Turbo Efficiency Discussion

[NateD4]

Joseph.

What are you trying to figure out exactly?

You are right that the turbo will always have back pressure while spinning. Mainly due to it's radial in flow and the centrifugal effects.

If you are talking A/F ratio... You will probably get some weird ratios on throttle cut.

I would suspect a rich mixture as you cut air with the BOF valve.

One thing to watch is shaft speed on the turbo as a BOF valve without enough back pressure reduces load on the compressor disproportionally with turbine load due to mass flow and energy differences.

I think aftermarket fuel computers have an option to cut fuel on deceleration. I would also open the waste gate to normalize the exhaust and intake pressures during throttle down.

[Joseph Upson]

Joseph.

What are you trying to figure out exactly?

You are right that the turbo will always have back pressure while spinning. Mainly due to it's radial in flow and the centrifugal effects.

If you are talking A/F ratio... You will probably get some weird ratios on throttle cut.

I would suspect a rich mixture as you cut air with the BOF valve.

One thing to watch is shaft speed on the turbo as a BOF valve without enough back pressure reduces load on the compressor disproportionally with turbine load due to mass flow and energy differences.

I think aftermarket fuel computers have an option to cut fuel on deceleration. I would also open the waste gate to normalize the exhaust and intake pressures during throttle down.

Beyond measuring exhaust back pressure to see if it was practical for me to drop down one turbine housing size, I wasn't trying to figure anything else. Holding the wastegate open off boost was something that originated from Corky Bell and an idea years ago when turbocharging wasn't near as popular as it is now and before I realized how outdated much of the info in his book is since fuel injection was still kind of new. I had given the idea some thought recently because with the boost control solenoids it would be pretty easy to accomplish and I like the technical aspect of mechanics and engine management physics.

I was getting good mileage with a turbo long before this, I got as much as 28 mpg with a low compression 3100 (iron heads on it) and the 4 spd muncie with high hwy rpm.

I have a wastegate for managing boost, the blowoff valve is to avoid compressor surge and shock wave damage and was only a problem during the brief period where a boosted run blew a poorly connected pipe out of the silicone coupler. The AFR on throttle cut depends on programming of decel fuel, I just realized yesterday that I don't have the occasional pop on hard run decel I had on the previous motor because the tune has changed in a manner that richened the exhaust on decel accorging to the history table.
I have a good size turbine housing so at cruise there's no back pressure issue. What I regret is not taking a brief moment to make a simple 3" dump pipe that I could quickly install for open exhaust since my muffler system is held on by a V-band clamp and a support spring at each tail pipe. I plan on installing an electric cutout though. The car is blazing fast on just 3 psi I can't stress it enough in comparison to when the engine was first built and to the stock motor. Degreeing the camshaft in is crucial because that's the difference between falling off at 4000 rpm vs pulling through to who knows what since I back off throttle due to engine sound since my rpm gauge isn't working.

[TGP37]

These motors do respond well to boost. First time I did a full throttle run at 5 psi, I was impressed. 8 psi just feels wicked strong. I ran 12 psi for 3 wot runs and it was too much for the 4t60e. Plus 12 psi just felt too stressing on the motor and needed timing pulled a little too far to prevent knock. 8 psi is a good line for me....just enough, not too much. Even at 8psi w/ a bov, I was blowing the intake off the throttle body when coming out of boost. I had to carve grooves for the silicon to bite into, worked great.

You raised a point about richer AFR during decel.....what is the ideal AFR for decel after boost? On one hand, more fuel would cool the pistons, valves, etc. On the other hand, mpg goes down slightly.


The electric cut out, I think is a great idea. Could have it open automatically by a pressure switch similar to meth injection. And have a master override relay to turn the system on/off. She will idle and cruise nice n' quiet, but opens up in boost. Effectively increasing the pressure ratio across the turbine.

Maybe design a circuit that opens the cutout when the pressure ratio across the turbine (downpipe back pressure increase) gets to a specific point. Functioning similar to a wastegate, it could keep turbo efficiency up by relieving excessive pressure in the DP when needed. That "should" help maintain a higher efficiency. While still having a muffled exhaust, it would just bleed out exhaust when needed.

[Joseph Upson]

These motors do respond well to boost. First time I did a full throttle run at 5 psi, I was impressed. 8 psi just feels wicked strong. I ran 12 psi for 3 wot runs and it was too much for the 4t60e. Plus 12 psi just felt too stressing on the motor and needed timing pulled a little too far to prevent knock. 8 psi is a good line for me....just enough, not too much. Even at 8psi w/ a bov, I was blowing the intake off the throttle body when coming out of boost. I had to carve grooves for the silicon to bite into, worked great.

You raised a point about richer AFR during decel.....what is the ideal AFR for decel after boost? On one hand, more fuel would cool the pistons, valves, etc. On the other hand, mpg goes down slightly.


The electric cut out, I think is a great idea. Could have it open automatically by a pressure switch similar to meth injection. And have a master override relay to turn the system on/off. She will idle and cruise nice n' quiet, but opens up in boost. Effectively increasing the pressure ratio across the turbine.

Maybe design a circuit that opens the cutout when the pressure ratio across the turbine (downpipe back pressure increase) gets to a specific point. Functioning similar to a wastegate, it could keep turbo efficiency up by relieving excessive pressure in the DP when needed. That "should" help maintain a higher efficiency. While still having a muffled exhaust, it would just bleed out exhaust when needed.

The amount of fuel on decel for me according to the datalog appears to be in the range of about 16-17:1 with a few areas near 14. I just know the popping is gone. My fuel economy still appears great and I've been beating on the throttle a little more.

I have a snow performance kit installed but I only run water through it now because methanol is infrequently available and an unnecessary expense if water is all it will take to get the job done for my boost level. I'm still looking forward to getting a liquid to air intercooler installed. Right now my air temps are running about 70 deg above ambient at 4 psi, which is about 135 deg.

I'm still amazed at how fast the car is now on 4 psi, a full 7 to 8 should really put it over the top as it covers asphalt really quick.

[TGP37]

The methanol also provides an anti-freeze effect. Where I live, it gets cold 1/3 the year.

8 psi is great. More then a factory turbo while still reasonable on a stock PCM.

[NateD4]

Water injection is actually better than methanol for cooling as it can absorb more energy and doesn't mess with A/F ratio. However methanol has some other benefits.

I might suggest trying a water/denatured alcohol mix (as denatured is ethanol with some methanol in it). Most hardware stores have it, however I'm not convinced the price is really worth it for mileage.... on the other hand distilled water is really cheap.

My build is a bit different, I'm going to end up at 3.3L with a totally forged bottom end. I plan on running a lot of water injection and up to 10 PSI boost maybe even 12 PSI (eventually). However I'm going to have a long road ahead of me to get all the electronics set up...

I might even try leaning on cruise to a 15 or 16:1 and use water injection to reduce EGTs and control piston temperatures.

Cam wise, I'm sticking to VVT, the torque band benefits seem to worth while to abandon it. I may even get a custom VVT cam made as well, though we'll see. I suspect the stock cam is plenty.


Any chance you can dyno your car and let us know what kind of numbers you have?

[Joseph Upson]

water injection to reduce EGTs and control piston temperatures.

You have oil squirters to control piston temps, just use a good oil cooler to keep oil temps close to 180 deg. Regardless they run cooler than pistons without the squirters.
I plan to take my car to the dyno or to the track after more tuning which has been delayed due to a freak accident that fried my ecm and my emulator. Don't let your USB plug touch a positive power source.

[NateD4]

I'm thinking with water injection I might be able to get into the high 30's mpg wise by leaning....



As far as your emulator is concerned what are you using it for?

I'm looking at an aftermarket ECU because the software is going to need heavy mods outside of tuning.

Maybe an emulator would help.

[Joseph Upson]

The emulator is about as good as it gets except for a dyno when it comes to tuning considering it allows you to change values while the engine is running for immediate review of results. It's even more beneficial when you're trying to sort out closed loop idle after a significant injector size change. You would end up burning hundreds of chips otherwise.

[NateD4]

Which ECM are you running?

I had been looking at a newer E42 or E67.

[Joseph Upson]

730

[TGP37]

. Don't let your USB plug touch a positive power source.

Ouch, now that's a good mood killer there.

[TGP37]

Interesting info I found pertaining to water/meth injection. As it is in physics, water has a molecular mass of 18 while air we breath has a molecular mass of 29. The water, h20, has two hydrogen atoms which is far lighter then oxygen and means water vapor is less dense then the air our engines ingest.

So injecting water vapor will actually reduce the density of the intake charge over all. The common misconception deals with waters inability to compress well due to it's state of matter regarding temperature.

So air will become less dense as humidity rises. I checked up on it, sure as anything it's true.

Injecting Water will not increase compression when the water is in vapor form. Maybe this is the mechanics behind the cooling effect it has? Seeming how temperature is proportional to density delta rate of changes (considering ambient losses over time). A drop in density would bring about a drop in temps. The air (higher density) will act on the water vapor (lower density) and results in the air decompressing against the vapor being compressed. So the water inherits temps and the air reduces temp in the balancing of differences. Then the water releases heat when condensing in the exhaust allowing to keep gases hotter for better velocity.

What do you guys think? It seems reasonable.

[NateD4]

Your understanding of thermodynamics appears to be off. Density and pressure are related but not in the way you portray. The mole mass may be lower, however the effect the energy density has on the pressure is not directly related to the pressure.

I think what you are getting at is combustion reagent displacement. Whereby the volume of water displaces a volume of air which means less fuel to mix with the air. However the fault I think I see is in the phase change density difference during/after combustion.

[TGP37]

I understand where I went wrong, lol. I was visualizing on a molecule basis and overlooked the density of molecules per cubic inch.

But still, researching more, I found objects in motion will travel further in humid air versus dry air, assuming other constants are the same. But I think that is because energy is required to change the direction of travel of a specific molecule and heavier molecules provide higher resistance. Like plowing through balloons versus lead balls. Instead of my earlier perception of density.

After reading more and started to learn Vanderwaals ideas. I wonder if the deviation from the Ideal Gas Law, roughly 1% for water and 0.3% for air, is expressing measurable results in the combustion process. As small as the deviation is, across billions of molecules, should express an even constant just as fission has measurable results from a seemingly random, chaotic process.

I got to think more on that.........