Jump to content

Twin SU Draw through turbo?


DatsWeird

Recommended Posts

Did a search on here and found some good info from datzenmike on draw through turbo setups. Question is, has anybody done a pair of SU's side by side or on a log style manifold, feeding into the compressor side of the turbo? Like maybe a Y style manifold? I like the turbo tom setup with a 4 barrel, but have a set of SU's ready to go. I've only seen this Rt8tr9X.jpg

Link to comment
  • Replies 20
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic

In theory, I would think drawing through the compressor would produce a more consistent vacuum on the carbs, making tuning fairly easy once jetting is perfected. The carbs are separated on a n/a, with each running 3 cylinders independent of the other but sync'd in vacuum to deliver an even fuel charge? I've never run su's, just an observation

Link to comment

From your research how much modification to the carbs has to be done? Also with dual carbs, I would worry about vacuum signal for a boost referenced fuel regulator. I know it's kinda oddball but I'm thinking of keeping it simple although it would be nice to have an intercooler

Link to comment

If the throttle shaft bushing is tight, you just need to provide boost lines to the float bowls and add a boost referenced fuel pressure regulator. The float bowls and regulator should be fed from your intake plenum.

  • Like 1
Link to comment

A draw thru setup has its draw backs, one is when you let off the pedal, the turbo air is cut off, yet the turbo is still spinning, eventually you will start sucking oil thru the turbo seal and it will start blowing a lot of smoke, I have a draw thru on my Datsun diesel, every time I pull the intake tubes off they are soaked with oil, but I have an SD series diesel engine, and have not figured out how to do it blow thru like I want, so I live with it the way it is right now as it runs real good, the first time I let off the pedal I seen negative 10psi.

Normally when I see a draw thru with an SU, it's only one SU, never seen a draw thru with two of them, even on a supercharger it is one SU/carb.

I have talked to some turbo specialists and they said they have draw thru setups(gas), when I asked them about the blow-off valve I used for my application, they said it should work, then when they found out it was a diesel engine, they said nothing works on a diesel engine to relieve the vacuum issue on a draw thru.

I would stick with one carb unless it is a blow thru, but that is way harder to do than one thinks, from what I have read it is a pain in the ass, would be better to use EFI, and they already did a turbo on the Z car engine.

 

By the way, the Nissan SD series diesel engines in the USA have a vacuum controlled throttle, so it is a real pain to turbocharge a vacuum controlled injection pump, the turbocharger screws everything up.

Link to comment

Not sure if Mike covered this in his write up, but I recall Julian, Okayfine on The 510 Realm did a draw through set up on an L and had issues with gas penetrating the bearing on the intake turbine. You may want to look into special sealed bearings (don't know the name) for what ever turbo you use. 

 

I got to thinking about the potential advantages of a two carb draw through set up. One problem with draw through is straddling good jetting for pre and full boost mixtures. If you ran both carbs into a single intake plenum then split that into all four cylinders you could jet each carb differently and use them like a true two stage setup with a boost activated gate on the rich carb. OR maybe I just smoked too much weed tonight.

  • Like 1
Link to comment

Not sure if Mike covered this in his write up, but I recall Julian, Okayfine on The 510 Realm did a draw through set up on an L and had issues with gas penetrating the bearing on the intake turbine. You may want to look into special sealed bearings (don't know the name) for what ever turbo you use. 

 

I got to thinking about the potential advantages of a two carb draw through set up. One problem with draw through is straddling good jetting for pre and full boost mixtures. If you ran both carbs into a single intake plenum then split that into all four cylinders you could jet each carb differently and use them like a true two stage setup with a boost activated gate on the rich carb. OR maybe I just smoked too much weed tonight.

I like the way you think, kind of like secondaries on a 4 barrel setup. I've heard that most turbo's from the 80's should have carbon seals, using this one 7vRNUQj.jpg

 

Why draw through? L-6s are fortunate enough to have the available turbo manifolds... blow through is safer, then when you get your AFR's right, you can help me out. 

I don't like them computin machines on my cars, all mechanical is my goal

  • Like 1
Link to comment

A draw thru setup has its draw backs, one is when you let off the pedal, the turbo air is cut off, yet the turbo is still spinning, eventually you will start sucking oil thru the turbo seal and it will start blowing a lot of smoke, I have a draw thru on my Datsun diesel, every time I pull the intake tubes off they are soaked with oil, but I have an SD series diesel engine, and have not figured out how to do it blow thru like I want, so I live with it the way it is right now as it runs real good, the first time I let off the pedal I seen negative 10psi.

Normally when I see a draw thru with an SU, it's only one SU, never seen a draw thru with two of them, even on a supercharger it is one SU/carb.

I have talked to some turbo specialists and they said they have draw thru setups(gas), when I asked them about the blow-off valve I used for my application, they said it should work, then when they found out it was a diesel engine, they said nothing works on a diesel engine to relieve the vacuum issue on a draw thru.

I would stick with one carb unless it is a blow thru, but that is way harder to do than one thinks, from what I have read it is a pain in the ass, would be better to use EFI, and they already did a turbo on the Z car engine.

 

By the way, the Nissan SD series diesel engines in the USA have a vacuum controlled throttle, so it is a real pain to turbocharge a vacuum controlled injection pump, the turbocharger screws everything up.

 

wayno, your diesel doesn't have a carb so how is it draw through and not just a blow into? Diesels don't have a throttle body? When you let off the gas doesn't this just reduce the fuel to the injectors? (bare with me I don't know much about diesels)

Link to comment

wayno, your diesel doesn't have a carb so how is it draw through and not just a blow into? Diesels don't have a throttle body? When you let off the gas doesn't this just reduce the fuel to the injectors? (bare with me I don't know much about diesels)

 

If that were only the case it would have been so much easier.

 

The SD22/25 series diesel engine throttle in the US is controlled by vacuum, there is no throttle cable going to the injection pump, the throttle cable goes to a throttlebody/carburetor looking thing, the TB(throttlebody) has a venturi on one side surrounded by a butterfly valve, when the butterfly is not open while the engine is running, it pulls a lot of air thru the venturi creating a lot of vacuum, this vacuum pulls the injection pump rack to the idle position, the more the butterfly valve is opened, the less vacuum the venturi creates, the injection pump rack moves to a richer position, when floored(butterfly wide open) the injection pump rack is in its richest position as no vacuum is created in the venturi, when the engine is not running, the injection pump would rest in the floored position except for the electronics that move the injection pump to the off position(fuel cut off), it's a very complicated system, but the inline injection pump used in these systems lasts a very long time, while the traditional VE type used in all other diesel engines does not last a long time, at least that is what I hear, the inline pump will last 300,000 miles or more.

A turbocharger messes up the venturi when it is before the venturi, it pressurizes the venturi instead of letting it make vacuum, so the injection pump thinks it is floored all the time as no vacuum is created, mine ran really hot when piped that way, and I mean smoke the engine hot if one wasn't careful, I could not drive it that way so it sat in my driveway for over a year while I thought about it, I tried another type of injection pump I had that had a cable going to the injection pump, but could not get it to run properly, so I gave up and made mine a draw thru, this fixed most everything except it created a vacuum issue that has been half fixed with a pvc tube, a couple pvc fittings, a spring and a ping pong ball. :lol:

If you really want to understand all of what I did, where the engine came from, what I tried to do to fix it, you will have to go to my build thread about it at NWD.

http://www.nwdatsuns.com/viewtopic.php?f=15&t=1013

There is a guy that says he succeeded in doing a blow thru, I talked to him about it, he is the reason my engine has real power now, something he said gave me an idea that worked, but mine is setup a different way, I might someday try to do it blow thru just to see if I can get it to truly work as I know more now.

Where I got this engine from was supposed to be a success also, but it was a failure too, that seems to be my calling in life, buy someones failure and make it work, that is how I came to have disc brakes on my work truck, I bought a failed idea back in 2002ish and made it work, but it took several years before it was a complete success just like this turbo diesel, several years.

Link to comment
  • 3 months later...

I can't say I have much knowledge on this idea for a turbo. but on my project, a single SU will be feeding an m62 supercharger in a draw through setup.

 

I have a Datsun friend who is running a similar setup to me, but with a bigger engine (4.0 jeep straight 6) and a bigger supercharger (eaton m90)

 

so i'd say one SU would work for your setup, however I can see the desire in two, and could see how it might help your fuel curve from idle to boost.

 

curious what you end up doing! good luck, and be strange, its what we do.

Link to comment

I have seen a 'Y' manifold to use a DCOE on a draw through system, probably in one of my many Peterson Publishing books.

So doing a twin SU is possible.

 

But I have always wondered how a turbo likes being at full tilt, and having the throttles suddenly closed ?

They are a little touchy on things like that.

Link to comment

The problem with SUs and turbos or superchargers is the lack of a physical accelerator pump.  They rely on a hesitation of the dashpot piston lifting to richen the fuel mixture.  This isn't enough for the volume of air you'll be pushing through it.  You'll go lean every time you kick it down.  That's why you'd be better off with a Weber, Carter, Edelbrock, Rochester, anything else that has an accelerator pump, and you may need to build a box around the carb to equalize air pressure at the carb vents with the boost, depending on how its set up, or it may not meter fuel properly.  

Link to comment

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.