A major consideration which is why I wondered about the BOV as a fail-safe. I guess I'm still wondering. Perhaps someone can clear that up for me.

A rotary stepper motor is capable of, say, 500 rpm or 8 rps. The waste gate rotates, say, 1/4 revolution. .25/8 = .03 seconds It's not that simple. It's more like a WAG but I think that would be quick enough especially when it's likely that the waste gate isn't moving from fully closed to fully open. Again, a WAG.

 

Point taken. It could be as simple as a manual choke and a psia gauge. Watch the gauge and push or pull the knob. That would work well under a steady load like pulling a step hill which is the problem I have. The only difficulty would be that I'd need to hold a steady throttle opening. I think my foot would be moving all the time as I back-off for corners and the floor-it to come back up to speed.

 

Thank you guys for signing up for this discussion. All the questions asked and criticism force me to think about things I might not otherwise in addition to teaching me the practical details of turbocharging.

 

My comment was a mechanical method to prevent boost at low elevation (basically a lock out), and then let the gate do it's job at elevation with a manual, or electronic boost controller to add boost as you climb. The ebc may be another avenue to look at, possibly altitude compensated.

 

I have searched the internet for any reference to a boost controller that was altitude compensated. I couldn't find anything. I've even called some of the boost controller manufacturers. So, if anyone knows of such a thing please post it.

 

I'm pretty sure it's something you'd have to build, or modify.

 

the boost control section of any megasquirt ECU should be able to do what you want...since it uses a MAP sensor you will be able to set it up for 14.7 PSIA...and has full PID control settings so you can set it up for you application/system...or you can run it in open loop as well with a specific solenoid duty cycle at any RPM/manifold pressure.....megasquirt can be set up to run 2 MAP sensors as well.....1 for manifold pressure....the other for BARO/ambient.

Use a compressed air tank/compressor and air solenoids. Ultra light wastegate spring (or no spring) and a large wastegate for the application......the air can force the wastegate open when operating at sea level so hardly any boost is produced...or only force it open under heavy load at sea level to prevent boost and conserve control air .

 

Which is exactly what I'm working on here.

 

I'll look into it.

 

A few points.
- You don't need an intercooler or water/meth injection. Straight water at fairly low rates will intercool to near ambient at the very low pressure ratios you propose. It will also add several octane points so detonation will definitely not be an issue. You can run leaner than factory at full load and save fuel when running WI. Control the WI based on manifold pressure, not compressor discharge pressure.
- If you really want to run zero boost, don't use a wastegate. Throttle the intake of your turbo using your stepper motor and a second throttle plate. Sense the boost aftyer the compressor but before the factory throttle plate. You won't need a carbon compressor seal because the compressor downstream side will never see less than atmospheric (provided your control system can react as fast as you say.)
- I would suggest about 15.7 - psia at all altitudes ie 1 psi boost at sea level. (Measured post compressor but pre-factory throttle plate.) Your stock engine management delivers enough fuel to support the very small (7%) power increase with WI.

 

I once had a 1979 Trans Am on which I installed a Holley WI system. I believe it worked as you describe. If I could find one like that NOS still in the box I could avoid the R&D process.

I like this idea. It eliminates having to deal with the effect of the excessive temperatures of the wastegate on the stepper motor. The failure mode of the wastegate is to dump all the pressure. The failure mode of of the second throttle plate has to be to close the second throttle plate to a certain minimum opening. Have to come up with that. An "idle screw" would set the opening. Is the rotation of an ordinary throttle plate biased to close under flow? If it wasn't it could be, I guess.

Any system that will function as I propose must register the controlling pressure immediately before the factory throttle plate.

 

I have not checked back on this thread for a couple of weeks but I am glad to see it is still progressing. One thing that you are always saying is staying in the factory parameters of the engine. I really do not know diddly squat about VW engines, but I will assume that they are like every other manufacturer out there and that means a very conservative rating of what the engine can actually handle. That being said, I would look at what other VW enthusiasts have done to these engines to get a proper perspective of what the engine can actually handle, you will never get that information from the manufacturer, only from others who have done it. There is a lot of very knowledgeable people here and it is nice to see ideas still coming up. Keep at it, a solution might be just around the corner.

 

Not sure why the OP should do that? He doesn't want to subject the motor to any greater stress than the original specifications.

I suggested above a 7% increase which is also so small that it is not even worth investigating its effect on durability.

OP.
The second throttle plate should probably be wide open under normal operating conditions and start closing as boost climbs (engine loaded). If you want a fail-safe perhaps a 2psi BOV would be the best solution.

The high pressure WI systems from Snow, AEM, Aquamist etc provide fine atomisation which is what you need.

 

That won't work. Remember, the pressure at the factory throttle body is to be a constant 14.7 psia. Going from sea level to the mountain tops results in a 0-5 psig range of operation for a BOV or wastegate.

 

The problem with added power lies with the transmission. It's known for failure.

 

I think I'd prefer a passive solution, that being an intercooler provided I can fit it under the hood. I intended to construct a model from cardboard to check that.

 

Yeah - not thinking. You would need an absolute pressure BOV. Something controlled via the MAP sensor signal.

An intercooler isn't going to do much. With such a small PR the turbo won't be heating the charge very much. WI will provide significant detonation suppression regardless of boost level. (I seem to recall you saying the engine is close to the detonation limit NA.) There are DIY WI blogs out there. Some higher pressure windscreen washer or headlight washer pumps can be sourced cheaply..

I was thinking about the advantages of the second throttle plate being located upstream and decided downstream would be OK too. Turbo sizing would be slightly smaller with the downstream alternative.

 

Check out this build link and click on the other link to another site. This guy was using a draw through setup before and was able to adjust boost from 0 on up. I realize it is a different system but there might be something you can use.

https://www.theturboforums.com/threads/1968-camaro-single-turbo.385521/#post-2023052

 

My research leads me to believe that the secondary throttle plate could be rotated 90 degrees (closed-to-open) in 1/2 second. Does anyone have a feel for for this? Is that quick enough?

 

That sounds pretty slow. OTOH it shouldn't need to move that fast. It won't ever be fully closed - most of the time it will be fully open. (Have it sensing pressure between the two throttles.
Worst cases would be:
1. Snap throttle closed at full boost (2nd throttle part closed to control boost). No problem here. Closing the primary throttle limits boost to the engine.
2. Snap throttle open with turbo spooled (fast upshift. Is it auto trans?). Turbo will wind down very quickly after closing a downstream throttle - max load on compressor and very little exhaust gas to power the turbine. Furthermore the "slow" secondary throttle will still be part closed in accordance with whatever pressure exists in the compressor discharge. Resuming power to the turbine will not spool the turbo faster than the secondary throttle can react.