Modifing standard Exhaust system

[Guest]

I have been thinking about removing the middle silencer box from my 84, 944. Just leaving the rear one in place. Will this be better or worse?
Maybe also fitting a competition back box.
I will be fitting a performance cone air filter so I was thinking along the lines of improved breathing.

 

[Indi9xx]

front box in a 944 is a resonator mostly, to allow exhaust pulses to expand into and join up into a more constant flow.

The result of this, is that removal of the box can make improvements to the pulse tuning of the exhaust (Complicated subject that I can go into more if anyone wants me to) but can result in the section after where the box normally is seeing large pulses of air, rather than a smooth flow, which may be restrictive.

In other words, the back box will see pulses rising to XX cfm of flow during each pulse, a flow of -X cfm between pulses at ay given rpm.

Where as with the resonator, you may see a more constant X cfm of flow with possibly only a slight reduction in flow between pulses.

So you may find that the pulse tuning advantages are reduced by the decrease in efficiency of the system after that point.

Noise wise you should not see a massive rise in noise, just perhaps a change in note and also some increase in noise at certain rpm.

The standard rear section isnt very restrictive as standard, but there can be some improvements that can be made.

All the best

Jon

 

[Guest]

Hi Jon and Mutley, if you would elaborate further jon I for one would be most interested. I had the front "muffler" removed on SCZ and a welded in tube now replaces it. Like a lot of these things alleged "performance" things increases are not noticeable on a Lux, except that it sounds faster! I had the noise metred yesterday, 89db at 3500rpm rising to 101db max by "blipping" the throttle up and down, between 1000-3500rpm. With the noise meter one metre from the tailpipe, so noise legality shouldn`t be a problem. Definitely a change in exhaust note with a deep note that shakes my neighbours windows apparently, sounds great going uphill especially with walls either side, and a nice top end "bark" to it. Its the noise that makes you drive with the window open in the freezing cold![]

 

[Indi9xx]

Exhaust gasses travel down the exhaust pipe at close to the speed of sound.

As each exhaust valve opens, it creates a high pressure wave which travels along the exhaust until it hits a sharp expansion, as the exhaust valve closes, the pressure wave ends... As the end of the pressure wave hits a sharp expansion (like the first box) if the rate of expansion is large enough to dissapate the wave a little, this then causes a negative pressure wave to head back up the exhaust towards the exhaust valves again.

If you get the length of exhaust correctly, with minimum areas of sudden expansion, or tune the length of the pipes leading to areas of expansion, you can get a benefit from the negative pressure wave hitting another exhaust valve while its open. So helping suck out exhaust gasses from that cylinder and reducing the amount of force required by the engine to push out those gasses.

Combine this with a camshaft with some good overlap, you can begin to get an exhaust system, that at certain rpms will help not only suck out exhaust gasses, but also help draw in air fuel mixture.

You also get harmonics so multiples of ideal lengths will also work.

For example, if you work out that you have a dip in your powercurve at 4000rpm that you would like to fill, and to get the benefit at that rpm from pulse length tuning you need a 7 foot run of exhaust before the first point of expansion, then also a 14 foot run would also work.

You can use some calculations to work out how wide the effected power range would be and what exact lengths to use.

The same theory also works with intake manifolds, but its quite tricky to have intake manifolds long enough to work... But there are other things that can be done to intake manifolds with length tuning.

Also to get pulse tuning to work, you also need to make sure the pipes are not too big, so as they weaken the strength of the pulses.

If your really cunning, you can calculate exhaust lengths between boxes as well as the length of the exhaust and distance between exhaust valve and first expansion, so you either have multiple harmonics of pulse tuning which either work together to combine or to give a wider spread of power increase.

On a race engine, you really want to try to make these improvements to work at gearchange points, so as to give a boost at points where the engine may normally be in danger of bogging down.

All the best

Jon Mitchell

 

[John Sims]

ORIGINAL: scz

.... Definitely a change in exhaust note with a deep note that shakes my neighbours windows apparently, sounds great going uphill especially with walls either side, and a nice top end "bark" to it. Its the noise that makes you drive with the window open in the freezing cold![]

It sounds like it is worth doing for that alone. []

Aston Martin have dynamic valves in their exhaust systems which reduce noise at mid levels for legal noise level restrictions. The valves are open when you start the car and when you give it some stick.

I have been wondering about a similar system for Beaky. I think WUF sounds the absolute sphericals but would be too loud for track days. This being the case I was pondering a Y pipe on the wastegate with one branch connecting with the original system and one open for general sensory enjoyment. The open pipe would have a threaded cap which could be applied for track days and long runs.

 

[Indi9xx]

Wastegate outlets are another subject where improvements can be made on 944 turbos.

If the tube exiting the wastegate has an anti reversion design, it can stop, or reduce negative pulsewaves entering the crossover pipe, which can shock load a turbocharger (reducing its life by a small amount) and in some cases and at some rpm can interphere with the crossover pipe flow and EGT at some rpm.

If you have a venting wastegate exit, anti reversion properties become even more important, as there would be an exagerated negative wave effect compared to the wastegate venting into the exhaust system (being turbocharged will have a reasonably smooth pressure wave)

The problem with not having anti reversion on the wastegate exit in the standard system, is that you can start to get really strange harmonic waves developing and colliding in the exhaust system and crossover pipe.. A bit like having two exhausts of different lengths connected into one system at the end of the exhaust, which would be a bit of a disaster in terms of exhaust design.

All the best

Jon Mitchell

 

[John Sims]

Exhaust tuning is something of a black art. I have been very much aware of the impact exhausts have on the performance of an engine ever since the addition of a Janspeed system transformed my Capri 20 years ago.

It is partly because of knowing what a difference a good(or bad) exhaust can make, and knowing that I haven't got a clue what would achieve either, that I haven't approached the plan as above.

I had assumed that, once the non turbo exhaust had past the wastgate it would have little or no effect on the remainder of the system. I can appreciate how this may well not be the case and Jon's explaination makes very good sense.

I am further intrigued by the use of separate wastegate exhausts on the 917 and would be very interested in any suggestions as to what might achieve improvements with regard to wastegate venting.

 

[Indi9xx]

A good non turbo exhaust relies on pulse tuning, good flow and just the right amount of back pressure.

A good turbo exhaust needs to have all the same qualities, except you need to exchange pulse tuning for good thermodynamic design.

Most people see a turbo as a windmill being blown around by exhaust gasses. But this is only 25% of the story... 75% of a turbochargers rotational force comes from the thermodynamic effect of hot high pressure exhaust gasses entering the turbo hot side, expanding into the exhaust housing and so cooling, then exiting the turbo, decreasing in pressure even further and cooling more as the travel away from the turbo. This effect causes the exiting exhaust gasses to be as important, if not more important than the gasses entering it.

So a good turbo exhaust needs to exploit this effect. Which is why turbocharged rice rockets respond so well to big bore exhausts and low resistance silencers (good flow)

However, you also want an exhaust system which, like a good dump valve, will actually help the turbo maintain rotational speed during gear changes.. So you want the exiting exhaust gasses momentum to keep pulling the turbine round, even when your off the throttle, so the turbo is spooling as fast as possible, once your back on the throttle again. So if you go too big on the bore, and you may find that lag after shifts actually increases.

By the same token, in a turbocharged system, you really do not want any gasses backflowing during gear shifts... as that could cause the turbo to naturally try to change direction... Although this can happen, its best avoided at all costs.. Which is when anti reversion designs can help.

The other effect thats really important to understand regarding turbocharger exhausts, is that during gear shifts, the exhaust side turbine, will be trying to act almost like a compressor turbine... as it will still be spinning, it will be trying to suck air up the crossover pipe... But due to the closed throttle, there will only be a small amount of available air to move.. Which is why you can use a second wastegate (possibly just the usual wastegate if your cunning) to relieve the low pressure in the crossover pipe and therefore reduce the forces (low pressure resistance) trying to slow the exhaust turbine. This effect has been used several times over the years in motorsport to decrease post gearshift lag. In effect, it works a little like an exhaust side dump valve.

There are many things that can be done to a turbo exhaust, and so far, I have not seen anything really effective done to a 944 turbo exhaust system as a whole package..not for much longer... We have improved on the standard system with some of our experiments in the last year and plan on making some of these experiments part of a complete 944 turbo exhaust package.

All the best

Jon Mitchell

 

[Steven944]

Have a look at www.scart-exhausts.com - thay have an intermediate bypass pipe for the standard N/A 944, and I've been trying to get a firm delivery date and order one for a few months now, to no avail. Maybe if a few of us got together and placed a multiple order, it might sway them !!

I've got the Dansk stainless back box, but want a polished front section to go with it, and a bit more noise wouldn't go amiss either, so this looks like just the ticket !

 

[John Sims]

Why not try Simon at Essential Styling? He might give you better service.

 

[John Sims]

ORIGINAL: Indi9xx

....We have improved on the standard system with some of our experiments in the last year and plan on making some of these experiments part of a complete 944 turbo exhaust package....

Excellent! I assume it will be spot on 89 dB's so that we can use it for track days? Wouldn't it be nice if were switchable so it might be a bit throatier (read louder) to enjoy around the deserted country lanes. []

 

[Steven944]

How freaky is that !!?? got in touch with Scart this morning and they will have stock at the beginning of April, and I've placed an order - I'll keep everyone posted as how this goes and sounds !

 

[Guest]

ORIGINAL: Steven Ferguson

Have a look at www.scart-exhausts.com -

That looks similar to what I did, and a reasonable price too. I spent about £100 on parts and labour. The back box they supply looks nice too but £450!!! Unfortunately the company appears to be French[][]