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PostPosted: Wed Jul 13, 2005 11:34 pm 
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Quad T88
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We can discuss laminar vs turbulent if you want, but its not noramally an issue when talking about exhaust systems.

Image

This is laminar flow. The center portion has the highest velocity obviuosly however when measuring the velocity you only get an average of the cross sectional area not the peak value of the center.
Laminar flow is only really seen in fluid transfer when its put under a steady flow. For example a water has laminar flow down the cylindrical hose Laminar flow is only acheived with smooth steady flow.

Turbulent flow starts as laminar flow. However as the velocity increases in an exhaust system the pulses over come the gases inertia and become turbulant. as there is no steady flow in an exhaust system its not possible to have laminar flow

The idea behind sharp and steady pipe changes is simple. A sharp abrupt change is pipe diameter should be avoided if possible.

A smooth transition does not effect the already turbulant gases velocity, however if you all of a sudden change the diameter you will effect the flow of gas in that section causing a turbulant section that will lead to back pressure, as the gas must get moving again after a sudden change.
This is mostly applied to headers and extractors because the sudden change when put into a dump pipe is usually very close to the turbine outlet where the gas turbulance is all over the place anyways.

BHDave you may remember when I had my dump made I was not happy with it because of the sharp change from 2.5 to 3inch then I had it re-made with a smooth transition and a few other minor changes. It never made any difference to the way the car drove. I still have both dumps from that comparison but I only use the smooth one as in thoery it should be better.

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PostPosted: Wed Jul 13, 2005 11:39 pm 
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Risking, I am seriously impressed with the amount of thought and length of the posts you've put in this thread. SilviaNSW.com always impresses me with the intelligence/fuckwit ratio.

On a side note, after I re-did the turbo gasket on my 93 SR20 180sx, I [obviously] didn't re-seal one of the joins in my exhaust (just before the cat, I think).
It's steadily gotten worse, so I took it to an exhaust place today where they sealed that biatch back up again (I just didn't have the time/tools and he's a mate of a mate so it was cheap as chips anyway)

All this leads me to my question - would a 'possibility for escape' of my exhaust gases before/after the cat influence performance? Or is this yet another case of I only think my car performs better now I've had a (minor) problem fixed?

It's a 3" dump back HKS system - if that makes any difference.


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PostPosted: Thu Jul 14, 2005 12:05 am 
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Quad T88
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You are right. In theory the leak will effect the gas flow but in real time you won't notice it.

Having said that if you have a leak at the manifold before the trubo then you are going to have upset the velocity of the manifold runners. Also you are going to be loosing presious hot exhaust gas. Something that has not been mentioned yet in this thread is the effects of heat.

Hot gas is actually lighter than cold gas and hense easier to move. Hot gas also has more energy than the cold gas. So if you can keep hot gas inside the manifold you will have more energy on tap to drive the turbine wheel and hense make more boost sooner. Besides to protect things this is the main reason behind exhaust heat wrap, to trap the heat inside the runners instead of leting it radiate to the atmosphere.

If you have a manifold gasket leak you will be either blowing out the hot air or drawing in cooler air depending on where the leak is and the scavanging effects inside the manifold. Both of these are a bad thing to performance and efficency of the turbo.
A leak further down besides having a very annoying noise won't have any major ill effects on performance.

There is so much in exhaust system design I could go on for days cat converter shapes, internal muffler designs flow vs noise the list goes on and on.

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PostPosted: Thu Jul 14, 2005 12:15 am 
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T04 Hybrid
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Thanks for that Risking... I didn't think it would've (in the real world) made much difference, except the last few days (I'm pretty sure the leak was getting progressively worse) my car would cough and splutter a little bit when I put my foot down hard, which I'm sure was a lot to do with this introduction of a temperature difference half-way down my exhaust.

In terms of the turbo bolts being loose (and associated demise of the turbo exhaust gasket) that definitely produced performance issues... basically the turbo exhaust wasn't blowing out through the exhaust system, part of it was blowing directly into the atmosphere, less boost to the engine, more noise, potential for damage to components and those around them (blasting hot air directly from a turbo can't be good for anything)

It's a hell of a job to fix, you have to remove basically one side of the engine to get to four bolts and replace a gasket, but it's well worth it if you want to learn more about your car - and the labour savings are most probably massive.
I do have pictures of the whole process and should write up an article, same with my coilover install...


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PostPosted: Thu Jul 14, 2005 10:57 am 
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Pig Magnet wrote:
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...


You are right and wrong... As the RPM / flow / turbulance changes constantly within the exhaust pipe the direction of the flow would also constantly change.

So the gas would have a direction but at the same time it may be for such a brief moment that it could almost be viewed as being directionless.

Hands up all those who would like to see Risking open up a exhaust shop :)

It would appear the only constant in this discussion is change.

I think at the end of the day I need to produce the most free flowing, least joins (another factor) possible system while maintaining optimum velocity and least flow restriction / back pessure possible with the pipe size that best achieves this.

Thanks for the imput guys there has been some great info / theories put forward. It never ceases to amaze me how unpredicatable air / fluids can be.

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PostPosted: Thu Jul 14, 2005 11:14 am 
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The direction of gas flow is always going to be realated to the sorce of the velocity.

In an exhaust system the source is the head of the exhaust valve. So the gas flow is always going to be away from the valve.
There are times when this is not the case such as when the valve overlap occurs on every cycle.
When valve overlap occurs it is possible for spent gases to re-enter the combustion chamber and "contaminate" the fresh air/fuel mixture. This will lower the detonation threshold of the mixture and also reduce the efficency of the cylinder due to the diluted ammount of oxygen.

The idea of scavanging around the headers is exactly what reduces the "reversion" of gas flow in an exhaust. A well designed set of headers will keep the waste gas moving fast enough so as to not let the valve overlap effect its direction. In a stright section of pipe there is no direction change as there is nothing to causee the change. Come to a bend the increased reistance can cause some hassles to flow direction but usually if the velocity/capacity ratio is well thought out there will be no change here either.

That said there is a constant change in direction as the gas passes through a muffler. Thats part of how a muffler works. As the gas changes direction it looses energy however we are not discussing mufflers so wil leave that for another day.

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I think at the end of the day I need to produce the most free flowing, least joins (another factor) possible system while maintaining optimum velocity and least flow restriction / back pessure possible with the pipe size that best achieves this.


Thats pretty much spot on for a road going exhaust.
Joins are a whole different kettle of fish. Their effects on turbulance are massive but in the grand scale its negligable.
You want to keep the ammount of pipe changes to a minimum, the number of tight bends (always use mandrels) to a minimum. also an over looked feature is the actual flanges they need to be as smooth as possible internally as well.

The best mufflers to use are ones where the 3inch pipe actauly fits inside the join and does not but up against it. This way there is no weld in the gas flow its on the outside and never protrudes inside the exhaust.
Same can be said for any join, flanges, cats etc etc.
Ill post pics latter to explain this better.

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PostPosted: Thu Jul 14, 2005 12:04 pm 
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Funny but the anti reversion headers of the 70's in the US never caught on, which really suprises me considering the benifits they gave. I was speaking post turbo, so really I should have said redirection rather than change of direction.

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PostPosted: Thu Jul 14, 2005 1:14 pm 
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Risking wrote:
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.


:-? i believe thats what i was saying... simply, i was saying that the more mass the exhaust can move with the less backpressure the better, and that velocity was a side attribute that was really something you wouldnt be chasing, because velocity can increased with smaller diameter orifice, but that increases backpressure, which means less capacity. or in other words capacity isnt always increased with an increase in velocity


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PostPosted: Thu Jul 14, 2005 2:16 pm 
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with velocity, you really want to keep it, but it's only important in the area of the exhaust that's "tuned"

What you want to avoid is upward changes in velocity. (forced velocity increase, which is due to a constriction of the pipe)

If you come out of the exhaust ports, into really fat manifold runners, then go into a turbo the exhaust gas immediately slows down, flows along the runner, then has to be sped up again in the turbine housing, which wastes energy and reduces turbo response.

It's most important in n/a extractors. If they're too fat then the gasses slow down and kind of "sit" in the pipes, needing to be pushed out.
If the pipe diameter is smaller and there are no sharp bends or messy joints the exhaust pulse will travel very quickly through the pipe with little resistance - leaving behind it the all important low pressure scavenge pulse with does the "extract" function of the headers.

My view of these things is that you want to conserve energy and velocity up until the first point of necessary restriction / velocity change, and after that you just want the least resistance possible.

In the case of a turbocharged engine, the turbo is a massive restriction, forcing the gasses through a small opening, around a housing, and through a turbine. It does multiple direction changes, and has its velocity increased by restriction.
After the turbo any "tuning" or velocity based benefits of the exhaust will be irrelevant, so all you want to do is get rid of the gas with minimal resistance. The easier it is to push the gases out the better it'll flow, so you need a large pipe.

On a naturally aspirated engine you want to keep the pulses well defined and at high velocity until they've done their job, which is at the end of the extractors. This is where you typically find the catalytic converter.
No matter how much pulse velocity you have through the headers the exhaust gases will suddenly be expanded (pressure and velocity drops), forced through a ceramic matrix, then reduced back to the outlet size (pressure and velocity increases due to restriction), which basically leaves the exhaust with a bunch of lazy exhaust gas that needs to be gotten rid of with the least restriction possible - ie, back to the large pipe being best.

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PostPosted: Thu Jul 14, 2005 2:23 pm 
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some good info!

anyhow while we are on the topic of air flow/pipe diameter, id like to ask...
what is the effect of increasing the size of the afm->turbo pipe. i assume here the compressor is making the velocity? what benefits/negatives can be had by say fitting a 3 inch pipe here. decreased capacity? or will this be negligable as the afm will be controlling the airflow anyway? na will be a differnt case?
anyhow, i have heard that around high boost levels the afm pipe being made out of rubber tends to squeeze in on itself, not sure if this is the case, as ther is a reinforcing wire coil inside of it. risking got any info on this?anyone?

cheers all

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PostPosted: Thu Jul 14, 2005 5:09 pm 
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I would hazard a guess and say the pipe "sucking" in was caused by apressure differential. A restriction at the AFM / air cleaner is the likely culprit.

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PostPosted: Thu Jul 14, 2005 5:21 pm 
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Quad T88
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There is no effect of going to big on the intake pipe to a turbo.

The rubber hoses do suck in because the compressor is able to consume more air than the pipe is capable of suppling. This causes a negative pressure inside that pipe and hense the colapse.

The pipes only tend to colapse when there is a restriction before it, like an AFM or blocked air filter.

Dumhed I know what you are saying about the rest of the exhaust being irrelevant however you still need to keep the gas moving or else you will create a section of little to no gas flow and thats backpressure.

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PostPosted: Thu Jul 14, 2005 5:26 pm 
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yeah that's why you need a nicely designed exhaust with straight through mufflers :)

After the cat I think it's easier to keep the gas moving slower through a 3" pipe than it is to speed it up and force it through a 2" pipe.

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PostPosted: Thu Jul 14, 2005 5:33 pm 
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It depends on the internal shape of the cat.

A well designed cat will speed up the gas flow even though it is a minor restriction, in that case you can keep the gas flowing quickly.

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PostPosted: Thu Jul 14, 2005 5:48 pm 
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PC another side effect of the problem you mentioned may be slower boost times, as the pressure differential will cause a descrepancy between what the engine wants and what the tube can supply.

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