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LB7 Turbo on an LLY

17809 Views 60 Replies 11 Participants Last post by  killerbee
Thanks Idahofox and Rick for the info on the LB7 turbo on the LLY.

LB7 on LLY This is the kind of thing we should be talking about.
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The IHI <> VVT swap should be interesting. Rick and I both expect a performance drop since the VVT does generate more/better boost across the rev range.

FWIW the VVT generates huge amounts of drive pressure compared to the IHI. For instance, at 20 PSI of boost, the IHI drive pressure is about 25 PSI and the VVT is over 40 PSI. At 30 PSI of boost, the IHI is around 40 PSI but the VVT is close to 70 PSI.

On the 6.6 Dmax, each PSI of backpressure costs about 2 HP in strictly mechanical terms. So by reducing the VVT's drive pressure to be 1/2 way closer to the IHI you could easily find 30 HP. That 30 HP, BTW, is equiv to about 1200 BTU/min.

That's all the theory. I'm looking forward to the results from the swap. It will give me confidence to push forward with the vane redesign if it works.
Engineering is as much as art as it is a science. You can exhaustively go over a problem and "engineer" a solution that fails because of unaccounted for variances in the application. Happens all the time. Knowing what to ignore and what to pay special attention to is the mark of a "good" engineer. Experience helps, but a humble attitude goes further.

Having said that, there is no substitute for engineering when it comes to Cheaper, Faster, Better improvements. You have to have a grasp of the numbers to see which way a design is going, or can go. Otherwise, you are left stumbling in the dark. But as the saying goes: "Even a blind squirrel stumbles across a nut once in a while" and some people get lucky in their attempts.

ME? I'm not an engineer. I just like to make my truck run better. :D
TxChristopher said:
All true, except the design of the head still comes into play as you must take into account the cylinder firing pressure.

Work out the mechanical HP to pump 3.3 liters of gas per revolution against a given pressure. THAT is the minimum HP the backpressure is costing in purely mechanical terms. That does not account for all the other things like scavanging of the chamber or the fact that the pressure in the chamber is alway higher than in the manifold. I used 2800 RPM as a base RPM.
TheBac said:
Jon, you have my apologies if I spoke out of turn on your experiment. I think its the best idea I've heard in a long time, and couldn't wait to tell the membership.
No problem Tom. I don't do it for the fame anyway.

This wasn't possible prior to tools like EFI Live, even though I thought of it long ago.
Diesel Tech said:

How about the internal EGR affect?
I called it scavanging, but the same thing.

Also, I would like to correct the pure mechanical HP loss at 2800 is 1.4239 HP/PSI of drive pressure. I rounded WAY up. My fault. Still significant.
killerbee said:
The turbo, in it's "free lunch" aspect, is driven by exhaust, a waste product (expanding exhaust gases that have left the cylinder), so it is not tapping power from the motor, at least not like a parasitic device, alternator for example.

This is where I am not seeing a benefit, but I am trying.:Thumbup: My limited forced induction knowledge is showing.

Are you saying that excessive turbo backpressure is causing less expansion (thus less work performed) in the cylinder? For that to be the case, drive pressure would have to be significant (vs negligible) compared to cylinder pressures.
A turbocharger is not a free lunch. Never was. The engine has to fight the backpressure to get the exhaust through the turbo, That takes WORK. Work over time is POWER. The nice thing about turbos has been they only consume power in pace with the work they do. Very one for one as apposed to the constant drag of a direct drive supercharger. The "better" the turbo, the better the ratio between the drive pressure and the boost.
We have two seperate topics here.

One is the cost, in HP, of the restriction caused by the turbo. The other is the how a turbo gets power out of the exhaust stream and why it is such a good thing on an engine.

First one first. Regardless of wether there is a turbo on the exhaust pipe or just a restrictive muffler, the backpressure takes real horsepower to overcome. In purely mechanical terms, this is the force over time needed to push the exhaust out of the chamber. I think everyone can see that. Reducing the backpressure allows the power to go to turning the driveshaft instead of to push the piston up against the backpressure. On the Dmax, this turns out to be 1.4xx HP per PSI of backpressure. In relation to the turbo, the less backpressure the better if only for the power it frees up in the motor.

All turbines get there power by changing the speed and/or direction of the fluid/gas they work with. In doing so, you end up with a Differential Pressure (DP) across the turbine. The greater the velocity of the fluid/gas, the greater the power imparted to the turbine when it changes the direction. Part of the efficiency of the turbine is characterized by the ratio of DP to the work being done. In the turbo charger case, how much boost you make compared to the drive pressure.

One of the things that can make TurboCharging so attractive is the "still expanding" gas affect. I think we have all seen the flames coming out of the pipes on drag cars. Yep, still expanding. Nearly all engines do a similar thing at full load. Maybe not so spectacular, but similar. Not enough to be useful in the engine chamber though. The flame is accelerating the gas even faster than it left the chamber. Since our turbo's power is directly related to the velocity of the gas it seems like a free lunch.

But the engine itself is what has to push against the "still expanding" gas to get exhaust into the pipe. The turbo is on the other end pushing toward the moter with its DP. IF you add them up, you find the power to run the turbo (turbo output plus drag, friction....) is exactly the same as the power the engine uses to overcome backpressure. No free lunch.
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The comp cam is much more agressive. Look how quickly it is opening/closing the valves compared to OEM. The area under the curves is roughly proportional to the window where gases can cross over. This graph is of the Exhaust-Intake transition near TDC, so the relative pressures are almost exactly backpressure Vs Boost.
We are a bit off topic here, but the problem with the agressive opening/closing rate isn't the OEM roller lifters. Moderately high reving engines will start to have problems with the pistons chasing the valves though. The upper valve train isn't stout enough, IMO, to control valve float over about 3500 RPM with the stock cam. Let alone a more agressive one.
Back on topic....

Rick's schedule and mine are not meshing. We both intend to move forward on the swap at some point. However, I only have 3 weeks till I hit the road for my fall professional obligations and will not have another window to do this till October.

Maybe a mid winter run to Kennedy's and use the treadmill to simulate OH again like PUSU.
Using the treadmill, it would be nice to:

TD-EOC shakedown
V2 shakedown
LB7 turbo on LLY comparesons
Modified turbo vanes testing.
Water Injection?
Water mist fogging of stack?
Tuning comparisons.

That schedule would take at least a week.

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Simply opening the vanes is not the answer
You will find the LBZ and LLY compressors are comparable in efficiency. The turbine side is another story.
killerbee said:
...will poor turbine efficiency, CREATE poor compressor efficiency?

This baby produces heat better than it produces boost.
No, it will not.
No, but I have been able to compare outlet temps on the IHI and VVT. They are nearly identical pound for pound. The VVT might be a touch better in the higher boosts, but not by much.
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