HTB2/64 vs. HTB2/66?

[TX Gooseneck]

I've currently got the 64 on my truck. I'm not having any of the problems with surging and the truck runs fine (my impeller is clipped, not sure if that makes a difference or not). Anyway, I'm going to be taking care of my fuel shortage issues with a FASS and putting another box on and was curious as to the benefits/issues with going to the bigger unit.

 

[JStieger]

... was curious as to the benefits/issues with going to the bigger unit.

Benefits: more top end cooling, more top end HP



Issues: possible surging, more low end smoke (=fuel $$)



The presence of surging depends on how you drive the truck and how you're truck is set up. The way I drive I'm kind of a lazy shifter and rely on the low rpm grunt of the Cummins to help pull me out of certain driving situations. In this case, surging occurs for me.



However, if you drive the truck like you stole it with the rpm over 2,000 or so all the time then you probably will not see the surging as much since the engine is spinning enough rpm to adequately consume the air volume that the turbo is generating and you are just "powering through" the surging if it was present at all.



So picture yourself in this scenario owning a 66 turbo. You're on a hill towing a monster load like in your reader's rig gallery. A guy passes you in a little car since he had a 10-mile run for this hill ( :-laf ). As you get 2/3 up this hill the guy in front starts slowing down and wishing he had a Cummins. You know you're nearing the top of the hill so you don't want to pass so you just let the rpms fall to say 1800 to match his speed to keep a safe following distance. The guy crests the hill and then you start rolling back into the throttle. All of a sudden you start feeling a shudder in the steering wheel with a cha, cha, cha, woof, woof sound.



Well this is what happened to me with a car trailer and jeep so imagine how much more boost and air volume the turbo would be generating with your typical loads and the fact that the CTD can only consume so much of this air volume at 1800 rpm.



Here's some links to help you decide:



https://www.turbodieselregister.com/forums/showthread.php?t=146186



http://www.dieseltruckresource.com/dev/showthread.php?t=84055



As you can see in the links above, I admit when I first got the 66 and was towing I didn't experience the surging. Then when I started towing around the cities or states with lower speed limits and higher traffic was when the surging started occurring.



You can always try it; these turbos are easy enough to swap out!

 

[KevinO]

i've got the 64/ clipped wheel and i can surge the turbo with no load if i let the auto shift at the wrong points. I have the old twins program now but before i had the old standard program it never did it but it also dyno'd at 417 hp with exact setup now with the twins program i dyno'd the other day at 491 hp.

 

[hasselbach]

Benefits: more top end cooling, more top end HP



Issues: possible surging, more low end smoke (=fuel $$)





However, if you drive the truck like you stole it with the rpm over 2,000 or so all the time then you probably will not see the surging as much since the engine is spinning enough rpm to adequately consume the air volume that the turbo is generating and you are just "powering through" the surging if it was present at all.

They surge not because the engine is running too slow to consume the air they are producing, but more of the fact that exhaust housing is too large to turn the heat from the exhaust into rotational energy on the intake compressor side. Causing a shudder as the exhaust impeller loads up against the flow of the gas, but stalls due to insufficient flow to drive the intake side.



I can make mine surge (woof woof) even at 1 or 2 psi of boost. So its not that the engine isn't consuming air. (btw, what the engine doesn't consume is shown at positive boost).



Add more fuel at the lower end of the rpm scale, or drive the turbo with a smaller exhaust housing and the surge goes away albiet higher exhaust temps on the high side.

 

[Forrest Nearing]

it is that the engine isn't consuming the air!



the engine can't take the air, so the air stalls, and like putting your hand over a vacuum cleaner hose, the compressor speeds up very fast and then slows back down really fast... and back and forth...



a larger turbine side will alleviate surging because the compressor will turn slower...



smaller tubine side will increase surging problems because the compressor is turning faster at a given load/rpm

 

[TX Gooseneck]

Ahhh... I experience the turbo bark on mine, sometimes even do it on purpose (in those cases when a riceboy is next to me with windows down, I'll leave the throttle on the floor while in 6th at 45mph and lift. Loud enough to make them slow down)



J, I don't pull anywhere near that heavy with this truck anymore. It gets a 1 or max 2 trailer load on short runs. I've only put 75k on it this year

 

[hasselbach]

it is that the engine isn't consuming the air!



the engine can't take the air, so the air stalls, and like putting your hand over a vacuum cleaner hose, the compressor speeds up very fast and then slows back down really fast... and back and forth...



a larger turbine side will alleviate surging because the compressor will turn slower...



smaller tubine side will increase surging problems because the compressor is turning faster at a given load/rpm

Nope, not the same as a vacuum cleaner. When you put your had over the vacuum cleaner, the motor speeds up because its not moving air (vacuum) and the impeller is basically cavitating. In a pure vacuum, the motor would rev freely without any load on it.



The turbo is stalling because of the load on the intake side of the compresser is too much for the exhaust side to power due to having a overly large exhaust housing (inefficient at low rpm flow) and when the intake side 'bites' the air, it stalls the turbo since the exhaust side doesn't have the torque or efficiency to drive it. Add more flow (heat) and the exhaust side will then power the intake compressor. Put a smaller exhaust housing on it (more drive at the same flow of exhaust, and the stall goes away. But you risk higher EGT's on the upper end.

 

[hasselbach]

Ahhh... I experience the turbo bark on mine, sometimes even do it on purpose (in those cases when a riceboy is next to me with windows down, I'll leave the throttle on the floor while in 6th at 45mph and lift. Loud enough to make them slow down)



J, I don't pull anywhere near that heavy with this truck anymore. It gets a 1 or max 2 trailer load on short runs. I've only put 75k on it this year

Bark and surge are two entirely different things. Bark is basically reversion in the compressed side of the intake track trying to flow backward when the exhaust drive pressure is reduced significantly (intake flow post turbo flows backward against the impeller, trying to stop the impeller. )



Surge is compresser cavitation caused by the load on the intake side overcoming the drive of the exhaust side, causing it to shudder as the exhaust flow isn't enough to drive the intake side.

 

[Cooker]

Bark and surge are two entirely different things.

It is my understanding that compressor surge and a "turbo bark" are the same phenomena.

 

[hasselbach]

It is my understanding that compressor surge and a "turbo bark" are the same phenomena.

Well, from what I have witnessed and been told. .



Bark is when you lift off the throttle quickly,,



Surge occurs at part throttle thoughout a specific rpm range. .

 

[PC12Driver]

Surge is compresser cavitation caused by the load on the intake side overcoming the drive of the exhaust side, causing it to shudder as the exhaust flow isn't enough to drive the intake side.

If the engine was sucking on the compressor, there would be a vacuum in the intake. The guys that get surge from large compressors are getting it while they are showing boost on the gauge, so it's not from the engine "pulling" on the compressor.



"Surge" and "bark" are the same thing. They result when the compressor pressure is too high for the current flowrate (the bottom side of the MAP). Basically, the turbo is trying to feed more air to the engine than it can accept. The angle of attack on the compressor vanes is exceeded, the airflow separates from the vanes, the compressor stalls (the wheel doesn't stop-the air just isn't "sticking" to it any more), and the airflow through the compressor is briefly reversed (again, just the airflow, not the wheel). Once the pressure drops the air "sticks" back to the vanes, pressure builds, repeat. Above a certain rpm the engine can accept the volume of air and the surging stops.



So with that in mind, it is too small of an exhaust housing that causes surge. The smaller housings will drive the compressor harder earlier in the rpm range, thus running it past the surge line of the MAP (like JStieger's scenereo above). A larger housing will slow down the compressor, allowing the engine to have more rpm (and able to move a greater volume of air) for a given psi of boost, thus minimizing/eliminating surge. Have you noticed that it's the fellows running the larger compressors (62mm and up HTBs for example) with the 12cm housings that are having surge issues, not so much the guys with the 14cm or larger housings?



The same conditions are what cause "bark". Turbo "bark" happens when there's a sudden drop in rpms while the turbo is still spinning hard due to its inertia. Same as above; the pressures are too great for the volume of air being moved, the compressor stalls, airflow reverses in the compressor until the pressure drops to a point where the airflow can re-attach to the vanes, repeat.



Forrest has the right idea with the vaccum cleaner; but instead of covering the intake, surging would be the equivalent of covering the outlet.



And fwiw, cavitation occurs when liquid is suddenly converted to a gas due to a sharp decrease in pressure. That would be an extreme form of water injection if we have cavitation in our turbos...