StuartBJ
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| posted on 24/11/10 at 05:38 PM |
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Exhaust outlet length....effect on Power? .......Blown Diffuser
Not to do things by half....... I have been discussing the feasability of installing a blown diffuser with Alan from AGM, which is something he has
considered previously but we have hit a question maybe someone with more knowledge can answer.
My exhaust would be as normal ....downpipes....silencer(back box) in a similar place as a 7........ but the actual exit pipe from the silencers would
be lengthened from a few inches to 1-2 foot to enable it to be routed to the diffuser.
Does anyone know the effects this would cause? My initial investigation seems to be telling me that only the sound will be effected as it is
downstream on the silencer (back box) and that I may actually see a widening of the torque band before peak torque is achieved as long as I have
sensible diameter pipes i.e 2.5-3 inches dia and not coffee cans.....but alas I am not an expert on this subject, I hope someone else is ????
Oh and when I say blown diffuser I don`t mean the nancy boy F1 type, I`m refering to the 1984 Renault (Re50) Lemans variety.
Food for though ehh 
[Edited on 24/11/10 by StuartBJ]
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Badger_McLetcher
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| posted on 24/11/10 at 06:30 PM |
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I can't imagine it would make too much of a difference downstream of the silencer, as long as it is free flowing. I'd be sceptical of the
torque band widening tbh, but stranger things have happened
However if the diffuser is actually working and creating a low pressure area, you may see some small gain at speed as the gasses are sucked out.
I'm afraid I'm no expert, this is just an opinion!
If disfunction is a function, then I must be some kind of genius.
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StuartBJ
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| posted on 24/11/10 at 06:46 PM |
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quote:
Originally posted by Badger_McLetcher
I can't imagine it would make too much of a difference downstream of the silencer, as long as it is free flowing. I'd be sceptical of the
torque band widening tbh, but stranger things have happened
However if the diffuser is actually working and creating a low pressure area, you may see some small gain at speed as the gasses are sucked out.
I'm afraid I'm no expert, this is just an opinion!
Thanks for the reply, I always find forums are good places to bounce idea`s off others. With regards to the diffuser I`m hopeing its fully functional,
its a flat bottom before that so there should be some decrease in pressure.... but this could well be optimised by sideskirts and/or some front
canards etc.
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MikeR
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| posted on 24/11/10 at 07:27 PM |
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if you make the outlet pipe from the can is larger than the inlet pipe - then you should be ok. Look up pressure drop over pipe lenght equations on
the web to figure out what size you need. Then when you've had enough - go back to my initial statement. I'm no engineer - but with the
diffuser sucking the gas out and the exit pipe being able to flow a lot more gas than the inlet, it will slow down but shouldn't cause back
pressure issues. If you want to have some real fun you could tune the final section so that it sucks the gas out of the exhaust can at your favourite
RPM.
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StuartBJ
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| posted on 24/11/10 at 10:03 PM |
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Ok so let's say (for example) I choose a 2.5 inch dia inlet pipe to the can then I would require a 3 inch dia exit pipe as it will flow more
than the inlet pipe (makes sense so far)...... Pressure drop from can to atmosphere will be as good as irrelevant ?
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RazMan
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| posted on 24/11/10 at 10:47 PM |
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Some cans are built the other way round to create a small amount of back pressure. Mine for example is 2.25" inlet and 2.5" out. A turbo
engine is best with no back pressure.
HTH
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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Chippy
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| posted on 24/11/10 at 11:00 PM |
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Mine is similar to RAZ's, on my BOA I have a 2.5" pipe with a 2" silencer, mainly because the BOA goes better with some back
pressure. Cheers Ray
To make a car go faster, just add lightness. Colin Chapman - OR - fit a bigger engine. Chippy
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coyoteboy
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| posted on 25/11/10 at 10:12 AM |
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It's not backpressure (which is effectively just resistance to flow, and therefore generally bad in any engine) that's your problem here,
you're looking at changing the length of the tube thus the resonant characteristics of the tube. At certain RPM your exhaust momentum will tend
to suck the gases out of the cyls due to pulse effect timed to co-incide with the valve opening IIRC, hence a change of length (which includes changes
of silencers or tube diams) will affect the resonant response (frequency and amplitude) of the exhaust and so may alter the scavenging effect of the
exhaust across the RPM range. I'd struggle to put any figures on it though. I've heard a few clio cup racers claiming that they lost
stacks of power when their backboxes have snapped off during a race, but I've no way of calculating it at the moment - one would assume that
their problem is caused by the removal of the backbox shortening the exhaust and thus raising the torque gains to above the RPM the engine can reach
in a race. I've some software at work that might be able to guestimate the changes but I've not got round to learning it yet and I'd
need to know everything from the bore/stroke of your engine, header lengths, intake arrangement etc, but it would be an interesting way of learning if
you're not in a rush for an answer.
[Edited on 25/11/10 by coyoteboy]
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MikeRJ
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| posted on 25/11/10 at 10:31 AM |
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quote:
Originally posted by Chippy
Mine is similar to RAZ's, on my BOA I have a 2.5" pipe with a 2" silencer, mainly because the BOA goes better with some back
pressure. Cheers Ray
Back pressure changes an engine fueling requirements, so reducing back pressure without compensating the fuelling may well give a drop in performance,
but this is not the same as back pressure being required for the best performance. Back pressure can only ever reduce the power capability of an
engine.
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StuartBJ
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| posted on 25/11/10 at 01:03 PM |
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quote:
Originally posted by coyoteboy
It's not backpressure (which is effectively just resistance to flow, and therefore generally bad in any engine) that's your problem here,
you're looking at changing the length of the tube thus the resonant characteristics of the tube. At certain RPM your exhaust momentum will tend
to suck the gases out of the cyls due to pulse effect timed to co-incide with the valve opening IIRC, hence a change of length (which includes changes
of silencers or tube diams) will affect the resonant response (frequency and amplitude) of the exhaust and so may alter the scavenging effect of the
exhaust across the RPM range. I'd struggle to put any figures on it though. I've heard a few clio cup racers claiming that they lost
stacks of power when their backboxes have snapped off during a race, but I've no way of calculating it at the moment - one would assume that
their problem is caused by the removal of the backbox shortening the exhaust and thus raising the torque gains to above the RPM the engine can reach
in a race. I've some software at work that might be able to guestimate the changes but I've not got round to learning it yet and I'd
need to know everything from the bore/stroke of your engine, header lengths, intake arrangement etc, but it would be an interesting way of learning if
you're not in a rush for an answer.
[Edited on 25/11/10 by coyoteboy]
I`m always a fan of putting a number to a problem, that way it isn`t open to speculation. I would be happy (and very greatful) to supply info when
you are ready to go. Tell me what you need
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coyoteboy
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| posted on 25/11/10 at 01:20 PM |
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I'll have a word with the engine simulation team and grab the software ASAP and see what I can do for you, I'll U2U you when I'm
ready to run.
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StuartBJ
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| posted on 25/11/10 at 10:04 PM |
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Thanks fella will look forward to it
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Chippy
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| posted on 25/11/10 at 11:07 PM |
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MikeRJ
While I don't proffess to know anything about exhausts and silencers, I would contest your statement as with the 2.5" silencers the engine
made 225 BHP and 234 Lbs torque. The R/R guy said to change to a 2" silencers, as the "BOA likes a bit of back pressure". The mod
was done and the figures after were 238 BHP 253 Lbs Torque. Not a huge improvement but non the less an improvement. Cheers Ray
To make a car go faster, just add lightness. Colin Chapman - OR - fit a bigger engine. Chippy
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coyoteboy
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| posted on 27/11/10 at 09:05 PM |
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There is absolutely no logical reason for an engine "liking" backpressure (backpressure=exhaust restrictions). All exhaust restrictions do
is create a larger exhaust port pressure meaning it's harder to get the waste gas out of the engine, reducing the charge intake. However
changing the diameter and length can massively alter the resonant response and therefore change the power. It's nothing to do with backpressure,
it's to do with tuning both fuel, oscillating air columns (both intake and exhaust) and choosing at what RPM you want your torque.
Changing one thing, say going for a larger diam than stock, can adjust the torque curve, sometimes in a negative way at the peak power point. Changing
down to a smaller diam may bring other parameters back closer to the optimal. Whatever you do, exhaust restrictions ALWAYS have a negative effect, but
you may incorrectly assume that changing to a "more restrictive" pipe helps when in reality you're changing a resonance point.
One interesting point is that expanding the exhaust into a pipe that's wide (rather than narrow) is that the gas cools more, and more of it
physically sits in the exhaust pipe. This in itself effectively creates a restriction as there's a "block" of cool (dense) gas that
needs shoving out and the engine has to work against that too.
It's all a compromise, finding the correct one is the one that gives you best performance.
Check out this for a bit of info - it's not perfect but it's a start:
http://forum.concours.org/index.php?topic=45090.0
[Edited on 27/11/10 by coyoteboy]
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Rocket_Rabbit
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| posted on 4/12/10 at 06:15 AM |
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quote:
Originally posted by coyoteboy
There is absolutely no logical reason for an engine "liking" backpressure (backpressure=exhaust restrictions). All exhaust restrictions do
is create a larger exhaust port pressure meaning it's harder to get the waste gas out of the engine, reducing the charge intake. However
changing the diameter and length can massively alter the resonant response and therefore change the power. It's nothing to do with backpressure,
it's to do with tuning both fuel, oscillating air columns (both intake and exhaust) and choosing at what RPM you want your torque.
Changing one thing, say going for a larger diam than stock, can adjust the torque curve, sometimes in a negative way at the peak power point. Changing
down to a smaller diam may bring other parameters back closer to the optimal. Whatever you do, exhaust restrictions ALWAYS have a negative effect, but
you may incorrectly assume that changing to a "more restrictive" pipe helps when in reality you're changing a resonance point.
One interesting point is that expanding the exhaust into a pipe that's wide (rather than narrow) is that the gas cools more, and more of it
physically sits in the exhaust pipe. This in itself effectively creates a restriction as there's a "block" of cool (dense) gas that
needs shoving out and the engine has to work against that too.
It's all a compromise, finding the correct one is the one that gives you best performance.
Check out this for a bit of info - it's not perfect but it's a start:
http://forum.concours.org/index.php?topic=45090.0
[Edited on 27/11/10 by coyoteboy]
Excellent and accurate post
In addition, if your blown diffuser does anything on a road car, i'll be amazed!!
Oh, and it'd be quite dangerous actually
[Edited on 4/12/10 by Rocket_Rabbit]
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Strontium Dog
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| posted on 4/12/10 at 02:23 PM |
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I'm with Mr. Coyote on this! There is only one place for exhaust back pressure and that's on a two stroke!
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Neville Jones
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| posted on 4/12/10 at 04:59 PM |
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Two things need answering.
First...Blown Diffuser.
The reason for 'blowing' the diffuser is/was to create more airflow and thus more downforce through the diffuser. This was done in F1 and
LMP's, then went out of fashion for various reasons. Mainly, at the time you want the downforce, in corners, the exhaust was not enough as revs
at this point were usually down.
The lower pressure of the diffuser area proved to have minimal effect on engine parameters.
Second...Exhausts.
With the engine that needed a smaller exhaust to give more power, the smaller exhaust gives a higher gas flow velocity, and more than likely this is
what increased the power due to better scavenging effects of the higher exhaust gas speeds. This would change with a change of cams, so is not the
case in all instances.
This is similar to opening head ports out too far, and losing power. There is an optimum, which is inlet AND outlet dependant.
Cheers,
Nev.
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Rocket_Rabbit
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| posted on 5/12/10 at 01:29 AM |
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quote:
Originally posted by Neville Jones
Two things need answering.
First...Blown Diffuser.
The reason for 'blowing' the diffuser is/was to create more airflow and thus more downforce through the diffuser. This was done in F1 and
LMP's, then went out of fashion for various reasons. Mainly, at the time you want the downforce, in corners, the exhaust was not enough as revs
at this point were usually down.
The lower pressure of the diffuser area proved to have minimal effect on engine parameters.
Second...Exhausts.
With the engine that needed a smaller exhaust to give more power, the smaller exhaust gives a higher gas flow velocity, and more than likely this is
what increased the power due to better scavenging effects of the higher exhaust gas speeds. This would change with a change of cams, so is not the
case in all instances.
This is similar to opening head ports out too far, and losing power. There is an optimum, which is inlet AND outlet dependant.
Cheers,
Nev.
Not bad, but the fact that was all said before in the thread.....
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SeanStone
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| posted on 21/4/11 at 08:15 AM |
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A blown diffuser is a pretty complicated system. I think there is just as much aero as there is engine and exhaust development in order to actually
keep a decent flow of exhaust gas on part throttle.
Looking forward to seeing what you come up with! Bare in mind your efforts may be negligable due to your ride height. you need to be pretty close to
the floor for a diffuser to rear start working it's magic!
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