Engine/Transmission (1998.5 - 2002) Boost Pressures?

[DLeno]

Hey this is good stuff. If I may, I'd like to ask some follow-up questions:

Originally posted by ynot

Changing turbine housings ALWAYS changes turbo efficiency and/ or pressures, even to the point of similar numbers. A 16cm wastegated and plugged will make different boost and spool than a 16cm non-wastegated.

Yes, I understand that the housing change will change the efficiency map. And, BTW, I appreciate the clarification though with respect to compressor efficiency as a specific term. But how does plugged wastegate housing behave differently from a non-wastegaged housing of the same geometry? The presence of the wastegate itself must affect the internal airflow.

Everyone changing wheels, face pitch, leading edge or role pitch, bleeds, etc. are trying to change the MAP range and pressure AT A GIVEN RPM (30psi at 48,000rpm instead of 62,000rpm or whatever). That will cause a slight MAP change, but does it matter? Turbo MAP efficiency is nothing more than keeping the turbo below a certain RPM, as to not make more compressor heat than boost.

. Or cause things to fly apart, right? I think a clarification of terms might be in order here: Given that the acronym, “MAP” refers to Manifold Air Pressure, are you using the term “MAP efficiency” to refer to a pressure versus efficiency curve of the turbo itself? I’m using the term “efficiency map” in a more general sense to refer to the turbo operating region defined by optimal ranges for three parameters: airflow (cfm), MAP or boost(lbs), and the resultant turbo efficiency(%). Since these three are interrelated (which is what the efficiency map shows), I was suggesting that there might be a cfm related issue in the form of higher EGTs as a consequence of operating a smaller turbo at a given boost pressure ("outside" of its efficiency map) versus operating a larger turbo at the same boost pressure ("within" its efficiency map). that is, the higher EGTs would result, not (necessarily) from the "inefficient" turbo itself contributing heat, but from the air flow problem that results from high boost pressures combined with low cfm air flow. Perhaps reference to the “efficiency map” in this context and for that purpose is not very technically accurate. I believe I am being consistent with the numbers you gave below, but I would value some expert feedback on this analysis to check my understanding, however.

Super heating the turbo takes running over the MAP for so long that the ATAAC "heat soaks" and ceases to cool inlet air. Has no real detrimental affect on the turbo itself at all.

ok, I think I follow you here. By “inlet air” you mean “inlet to the ATAAC/intercooler itself”? By “running over the MAP” do you mean “operate outside the optimal pressure-efficiency operating region”? Or what do you mean by MAP here? As for heat soaking, any time the turbocharger delivers air that is too hot, the intercooler may not be able to cool things down enough. However, from a thermodynamics standpoint, I note that: (1) the greatest rate of energy transfer occurs when the temperature difference is the greatest. The hotter the air coming into the intercooler, the greater the temperature difference between this hot air and the air crossing the intercooler fins. Therefore the intercooler itself will (under steady state conditions) never be warmer than the air coming from the turbo and so will never add energy as a heat soaking condition would -- at least as I understand the term. (2) if the intercooler itself is indeed hotter than the air coming from the turbocharger then probably two things are the case – (a) the turbocharger is no longer running hot (you just stopped pulling hard), and so (b) the condition will not last very long.



To be sure, there is a thermal resistance (case to ambient) issue for the intercooler – it can only dissipate energy at a certain maximum rate. So indeed there could be a condition where the intercooler can’t keep up, and the end result is that air entering the intake is too hot. So if that’s what you mean by heat soak (dumping energy into the intercooler faster than it can dissipate it) then I see your point.

If it takes 50psi+ or whatever to role down the road for extended periods, it may be time to reconfigure more than the turbo anyway. The only thing that really kills a turbo is so many RPM the wheel splits (105,000rpm- 120,000rpm+), a balance problem or shaft/ bearing coking in which the turbine pushes too hard trying to spin the compressor, thereby twisting the shaft.

ok, then in your above statement, “… no real detrimental affect on the turbo itself at all” you must be excluding high turbine rpm’s and excessive temperatures, either of which could be (but not necessarily are) a consequence of operating outside the efficiency map. That is, it isn’t the inefficiency itself that is problematic, but bearings at 500 degrees and a turbine at 120,000 rpm might be. Either of these conditions could result from “operating outside the efficiency map”, for example if you push an HY35 to 50 lbs.

I believe the original reference was to changing turbine housings. End result is about 2- 2. 2psi drop per size. 30psi with a stock 12cm would be 25+ with a 16cm, with a 200degree (or so) correlating drop per size (hot side) in EGT's over 30psi boost or 1000*(i. e. 1350* would then be 950*/ 1000* hot side)... T

If I interpret these numbers correctly, the 16 cm housing on an HX35 will result in lower boost pressures and lower EGTs for the same engine horsepower output. That is, HP depends on airflow and fuel, not boost pressure and fuel, except to the extent that pressure drives air flow of course. The 16 cm housing, then, allows greater air flow at lower boost pressures, thus allowing greater engine horsepower at lower EGTs while staying inside the turbo efficiency map.



does the larger housing cause a slower spool-up?



thanks for indulging my questions -- doug

 

[Diesel Freak]

The term "Map" with regards to a turbo charger is not an acronym. it is an actual "map" with efficiency islands, compressor RPM, mass flow rate, and pressure ratio.

 

[Diesel Freak]

does the larger housing cause a slower spool-up?

Yes!



This is a turbo-compressor map. In this case it is for a T-66.

The 16 cm housing, then, allows greater air flow at lower boost pressures

Changing the exhaust housing on this turbo (or any turbo) will not change the mass flow rate of the air going through a turbo at the same engine RPM, turbo RPM and Pressure ratio. (disregarding intake and exhaust valve overlap).



The ONLY time you can get greater air flow at lower boost pressures is if the air entering the engine at a lower pressure is cooler than the air at the higher pressure. (read "better intercooling")

 

[DLeno]

Originally posted by Diesel Freak

Changing the exhaust housing on this turbo (or any turbo) will not change the mass flow rate of the air going through a turbo at the same engine RPM, turbo RPM and Pressure ratio. (disregarding intake and exhaust valve overlap).

well that makes sense, but what I'm trying to get a handle on is the actual performance changes and the effect of changing one of several critical design parameters of the unit. that is, at the same engine RPM (and a larger housing installed) I doubt that the turbo RPM and pressure ratio will stay the same. So in practicality, the mass flow behavior (over the engine RPM range) will change because the operating parameters of the turbo have changed. We've talked about the potential effect of boost (lower) and EGTs (lower) but not about the HP versus RPM performance of the engine with the different turbo.

the ONLY time you can get greater air flow at lower boost pressures is if the air entering the engine at a lower pressure is cooler than the air at the higher pressure. (read "better intercooling")

ok, pardon my question, but doesn't the larger housing reduce the "back" pressure seen by the exhaust manifold and thereby accomplish better air flow through the engine? that aside, it seems that the larger housing would result in a lower turbine rpm and lower boost (for a given engine RPM) which would shift the HP peak to a higher RPM. Thus, the result of putting the 16cm housing on an HX-35 would be a higher usable HP potential due to the lower EGTs, but in order to realize this higher usable HP, the engine would have to rev higher. for a 6-speed especially, my guess is that such a turbo would be pretty doggy around town but good on a long hard pull.