final exit for a 1275 and upwards and 1.625" internal for a 998.

[Pooky]

If you do that you'll lose power and increase fuel consumption. A guy I know calls twin final exit pipes "noise boosting power sappers".
The diameter of the exhaust pipe should be the same all the way through. If the cross-sectional area increases at the final exit, the gas exit velocity is suddenly reduced and back-pressure increases (it's technically known as 'Bernoulli's Theorem). The ideal size is 1.75" internal from end of manifold to final exit for a 1275 and upwards and 1.625" internal for a 998.

I find this fascinating. I've never understood how the last 6 inches (referring to the final tip of the exhaust out of the back box) of an 8 foot long exhaust can make so much difference to a system. I can understand that if the whole system's diameter is too big you'll lose power but the final exit diameter the result must surely be so slight it makes almost no difference?

When exhaust gases leave the tip of the exhaust is this not similar to an infinitely large tailpipe? Would having a 2" or DTM tailpipe on the end of a 1.75" back box make a significant difference to power?

Cooperman, I'm not doubting what you say, this is something I've always wondered.

Edited by Pooky, 22 June 2010 - 02:23 PM.

[Dave33]

As cooperman said,a larger silencer will only increase noise.the 1.75" diameter pipe and silencer baffle are known to be good for around 120hp.
If you engine say was putting out more than this you would need to consider going up in diameter from the cylinder head backwards.
Theres alot of things a well designed backbox like the rc40 does well,like reducing back pressure.
if you were just talking about the tailpipe exiting the back box then as long as theres no steps,then increasing this size wont effect anything other than noise.
This is probably why you can get lots of different rc40's that have different tailpipes,but the same size internal baffle.
hth
dave

[Cooperman]

It's to do with the velocity of the gas molecules, which do, of course, contain some solid particules.
What you want is for the inertia of the moleules, i.e. the speed x the weight, to be constant all the way along the pipe, so that at final exit it 'hurtles' out at the optimum speed, rather than slow down suddenly and cause an inrease in local pressure. This increase in local pressure 'screws up' (to use a technical term ) the extraction effect of the gas velocity. Once it has actually cleared the end of the pipe into the atmosphere, it no longer affects the performance of the car, but whilst it's still in the pipe it does. Think about the end of a hose pipe. If the last few inches was increased in diameter the water would not exit so efficiently.
Bernoulli's Theorem states that when a fluid passes along a pipe, if the cross sectional area increases, the velocity of the fluid decreases and the pressure increases. A spray-gun works as the cross sectional area decreases, the velocity of the air increases, the pressure decreases and the resultant low pressure at the lower cross-sectional area sucks the fluid into the spray gun nozzle.
So, for optimum engine efficiency there is the need to keep the gas flow along the exhaust pipe at a constant and optimum speed all the way along, right to the final exit.
The 1.75" internal diameter has been shown (not by me!) to be the best size for a 1275 engine operating between 4000 and 5000 rpm. A full-race engine operating at 6000 to 8000 rpm will need a slightly larger, say 1.875" to 2" ID
For a forced-induction engine, the cross sectional area should be increased by the proportion of the boost added. That is, if the boost is +7.5 psi, the pipe area (not diameter) should be increased by 1.5 as the increase in inlet pressure is up from atmospheric at 15 psi to 22 psi.
I hope I've explained this OK, sorry if it's still not clear.

[Pooky]

Crystal thanks very much

[Cooperman]

Crystal thanks very much

A pleasure, as always!

[Dave33]

Cooperman dont take this the wrong way,i find what you say very interesting.
But is there any actual proof that having the tailpipe area tapering to an increased size actually has a negative effect?
Obviously a smaller exit would increase back pressure,but a slightly larger diameter?
Im wondering,because you would have thought the makers of the rc40 that are now available with different outlets,would want the exhaust system to perform as well as it was designed to do.
I can understand by what your saying,that in theory it will perform worse,but sometimes this doesn't apply to what actually happens.
Dave

[Cooperman]

I can only state what the physics as I learned them are.
An increased cross-sectional area causes the gas velocity to DECREASE and the local pressure to INCREASE thus reducing the gas molecular velocity with an adverse effect on gas exit speed and a reduction in exhaust efficiency.
Take a look at the successful works racers and rally cars of the most competitive period. They all had single exit pipes of the same diameter as the main part of the exhaust and this was for that reason.
As an example, if you go from a 1.75" ID pipe to a 2.5" ID pipe, the cross sectional area goes from 2.4 sq.in to 4.9 sq.in. This will slightly more than halve the gas molecular velocity and reduce the exit efficiency by a similar amount.
A less efficient exhaust = less power and more fuel used.
If you take the Maniflow exhaust as an example, their 1.75" ID centre exit system is just that, 1.75" all the way, except for the straight-thru' silencer boxes, which reduce back to 1.75" after deadening the noise level with the main internal diameter remaining at 1.75".
My son, who works in F1 as an engine design and installation design engineer has always insisted that this is the case, even on F1 cars, which always have constant diameter exhaust pipes. The required diameter is a function of the engine capacity, the optimum rpm and the optimum gas velocity for best inertial extraction. It's called computational fluid dynamics.
Don't be misled by the sales organisations who want you to buy their fancy looking pipes. All they are interested in is the money.

[Cater_Racer]

Scott,

If you're interested in trying it, I've just taken a 2" LCB , downpipe and rear cetre exit box off my car.

Now got a cooper freeflow 1.75, 1.75 downpipe and RC40 on (today).

Not tested it yet, but the 2" is up for grabs.

Cater

[Cooperman]

Scott,

If you're interested in trying it, I've just taken a 2" LCB , downpipe and rear cetre exit box off my car.

Now got a cooper freeflow 1.75, 1.75 downpipe and RC40 on (today).

Not tested it yet, but the 2" is up for grabs.

Cater

That'll be 2" OD, which is 1.875" ID and that's fine for a modified 1275 or bigger. Sounds like a good deal to me.