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 Post subject: Exhaust Question
PostPosted: Wed Jul 13, 2005 2:29 pm 
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Hi Guys,

After speaking with DumHed the other day I have decided to give my standard exhaust system a bit of a rethink.

I was always under the impression that if you go for too large of a exhaust system it would effect back pressure and reduce low end torque.

This is what I have found from personal experience so far, but Dumhed assures me that turbo engines don't suffer such problems especially if they are fuel injected.

So with that in mind I am looking at improving the flow of the exhaust system.

I have noticed on ebay there are two distinct trains of thought when it comes to dump pipes.

The first is a single 3" pipe straight off the turbo exhaust.

The second is with a seperate tube for the wastegate.

Now I have heard theories that the seperate pipe is better because it cause less flow turbulance but looking at the ones on ebay they rejoin at right angles (or close to it) to the main pipe and also appear to use a smaller pipe before the flange which would reduce flow.

Has anyone tried both types and can tell me how it effected torque / HP?

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PostPosted: Wed Jul 13, 2005 4:04 pm 
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You are not going to feel any difference on the street when trying to compare a single dump to a split dump. On the dyno you may see very slight variances but really its nothing to get all excited about.

As long as the dump is as big as you can fit then it will do the job.

People always tend to get to wrapped up in the theory of things and forget that in the real world it may not actualy make that much difference.
For example I hate listening to people that bolt a strut brace on there car and then say oh it handles soooo much better. BULLSHIT!! They think it handles better because it now has a strut brace. Or they remove their back seat and say oh the car is way faster because I remove 5Kgs worth of seat.

Yes if you fit a strut brace, swaybars, coilovers it will feel different but a brace on its own is negligable. As to is the back seat. Strip the car save 50Kgs then yeah you may feel a bit of difference.

So anyways back onto exhausts, Try not to get all caught up in theory about how well each flows. They are both going to flow enough for what you want them to do.

If you want to talk theory then okay.
In an exhaust system you need 2 things Gas velocity (how fast the engine can expel gas) and capacity (how much gas can be fitted into the pipe)
Now a big pipe will have great capacity but a low velocity. A small pipe will have great velocity but low capacity.

So lets say joe blogs fits a 3.5inch system to his NA silvia. He looses power. Now he thinks okay I have reduced the backpressure and Ive lost power so hey I need more backpressure. INCORRECT!!!
He has reduced the velocity of the exiting gas which is what has robbed him of power. The velocity is what causes the scavanging effect in an exhaust system.
So the best exhuast for blogsy is one with the greatest velocity without compromising capacity (remember to little capacity leads to backpressure)
Idealy a small diameter will create higher velocity at low RPM but unaceptable backpressure at high RPM.
A larger pipe will have lower velocity at low RPM but more capacity at higher RPM.
So you need to determine what sort if power band your engine has to determine what size exhaust you need.

Now in a turbo setup. The actual exhaust housing and turbine wheel of the turbo is what creates the velocity in this system not the pipe size. As the turbine rotates it not only creates velocity but also pressure.
Now what do we need in an exhaust system? Velocity and Capacity.
Velocity we have because the turbine is making it however we also have the opposite of capacity which is pressure. What do we do to get rid of the pressure? We increase the capacity buy using bigger pipes.
So with a turbo in place you can see we now have to use a large pipe to reduce the pressure the turbine creates and we can also retain the velocity as we are no longer leaning on the pipe diameter to create it we are now using the turbine to do it.

Hope this helps.

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PostPosted: Wed Jul 13, 2005 4:22 pm 
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Everytime i learn somthing new it pushes somthing old out of my brain :P

I dont see why it needs to be called Gas velocity, you could just call it Gas speed as its not specifying a direction.

But velocity sounds better i guess 8)

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PostPosted: Wed Jul 13, 2005 4:44 pm 
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The gas will always take the path of least resistance so effectively it does have direction.

Velocity and speed are not the same thing. velocity is the rate at which the gas excelerates, speed is the distance covered vs time.

Thanks for that Risking, turbos are a whole new world to me at the moment. All of the N/A information I had from the past seems to go out the window i.e gas reversion, back pressure etc.

Theorectically the twin pipe arangement should have the best velocity out of the turbo provided turbulance rentering the pipe doesn't cause a negative effect or restict the flow when trying to re-enter a smaller diameter pipe.

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PostPosted: Wed Jul 13, 2005 4:51 pm 
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well in a turbo system you dont need velocity, and in n/a after the extractors you dont need it either - the more mass you move the less backpressure and the more power you make

with a turbo, its alot simpler though, you simply need to have as little backpressure as possible on the exhaust side of the turbine, this increases the pressure difference between the exhaust side and manifold side of the turbine, which makes the turbo more efficient and ends up making more power. also helps with spool as there is more force turning the turbine, and less backpressure slowing it down or acting against it.
so velocity doesnt really matter here, its just a matter of expelling the gasses as quickly as possible.

search as well, and you will find this has been covered a few times before with good detail


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PostPosted: Wed Jul 13, 2005 4:56 pm 
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182Go wrote:
Theorectically the twin pipe arangement should have the best velocity out of the turbo provided turbulance rentering the pipe doesn't cause a negative effect or restict the flow when trying to re-enter a smaller diameter pipe.


you've pretty much got it there, its a compromise between having a split dump/turbine and then having some sort of merging which causes turbulance, or you have single style one which merges at the start which may cause some turbulance but then smooths out and stays that way

it really depends on a case by case basis, the HKS dump is split but not well designed which probably flows less overall then a well made single, but at the same time the CES is also split, but of much better design which alot of people claim is the mother of all dump pipes


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PostPosted: Wed Jul 13, 2005 5:35 pm 
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I guess the best compromise would be a single with a thin devider which extends a few inches.

CES... does anyone have pics of one and approximate cost??


Ok found CES.. that was what I was thinking would work just need to find a price.

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PostPosted: Wed Jul 13, 2005 6:13 pm 
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so velocity doesnt really matter here, its just a matter of expelling the gasses as quickly as possible.

You are right yet wrong in the same sentence.

EVERY system needs good velocity to expell gas at a rate that will allow enough capacity for the next round of exhaust pulses.
However in a turbo setup you need not worry about velocity as it is produced by the turbine wheel and volute of the exhaust housing.
So you are right in saying that with a turbo exhaust you need only worry about capacity, however don't disragard velocity.
To expell gases as quickly as possible you need velocity NOT capacity. So if you happen to to large on the exhaust side you will effectivly loose the natural velocity created by the turbo and be back to square one again.
The turbo is expelling its own gas you just have to provide it with adequate capacity to do the best job it.

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PostPosted: Wed Jul 13, 2005 6:21 pm 
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With split dumps think about this.

What have you learnt so far?

Small pipes have greater velocity yet lower capacity true?

So the smaller wastgate runner is going to have a greater velocity than the main exhaust, so in thoery when the gated waste enters the main exhaust stream it will be traveling faster than the other exhaust and hense should actually create a lower pressure in the section before the wastegate enters the pipe.

Now thats all well and good if the wastegate has a smooth transition into the main pipe, this is not noramally the case though, so you may infact be creating turbulance inside the pipe before and after the wastegate entry which throws the whole theory out the window.

If as you say the wastegate pipe feed was to travel down the inside of the main pipe and exit in the same direction as the flow it would work pefectly however at tha same time upset the velocity of the main exhaust gas as it has to change around the wastgate pipe.

Exhaust designs are all about compromise and what works best on the individual car.

CES designs seem to be quiet good though.

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PostPosted: Wed Jul 13, 2005 7:03 pm 
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Risking wrote:
Quote:
so velocity doesnt really matter here, its just a matter of expelling the gasses as quickly as possible.

You are right yet wrong in the same sentence.

EVERY system needs good velocity to expell gas at a rate that will allow enough capacity for the next round of exhaust pulses.
However in a turbo setup you need not worry about velocity as it is produced by the turbine wheel and volute of the exhaust housing.


the velocity of the gasses at the basic level would be related to the amount of fuel/air burnt in one cycle, and then would change according to the size of the orifice/piping that its exiting from, right ? the exhaust gasses spin the turbine, so in affect the turbine actually slows down the gasses and creates backpressure, and by decreasing the size of the exhaust off the turbine, you will increase velocity relatively to a larger diameter exhaust, but the larger exhaust piping will move more mass for any given period of time and cause less backpressure. this is what i meant by velocity doesnt matter

i would go far enough to say the best exhaust (though not practical) for power, would be just a bellmouth hanging off the turbine and having the same diameter as the turbine


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PostPosted: Wed Jul 13, 2005 7:11 pm 
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[quote="To expell gases as quickly as possible you need velocity NOT capacity. So if you happen to to large on the exhaust side you will effectivly loose the natural velocity created by the turbo and be back to square one again.
The turbo is expelling its own gas you just have to provide it with adequate capacity to do the best job it.[/quote]

hey risking,
the more post combustion gas going from the engine would create more pressure?
so does the size of the most efficient (efficient as in good velocity)exhaust pipe relate to the size of the exhaust turbine/housing?
also u are saying the velocity of the wastgate seperator on some dump pipes is different from the larger pipe, and thus will affect airflow unless it was connected at the exact same direction of airflow. would this be possible though? i guess there would always be an offset in air flow direction, with the difference in velocity, and direction? is ther a way of ensuring the 2 pipes join up perfectly?

how about no exhaust? screamer pipes?
chers bud

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PostPosted: Wed Jul 13, 2005 7:11 pm 
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182Go wrote:
The gas will always take the path of least resistance so effectively it does have direction.


Yeh but the direction is not specified,

if you were to say...3m/s south, thats a vector, velocity

where as 3m/s is just speed or a scalar...

and the only time velocity would be used as acceleration would be an increasment in rpm of the motor, giving a plus or minus squared.

and velocity is calculated the same as speed btw, they are exactly the same thing only velocity gives a direction

Correct me if im incorrect...

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PostPosted: Wed Jul 13, 2005 7:18 pm 
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182Go wrote:
I guess the best compromise would be a single with a thin devider which extends a few inches.


genuine greedy dump pipes have this exact design, and exhaust gasses from the turbine and wastegate merge parallel, which is good for flow, but it still has its compromises and design downfalls. heres some pics of mine that show this,

Image
Image

the best is probably CES, although reliability was lacking in previous versions that cracked, which was pretty pathetic considering there price. my next option was tomei or greedy, i eventually decided greedy because i couldnt justify the expense of a CES over a greedy as i didnt think it would make much difference on a gt2530


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PostPosted: Wed Jul 13, 2005 10:26 pm 
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so does the size of the most efficient (efficient as in good velocity)exhaust pipe relate to the size of the exhaust turbine/housing?

There is no realation ship between the ideal A/R of an exhaust housing to the ideal diameter of the exhaust size no.

Generaly though the larger the turbo exhaust housing the more velocity they are capable of producing once they are in there efficency range so the larger the exhaust diameter you can use without effecting the velocity.

Im yet to see a formula to work out the perfect exhaust diameter for a given turbine housing so to speak.

mokompri I see where you are coming from, the smaller pipe will increase velocity however you need to have a large enough pipe to keep the capacity, If you can't flow the capacity required you will get backpressure which you don't want.
The manifold runners are smaller than the rest of the exhaust, reason being they need the velocity to get the turbine wheel spinning. Ideally on an SR0 you want 1 1/4 inch runners. Yes the wheel is a restriction to the gas flow however a well designed exhaust housing will keep the gas velocity up and actually increase it. As I explained a turbo will create its own velocity .

Quote:
also u are saying the velocity of the wastgate seperator on some dump pipes is different from the larger pipe, and thus will affect airflow unless it was connected at the exact same direction of airflow. would this be possible though? i guess there would always be an offset in air flow direction, with the difference in velocity, and direction? is ther a way of ensuring the 2 pipes join up perfectly?


Seperate wastegates have a higher velocity due to the smaller pipe diameter.
I have seen Dump pipes built by exhaust shops that do exactly what you have asked.
The gated gas runs into the exhaust and travels along inside for a few inches before finally being vented in the same direction as the exhuast exits. While this works its a compromise for the main exhaust flow which is disrupted by the 1 inch waste gate pipe thats inside its 3 inch pipe.

There is no way of joining them perfectly without compromise no. Having said that the further down the exhaust the waste gate joins in the less of an ill effect it would have on the flow.

While this thoery is all well and good remember in real life situations you are really not going to notice a hell of a lot of difference having the wastegate merging at different angles and different sections etc.

Any 3 inch mandrel bent dump pipe will do the job any of us would ever need. I say mandrel as think about the way a press bent exhausts are made. The bend actually gets smaller where its "kinked" When the pipe gets smaller the velocity increases but the capacity decreases.
A mandrel keeps everything realative to the rest of the system.

You don't even get right into exhaust technology when talking about F1 systems. Sure they are superior and Very well designed but in thoery even they can be better.

As long as you are learning something seohyunjin then its worth posting.
I spent a fair chunk of time studing gas physics in particular exhaust technology a few years ago so its good to re-fresh my memory and have a civil discussion with people over it as well.

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PostPosted: Wed Jul 13, 2005 10:46 pm 
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I cant believe you've gotten this far into an exhaust thread and not mentioned laminar vs turbulent flow.

Cant remember the specifics but turbulent flow is considered better in an exhaust for some odd reason, hence the sudden step you see in some dump pipes right after the turbine outlet. Though that could be something to do with the turbine outlet generally being about 40mm or less and a 40mm bit of pipe being a major restriction.

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