Thanks for all the great info guys i really appreciate it. Is there such thing as a wastegated 1.15?

 

The picture Roland posted was the front of the housing and the one I posted was the back. You won't have one with out the other and all 38R's have a ported housing on them.

 

Gotcha. It just didn't look like his was ported but I couldn't tell from the pic.

 

From the viewpoint of a "turbo" the "diesel engine" it's attached to performs two main functions... (1) the engine resists the inflow of intake air and this makes the "compressor" work at a given "pressure ratio" which in turn causes a given load on the turbine's output shaft ...and (2) the engine combusts fuel which provides enough "waste heat" in the exhaust gas to power the turbine and supply the needs of the compressor.

Since the volume airflow through an engine is equal to one-half of its swept displacement volume for each revolution of its crankshaft the engine provides a resistance to intake airflow that's inversely proportional to the engine's RPM and this means producing a given intake airflow at a lower engine RPM requires the compressor work at a higher pressure ratio than would be needed to provide the same airflow at a higher engine RPM.

I'll give a basic explanation of "compressor surge" by using a "shop vacuum" analogy where the shop vacuum inlet corresponds to the compressor inlet and the shop vacuum blower fan corresponds to the compressor wheel and the shop vacuum canister corresponds to the compressor housing and the shop vacuum motor corresponds to the turbine.

If you start a shop vacuum and place your hand over part of the exhaust port you'll feel some pressure build up in the canister and the amount you restrict the exhaust port corresponds to the amount of resistance to intake airflow provided by the engine at a given RPM. As you progressively restrict the exhaust port with your hand you produce more "boost pressure" in the canister and the amount of airflow diminishes and eventually you reach a point of "maximum" boost pressure and "minimum" airflow and then the blower fan "stalls" and you note a change in pitch of the blower motor as its load is reduced.

As depicted below a stall condition is caused by a "disruption" of the airflow-blade angle geometry...



...and the graph below indicates that there's a tradeoff between airflow and pressure similar to what I described above.



Now compare the shape (pressure vs airflow) of the fan characteristic curve above with the shape (CPR vs MAF) of the GTP38R compressor wheel "speed curves" below at various compressor wheel rpm values and look in particular at the speed curve for 97,000 rpm which starts at the point CPR=1.9 & MAF=68 lbm/min and moves up and to the left to the point CPR=2.8 & MAF=53 lbm/min and then flattens out as it reaches the "surge line" at the point CPR=2.9 & MAF=37 lbm/min and note that this point is also where the "engine load line" for RPM=2000 intersects the surge line.



The above graph which is for the "standard reference" compressor inlet condition of 13.95 psia and 85*F allows any user of a GTP38R turbo to predict the operating conditions that will cause compressor surge and the example I just gave says the operating point CPR=2.9 & MAF=37 lbm/min defines the maximum possible load that can be placed on the engine at 2000 RPM without producing a "compressor stall"!

In general if you're operating at any combination of CPR & MAF that places you on the "surge line" and then apply some additional throttle (which produces more exhaust heat) or close the wastegate some by tightening the actuator rod (which produces more exhaust pressure) you increase the drive power to the compressor wheel and this increases the rpm of the compressor wheel and this pushes the operating point up vertically past the "surge line" to a higher rpm "speed curve" and this results in a "compressor stall" which allows a momentary back flow of boost air past the compressor blades. This back flow momentarily reduces the CPR enough to place the operating point back on the "surge line" but if you keep your foot on the throttle you cause repetitive stalls which is called surging!

 

What he ^ said....

 

Wow... that's awesome! Makes sense now just what exactly is happening. So, does all boil down to our driving habits or improper equipment? Also, are you saying that the reason it doesn't surge when it down shifts into 3rd is because it brings the turbo's ratio back to where it needs to work? I was reading that the H2E has a lower ratio... I'm wondering if it would behave like the 38R?

 

Folks, that is why manufacturesrs worth a crap post those efficiency charts. Design here has a bit of a trade off when seeking response, total power/flow, and surge and stall. Some designs are better than others. All are a compromise one way or another.

These charts will enable you to predict the unit's stall and surge envelop as well as total power they can produce if you are willing to do the math for the 7.3 or whichever application you are considering.

As for me, I did the math, scratched my head and then listened to Swamps and got my setup. Numbers are great but experience with the actual product is king.

BTW Gene, that was a good explanation. You are getting better at this. Hope you are doing well.

 

I'm assuming that... "the turbo's ratio" ...means the CPR=Compressor Pressure Ratio which is the vertical axis on the graph of any compressor map and the CPR is equal to... CPR=(AAP+BP+ICPD)/(AAP-AFPD) ...where AAP=Ambient Air Pressure psia, @ operating altitude, BP=Boost Pressure psig, ICPD=Intercooler Pressure Drop psi, and AFPD=Air Filter Pressure Drop psi ...and as you can see the definition of CPR doesn't depend on the characteristics of the turbo.

The MAF is the horizontal axis on the graph of any compressor map and the MAF is equal to... MAF={(VE)(CID)(RPM)(AAP+BP)}/{(MAT+459.67)(1,278.46)} lb/min ...where VE=Volumetric Efficiency a ratio from 0 to 1, CID={(Nc)(Pi/4)(Bore)^2(Stroke)} in^3, Nc=Number of Cylinders, RPM=Crankshaft revs/min, AAP=Atmospheric Air Pressure psia, BP=Boost Pressure psig, and MAT=Manifold Air Temperature *F ...and as you can see the definition of MAF doesn't depend on the characteristics of the turbo.

Since the definitions of CPR and MAF don't depend on the characteristics of the turbo why choose one particular turbo over another? This isn't a "trick question" and I'm not trying to embarrass anyone by asking it but rather I'm trying to get a "turbo discussion" going because changing out the stock GTP38 turbo is one of the most commonly talked about mods on FTE!

Here's a hint to kick off the discussion... "the H2E has a lower ratio" ...doesn't make sense because one reason to buy a H2E or GTP38R is because these turbos can operate at a higher value of CPR than is possible with a stock GTP38!

 

Wait, I think I've got it backwards... I want a higher ratio. In other words, MORE Boost to the back pressure. But you're telling me that it's characteristic of the engine? I'm not trying to be dumb, but you lost me a bit back there. Why would it surge in 4th while lugging it whereas bumping it down to 3rd will allow me to build more boost without surge... is it a difference in exhaust flow rather than exhaust pressure? Just trying to grasp this... I'd almost PM you and ask you to call me!

 

You guys & your fancy-pants turbos..... Never seen surge once with mine.

 

I think it's not so much a DIFFERENCE but rather an INTERACTION between pressure, engine rpm, and compressor rpm.
If you want 20 psi boost at 1500 rpm without any surge, then get a smaller turbo that will spool up a higher wheel rpm at a lower engine speed.
But that setup will quickly choke out at higher engine rpm because it's too small for the job at that point in time.
.
The ultimate setup with ideally be dual turbos. The smaller one would spool up fast, then as engine rpm/load comes up, the bigger one would start to take over where the smaller one could not keep up.

 

Exactly -- the smaller housing favors low end (spools fast), large housing favors top end (much lower backpressures). Mine has the 1.15 non-wastegated housing and it's a little laggy, yet moves enough air in the mid-range, and isn't nearly the restriction the .84 a/r housing was at the top end.

Any single turbo setup is a compromise -- compounds are the ultimate fix, like Dan said....

 

This is still confusing... I'll have 160cc SS's with 100% nozzles. I don't want a turbo too small that'll surge too easily, but I don't want one so big that it smokes like a coal plant until the RPM's come up either.

 

Great information and explanation, thanks for taking the time to post it.

 

Below I give some additional discussion of the "surge topic"...

...and at the end of this post I give a link to more than you probably want to read about the BP vs EBP topic!

If you used an "electric motor" to power the compressor shaft instead of an "exhaust driven turbine" you'd still get the exact same "compressor map" and the point CPR=2.9 & MAF=37 lbm/min that I gave in my previous example would still define the operating point at 2000 RPM that results in a "compressor stall"!

The CPR=(AAP+BP+ICPD)/(AAP-AFPD) and it's the "compressor" that generates the output pressure to produce a given BP which in turn results in a given CPR and the MAF={(VE)(CID)(RPM)(AAP+BP)}/{(MAT+460)(1,278.5)} lb/min and it's the "engine" that uses this BP at a given RPM to produce a given MAF! Since the compressor will stall at the exact same values of CPR & MAF no matter what's driving its input shaft the repetitive "compressor stalls" which is called surging don't have anything to do with the particular choice of "exhaust housing" per se!

To generate a given BP it takes a given CSHP=Compressor Shaft HP and this CSHP must be supplied by the turbine as an equal amount of TSHP=Turbine Shaft HP and it takes a particular combination of EGT & EBP to drive the turbine hard enough to generate the required TSHP because...

TSHP={(EGT+460)(1-TPR^-0.257)(1+1/AFR)(MAF)(TTE)}/(158.7) hp

...and if everything else remains the same the TPR=Turbine Pressure Ratio is determined by the EBP via... TPR=(AAP+EBP)/(AAP+PTPD) ...where AAP=Ambient Air Pressure, psia @ operating altitude, PTPD=Post Turbine Pressure Drop psig, and TTE=Turbo Turbine Efficiency % ...and the bottom-line is the above TSHP equation says that a higher EGT will produce a given BP with a lower EBP and vice versa that a lower EGT will produce a given BP with a higher EBP!

If you use a non-wastegated exhaust housing the choice of A/R determines the tradeoff between the EGT & EBP that's needed to drive the compressor hard enough to produce a given BP. Changing from a larger A/R to a smaller A/R is like closing the wastegate some on a wastegated housing and this has the net effect of increasing the EBP and decreasing the EGT that's needed to produce a given BP. Changing from a smaller A/R to a larger A/R is like opening the wastegate wider on a wastegated housing and this has the net effect of decreasing the EBP and increasing the EGT that's needed to produce a given BP.

At a given RPM the overall performance of a turbo-diesel depends on the particular combination of EGT & EBP that's employed to produce a given BP and having a wastegate allows for more flexibility in coming up with the most favorable combination of EGT & EBP which is the one that provides more FWHP and a higher efficiency for a given fuel flow.

Without a wastegate you have a fixed exhaust flow area and the EBP increases as a function of the MAF and you need to choose a flow area that's large enough to accommodate the maximum MAF desired and this approach can only be even near "optimum" at high RPM where the MAF is near its maximum value. With a fixed flow area there's no EGT vs EBP trade space available like there is with a wastegate.

If anyone's interested I can add more to this thread later after I find out why my email program keeps getting a "script error" that locks up the entire program and then requires using the "task manager" to shut down the program! In the meantime if anyone's interested in some additional discussion of how a turbo-diesel works here's a quote from this thread... Is a wastegated turbo really required? - TheDieselGarage.com ...which I started contributing to in post #6 and if you get around to reading some of my explanations there for how a turbo works let me know if they're any easier to understand than the ones I gave on FTE?