Turbo w/twin SU's??

[veritech-z]

I really think it could be done, so long as you run some vacuum lines to the float bowls to equalize the pressure, and use a higher than stock pressure fuel pump. While you may in fact be crazy, that doesn't rule out the possibility of a twin blow-through SU setup And just because we've never seen it, doesn't mean it can't or even hasn't been done, right? (also, just because it's on the car and connected, doesn't mean it worked or ran-this car IS dead in a barn, after all!)

[roninjiro]

I really think it could be done, so long as you run some vacuum lines to the float bowls to equalize the pressure, and use a higher than stock pressure fuel pump. While you may in fact be crazy, that doesn't rule out the possibility of a twin blow-through SU setup And just because we've never seen it, doesn't mean it can't or even hasn't been done, right? (also, just because it's on the car and connected, doesn't mean it worked or ran-this car IS dead in a barn, after all!)

 

in that case i will try it with my spare motor just b/c i love the su's. i believe you can make it work but we wont know until someone has done it. it would be even better if it does better than everyone thought.

 

btw, veritech. as far as equalizing pressure in the bowls; what do you think would be the best way to do that when connecting the vaccum lines

[Tony D]

How will you bias the float bowl pressure against plenum pressure to enable on-boost enrichment different than what is available in a standard SU Jet Diameter Minus Needle Diameter fuel metering?

Most proper blowthrough setups use a biasing to up the float bowl pressure incrementally to enrichen the mixture by raising the float bowl fuel level, modulating the pressure on the inlet of the carb may affect the suction piston operation.

[roninjiro]

veritech-z?.....

 

u would figure by possibly modifying the slope beneath the piston to allow pressure to force the piston up as the boost rises thus if using custom ground needles the enrichment should be good enough to run properly and add much more power. but, you do indeed need to have much higher fuel pressure as well as some type of vaccum line going to the fuel bowls from inbetween the turbo and intake that can be regulated to pressurize it.

 

but this is just an idea at the moment, as i never boosted b/f. so input is greatly appreciated

[veritech-z]

My SUs have a little overflow hose fitting in the top of the float bowls, I was thinking that if you pressurize them using a boost reference line sourced off whatever you are using as a plenum, it should keep the fuel from backing out of the bowls under boost...I'm no SU expert either, I always thought that the fuel enrichment on those was controlled by the slope of the needle, something like every 1/8" equalled 500rpm of engine operation or something like that. Shouldn't you be able to fiddle with the needle specs to get the enrichment curve you need? I spoke to a guy on the phone at a place that does turbo Mini's using a single Jaguar SU (2" inlet instead of 1 3/4" like on the Z), and they said they just bolt them on as is and slap the turbo on, but I didn'tthink to ask him if they were draw through or blow through at the time...In retrospect they were probably draw through...I'm very curious about this mystery car that phantaz found, because I've never heard of SUs being run this way. If it were as easy as all that, or worked as well as all that, I imagine it would have been done more often...

[OlderThanMe]

the needles would bepretty hard to do I think. If you have looked at them the angle on the needles is very slight. If you had a micro sized lathe and didn't mind pulling your carbs apart 500 times to regrind the needles to the proper size then go for it! Then the problem of getting bowls set up right... Anywhere over 8-10 PSI would be hard to tune I think. Whenever you wanted to go up on boost you would have to change the needles.

Good idea I think and good luck!

[veritech-z]

TonyD, would you want more, or less air pressure in the fuel bowl under boost? Do you need air pressure at all, or could you get away with just running an FMU and a block off in the overflow? What about using two supplemental injectors on a hobbs switch right behind the carbs, and leaving the rest normal?

 

OTM, there are SU needle size calculators all over the net, the problem would just be finding a source for them. The Mini Cooper guys have a lot of resources, but I hear that the Japanese made SUs aren't exactly compatible with the British made ones...

[Tony D]

Air pressure in the float bowls is regulated by a modulator ring at the entrance to the main carb throttle body to give incrementally more pressure in the float bowls, compared to the throttle bore itself.The suction piston still works THE SAME as on a non-blown through carburettor, as there is STILL a pressure differential between the front of the carburettor and the back. Needle diameter is not done on a lathe unless you are making a new needle from brass stock. You have to chuck it in a lathe or drill, and use crocus cloth on the various jet-metering-steps to remove material. On an SU you HAVE to go from a state of LEAN to RICH while tuning the needle taper---not exactly what you want to do on a turbo car.

 

I went through hell one summer in 79-80 tuning a single draw-through SU for a 2.6L car...... I can tell you from experience, it's FAR EASIER to do this via standalone EFI.

 

The needle stations do NOT correspond to rpm! The correspond to load / manifold vacuum referenced points. It may be at one station at 1500rpm, and the same station at 4000rpm---the difference being throttle position and load imparted to the engine. You have to determine which station you are at by a tool that sticks up therough the damper cover, which shows your stations scribed in lines...you makre load and rpm -vs- AFR and make your needle adjustments from there.

 

Like I said, the needles will be done from Lean to Rich, so if you go too far, you are over-rich, and get to start all over. Hopefully you are good taking notes and know the diameter you started with, and where you went too rich, and can go back on your next needle to the righ diameter. You get real good with light touches using 600 grit crocus cloth dabbed with mother's polish to take .0005" or less off the needle at a time.

 

Oh yeah, and each time you do the stations, don't get that cloth on the previous station, or you screw it up.

 

And make sure you put that needle back into the piston EXACTLY where you took it out from, relative to total length extended into the jet body or you will be off---making you sand the wrong position the next run...

 

And yeah, you gotta do TWO of them EXACTLY the same instead of just one...

 

And that means you will get really really good at pulling the suction domes and pulling the needles out and putting htem back in...

 

And we are pursuing this WHY again?

 

Suffice to say, you can blow-through an SU. The jetting does not have to be operated much differently that the N/A taper IF you use the modulator ring and riase the float bowl level. (I wish I had known about this back in 80 for my drawthrough as I could have used an adkustable FPR to raise float bowl level on-boost and keep the needle a bit fatter for better off-boost fuel economy) Problem is you will be estimating how much fuel you can pump through an open orifice at incremental pressures as the boost goes up.

 

At some point you will run past the original SU inlet nozzle's capacity to fill the bowl and keep the fuel level steady, your float bowl level goes down, and you run lean on-boost.

 

BOOM!

 

For all the effort to get them half-assed working correctly, you would be WORLDS AHEAD gutting the SU's, putting a 1.5" spacer behind each of them, installing a 1000CC/Min injector facing forward towards the throttle plate, and drive them using a Megasquirt.

 

I 100% guarantee that the Megasquirt setup will deliver MORE power, BETTER mixture control, and take LESS TIME TO SET UP than working with the SU's.

 

On an N/A car, dialing in SU's is one thing. Go turbo, and they start falling short. They weren't designed to work that way, and you are really trying to make a hammer do the job of a scalpel during brain surgery!

 

Oh yes, and on the "foam filled floats" issue. The stock brass floats tend to collapse around 10psi. You guys are working way to hard to make a BAD idea work poorly (at best)...

 

Been there, done that....never again!

[OlderThanMe]

I agree...

You can get megasquirt and a WBO2 sensor for a decent $600 and get the car making WAY more power in less time...

I plan on using stripped out SUs just for giggles with injectors underneath each of the intake runners...all the EFI stuff will be under heat shielding and hidden...

[roninjiro]

so it that much bad of an idea huh?, ok.

[Lunar240z]

I agree...

You can get megasquirt and a WBO2 sensor for a decent $600 and get the car making WAY more power in less time...

I plan on using stripped out SUs just for giggles with injectors underneath each of the intake runners...all the EFI stuff will be under heat shielding and hidden...

 

what about gutting the SU's and putting some injectors in there? not exactly optimal... but kinda cool

[Tony D]

Well, you can do what I said... to make the carbs work. They might do it. But you will be limited to 10psi from those brass floats. For 10psi, why bother? IMO for that amount of boost, you will get more reliable response from the stock L28ET setup, using Megasquirt for fueling.

 

As olderthanme says, for the SU's on a N/A setup, you can make some serious "stock looking" power, yet have all the advantages of EFI.

 

For olderthanme, here are some interesting thoughts:

The crossbar linkage on the balance tube has an ID slightly larger by about .010" thatn a common hardware screw. If you get this screw with a shoulder on it, it is the EXACT same diameter as the stock Nissan "D" Shaft sticking out of the side of the T/B. Meaning file a flat on it, and it rotates in the same direction as the stock Nissan Throttle Position Potentiometer, so with a small plate on that front casting boss, and the screw stuffed into that balance tube crossbar, with the screw pinning it form roatating, you have your self a nice TPS setup for your "gutted" SUs... Using a tube and cutting the bores out of the SU body on a lathe using a boring bar makes for a nice insert for a 45 or 46mm Straight-Through throttle bore, which can utilize the stock air horns inside the stock air cleaner. If you do it F-1 style, and put your two large 550cc injectors in the air horn using a device similar to the Ford TBI setup on the 5.0 HO engines in a Mustang, or Crown Vic, you have a system that supports at least 215 HP, and all the tubing can be concealed in vacuum tubes...

 

Not that I have given any thought to hiding a system in stock SU's, or gone through preliminary mockup or anynthing.... LOL

[OlderThanMe]

Lunar: that COULD work..and I have seriously considered it...

but TPI is more accurate than TBI...plus with a good aftermarket EFI and TPI you can individually tune each cylinder to what it needs.

 

Tony_D: I am going to be building mine for a turboed motor...for the fact that that SUs won't work very well(for a turbo) and will have people scratching their heads.

What is the stock ID of an SU body? I think I measured mine at about 43mm but that was a while back.

THANKS for the info about the TPS!! that would work wonderfully! So would I use the stock butterfly valves in the SUs? that seems like it should work.

I'll get my SU parts out and put my carbies back together to figure out how to set this up..

Thanks Tony

and merry Christmas everybody!!

[roninjiro]

sry to bump a thread back to carburetrion but i have read and read and it is very possible and CAN be very effiecient to boost the su carbies.

 

if you really study in comparison which is better, draw through, blow though you might see where im coming from.

draw through:

1. carb must be sealed & pressurized (yes you can get a solid float, or mod the old one)

2. ability to use an FMIC

3. throttle response instant

4. must use a boost referenced fuel pressure regulator

5. YOU CAN BOOST HIGHER THAN 10PSI

6. You will have to go with about a 14 psi fuel pump (see 4)

 

draw though

1. easy, factory method

2. cant use any type of air cooler

3. tuning is more critical

4. fuel to all 6 cylinders with one carb evenly is unheard of

5. throttle response is sluggish at first.

6. no big worry of carb vacuum leaks

 

i read and read and read through a turbocharging book that had turbocharging with carburetors (Maximum boost) and the hot rod mag site, and the long search through google.

 

anyways the conclusion is that what you did, draw through, is TOTALLY different than what a blow though set up. sry, if im out of line about the way i addressed this b/c maybe some of us (me) do want to pursue this and when your one a "hybrid" z site, you figure people would be more open minded about it. but then again what do i know, i guess i read too much

[OlderThanMe]

open minded means accepting the good and bad...

draw through is definitely not what you want.

If youdecide to do the blowthrough SUs the more power to ya' buddy.

It just seems that with the highy accurate programability of EFI can adjust for any boost and KNOW you aren't going to hit a lean spot at full boost and instantly detonate your motor.

[roninjiro]

i understand what you are saying, but what D said "And we are pursuing this WHY again?". that is IMO an atempt to shut someone down.... see what im saying? of course EFI is easier, but what about uniqueness? just to be different and successfull is how i operate, the "odd one" you might say. but i challange you on lean spots OTM. when you have EFI even in testing you will hit lean spots if you have little experience with setting the system up; as with carburetion. take pros from both catagories and i pretty sure that both can get almost the same results w/ different methods of getting those results.

 

what i am trying to say is, "anything is possible, but are you good at it is the question." this seems to be territory were skill comes into play. can you design, build, modify, tune, and be successful? thats basically what it comes down to in the end.

[veritech-z]

I think Tony D is whay you might call "the bitter voice of experience" in this situation. I know I had a few ideas about how this might work, and OTM might have done, but I really didn't know much about the nuts and bolts. Although I did have the same idea about firing injectors down the throttle bore...

[OlderThanMe]

ah... but with EFI if you hit a lean spot you make a few clicks and you have no more lean spot...even with a WBO2 it would take forever to get your SUs not to hit lean spots...if you had an easy way to work on SU needles then sure go ahead. It seems all hit-or-miss with trying to tune carbs for boost fuel addition. And if you wanted to up the boost you would need to change needles again.

If it were me tuning the SUs on a turbo motor by the time I had them close to tuned I would have melted pistons...If you were getting close but went too far on a needle you would have to start over...

 

Veritech's idea of putting the injectors pointng down the throat would work great. An injector under each intake runner would be even better for a high performance engine that you could tune.

If you want to do something different and unique why not do a SINGLE 4 barrel carb and turbo it? V8 guys have been pushing and pulling air through them with forced induction for years!!! Just hardly ever on an L6 though.

[Tony D]

I think Tony D is whay you might call "the bitter voice of experience" in this situation.

 

Indeed, feel free to tread the exact same path as other have done in the past, and call your self unique, simply because you are ignorant of other people's accomplishments. not throwing stones, here, just shaking my head and laughing at the "to be unique" argument. Veritech speaks the truth, my question was, reading everything you have to go through, logic should start coming in to the equation somewhere and you have to start weighing the amount of work versus the amount of return.

As for you 'plan' and bold comments, it sounds like someone more intent at being upset rather than listening to advice from someone who has been there.

 

In particular, this was kind of funny:

"take pros from both catagories and i pretty sure that both can get almost the same results w/ different methods of getting those results."

May be, but one using stone knives and bearskins will take a bit longer to tune your vertical hold than the one that comes iwth a diagnostic scope and a full modern set of TV Tuner's tools. At the top levels of racing, you will find when you look closely that most come to the SAME conclusion albiet through different paths, but what they end up running in the end usually looks identical to the other guy because some stuff really does work better than other stuff does. What you are proposing is a sub-optimal setup, you will work a lot to achieve results similar to what you can get on stock EFI manifolds and a boost controller. Prove me wrong, happy to see innovation...but don't kid yourself you are unlocking some overlooked performance secret!

 

So, before you blow up your engine, let me point out a few things you have missed in your latest plan, or downright have wrong, I'll take your points as you proposed them:

 

BLOW through:

1. carb must be sealed & pressurized (yes you can get a solid float, or mod the old one)

>>>>Carb needs a bonnet over the throat and accompanying float-bowl pressurization. Unless your throttle shafts leak like seives, sealing them in unrequired. If you must, running small tubes from your plenum to the throttle shafts to make a 'clean air buffer seal' arrangement is relatively easy.

 

2. ability to use an FMIC

>>>>True

 

3. throttle response instant

>>>>drivability like that of a N/A car throttle response wise, but also possible to boiil your fuel after long on-boost episodes (especially if non-intercooled).

 

4. must use a boost referenced fuel pressure regulator

>>>>wise in either case

 

5. YOU CAN BOOST HIGHER THAN 10PSI

>>>>in all caps, O.K., read below to get even more mad, and to start shouting even louder against the laws of physics

 

6. You will have to go with about a 14 psi fuel pump (see 4)

>>>>Plain WRONG! If you run a 14psi pump, you will be limited to 10psi. I call tell you FIRSTHAND that when you boost to within 1-2 psi of the fuel pressure (you say 14psi at the pump---what at the fuel bowl...12?) you end up RUNNING LEAN and the boost pressure pushes your fuel back up the pipe and deadheads you pump. You need an EFI pump, with the ability to keep AT LEAST 3psi ABOVE your boost pressure, or you WILL blow your engine from running lean. If you do not change your floats (the subject was SU's, so unless you are using the 73-74 Flat Tops with Composite Foats-----HEY, you said unique, maybe that is the EXACT carb setup you are using so you are truly "unique"!) you WILL collapse your float at 10psi. You will collapse Weber Brass Floats at 10 psi. Rochester Floats make it to 12 psi before collapsing. Where are you getting these composite floats for SU's again? Because it's a trick item, and I want some. Not throwing stones or being sarcastic, but countering people making assumptions and theorizing about stuff that they haven't tried yet, and may not know the whole story about. If ther eare composite floats available for SU's, then I don't know about them. (Oh, BTW, those Plastic Hollow Floats crack, take on fuel, and sink...they may not collapse like the brass units, but the fail in short order....) And the stock brass units collapse at 10psi, meaning that WILL BE your limit without changing the floats. If you don't change your floats, using a 14psi pump (oh, like a Holley Red, perhaps?) and 10-12PSI will be your limit before detonation kills your engine. I'm not arguing, I'm relating facts from firsthand experience and observation...

 

DRAW though:

1. easy, factory method

>>>>If you have a Buick, or other GM product from the 60's. BLOW THROUGH was the standard by the 80's on European Vehicles like the Lotus and Maserati BiTurbo.

 

2. cant use any type of air cooler

>>>>This does not mean you can't forestall detonation through other means of "charge air cooling"---such as methanol injection, or simple water injection. I personally have run over 20psi on non-intercooled blowthrough and draw-through systems. An intercooler is noice, but in some applications you simply don't have the room, and it's no big deal, you work aroudn it with alternate types of 'charge air cooling'.

 

3. tuning is more critical

>>>>If you think tuning is easier on one than the other, you are mistaken. If anything, tuning on a drawthrough is easier because of using the carburettor in the same state it normally is, with a vacuum source underneath it! Though once you go on-boost with an SU using a needle and no supplementary injection of fuel, you're screwed. Limit your boost, as there is only so much fuel you can pull through that jet, and unless you run a boost-indexted FPR (yes, on a draw-through) or some other way of raising the float bowl level HIGHER than normal while under boost, you will have one HELL of a time getting the on-boost AFR correct after 12psi. Using a Quadrajet, no problem. A pair of SU's might be easier to get the fuel, but I feel for the poor bastard who will have to calibrate them and make sure they are giving the right mix.... I shudder at that.

 

4. fuel to all 6 cylinders with one carb evenly is unheard of

>>>>ABSOLUTELY! But Fuel THROOUGH A TURBOCHARGER to all six cylinders is EASY! As Smokey Yunik said of the "Turbocharger"---it's not really so much a turbocharger as a "homogenizer"---AFR out of that turbine blade is VERY consistent. Getting the air to the six is the easy part.

 

5. throttle response is sluggish at first.

>>>>First? As opposed to later? I don't know what that is supposed to mean, but Throttle Response is directly related to intake length and velocity through the system. This should say "Throttle Response worse than blow-through" and that's about as far as you can take it.

 

6. no big worry of carb vacuum leaks

>>>>HAH! Vacuum leaks on this draw-through will equal DETONATION through running lean. On a Blow Through, the vacuum leaks will turn to (and that probably should say 'might') pressure leaks, which means fuel-air may leak out. On a draw through, you just run lean and detonate.

 

To sum it up, no, I'm not trying to shut anyone down, but give me a logical reason to go through ALL THIS when you can have the EXACT SAME LOOK, proper fuel distribution, and SAFE engine operation with far less work?

 

You feel free to make up anything you want, I encourage it. But to make a comment like this: "anything is possible, but are you good at it is the question.this seems to be territory were skill comes into play. can you design, build, modify, tune, and be successful? thats basically what it comes down to in the end."

 

Well, to put it mildly, it's downright insulting! Impugning someones' experience through an arrogant claim to say you are good at somthing...well let's just leave it at this: I've done this setup, I've made it work---you haven't as of yet. My question of WHY comes directly from that experience. You claim uniqueness. Well, fine. Then performance isn't the goal. Cool. Go for it! But don't think this setup is anything new, and if you think you can argue or change the laws of physics, then you are, indeed skilled to a godlike level, and we should all bow before you and drink of your intelligent succor when it comes to making 1930's techonlogy work the way it did in the 1930's.... Bravo!

 

There is a difference between being "open minded" and simply choosing to ignore some hard-learned facts from those who have been ther ebefore you...you are correct ronjiro, for someone posting here, I would have expected you to be a bit more open minded...

[Tony D]

If you want to do something different and unique why not do a SINGLE 4 barrel carb and turbo it? V8 guys have been pushing and pulling air through them with forced induction for years!!! Just hardly ever on an L6 though.

 

Richard's Z&ZX Service was in the low 11's in the early 80's in a full bodied ZX with a HOlley 650 Draw-Through Setup. My 73 had over 350HP on a similar setup using a Rajay Turbocharger, and External Wastegate. See the post below on "Turbo Toms" Information---the four-barrel turbo L6 was actually quite a popular conversion into the 80's here in the USA. In japan, they went with blow-through mikuinis in the 70's, and went to EFI by 1985---if you were a serious competitor. Till 1989, the Carboy L-Engine Shootout still had a majority of top competitiors running blowthrough Mikuinis or OER carbs. There were only 3 EFI drive teams at that one... The next year it all changed, and the tide swung forever to EFI setups.

 

Both of those turbo-Z's I had were daily driven. The Mikuini car got the nod towards drivability. It averaged 17mpg at that power level (though when using power, it's mileage was more like 5mpg). The suck-through Holley wasn't that great on boost onset, nor drivability when cold compared to the Mikuini car, but on-boost they were about equivalent.

 

My Triple Mikuini Blowthrough was detuned with a smaller turbo and lower boost (as well as selling off the FMIC) to around 300-350 and was dialy driven 26K miles a year in Commuter Traffic in L.A. from 90-94.

 

Then EFI came Down the Pike that was affordable. And after dealing with suck-through and blow-through carburetion since my first Turbo Corvair upgrade in 1979, I BID CARBURETION A FOND FAREWELL AND NEVER LOOKED BACK!

 

So what do I know, eh? Feel Free to Carburate. Want to buy a four-barrel-to-turbo adapter, how about SU-to-turbo Adapter? I got all sorts of stuff laying around the containers out back. My goal for them is to construct a "period conversion" car for shows. Not to drive, gads compared to EFI vehicles they are HIDEOUS, but just to show the kids who never saw a carburetted turbo setup what it was like back in the 70's...