Engine/Transmission (1998.5 - 2002) Fundamental turbo questions - Spool, Lag, Smoke

[R-N-R]

I tend to think of the turbo primarily in terms of EGT control, with a larger turbo providing more air (cooler), and more lag in spool up (takes longer to "pressurize" the housing???). A smaller provides faster spool but less air to burn with the fuel resulting in higher EGTs.



Doesn't the smoke clear up when boost increases? Why do the big HP trucks start smoking heavy when the turbo "lights"?



If you are able to reduce spool lag (AFE, EZ, Mad ECM, etc... ) why wouldn't you be inclined to use a larger turbo to gain the top end advantages? I understand rollng into the throttle as opposed to stabbing it, but doesn't this principle apply to all sizes of turbos?



It seems that if you are running higher hp (larger injectors/more fuel) then you need a larger turbo to keep EGTs in check, unfortunately this comes at the expense of spool lag.



Flipping the coin over, how about using a smaller turbo to get faster spool up with a water/meth injection system to help reduce EGTs?



Sorry for the rambling but I am (obviously) confused... Would anybody care to try and explain the basic spool/smoke/HP relationships of the turbo (in simple terms if possible)?



TIA,

Bill

 

[Beast2B]

I think the magic answer is twins! I'll wait to read some wise words, too, but I think the broad picture of what you describe is accurate.

small turbo = fast spool, less EGT control

big turbo = slower spool, better EGT control

twins = better in both categories, not sure of the downside (other than cost!)

 

[R-N-R]

Twins! I absolutely agree, that seems to be the solution. Unfortunately I need to find an intermediate solution before I am ready for that step. I have been curious about the boost/smoke thing though, especially with the high HP trucks "lighting" the turbo.

 

[Beast2B]

My measly single B1 does what I would expect - if I get on it with low boost, it smokes like a freight train but clears up as boost comes on. Worse (or better ) at higher levels on Comp. Seems to follow the simple equations above...

 

[R-N-R]

I think I am probably screwed for finding a "good" solution now that I will be able to use on top of a twin setup later.



The vendors that I spoke with all recommended twins for my application. Perhaps someday, but for now I have been considering the HTT Hybrid, PDRHX40, B1-2, and the HTB2. I have read too many bad things about the HX failures (not necessarily the fault of the product), the HT2B is still failrly new and the price change did not amuse me either. I am under the impression that the B1 series is a bit "big" for my setup. Kinda puts me back to square one...





... still hoping one of our gurus can help explain the spool/smoke thing...

 

[Beast2B]

The Piers HX40 is used as one of the turbos in his twin setup ( http://www.piersdiesel.com/DodgeTwins.htm ). If twins are in the future, go for it! I know there are several options for the B1's also depending on truck, trans, elevation... With the DTT and your other mods, I'd think the B1-2 would work, lower EGT than KwikSpool, but less lag than the full on B1. Check with Piers or track down KWIKKURT here (he helped me decide and was great before/during/after the purchase)

 

[KLockliear]

I tend to think of the turbo primarily in terms of EGT control, with a larger turbo providing more air (cooler), and more lag in spool up (takes longer to "pressurize" the housing???). A smaller provides faster spool but less air to burn with the fuel resulting in higher EGTs.

Yes, but remember that bigger turbos use bigger "wheels" - which means more inertia to accelerate, and usually more aggressive trim(for the sake of conversation lets think of it as pitch - like a propeller). More pitch, more time to spin it up too.

Doesn't the smoke clear up when boost increases? Why do the big HP trucks start smoking heavy when the turbo "lights"

As boost comes up, the aneroid in a P-pump truck, or the computer in an electronic truck, will add more fuel.

It seems that if you are running higher hp (larger injectors/more fuel) then you need a larger turbo to keep EGTs in check, unfortunately this comes at the expense of spool lag.

Yes... much like a cam in a gas motor - you take from the bottom to give to the top. One thing that is a win-win (other than the cost) is a good cam. Quicker spool, more air so less smoke and EGT, and some mileage.

 

[Hohn]

I tend to think of the turbo primarily in terms of EGT control, with a larger turbo providing more air (cooler), and more lag in spool up (takes longer to "pressurize" the housing???). A smaller provides faster spool but less air to burn with the fuel resulting in higher EGTs.

Doesn't the smoke clear up when boost increases? Why do the big HP trucks start smoking heavy when the turbo "lights"?

If you are able to reduce spool lag (AFE, EZ, Mad ECM, etc... ) why wouldn't you be inclined to use a larger turbo to gain the top end advantages? I understand rollng into the throttle as opposed to stabbing it, but doesn't this principle apply to all sizes of turbos?

It seems that if you are running higher hp (larger injectors/more fuel) then you need a larger turbo to keep EGTs in check, unfortunately this comes at the expense of spool lag.

Flipping the coin over, how about using a smaller turbo to get faster spool up with a water/meth injection system to help reduce EGTs?

Sorry for the rambling but I am (obviously) confused... Would anybody care to try and explain the basic spool/smoke/HP relationships of the turbo (in simple terms if possible)?

TIA,
Bill

There are a couple of variables involved in your question:
1) turbo size, which means a) compressor size b) turbine size c) turbine HOUSING size

2) overall airflow capability

3) EGT, which is a function of airflow and fueling

4) Fueling

A turbo is an inheren't compromise because it can ONLY be optimized for a given circumstance of fueling, engine load, RPM, etc. If we didn't have variable engine load, variable fueling, variable RPM, etc then designing the "perfect" turbo would be easy.

Since it's ALWAYS going to be a compromise, we have to choose what KIND of compromise we can live with. Do we want a narrower useful operating envelope with more peak performance within it? Or do we want a broader operating envelope that will yield less power, but a more useful RANGE of power?

If it's a daily driver, you'll want to favor a broad useful envelope that sacrifices peak HP. YOUR usage profile and truck options will determine where you draw the lines of spoolup and EGT control. Do you tow? How heavy? Auto trans or stick? How tight is the converter? On and on and on.

The same restriction that kills top end HP and EGT control is what is responsible for rapid spoolup.

One more thing to think about-- How much boost do you need? As far as I'm concerned, boost is a useless number in itself. Boost pressure does NOT determine airflow by itself! If we have 40psi of boost and 60psi backpressure, would this even support 200hp? Probably not-- there's too little airflow.

There are MANY examples of HP increasing when boost is LOWERED. Same for EGTs-- many times you will see LOWER EGTs from LOWERING boost pressure.

It's all about the airflow. The stock turbine housing is too small, even for stock fueling! Now we come along and add 100hp and wonder if the turbo is the right size? IMHO, even modest HP increases warrant the change to a 14cm housing. More fuel means 16cm housing.

Keep in mind that we want to stay focused on OVERALL AIRFLOW. So if you have a large wastegate that can flow easily, then you can be very flexible with your turbine sizing. Non-wg housings must be matched more carefully. 16 for sticks, 14 for autos is usually a good starting point. If you need more than a 16 housing can give you, you should probably consider a larger turbo or 35/40 hybrid.

Keep in mind that a larger compressor needs larger turbine side A/R (or high-flow WG) to match its higher flow capacity.

Justin

 

[R-N-R]

Justin,

Thank you for the awesome explantation! Please indulge me with a few more more questions.



How is back pressure determine (what is it?) and how does it effect turbo choice?

How does the torque converter factor into the turbo equation?



Thank you again I appreciate your response,

Bill

 

[Jim Fulmer]

... still hoping one of our gurus can help explain the spool/smoke thing...



With a 12V it's a balance act, if you run a big pump and big injectors like me then you have to hold off the max fuel till there is enough boost or it will lag more or in some cases almost blow the fire out, on my truck there is quite a bit of fuel from the onset but flows much, much more when at higher boost sets in, by doing this the chargers actually light faster and hit harder and all of this is in the settings as the foot is still in the same place... ... ... on the wood.



Jim

 

[ESorensen]

I agree with the compromise explained earlier. However, I believe you will soon see a large variable geometry turbo available to help narrow the compromise window. At low boost, it will act as a "small" compressor while at higher boost pressures, it will perform as a "large" turbo.

 

[KYLE4]

I got one for the bd power super b turbo it was designed by bd's engineers it has a small impeller but the fins on the impeller have been extended so you get small turbo spool with large turbo volume this will support 450hp. This turbo is also offered in a twin setup from bd and you can later buy another and the piping to have a twin setup. Its a great setup for everyday use not everybody wants to spend 4500 on a set of twins

 

[Beast2B]

Yeah, I can't wait to see the variable stuff, should be a great way to narrow the gap between lag and response.

Now that folks (BD and others?) are customizing/building parts instead of just mixing and matching existing parts we should begin to see more dedicated applications as well as more options for configuring turbos to a users needs.

 

[Hohn]

Justin,
Thank you for the awesome explantation! Please indulge me with a few more more questions.

How is back pressure determine (what is it?) and how does it effect turbo choice?
How does the torque converter factor into the turbo equation?

Thank you again I appreciate your response,
Bill

Bill--

Backpressure is simply ALL the work the engine has to do to push the exhaust gases out of the engine. So you can lower backpressure a million ways-- larger exhaust valve, more valve lift (larger cam), enlarged exhaust ports, larger manifold passages, larger turbo, larger exhaust pipe, etc...

Backpressure is a necessary evil on a turbo engine. This is because if the turbo doesn't cause a restriction of some kind in the exhaust, it can't extract any work from the exhaust. No backpressure, no turbo boost.

A comparable situation would be how the fuel economy of an engine lowers as we make it do more work. We could get GREAT "economy" if it wasn't doing any work, but then why would we even have the engine if it wasn't giving us any work? An idling engine uses very little fuel, but because you aren't moving it's giving you ZERO MPG.

So we want to balance the turbo's restriction to where it it pulling JUST ENOUGH energy out of the exhaust stream to do it's job. Too little, and we won't have enough boost and we get high EGTs. Too much, and we are restricting the airflow out of the engine too much, and we get high EGT.

So you can see that there's a whole spectrum ranging from very-little restriction to a LOT of restriction. On the "loose" end, you'll have very laggy turbo response and the possibility of lots of smoke, but if driven to accomodate that, it should give you better mpg and lower PEAK egt, even if SUSTAINED egt is higher.

Dodge tried this on the early 1st-gen trucks with the 18cm and sometime 21 cm housings. These flowed a LOT of air, but had poor driveability and felt gutless.

On the other extreme, Dodge has tried the HY35-9 with its 9cm housing which is VERY tight. Spoolup was almost instant, but airflow suffered badly and peak EGT can get VERY high when hp is uprated.

It's also interesting to note that Dodge fitted 18cm housings to engines rated only 160hp, but when ratings went up to 235-245hp, then switched to the smaller 12cm and 9cm (01-02 auto trans trucks) housings.

This is actually the OPPOSITE of what you would expect, because more HP means more fuel, and that means you need more airfllow to burn it, so the high hp engines should have LARGER turbine housings.

Why is this so messed up? EMISSIONS. The best way to reduce particulate emissions (soot/smoke) is to have rapid spoolup so that the engine almost NEVER runs in a rich condition.

So, Dodge/Cummins designs the stock turbos to run as close as they can to the "tight" side of the spectrum and still control EGT AT STOCK FUELING. This is important. The stock housing is designed to provide 21psi of boost AT STOCK FUELING. So if you come along and increase the fueling (hp) of your engine, then the turbo has gone from being "just big enough" to "way too small".

Torque converters on an auto transmission have a HUGE impact on selecting the proper turbo because they determine HOW the engine is loaded and WHEN.

With a stick, you only have a clutch (on-off basically) and 5 or 6 gears. Not much in between.

With the auto, you have an infinitely variable loading situation for each of the 4 gears.

The factory converter is very "loose" to the engine won't see much loading right away. If you replace it with a "tight" aftermarket converter, then the engine WILL see MUCH higher loads at much lower RPM. This means you'll need a turbo that spools sooner to ensure you can burn the fuel at lower RPM. It also means that the turbo will run out of steam on the top end.

It's VERY difficult (almost impossible) to perfectly match turbo and converter because as you change the turbo, you change the tq delivery at lower rpm, and this changes the stall speed of the converter that you're trying to match.

So you're basically aiming at a moving target-- one that moves the closer you get.

All that to say that turbo selection is even more critical on an AUTO trans truck than on a stick.

I'd suggest you start first at fueling level (hp) that you want.
THEN, determine how much air you need to burn that fuel.
THEN, determine whether you want spoolup or top end performance more
THEN, match converter to turbo. The tighter the turbo you run, the tighter the converter you can run.

Justin

 

[jjohn]

Nice explanations Hohn,



Bill,



Comparing your set up to mine and not knowing what % TC you have…



('02 2500, 2WD Auto 4. 10, DTT full upgrade, Turnbuckle; Drag Comp; DDII)



Mine (02 2500 4wd Auto 3. 55, DTT full w/ 89H TC, modified HX3514cm waste gated, Comp, EDM M1. 5)



I would say you would do well with a HX40 or equivalent.

I have a modified HX35 14cm at 5000 feet and up. I am very happy with spool up and top end egt’s for my use. No towing to speak of.



My neighbor has the same Comp box, DD2’s and 89H DTT. He tows 10K range in the mountains with a Piers HX40 16cm. He is a little smokier off the line... but not bad. He loves it.



Assuming you are at a much lower altitude (650 feet range)-more air, and less weight 2wd, 4. 10 gears- more freedom to spool up, Drag Comp and DD3’s-more fuel, I feel HX40 with a 16cm or equivalent would be the ticket.



JJ

 

[R-N-R]

Thank you Keith and Jim, your input is greatly appreciated. Justin, your more than excellent explanations of what back pressure is and how torque converters affect turbo selection adds considerable clarity to why selecting the "best" turbo per profile can be difficult, thank you again.

I'd suggest you start first at fueling level (hp) that you want.

In Aug of '03 I dyno'd 460/1050 on a mustang by a reputable company. However I have to believe that something was misconfigured as these numbers seem too high for my setup. Hopefully the results were at least in the ballpark, so I would like to shoot for the 500rwhp club. This adds to my dilemma as the turbos seem to be rated up to 450hp in the "smaller" group and over 550hp in the "larger" size group. I am concerned about pushing the HX40 arena too hard, but not having enough fuel (hp) for the B* level. I expect that this is one area where twins fit into quite nicely, however once again I am not ready for that step yet. Working in a Info. Tech. career leaves me not too concerned about "vapor products" as there is always something better "coming out". This is not a slam, just in today's fast paced and very competitive world, products are being unveiled with exponential regularity.

THEN, determine how much air you need to burn that fuel.

If I am understanding correctly, this seems to be one of the more elusive answers.

THEN, determine whether you want spoolup or top end performance more

On the top end my primary concern is containing EGTs while playing, more so than dyno numbers. Since play time is limited, I am leaning toward spool up as the priority.

THEN, match converter to turbo. The tighter the turbo you run, the tighter the converter you can run.

Hopefully I can reverse engineer this one as I would rather not replace the torque converter at this point.



JJ,

I have the 91% TC. Thank you for the recommedation, putting things in descriptive terms makes it easy for me to relate with. What are your thoughts on the 35/40 hybrid?





The fog is lifting, but very slowly it seems... .

Boost clears up smoke, but boost also allows more fuel that creates more smoke...

Tighter torque converter allows for a tighter turbo. Tighter turbo meaning Faster spool? Smaller housing?

Larger turbo (compressor/housing) reduce EGTs, but can also increase EGTs...



I seriously appreciate everybody's input and envy those of you that have a solid grasp of turbo dynamics. While I am not out to beat this to death, I would much rather make an informed decision than throw mud at the wall to see what sticks... . I did not start this thread with "help me select a turbo" in mind, however that is certainly what I am doing so I hope these questions and answers are following the "Fundementals... " theme of the thread topic and of course that the thread helps others as well.



Bill

 

[Beast2B]

Bill, that whole dilemma was one of the reasons I chose the B1-2. According to the shop I purchased it from, it's an easy change to a full B1 housing if desired, but the -2 housing is a little tighter so spoolup is sooner... That and the longevity we mentioned in the PM's made me feel that the benefits outweighed the cost. You've inspired me to complete a fuel system and get a dyno run soon...

My one further (group of) questions is this: If I move up to the standard B1 exhaust housing I will get more lag, correct? Does that also equate to less drive pressure? ... and what will that do to my boost (I can peg my current 35lb gauge and then some)? ... or am I better off with the current housing and adding a wastegate?

 

[jjohn]

JJ,

I have the 91% TC. Thank you for the recommedation, putting things in descriptive terms makes it easy for me to relate with. What are your thoughts on the 35/40 hybrid?

Bill

Talking with the few guys i've meet who run them at Dyno day events… I never heard they perform significantly better than of a good HY35. That is my very limited exposure to them. I wish I knew more about them myself, Just no real first hand knowledge, sorry.

 

[Hohn]

Crap! Lost my lengthy post!


Anyway, B2B:

If you have the B1-2, then it's the full sized housing and smaller wheel. The small housing/bigwheel combo is the KWIKSPOOL B1. Within wheel sizes, you can swap housings easily (Full B1 to Kwikspool, and B1B [AKA B1-2] to KSB1-B Lightning [AKA KSB1-2]. Swapping one wheel size to another isn't practical, but it can be done with $$ and machine work.

I'd suggest going to an external gate if you want to relieve backpressure. A Tial 40mm should be big enough. I'd just machine a spacer for the turbo and mount the gate to that.

JLH

 

[Beast2B]

Thanks for the clarification Justin. I think the wastegate may be the way, I see my EGT stay pretty calm until my boost goes over 35psi- then it climbs more rapidly.