Post turbo exhaust tuning important? 1

[Viper488]

I've been searching the internet and can't any info on choosing pipe diameter, flow requirement calculations etc for a turbocharged car. Plenty of theory for n/a cars of course.

People in the car forums I hang out in say it 'doesn't matter'. Could be I suppose. With the turbo blocking so much of the exhaust's strength, maybe post turbo exhaust details aren't so important?

 

[Lou Scannon]

Quite the contrary... here's why. Turbine shaft power is tied to the pressure ratio from the turbine inlet to outlet. The required shaft power is determined by the boost pressure being delivered by the compressor. If pressure at the turbine outlet can be reduced, the required pressure ratio across the turbine will not appreciably change, therefore the pressure at the turbine inlet will be reduced by an amount equal to the turbine outlet pressure reduction times the pressure ratio. So for instance if the turbine pressure ratio was 2:1, a 1 psi reduction in turbine outlet pressure would yield a 2 psi reduction in turbine inlet, or exhaust manifold pressure. This is with boost pressure held constant, so it's easy to see that power will increase by virtue of the improved pressure ratio from intake manifold to exhaust manifold resulting in improved flow of fresh charge into the cylinders.
So just as with N/A engines, but with even greater effect, turbo engine breathing and hence power are greatly influenced by exhaust system backpressure.

 

[Viper488]

I would think so. Yet there just doesn't seem to be much general info or tech articles on the internet about post turbo exhaust tuning etc..

 

[Lou Scannon]

Well, both, really. The turbine converts kinetic energy from exhaust flowing at high speed past the blades into shaft energy. The kinetic energy comes from two sources: (1), the kinetic energy in the exhaust flow due to the motion imparted from the blowdown phase and exhaust stroke, and (2), the acceleration of the gas through a nozzle (i.e. the turbine scroll). The energy for (2) comes from the pressure at the turbine inlet, which gets converted to kinetic energy, and far outweighs (1), the contribution from kinetic energy of the flowing gas through the exhaust ports and manifold.

 

[SomptingGuy]

Tuning generally refers to attempting to make use of pressure waves to increase volumetric efficiency. Lengths, area changes and volumes are all important for that. Given that your exhaust valves are "behind" the turbine, I doubt if tuning the exhaust (using this definition) can be worthwhile.

 

[fabiom75]

What would be an optimal pressure ratio? Actually we have archived 1:0,67 (1 in the intake - 0,67 in the exhaust)

 

[Lou Scannon]

Viper488, tuning really isn't an important consideration post turbo. The cylinder pulses are heavily dampened due to the expansion in the turbine. The overriding criterion is minimization of backpressure. That dictates a system with the largest cross section, minimum length, and minimum bends or other sources of friction, achievable within the application constraints (i.e. space, cost, noise, exhaust aftertreatment). Good piping design practices should be followed as well, such as avoidance of sudden changes in cross-section, and minimization of surface roughness/irregularities.

 

[Lou Scannon]

fabiom75, I think you're saying that your ratio of absolute pressure in the intake manifold versus exhaust manifold is 1.5:1
If that's so, that's pretty good, in fact the highest I've heard of. Ideal would be infinity, but that isn't realistic.
If you're actually talking gauge pressure, you should start working in absolute pressure for this kind of analysis.

 

[fabiom75]

Hemi,

actually I have a gauge in the exhaust side of the turbine.
At 39psi of boost we read 26psi in the exhaust. I think it's pretty good. But I was wondering if it could be worth to use a smaller exhaust A/R to raise a little bit the backpressures and make the turbo spool quicker. Without losing that much power...

 

[Lou Scannon]

...That's the trade-off. You've got to decide for yourself what is the best combo. Usually that means testing (or modeling, if you have the resources) all the options, then selecting the best option based on the results.

Are you sure those are both absolute pressure gauges you're using?

 

[fabiom75]

yes sure 100% why? We use a lot of boost...

 

[Lou Scannon]

...that's a pretty decent pressure ratio alright. Any idea what your turbine outlet pressure is?

 

[Viper488]

Well I'm confused then. Hemi says "tuning really isn't an important consideration post turbo. The cylinder pulses are heavily dampened due to the expansion in the turbine. The overriding criterion is minimization of backpressure."

Then, subsequent replies here go on to talk about raising backpressure so the turbo spools faster etc.

Off the top of my head I'd guess that a turbo would spool quicker if flow was going through it faster (less post turbo back pressure). I mean heat alone isn't going to spin anything if that hot air isn't flowing is it?

Warning: I have no turbo experience

 

[Lou Scannon]

The backpressure you want to minimize is post turbo (I don't refer to the turbine inlet pressure as backpressure).
It is the net pressure drop across the turbine that provides power to the shaft as described above. So higher turbine inlet pressure or less turbine outlet pressure (backpressure) will both increase available shaft power, all else being held constant. That is where a smaller turbine housing comes in. By virtue of its smaller nozzle, turbine inlet pressure is increased and (assuming for the moment zero wastegate flow) higher kinetic energy is produced in the nozzle. This creates more shaft power which is converted to higher boost pressure in the compressor. That is why spool-up is improved when changing to a smaller turbine housing. By the same token, spool up will improve if turbine outlet pressure - backpressure - can be reduced. But whereas a smaller turbine housing improves spool up but reduces maximum power due to the adverse effect on exhaust manifold pressure from the standpoint of cylinder gas exchange, reduced flow restriction post turbo improves performance across the board. However, bear in mind that the spool up gains available by reducing exhaust system restriction are typically much less than those obtainable via a smaller turbine housing.

 

[jbthiel]

--Viper 488: Well I'm confused then. Hemi says "tuning really isn't an important consideration post turbo. The cylinder pulses are heavily dampened due to the expansion in the turbine. The overriding criterion is minimization of backpressure. Then, subsequent replies here go on to talk about raising backpressure so the turbo spools faster etc.--

Two different ideas. "tuning" usually means picking a header tube diameter and length to maximize scavanging during the exhaust stroke. This is not much of an issue on street cars.

Reducing back pressure changes the pressure drop accross the turbo and thus the power potential. I used to have an article from a WRX tuner on turbo exhaust design, but I lost it when my hard drive failed. But it basically said what "Hemi" already stated above. smooth transitions, large area, etc.

 

[jbthiel]

A small correction --- This is not much of an issue on TURBO street cars.

 

[Viper488]

You can still optimize HP, at least on a n/a car, by chooing the correct exhaust plumbing at all points to the exhaust system exit. Sounds like tuning. Anyway-

Why isn't it an issue on street cars? Maybe you mean OE cars with turbos, and the typical manufacturer not particularly concerned with max HP..more so emissions and other govt rules?

Because there's plenty of tech articles on choosing optimum exhaust system components for street cars with the exhaust components having enough CFM ability to not restrict your car's HP by choking it down somewhere along the pathway. David Vizard's much copied series on that comes to mind.

I tend to think of course post turbo exhaust component selection IS just as important as in a n/a car. At least in a performance turbo car; street or not.

What qualifies as a turbo street car btw? A TT Viper that goes 7.99 at over 170mph with a full interior, AC, etc? Or just the bit more plentiful 8.50sec TT Vipers?

Aside from splitting hairs, can anyone point to published tests, or personal involvement where post turbo plumbing sizes have been tried?

 

[BadBoyRacing]

I have heard 15% bigger than the turbos outlet,but that would be a subject to how high you boost

For a Turbo Engine street or not, Bigger and shorter exhaust system lenght is BEST for HP

jbthiel Forgot to Say TURBO street cars

The least backpressure is the best on both TURBO and N/A

On N/A you need to control the exhaust air to get the fifth cycle for increased power as the EV closes,that way you get more air in the cyllender and the engine breathes like a 385 instead of 350cid for example,you make the soundwaves and air weight/movement make Vacume to suck the hot air from the combuston chambers of the head witch the piston can not push out without help

 

[Lou Scannon]

Viper488 asks: "Aside from splitting hairs, can anyone point to published tests, or personal involvement where post turbo plumbing sizes have been tried?"
Well, this is not firsthand, but there is a wealth of cumulative experience on the Turboford site

http://www.turboford.net

Try the FAQ and Techboard, search on keywords of interest.
Not only theory, but my own experience and everything I've read support the principle that bigger/shorter/smoother (i.e. less restrictive) is better, when it comes to post turbo plumbing.

 

[Rob45]

I see a bit of misinformation on this thread;
What drives the turbo is a thermodynamic quantity called enthalpy, and it is a function of the pressure and the temperature of a gas or mixture of gases.
Thus, it would be correct to say that the total energy of the exhaust gas drives the turbo, and both temperature and pressure drop across a turbo while under boost.
SO: the hotter the exhaust before the turbo, the more energy can be recovered from it to drive the turbo.

And BTW: the reason downstream backpressure isn't so important on turbocharged cars is that they typically have much less valve overlap than comparable N/A cars, and so have much less backpressure sensitivity (loss of power with increase of backpressure)>