TECH: Back-pressure and Exhuast sizing.......

[toysrus]

The TC06 Exhuast Housing only requires a 2 1/4" Exhuast pipe size to mate up fine.....

Now the question is related to back-pressure and how we know it robs power, but the fact of the matter is that this 'back-pressure' is going to be pre-determined by the 06 Exhuast Housing itself, so in those terms, why would someone want a 3" exhuast diameter when the 06 Housing outlet only dictates 2 1/4" ??

Or is back-pressure increased by the increased area of restriction, i.e. the 06 Housing compared to the 06 Housing + 2 or so metres or 2 1/4" piping ?

Thanks :P

[JD_Stazza_Brendan]

When i bought my starion i had to replace the rear part of the exhaust as it started getting small holes in it from rust. About the last 4 or so feet was done in 3inch mandrel bent... I wasnt expecting to much of a difference,, probable a little louder, but it made quite a bit of difference.That was using the stock dump pipe and front part of the exhaust.

I read in a book a year or so ago,, 20th century performance, or something similar. A guy dynoed his car (RX7 i think) then put on a slightler larger tip on the exhaust and gained a few KW. by a few i think it was 3-4kw.

So id say there is restriction all the way through the system.

Hope thats any use.

[WidebodyWoody]

I was informed by so far three performance places that back pressure takes no part in turbo cars as the turbo creates the pressure for us. And that back pressure is only really an issue with NA cars. So for turbo cars the bigger the better as less restriction allows more air flow, and more power. But usually with NA cars, the more restricted it is, the less power you get, but the if you go too far, there isn't enough back pressure and in turn you loose power again.

[JD_Stazza_Brendan]

You can go to large with exhausts on some turbo cars.
Apparantly the larger the exhaust the more bottom end lag there is.

[CussCuss]

im on the side there is no such thing as to big, that is to say, whatever creates the least amount of pressure on the other side of the turbo, because thats what makes it spool, a pressure difference.
So the question is, will a 3" be better than a 2 1/4"... yes
would a 4" be better than a 3"? maybe... does the exhaust itself in an appropriate size help flow and create less pressure than having the air just sitting there? i dunno, theres a possiblity i did fluid dynamics in uni, but i forget all of the work i did in physics
is a very short (as possible, like the drag cars, 1-2') large (as possible) exhaust better than a 3"... hell yes

[Cordzboy]

In a practical situation, you always get friction with fluid flow through a pipe. This friction causes a boundary layer to form, which basically means the air at the edges of the pipe is flowing slower, whereas the air in the centre is flowing faster.
What this really means is that for any pipe after the exhaust housing, you're still going to get more of a restriction. By using a bigger pipe, you're increasing the cross-sectional area flowing at a higher speed => less restriction.

[mrb1]

The TC06 Exhuast Housing only requires a 2 1/4" Exhuast pipe size to mate up fine.....

Now the question is related to back-pressure and how we know it robs power, but the fact of the matter is that this 'back-pressure' is going to be pre-determined by the 06 Exhuast Housing itself, so in those terms, why would someone want a 3" exhuast diameter when the 06 Housing outlet only dictates 2 1/4" ??

Very good question.

Or is back-pressure increased by the increased area of restriction, i.e. the 06 Housing compared to the 06 Housing + 2 or so metres or 2 1/4" piping ?
Thanks :P

The back pressure in the exhaust manifold will always be way higher than anything you would measure in the exhaust itself. However they do add together. So a more open exhaust always helps.
I have measured the backpressure in the exhaust manifold with 1.1 bar boost on the inlet it can reach 1.8bar in the exhaust manifold!
Lots of data log graphs on my Starion web site to look at.

[toysrus]

Okay, to clear some things up, Back-pressure affects power on the camshaft 'overlap'.....Turbo camshafts always have less overlap then their N/A siblings which is why it affects N/A Engines greater then Turbo Engines.

So for turbo cars the bigger the better as less restriction allows more air flow

The problem with this is that the 06 Housing has predetermined the Outlet size to be 2 1/4".....

This friction causes a boundary layer to form, which basically means the air at the edges of the pipe is flowing slower, whereas the air in the centre is flowing faster.

Now, the actual Inducer size on the 06 Turbine is 2.17", so how thick/thin is this boundary layer ?


I guess the real question now is, does the Piping restriction itself add/impact on the Pressure at the Manifold & Exhaust Housing ? mrb1 ??

[Cordzboy]

This friction causes a boundary layer to form, which basically means the air at the edges of the pipe is flowing slower, whereas the air in the centre is flowing faster.

Now, the actual Inducer size on the 06 Turbine is 2.17", so how thick/thin is this boundary layer ?


I guess the real question now is, does the Piping restriction itself add/impact on the Pressure at the Manifold & Exhaust Housing ? mrb1 ??

Trying to determine how thick/thin that boundary layer is almost impossible to do, given the flow irregularities in the exhaust and the generally crap nature of fluid mechanics.

To answer your question (although myself and mrb1 have already said this), the piping restriction will add to the pressure at the manifold and exhaust housing. So, larger piping is definitely better.

[WidebodyWoody]

I think the majority of people are under the miss conseption that too big is not good and we shouldn't go over 2 1/2 but this is not true

[toysrus]

8 cm2 = 0.57 A/R

06 Turbine- Inducer: 2.175" Exducer: 2.560" = 65.024mm

Using Garretts' data on a GT3271 with a Turbine A/R of 0.78 and Turbine exducer of 64mm

13-18lbs/min Flow @ 1.25-1.75 PR
18-20lbs/min Flow @ 1.75-2.25 PR
20 lbs/min Flow @ 2.25-3 PR

Now because mrb1 didn't dyno the TD06-19c up to 30psi... :x :P I'll assume a max flow of 20lbs/min for a 0.73 A/R (modified 06 080 Exhuast Housing)

The problem here is that 20lbs/min Flow is a bit low......i.e. keeping the Exhuast temp @ 1000F gives a velocity of only 175 ft/sec with a 2.25" diameter and going any larger would reduce the velocity even further....

Now WTF does all this mean for those saying larger exhuast is better ? The only thing which a larger exhaust does is reduce Reynolds' no which means the flow is less Turbulent, hence less pressure correct ?

[mrb1]

8 cm2 = 0.57 A/R

06 Turbine- Inducer: 2.175" Exducer: 2.560" = 65.024mm

Now because mrb1 didn't dyno the TD06-19c up to 30psi... :x :P I'll assume a max flow of 20lbs/min for a 0.73 A/R (modified 06 080 Exhuast Housing)

My TD06-19C has a 12cm2 turbine housing.
Remember also that the exhaust is pulsed flow. Also the gas has inertia (mass). If it has to change direction there will be resistance.
Generally exhaust flow is completely turbulent it's basically a velocity thing, less velocity, less loss especially when it has to change direction like bends and going through mufflers.

[toysrus]

My TD06-19C has a 12cm2 turbine housing.

Well that throws a spanner in the works :x .......welcome to the 0.89A/R club btw :beer

So the question is why not run twin 3" Exhausts off a 4 1/4" dump ??

[mrb1]

So the question is why not run twin 3" Exhausts off a 4 1/4" dump ??

Because they won't fit :P

[logik]

because your car would turn into a lancer