"Backpressure" Where does the most benefit lie?

[GNDriven]

I am caught up in the decision process of selecting a turbo. Focusing on the issue of backpressure, I am wondering where does most benefit lie. The inital backpressure that it takes to spool the turbo or the unwanted backpressure once up on boost.
Can anyone give me some concrete figures of which turbo had what backpressure over time for a given run?
I am interested in the difference of the P-trim and the Q-trim turbos.
Also, can anyone calculate power losses caused by the exhaut stroke given different levels of backpressure?
Frame of reference would be right before the turbo;however, other points of measurment and reference are welcome. Thanks

 

[TurboTR]

Well... the short answer is- there is no benefit to backpressure But it is there when driving a turbine. The GT series turbos seem to do much better than the p or q trim predecessors. You may see upwards of 50 psi backpressure on say a 76 q-trim .81 a/r, depending on what you are doing with it. Change to a GT turbine side and the power goes up, which seems to imply that the backpressure goes down. Have not measured it on mine with the GTS76 yet though...

Best advice prolly, seek a GT series hybrid if available for your application

TurboTR

 

[ronbros]

A/R and backpressure

I do know this does not quite fit in , but I wish I knew the answer, I run a rotary mazda it has a swept volume of 80 cu.in its a small engine but i run 1.15 ar turbine housing with a P wheel it spools by 3500 rpm and boost is 18 psi max 20. what dynamics must be taking place here? makes about 450 rwhp 2650 lbs. yes i have a buick also. ROn in Daytona

 

[GNDriven]

TurboTR ,
There has to be some benefit to backpressure e.g the quicker spooling times between the smaller exhaust housing and the larger .82 housings. In an ideal application you would like to start out with a small exhaust housing and then open it up to a larger one once the turbo has spooled up to relieve the backpressure so the motor can get rid of its exhust gases. I was told a good point, that some people make the mistake of making everything larger on the exhaust side and give up many tenths on the way to full boost. Sure many people will say there turbo is a blurr after 5 psi, but 2 to 3 tenths of a second is a blurr. I am trying to find where in the spectrum yields the most gain. Help wanted.

GNDriven

 

[TurboTR]

Oh I totally agree... Is why I raved about the Garret VNT turbos years back They literally do just that- start small, then open up big. No wastegate required. But IMO backpressure is best described as a necessary evil, not anything beneficial Just mixing our terms here. I totally agree, choose a turbine hsg size that will allow you to spool and accept the pressure/flow tradeoff, rather than impress your fiends at the drive- in with a giant housing. But I kid our Supra friends...

TurboTR

 

[CTX-SLPR]

Isn't the downside of the VNT turbo its overcomplexity and need ro some sort of management system to properly contol the position of the vanes? I really like the idea behind them however the Murphy factor and need for yet another electronic gizmo seems to be weighted against the setup.
On a side note, is a GT hybrid better than a pure GT and is the hybrid a hot hybrid or a cold hybrid?

Thanks,

 

[TurboTR]

Good lord no... It is actually very, very simple. The Garret ones that is. Beautiful. One downside might be the extra weight- the exh hsgs tend to be large.

When I worked at the research inst I was tasked with building a closed loop exhaust *backpressure* control system for a project diesel engine Using a VNT turbo as the "actuator". Hey I thought it was fun It also incorporated some anti-lag features, such as when the driver would step on the gas, it could immediately close the VNT nozzle size down at first to bring the turbo up quicker. Awesome stuff really.

TurboTR

 

[CTX-SLPR]

sounds cool, you wouldn't be working in Pickel would you? How did the control system work?

PS what part of Austin are you from?

Thanks,

 

[TurboTR]

No, at the time I was in San Antonio. There is a very large research inst there, everything from nuclear waste management to covert electronic stuff to fire safety to engine research and between I've applied to and have been able to tour Pickel though. Saw some awesome E-M rail gun and flywheel energy storage stuff in there- located behind 3' thick concrete bunker walls I might add, LOL... Currently I work at AMD and live S.

The control system worked well, surprisingly enough (lol) Had prettty standard feedfwd and PID loop control. And some mapping of sensitivities vs speed/load, and other tweaks to make it work.

TurboTR

 

[CTX-SLPR]

that would be SWRI.... I've worked with those guys before. Dr Light is a pretty smart dude. I grew up in Pflugerville so Austin is home territory. If I go home this summer to visit family I might have to see if there's come Austin Area Buick stuff brewing.

 

[TurboTR]

Cool... I'll fill up the nitrous bottle and take ya for a "hunt" down Mopac lol...

TurboTR

 

[GNDriven]

Thanks for the reply guys,
I know for sure that there are those that have been doing this for a while, and have a sensor in every pipe of there cars. I ask those people have they performed any expirement in which they changed exhaust housings, monitored backpressure, and measured a performance lost or gain.

I wonder if anyone can tell me if they have done any data logging on the time it takes to spool there combination up from a certain starting psi to target psi, and would not mind sharing that information.

Also, does anyone know how to calculate the force that backpressure puts on the crankshaft during the exhaust stroke.

 

[TurboTR]

Well the force part is pretty straightforward in concept, it's basically the area of the piston x the backpressure in the cyl to get the force generated; then that result has to be resolved through the mechanical linkage to find the tangential force on the crankshaft. Of course that mechanical linkage also changes with crankshaft position so is a function of crank angle too And so is the backpressure for that matter, it does not level off to a steady state value until the exh process has blown down. Taking cylinder pressure measurements on mine (with small, production based exh ports and valves), that blowdown can last all the way to about halfway into the exh stroke *gulp*. But is pretty much a linear looking curve, so you could model it as one pressure line for about half the exh stroke, then another with different slope for the latter half. Then you'd have to know what the starting pressure would be as the exh stroke begins, which if I recall on mine was maybe about 150-200 psi or so, roughly. Then by about halfway up the exh stroke it levels off to about 40-50 psi, whatever the steady state exh backpressure is at the time. Better set up a good spreadsheet if you want to calculate all this

IME you can change housings and see perf and response differences, but the backpressure measurement is complicated a bit because of the heat involved and attaching instruments, one, but also because it changes with engine speed. Does not sit at a constant value like wastegated boost does, but keeps rising.

There are many here who have FAST systems and probably data logs that show spoolup time. Better pick a particular combo you are interested in.

TurboTR

 

[VadersV6]

The greater the pressure differential between the 2 sides of the exhaust turbine (lower on the downpipe side of course), the quicker the spool up. Obviously you reach a point of diminishing returns if you go too far. Obviously, this pressure differential is dynamic. If you reached a 1:1 ratio, the turbine wouldnt spin.
You need backpressure before the turbine. Its just a matter of how much you want, for whatever given behavior you're looking for out of the turbo, at a given rpm.

 

[GNDriven]

Thanks for the insight,
I'm starting to realize the complication of my questions.
I just do not like making decision based on gut feelings.
I am looking to produce 650-700 flywheel horsepower.
I have champion ported iron heads and matching intake., 3in downpipie with a 218/218 hyd flat-tappet cam, Eastern front mount, Art Carr 3200.
The two turbos and exhaust housing combos that I am considering are LT 70Ptrim vs TE45A Qtrim, .6x housing vs .8x exhaust housing. I am looking for the most efficient match. I do not want to pick one just becaue "someone likes it alot" or it "pulls like a train" .While this might not be feasible I would like the decision to be based on some scietific or engineering concepts.

 

[VadersV6]

I'm sure Lou Czarnota would know what turbo you're looking for. Call Lou's Garage in Lake Forest, Ca.
He wont be able to work on your car, but he could offer solid advise.