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Old 09-27-2013, 11:49 PM   #1
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Default Lets argue about VGT/VNT Diesel Turbos on a gas motor.

OK, so this is not *Miata* related, but it is *turbo* related so technically I'm not going way too far afield here.. I've been doing some reading on using regular old wastegated diesel turbos (IE the Holset HX55) on a gas motor, and have become really fascinated with the Variable Geometry turbos.

As discussed in another thread, I'm building a LT1 engined Mercedes 450SLC for the GRM $2013 Challenge. I want to build some cheap boost, but there are also valuable points to be had for doing cool, unconventional things. (Like, stuffing a rear-set LT1 into an old Benz..) so I'd like to try something sort of different. Well, different-ish.

I am aware of the concerns regarding EGT's in a gas vs diesel motor, and am comfortable that I can hold it together for what is likely to be a total future lifespan of maybe (*maybe*) a few hundred total miles. I mean, even if we do use it post event as a track pig or hill climb car or Super Dorifto Hooliganism machine, it's never going to see street use again. So turbo longevity isn't really a primary concern. Beyond that, there ARE people who run them on gas motors with no real problems. Maybe the key is to leave a little horsepower on the table and tune a little on the rich side? (Could it be that people who report incinerating the turbos are also crazy lean because of the Ridiculous Boost and Violently Fast Spool these things can make??)

Anyway, take that EGT stuff out of the discussion for now. Lets pretend it doesn't exist. For the purposes of this exercise lets assume that the motor and turbo in question need to have a total future lifespan of 20 hours. K?

Most gas-motor VGT's I'm seeing adapt an external wastegate controller to act on the linkage to the VG vanes, since they're effectively your WG in this no-WG turbo. Seems simple enough, but there's apparently a lot of experimentation and bullshittery involved in dialing it in. And, as we all know, a non-dialed in WG is usually not awesome. Amiright? So riddle me this:

What if I used a good sized conventional external WG ahead of the VGT to control boost? Could I not then just run a pull cable to the VG vane linkage, allowing me to select from anything from a tiny to a huge AR?? Like, shut the vanes for the autocross so it's literally boosting at idle and will spool immediately at the relatively lower Autocross RPMS, and then open it up for the drag strip so it doesn't come on so hard when I'm launching but then builds Goofy Boost as our train gets to rolling.. ?

Discuss.
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Old 09-27-2013, 11:55 PM   #2
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When you put it that way, it just might work.
Most discussions on this here involved reliability conscious highly (or hard, ie: track) driven cars which would also want it all electronically controlled, etc. For a couple hundred miles, and witha simple cable operating it, I don't see why it wouldn't work.
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Old 09-29-2013, 05:34 PM   #3
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It will work that way. Just be careful not to overboost. Those will hit 40psi pretty effing quick with the vanes trimmed to spool. And it will require a big wastegate when set that way.
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Old 09-30-2013, 10:13 AM   #4
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Adding an external gate to a VGT turbo is like building a miata into a drift car, yeah it'll work but you just took away the thing that makes it excellent. If you're worried about the EGTs just run E85.
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Old 09-30-2013, 11:12 AM   #5
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Quote:
Originally Posted by Leafy View Post
Adding an external gate to a VGT turbo is like building a miata into a drift car, yeah it'll work but you just took away the thing that makes it excellent. If you're worried about the EGTs just run E85.
I disagree; adding an external waste gate to a VGT allows me to have both a small and huge turbo in the same package, not to mention one which costs a fraction of what a comparable conventional turbo would cost. Granted, if not for the severe budget constraint associated with the GRM Challenge, I probably wouldn't have considered a VGT, but now that it's on the table I'm excited to mess around with one a little. Part of that "messing" process will certainly involve adapting a WG actuator to the VG Vane linkage, but with potentially 40 psi plus on tap, having an external WG to act as a "safety valve" while we figure out what the turbo does at different vane positions seems like a good idea. If we nail down the vane operation we can always just ditch the external, or set it to a high psi to keep it operating as a safety.

And seriously, why the Pervasive Hatred of drifting? There are 1000 different ways to enjoy a car, what is it about deliberate oversteering that sets people off? If some dude wants to slam his car and spend time sideways, and that makes him (or her) happy, how is that not then the thing that "makes it excellent" to them? Y'know? Sure, the whole Dorifto culture has elements that I find stupid, but I enjoy a bit of hooliganism as much as anybody else (full disclosure: probably a little more..) and lurid tire-smoking power slides definitely fit that behavior. The whole effort to turn that into some sort of judged figure skating event seems preposterous to me, so i dont participate, but driving at the edge of control sounds good in my book. Plus, have you seen the babes?? Those drifter kids have the Babes Thing nailed.

I guess what I'm trying to say is "can we all just get along?!?". This is the "Miata Turbo forum"' not the "Miata Turbos That Run 15" 6ULs, Have The Accepted Front Lip And Are Used Only For Operation Up To But Not Exceeding A Certain Slip Angle Forum". Isn't it?
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Old 09-30-2013, 11:17 AM   #6
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If you want a safety valve for overboost, put in a safety valve, it'll be much cheaper than an EWG, easier to implement and you can buy one from mcmaster for extra levels of GRM legitness.
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Old 09-30-2013, 11:33 AM   #7
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Wow, neg propped for disagreeing with you?

We live in savage times Ladies and Gentlemen, savage times.
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Old 09-30-2013, 11:34 AM   #8
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Quote:
Originally Posted by Pinky View Post
Wow, neg propped for disagreeing with you?

We live in savage times Ladies and Gentlemen, savage times.
I didn't neg you. It probably brain or 18psi for the drift loving. Personally I love hanging the *** end out, especially when I dont own the tires.
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Old 09-30-2013, 11:56 AM   #9
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Quote:
Originally Posted by Leafy View Post
I didn't neg you. It probably brain or 18psi for the drift loving. Personally I love hanging the *** end out, especially when I dont own the tires.
10-4, thanks for clarifying. Petty retributive neg retracted. And yes, you're right, a simple pressure safety valve should be given consideration in light of the budget. Thanks for the idea.
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Old 09-30-2013, 12:31 PM   #10
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How about rear-mounting it to reduce exhaust temps at the inlet?

P.S. I dorifto on my daily commute.
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Old 09-30-2013, 01:30 PM   #11
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There's been a couple people that have run these on MX6s. One of them just stuck the turbo at a spot that seemed to work well. The other experimented with springs and such.

Another tried to control it with megasquirt (was very promising), then decided he didn't care enough.

There are aftermarket standalone controllers for these things.


Honestly, i view it as too much work to bother, but if you're electronically-minded, i might see if you can figure out what sort of circuitry the standalone controllers implement, and copy that ****.

Otherwise, a run of the mill HX40 or 45 might be a better fit.
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Old 09-30-2013, 01:53 PM   #12
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If you set up a WG actuator to control it, could you then use the Megasquirt's EBC to control it?
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Old 09-30-2013, 02:06 PM   #13
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Quote:
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Personally I love hanging the *** end out, especially when I dont own the tires.
Is that why my tires looked so terrible after they came off your car?
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Old 09-30-2013, 02:07 PM   #14
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Wait. Is this the variable vane type or the sliding ring type of turbo?
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Old 09-30-2013, 02:19 PM   #15
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Quote:
Originally Posted by ch blank View Post
Is that why my tires looked so terrible after they came off your car?
Lol, yes thats what you had horrible outside should wear on the fronts, too much drifting on course. Now those all season's you lent me when I bought the car..
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Old 09-30-2013, 05:41 PM   #16
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Quote:
Originally Posted by sixshooter View Post
Wait. Is this the variable vane type or the sliding ring type of turbo?
Vane type. The nozzle ones all seemed to have big scary looking electronics on them, where Holset VE's just had a pneumatic cylinder acting on an actuator arm.
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Old 09-30-2013, 05:44 PM   #17
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Quote:
Originally Posted by concealer404 View Post
There's been a couple people that have run these on MX6s. One of them just stuck the turbo at a spot that seemed to work well. The other experimented with springs and such.

Another tried to control it with megasquirt (was very promising), then decided he didn't care enough.

There are aftermarket standalone controllers for these things.

Honestly, i view it as too much work to bother, but if you're electronically-minded, i might see if you can figure out what sort of circuitry the standalone controllers implement, and copy that ****.

Otherwise, a run of the mill HX40 or 45 might be a better fit.
I was thinking about a simple servo hooked to a WOT switch. When you mat it on the dragstrip it goes to the big boost setting.. Remember, LOW BUCK..
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Old 09-30-2013, 05:50 PM   #18
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Quote:
Originally Posted by Pinky View Post
I was thinking about a simple servo hooked to a WOT switch. When you mat it on the dragstrip it goes to the big boost setting.. Remember, LOW BUCK..
Thats not how it works. The vanes more or less change the AR of the housing, and you need to vary the location of the vanes to control the amount of boost. You can easily over spin the turbo if you leave it on the tiny turbo setting, and it'll just be a hairdrier with an overly restrictive turbine housing.
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Old 09-30-2013, 06:15 PM   #19
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I think that's the point of using an EWG along with the VGT, so you could run autocross with the AR statically set lower. I'm not sure how much difference it would make though.
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Old 09-30-2013, 06:20 PM   #20
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Originally Posted by thenuge26 View Post
I think that's the point of using an EWG along with the VGT, so you could run autocross with the AR statically set lower. I'm not sure how much difference it would make though.
But then you should just run a smaller AR turbine. The point to the VGT is so that the AR is constantly varying as engine conditions change so you make the most power all the time. The best way to make the most power all the time is to have a fast spooling (small AR) and high flowing (big AR) turbine, assuming you're keeping the compressor constant. If you just kept it in tiny AR mode it will spool fast but run out of steam up top, even running the EWG. If you kept it in massive AR mode (like pinky describes a drag race), it will take forever to start building boost and might not even hit your desired peak boost at all. If you move it from tiny AR to massive AR to maintain the boost curve you want you'll make more power everywhere.
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