• From: Life's great in 21158
  • Registered: 2009-05-01
  • Posts: 3,361

Topic: Draw thru Turbo w/ Carb Question, Disappointing Replies

So this morning I posted the following to *another* automotive forum:

"I've been musing about draw-thru turbo carb setups...one of the biggest obstacles seems to be the following issue: Under a full boost/ WOT situation, when the throttle plates (on the carb) are closed, you end up with a vacuum between the carburetor and the turbo. This vacuum can suck the oil out of the turbo, turning your hot turboed creation into a mosquito killing smoke machine. The solution everyone on teh webs seems to agree on is carbon seals in the turbo, to prevent oil sucking into the intake charge stream.

However, carbon seals can fail, and not every turbo has them. So, I had an idea: what if you were to move the throttle plate from the carburetor to between the turbo and the engine? Or, possibly, run two synchronized throttle plates- one at the carb, one between the turbo and the engine? Thoughts on such a setup? "

So far, 3 out of the 4 replies told me I was stupid for wanting to do a draw-through setup.  *sigh*  This is what I get for turning to the 'net for my answers.

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  • From: DFW
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Re: Draw thru Turbo w/ Carb Question, Disappointing Replies

Web forums can be great or terrible. Sometimes they are both.

I did a t3 turbo swap on an 89 Mx6 that we ran as our first car. I bought a couple ebay flanges and fittings and did it for like $80, including the junkyard turbo. The Mx6/probe forum told me it wasnt possible, wouldnt work, blah blah blah. I was then thier god for knocking it out without any issue, and then I sold the whole setup to a member of that forum in SPAIN of all place for $500 plus shipping.

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  • From: Tustin, CA
  • Registered: 2010-12-30
  • Posts: 3,275

Re: Draw thru Turbo w/ Carb Question, Disappointing Replies

I seem to recall hearing that bad things can happen when the throttles slam shut but the turbo was still supplying boost.  Something about the shock wave damaging the vanes.  It seems to me that this is where the blowoff valve comes into play but maybe it's not quick enough or something.
**Disclaimer** This tidbit is recollected from many many years ago.  I've slept lots since then so perhaps time has eroded my memory and technology has changed.

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  • From: Alton, IL
  • Registered: 2012-08-09
  • Posts: 105

Re: Draw thru Turbo w/ Carb Question, Disappointing Replies

The problem with that setup would be the blow off valve. It is a must have with the blade between the turbo and engine, but that is premixed air. So you would need a safe way to recirculated the "blown off" air/fuel somewhere. Other than that it will work fine in theory.

  • From: Central NC
  • Registered: 2009-05-05
  • Posts: 1,681

Re: Draw thru Turbo w/ Carb Question, Disappointing Replies

If the carb is upstream of the turbo it can't have butterfly valves in it for air control. Air control would have to be downstream of the turbo. The issue with putting a restriction like a set of butterfly valves upstream of a turbo is when the valves close it unloads the compressor. There is a lot of stored kinetic energy in the spinning wheel that suddenly has nowhere to go and that energy gets turned into angular momentum of the wheel itself. That translates in a sudden acceleration of the wheel to potentially destructive levels.

So you can draw through a modified carb but the throttle must remain between the turbo and the intake. When the throttle closes the air must be recirculated between the outlet and inlet of the turbo to prevent compressor stall.

The better choice if you can swing it is to do a blow through setup versus a draw through.

  • From: Central NC
  • Registered: 2009-05-05
  • Posts: 1,681

Re: Draw thru Turbo w/ Carb Question, Disappointing Replies

If you are going to run no more than about 5psi of boost then you can likely get away with doing a draw through setup.

Just make sure the seals are in great shape.

  • From: Columbus, OH
  • Registered: 2009-05-26
  • Posts: 2,839

Re: Draw thru Turbo w/ Carb Question, Disappointing Replies

Sometimes it's just a stupid idea.

I keed, I keed... ;-)

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8 Reply by Bayley (edited by Bayley 2013-05-28 11:53 AM)

  • From: Orion Twp, MI
  • Registered: 2011-04-18
  • Posts: 776

Re: Draw thru Turbo w/ Carb Question, Disappointing Replies

A close friend of mine (guy who actually drives the camo'd Focus in the mid west) built a draw thru turbo L28 Datsun motor for his 240Z.  Just like you, we would sit back and listen to every "expert" describe how this wasn't going to work.

Unfortunately, the carbon based seal really is your only option in this endeavor.  However, it works surprisingly well.  You don't need a blow off valve, the seals will last ten times longer than you expect them too and you can easily run double digit boost numbers.  The old draw-thru Datsun motor would run 10 - 13 psi no problem.  Driveability wasn't the greatest as power valves and vacuum secondaries would change engine load without changing throttle position.  Cold starts are tricky and take a lot more finesse to get motivated, but you get used to these things.   

Heck, the two of us drove his 240Z from northern Michigan down to Galveston Texas and back on a weekend road trip.  She purred along nicely while cruising at 120 mph.  smile

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  • From: Staunton, VA
  • Registered: 2012-05-16
  • Posts: 606

Re: Draw thru Turbo w/ Carb Question, Disappointing Replies

Is a blow-through not an option?

  • From: Land Of Subarus, Colorado
  • Registered: 2009-04-30
  • Posts: 9,416

Re: Draw thru Turbo w/ Carb Question, Disappointing Replies

http://www.murileemartin.com/UG/LNY13/042-UG-Monticello.jpg

Don't pay attention to anything the Internet Car Experts say on the "real" car forums. Instead, go with real-world LeMons-veteran advice. I can think of three draw-through turbo setups that have raced in Lemons (the Morrow's Auto Snoopy Van, the Turbo Schnitzel 351W-powered XR4Ti, and a 302-powered Benz a few years ago in California), plus there was the twin-Eaton supercharged draw-through setup on the Speed Holes Marlin a couple years back. All of those cars had a lot of problems(catastrophic backfire/blower explosion on the Marlin and a bunch of fried turbos on the Snoopy Van), but I don't think the vacuum-between-turbo-and-carb issue came up. I don't recall seeing any gigantic smoke clouds from oil-sucking problems on those cars.

The best-working carbureted turbo setup I've seen in Lemons was the Rally Baby blow-through setup, though. I figure, hey, if Rally Baby can make it work, ANYONE can.

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11 Reply by shredator (edited by shredator 2013-05-28 12:56 PM)

  • Registered: 2011-10-15
  • Posts: 94

Re: Draw thru Turbo w/ Carb Question, Disappointing Replies

what about pulling an equal vacuum on the other side of the compressor seal when the throttle gets closed?  im sure you could connect the oil drainback to the compressor inlet through some network of checkvalves and oil separator cans such that it should work in theroy.

  • Registered: 2012-04-01
  • Posts: 21

Re: Draw thru Turbo w/ Carb Question, Disappointing Replies

Draw-through turbo systems work well enough.  I had a Datsun straight 6 N/A motor I put a T3 turbo on, made an adapter for a 600CFM Holley (vacuum secondary) to adapt to the compressor side of the turbo, and ran it at 12-15 psi of boost (8.8 CR, 93 octane fuel) for about 70,000 miles.   Most of the older, pre-90's turbos had the correct seals in the turbo.  I never had any sort of blow-off valve or anything. No carb mods. The great thing is that since the throttle plates are before the compressor, when you let off the gas for a shift, the turbo shaft is still happily spinning away (as opposed to blow-through where you surge the compressor), ready for the upshift, no turbo lag during gearshifts!

The drawback is that they can't be intercooled beacuse the evaporated fuel drops out of suspension when it goes through the intercooler, plus the intake tract could be HUGE, especially if front-mount.

My 240Z went from a 15.1 quarter mile down to a 13.2 quarter mile, just from the draw-through setup!

I'd do it again. t-3 turbos with the right seals are a dime a dozen, heck I threw away a few from a 300ZX turbo - 80's vintage, a few years ago because their street value is nothing...

I don't know what all the other people (trackgeek, Debodaddy) are talking about on this topic.   Something about having to have throttle blades after the turbo, HUH?  I would have thought this forum had more than the typical internets "I heard from the NANCY BOYS that it could not be done"  blah blah.   Case in point: The factory Pontiac 301 turbo V8 (My Lemons dream) was a draw-through, the factory Buick 231V6 (pre Grand National) was a draw-through, the Maserati bi-turbo was a factory draw-through, the early Ford 2.3 Turbo (1979-1982ish) was a factory draw-through.   Plenty of examples exist, probably even in your local junkyard or craigslist so you can scavenge parts...

Also, if you feel like intercooling, plenty of fast drag cars are blowing-through slightly modified carbs with good results.

So do it!

Jer

  • From: Orion Twp, MI
  • Registered: 2011-04-18
  • Posts: 776

Re: Draw thru Turbo w/ Carb Question, Disappointing Replies

blackgt wrote:

Draw-through turbo systems work well enough.

...but was it any match for an 86 - 87 Intercooled Grand National if you were racing along I-35 thru downtown Duluth?

Oh yeah, what another Intercooled Grand National somewhere near the Ambassador Bridge on I-75 thru Detroit?

Dang, those pesky Grand Nationals sure liked to pick on you in the day.

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  • From: Central NC
  • Registered: 2009-05-05
  • Posts: 1,681

Re: Draw thru Turbo w/ Carb Question, Disappointing Replies

Draw-through setups work fine as long as when you choke the inlet of the turbo the kinetic energy in the blades is less than what is needed to reach the critical speed of the compressor or turbine.

  • From: Alton, IL
  • Registered: 2012-08-09
  • Posts: 105

Re: Draw thru Turbo w/ Carb Question, Disappointing Replies

blackgt wrote:

I don't know what all the other people (trackgeek, Debodaddy) are talking about on this topic.   Something about having to have throttle blades after the turbo, HUH?  I would have thought this forum had more than the typical internets "I heard from the NANCY BOYS that it could not be done"  blah blah.   Case in point: The factory Pontiac 301 turbo V8 (My Lemons dream) was a draw-through, the factory Buick 231V6 (pre Grand National) was a draw-through, the Maserati bi-turbo was a factory draw-through, the early Ford 2.3 Turbo (1979-1982ish) was a factory draw-through.   Plenty of examples exist, probably even in your local junkyard or craigslist so you can scavenge parts...

Also, if you feel like intercooling, plenty of fast drag cars are blowing-through slightly modified carbs with good results.

So do it!

Jer

The OP was the one wanting to do the blades post compressor. As for your examples, the OP is wanting better performance. The handful of examples you listed were absolute dogs.

If you find a carbon seal turbo I'd say do it. But if you are doing things in true Lemons form (grabbing stuff off the shelf in your garage), i'd recommend blow through. I modded an AFB for blow through and ran 18lbs through it all last summer. Ran good but heatsoak was always an issue in the carb body. And mine was intercooled as well...

  • From: Land Of Subarus, Colorado
  • Registered: 2009-04-30
  • Posts: 9,416

Re: Draw thru Turbo w/ Carb Question, Disappointing Replies

I hope that we're talking about a turbocharged 460 in the Lemons LTD. Right?

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  • From: Buxton NC or Truckee CA
  • Registered: 2013-03-25
  • Posts: 836

Re: Draw thru Turbo w/ Carb Question, Disappointing Replies

So the question then becomes, "how will a carb run if it's throttle body is well downstream?" The carb will always be under vacuum(hooray drawthrough) so it should continue to behave like a normal carb. My only worry with a downstream butterfly is that when you let off the gas the turbo will suck through a bit more fuel/air because the compressor will still be spinning and the engine will rich out for a second. That sounds right to me.

I'm a fan of Drawthroughs. I was an ACVW guy for a while and the drawthrough is a very common turbo setup on VWs. Most guys just run a T03 which is a carbon seal turbo.

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Re: Draw thru Turbo w/ Carb Question, Disappointing Replies

You cant pull equal vacuum on the "other side" of the turbo because that would mean a strong vacuum in the crankcase.  This can suck the crankcase seals in from the crankcase vacuum trying to even out with atmospheric pressure outside.

Throttling the engine in a spot besides where the carburetor is will most likely lead to lean spots, the air going past the partially closed throttle plates working with vacuum helps draw fuel in.  With them wide open there isnt much air velocity, it will probably go extremely lean.  I'd say it would probably have some/ a lot of drivability issues from that.

Not all t3's are carbon.  Its optional.

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19 Reply by widgetsltd (edited by widgetsltd 2013-05-28 06:01 PM)

  • From: Corona, CA.
  • Registered: 2009-08-03
  • Posts: 359

Re: Draw thru Turbo w/ Carb Question, Disappointing Replies

blackgt wrote:

Draw-through turbo systems work well enough...the Maserati bi-turbo was a factory draw-through, the early Ford 2.3 Turbo (1979-1982ish) was a factory draw-through.   Plenty of examples exist, probably even in your local junkyard or craigslist so you can scavenge parts...

Also, if you feel like intercooling, plenty of fast drag cars are blowing-through slightly modified carbs with good results.

So do it!

Jer

The Maserati biturbo was a blow-through with a Weber 2-barrel carb-in-a-box, which was strikingly similar to the Rally Baby AMC.

  http://www.maserati-alfieri.co.uk/18valve/18-valve-24a.gif

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Re: Draw thru Turbo w/ Carb Question, Disappointing Replies

Mike - talk with the Turbo Schnitzel guys.  While the xr4ti is no longer being LeMonsed, they report that they finally "figured it all out" with the blow-thru.

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  • From: Life's great in 21158
  • Registered: 2009-05-01
  • Posts: 3,361

Re: Draw thru Turbo w/ Carb Question, Disappointing Replies

See, these were the sort of replies I was hoping for with the original post.  I shoulda just asked the question here, first.  You guys are great.  *group hug*

Sounds like carbon seals are the way to go, like any engineer, though, I'm always thinking of some other, better, possibly easier and cheaper way to do something.  I cannot abide people who tell me an idea sucks and will never work and I should just do it the good ole way.  THis is why I run Lemons.

Phil- I actually sold the dun blowed up 460 to a guy in SC for $200 who was going to rebuild it and put it into a 4 wheel drive F150.  THe LTD has it's stock 400 in it now..which would respond surprisingly well to turboing, I think, with it's low compression heads.

This past weekend I was at a car show with Katie, and as we wandered around the over-restomodded Camaros and candy apple red 4 door Bel Airs, a primer black early-70's Dart Swinger caught my eye.  I wandered over to the father and son standing next to it, and peeked under the hood to find a bone-stock 318 with a homebrew blow-through turbo setup, complete with intercooler and a carb hat.  I chatted them up for a while, really good guys, said it was their first turbo setup.  The exhaust dumped in front of the right front wheel.  Looked awesome.  The son said he was running low 13's in the 1/4, and it was a blast on the street.  Best folks I met at the show, real grassroots types.

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  • From: oklahoma city
  • Registered: 2009-10-08
  • Posts: 1,586

Re: Draw thru Turbo w/ Carb Question, Disappointing Replies

blackgt wrote:

The factory Pontiac 301 turbo V8 (My Lemons dream) was a draw-through,

one of the guys in my shop is fully building a turbo TA pace car, and i'm pretty sure he wants to sell the stock running motor at some point.

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  • Registered: 2011-10-15
  • Posts: 94

Re: Draw thru Turbo w/ Carb Question, Disappointing Replies

jrbe wrote:

You cant pull equal vacuum on the "other side" of the turbo because that would mean a strong vacuum in the crankcase.  This can suck the crankcase seals in from the crankcase vacuum trying to even out with atmospheric pressure outside.

Does this actually happen? I thought people pull crankcase vacuum all the time with dry sump systems.

  • From: Orion Twp, MI
  • Registered: 2011-04-18
  • Posts: 776

Re: Draw thru Turbo w/ Carb Question, Disappointing Replies

shredator wrote:
jrbe wrote:

You cant pull equal vacuum on the "other side" of the turbo because that would mean a strong vacuum in the crankcase.  This can suck the crankcase seals in from the crankcase vacuum trying to even out with atmospheric pressure outside.

Does this actually happen? I thought people pull crankcase vacuum all the time with dry sump systems.

No, it doesn't happen.  People (i.e. drag racers) pull 20 - 30 in/Hg vacuum in their crankcases all the time.

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  • From: Central NC
  • Registered: 2009-05-05
  • Posts: 1,681

Re: Draw thru Turbo w/ Carb Question, Disappointing Replies

Bayley wrote:
shredator wrote:
jrbe wrote:

You cant pull equal vacuum on the "other side" of the turbo because that would mean a strong vacuum in the crankcase.  This can suck the crankcase seals in from the crankcase vacuum trying to even out with atmospheric pressure outside.

Does this actually happen? I thought people pull crankcase vacuum all the time with dry sump systems.

No, it doesn't happen.  People (i.e. drag racers) pull 20 - 30 in/Hg vacuum in their crankcases all the time.

The real danger is not the crankcase vacuum but in putting the compressor wheel under vacuum with too much momentum in it. Putting the crankcase under vacuum has positive benefits to the engine.