RSR exhaust box

[Bruce M]

Exhaust tuning is not unlike inlet tuning.

When it comes to the size of the pipework, max velocity of the gas is the key. If the pipework is too narrow, it slows the flow of gas and creates excess back pressure which the engine has to push against (reduces power output). If the pipework becomes too large the exhaust gas slows down and stumbles about, also reducing the flow of gas and the draw on the next cylinder to fire (less important after a turbo though). After that itis about the length of the primaries and secondaries. Because the exhaust gas is not continuous but in pulses (ie sound) it is sensitive to the harmonics of the pipework.

A little bit of back-pressure is ideal for a production car. It helps to stop the overrun pops.

If the exhaust is changed on a car, to maximise the performance (gain hopefully) the fueling / timing should be re-calibrated. Putting a free-flowing exhaust on a stock engine could reduce power if the engine goes horribly lean as a result of the increased flow.

[911hillclimber]

Thanks for the details!
I've change parts on my 3.2 and it has been suggested that the loss of back-pressure could explain some performance issues.

[Mike]

Work begins now on the MKII version.........in stainless steel!

[Mick Cliff]

You're having 7 back boxes Mike?
Bit of a weight penalty there after all the work you've done to date.
It should get under 105dB though!

[911hillclimber]

If these run to sub 102 dB I'm interested. Are you sticking @ stainless steel or will do steel with alum coating? (thinking of the 'note' difference with Stainless).

Presume 2 in 3 out or 2 in 2 out?

[Mike]

If these run to sub 102 dB I'm interested. Are you sticking @ stainless steel or will do steel with alum coating? (thinking of the 'note' difference with Stainless).Presume 2 in 3 out or 2 in 2 out?

These will be exactly as the previous 'flapper' box so to RSR shape 2 in 2 out. The stainless shouldn't make it any louder just a slightly different tone, still hoping for the 104 db again.
We are using some different internal padding this time which might tone it down a little more!

[oliveR]

Hi Mike,
very interesting, do you already know the weight vs steel ones ?

[Mike]

Hi Oliver, hoping for it to be just a little lighter than the MK1 version as we are fabricating the whole thing this time,
as opposed to adapting the existing one.

[mrg3.6]

Another great project Mike - any thoughts on cost yet?

Thx

Martin

[squirdan]

the Mark 1 version in ceramic coated steel was £1700 + vat from Neil

[Jasper]

I have an article from Pelican or Porsche on how to make a dual exhaust, must dig it out.

We had a Porsche & Model T run on Sunday and the Porsche nuts came back to my garage and swarmed on Blu Max, its a 71 RS look ex New Mexico, with a 87 3.2 engine with 964 heads on twin triple 48's, what we
are trying to figure out what kind of motor sport was it built for ?
Autocross, Rally, or Track, it seems too high for track have a look at the photos ? The Cage and Seats would not pass Scrutiny here, but has Passed scrutiny stickers on the Screen in the US?








rds

[Mike]

Just to finish off this thread, here are the results on the engine dyno of the new stainless steel production version of the 'flapper'
exhaust box. Amazingly, only 3hp difference between flaps closed and flaps open and no loss of torque at all!

So no real penalty for stopping the noise down from 116db to 105db!

http://youtu.be/RoxDk4eQGCI

[Mike]

A couple more pictures, this one will eventually go off for a grey heat coating. but for the time being staying au naturale.

[911hillclimber]

Very nice note, and open at 8K is just a bit special.
Engine would be FAR better off in my Lola...

Scared to ask, but are they for sale?
Will you do a flap-free version as hillclimbs are 108 dB max so open version not good except on the road.

[Mike]

Last pictures, in situ it really shows the bypass pipe setup going through the box.