The wastegate actuator opens a little after 9 psi now, but I would like to get it to open up around 5 psi. Is there a way to make this change by putting in a new spring, or is there another place to buy a different actuator?

Derek,

first of all you can lower the spring preload by lengthening the actuator arm. Typically there is a long piece of hex stock that has an internal left and opposite right hand thread fixed with jam nuts. Just loosen the nuts and turn the hex piece in the direction that loosens it. I don't think you will get it set down to 5 psi, but you may be able to drop one, two or even three psi.
There are a few drawbacks to run either very little pretension or a soft spring. The boost buildup can be drastically delayed by the wastegate opening too early, even though you might bleed off the control air for the actuator. Just the exhaust gasses pushing against the valve could create a small leak with too little spring preload.
Another option to lower the boost pressure is to pressurize the opposite chamber of the actuator box. So basically you could use two solenoids, one to guide pressure to the chamber opposite of the spring and one to the side with the spring. With the correct setup you will be able to widen the adjustable range of boost above AND below the spring setpoint. Most likely the actuator that comes with your turbo does not support this type of setup.

That makes sense about the exhaust gasses pushing the valve open. I would like to lower the actuator activation pressure from 9psi at least some. Otherwise we'll be choked at the restrictor at around 10,000 rpm because I can't lower the boost pressure any more to prevent a vacuum from forming if any of my theoretical calcs are remotely accurate.

I'll mess with the preload and see if I can't get it to at least drop some. If I could get it down to 7psi I would be pretty happy with it since that should get us closer to 11500 rpm before we're completely choked again with theoretical calcs.

If i may ask a stupid question, what's wrong with choking at 10,000 rpm? Is it that you're setting a fixed boost and you're worried what will happen above choke? Ideally you don't want your set boost to be unachievable due to the restrictor because the wastegate will shut and the turbo will scream its little head off, slaughtering the top end and potentially damaging the turbo.

If you can solve that problem though, choking lower is good. We choke just below 7,000 rpm. We use an all electric actuation system, which is more work to set up, but extremely flexible.

I would like to never have the restrictor choke. We will be dropping boost starting around 7000 rpm to achieve this though. I would like to end up at quite a bit higher intake pressure at 7000 rpm than what we will have at 11,500. I do not want to have the compressor spinning in a vaccuum because it only creates heat with no other benefit and while I haven't tested it yet, it seems like it would be much harder on the seals behind the turbine.

With that said we have an air controller to do this, but it can only mimick whatever amount of boost you would like to have going to the waste gate. So if I would like the system to have anything lower than 9psi at upper rpms then I'm stuck because the wastegate won't open below that as it sits now.

None of the seals will care, they see positive pressure when you're at full throttle. The only real worry is overspeed. If you have a 600 cc engine at 11 grand, and you tell the turbo to run 9 psi (unachievable), the thing will spool its tits off trying to achieve that boost, until it either reaches an equilibrium (air flow is limited, therefore energy to the turbine is limited) or it comes apart. In our experience, the former usually happens first. We've run some fairly insane turbo speeds in the past due to mistakes (284,000 rpm is the record) and the turbo didn't fail (I don't think you'd manage to get a non-variable vane turbo to go that fast though). Even if it doesn't fail, the overspeed will slaughter your power.

There are other approaches though. Although i've never tried it myself, i've heard that using a dual port actuator and tying the other port to the restrictor near the compressor inlet works well. The idea is that as vacuum in the restrictor builds, it is effectively treated as boost and the wastegate will open. So say the restrictor chokes at 5 psi of boost at a particular rpm. If the actuator is set at 9 psi, then it will regulate it to a 4 psi vacuum in the restrictor.

That's actually a pretty good idea, but I'd like to find a couple more actuators to tear apart before I even tried experimenting with it.

But I know about the overspooling, that's what I'm effectively trying to prevent by the exhaust pressure so it doesn't try to spin the compressor any faster. How do you guys measure turbine speed?

So evidently there are actuators for the application I would want already being made. They're a little expensive, but they're also good down to 4psi. Thanks for the help Pete.

Commercial turbocharger speed sensors exist these days. The older style relied on a notch in the shaft or something like that. The newer style sense compressor blades, so all you have to do is drill and tap your compressor housing. They are not exactly cheap though, we got ours sponsored.

Yes, we use both shaft and blade speed sensors. The shaft sensors read a flat machined in between the bearings. The blade counters read through the compressor housing and divide by the number of blades.

Unfortunately we don't have the resources to offer these parts with our program. Our aftermarket group did just release a blade counting speed sensor, so you should be able to order one of those through a Garrett distributor such as ATP or Limit Engineering.

I did my own personal layout for the GT12 a while back, so I know it's possible. I don't know if the aftermarket group did the GT12 for their speed sensor project, but it's certainly worth a look.

-Kirk

Here's the blade counting sensor for reference...

http://www.turbobygarrett.com/turbobygarrett/products/A...ue_speed_sensor.html

The aftermarket "Pro Kit" skips the gauge (gauge doesn't read high enough for a GT12 anyway) but includes the sensor, installation and signal instructions needed. It outputs a fixed voltage/duration square wave whose frequency is proportional to turbo speed.

This message has been edited. Last edited by: Dan G,

That's the sensor we use. We don't use the gauge though, we just run it straight to our ECU.

why don't you make a dual port actuator and run a solenoid? If your EMS has provisions for boost control you should be able to control ramp rates, gear dependant boost, etc.

To the OP...if you want to lower your boost and not do any hard work... Move your boost signal line from the compressor cover of your turbocharger to your Intake manifold. The pressure drop across your IC/IC piping will give your actuator a lower pressure reading.

(PS you should have a restrictor in the line to the actuator)

why would you put your boost control line at the compressor cover??? That's not the boost the engine's seeing, so that seems pretty obvious to me.

I've already got a pretty good idea of what I'm doing, but I did look at the dual port actuators. I'm just curious how you propose to actually make one work with only one solenoid? Where does the other side of the actuator plumb into that makes it any better? If I wanted to make my setup more accurate, but wayyyy more complicated I would have gone with it, instead I just got a 6lb actuator which should be good to about 12k for me and that's pretty good as I don't see us being up there very much.

Derek,

I have never seen an OEM that did not put the wastegate signal line right off the compressor housing. All the turbos I have seen come plumbed with a nipple on the compressor housing just for that purpose, to run right into the actuator if no controller, or into a solenoid if regulated.

Good point about not being the same pressure that actually enters the engine though, and my guess would be that the slight additional pressure at the compressor would be able to do a better job at holding the wastegate shut when running higher boost levels than we do. You could run into fluctuation problems with not enough pressure at the signal line. Does that make any sense?

Best,
Drew

Well all of the fluctuation problems occur from choking the restrictor and over spinning the turbo. I imagine that you could solve the problem pretty easily by simply messing with the tune and having it open the actuator a little sooner if the pressure does start to fluctuate on the top end.

Exactly, for the restricted application you're going to be really unlikely to have control problems because the pressure is much lower than what the turbo was intended for.

I was only speculating as to why the signal line usually comes off the compressor housing, and not the intake manifold.

Do you see fluctuations when you hit choke and the turbo starts making lots of noise and heat, and not any more air flow? Share how much?

I have no experience doing forced induction with a restrictor yet, so good things for me to be absorbing.

Best,
Drew

Unfortunately I don't have any experience with it yet either. The car was originally supposed to be running this week.... but it got pushed off. Hopefully we'll be up and running in another week or so once everyone gets back to school.

My understanding is that the wastegate signal is taken as close to the compressor as possible because it's a control system. You generally want as little delay as possible in the feedback circuit as it makes stable control harder.

On the other hand, it may be done simply for neater packaging. Either way, unless you have a super restrictive intercooler or something, i don't think it'll make much difference.