Post turbo exhaust tuning important?
Post turbo exhaust tuning important?
(OP)
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?
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?





RE: Post turbo exhaust tuning important?
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.
RE: Post turbo exhaust tuning important?
RE: Post turbo exhaust tuning important?
RE: Post turbo exhaust tuning important?
RE: Post turbo exhaust tuning important?
RE: Post turbo exhaust tuning important?
RE: Post turbo exhaust tuning important?
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.
RE: Post turbo exhaust tuning important?
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...
RE: Post turbo exhaust tuning important?
Are you sure those are both absolute pressure gauges you're using?
RE: Post turbo exhaust tuning important?
RE: Post turbo exhaust tuning important?
RE: Post turbo exhaust tuning important?
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 :)
RE: Post turbo exhaust tuning important?
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.
RE: Post turbo exhaust tuning important?
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.
RE: Post turbo exhaust tuning important?
RE: Post turbo exhaust tuning important?
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?
RE: Post turbo exhaust tuning important?
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
RE: Post turbo exhaust tuning important?
Well, this is not firsthand, but there is a wealth of cumulative experience on the Turboford site 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.
RE: Post turbo exhaust tuning important?
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)>
RE: Post turbo exhaust tuning important?
I have some experience that you may find interesting:
1. In the '80s, I was involved in altitude chamber testing of an aircraft engine that was turbo normalized. We achieved a test point with 30"HgAbs intake manifold pressure and 18"HgAbs pre-turbo system pressure (the turbo system was a two-stage system). The high positive delta P across the engine helped achieve a BSFC of 0.36 lb/bhp-hr, which is relatively good economy for a spark ignition avgas engine. However, the engine/turbo system was designed for one operating point, and transient response was not good. There is a lesson, though. Consider the whole four stroke cycle and the integrated PdV for the whole cycle when modeling the indicated work.
2. A little later, I was involved with the application of a VNT turbo to a passenger car engine. We were testing in the lab with a simulated exhaust restriction when we ran into a problem and had to remove the exhaust restriction. Engine bhp rose from 175 to 225 (with the same intake MAP and fuel flow) when we ran without the production (cat and muffler) restriction level. Some production exhaust systems can be very restrictive.
3. If you look at the turbo installation in large stationary engines, you will note exhaust diffusers that diffuse the turbine exit velocity prior to an atmospheric dump. This is done to reduce the turbine out static pressure, because it has a significant effect on engine system efficiency.
The bottom line is that careful turbocharger exhaust design can improve overall system power output amd thermal efficiency. However, this is rarely done in passenger cars (including performance automobiles) for two reasons:
1. exhaust diffusion requires space and can be bulky
2. transient response demands usually require the turbine expander to be undersized for most of the engine operating points and a lot of entropy is generated by the mass bypass control (wastegate) anyway, so why bother perfecting the exhaust?
I hope this wasn't too wordy.
PS. The many comments regarding turbine work are good but not quite complete. Turbine work is related to the mass flow times the actual enthalpy change across the stage. So it is related to mass flow, available pressure ratio, inlet temperature, gas composition, and turbine efficiency. The turbine expander work conditions are easy to quantitatively understand in a steady-state test rig, but can be more complicated in the dynamic environment of a recip engine exhaust stream.
RE: Post turbo exhaust tuning important?
Why do you think there is a total lack of participation in this thread by the almost all the established respected professional members of this forum.
I'm surprised it's still here. I guess it survived because despite the poor science exhibited, everyone has behaved in a respectful manner, so others have just ignored it.
I guess if someone reacts emotionally to this post, it might then get red flagged and go.
Regards
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RE: Post turbo exhaust tuning important?
--I agree, thats what I was trying to say.
My "TURBO street car comment" was aimed at PRE-turbo exhaust system tuning. In street cars (or street cars modified for race use) there is a significant space and dollar restriction on the installation of an all-out equal lenth header feeding a turbo. Especially when you consider the minimal power increase for all the pain involved. In street cars you can just up the boost to make up for the losses. But if you consider the case of F1 or Indy they have to take every opportunity possible, and this includes proper headers. But they have the benefit of time, money, fewer interferences, etc.
As for pipe diameter, POST-turbo, I would think that could be estimated from the total CFM your combination is using/expelling. But I still agree with "Hemi" on the overall design philosophy. Smooth transitions, etc.
I also found a rule of thumb that says " 1 inch Hg backpressure = 1 HP lost" ((For reference, we have the following conversions factors: 1 ATM = 14.7 PSI = 76 cm of Hg = 29.921 inches of Hg = 1.013 bar)) Use at your own peril :)
RE: Post turbo exhaust tuning important?