Big SU's?

[260pos]

Yes, I am a pain!

I have a question about SU carburetors, the real ones.

I found that there are 2in su’s. Model no’s are H8, HD8 & HS8.

I was speaking with “Dtsnlvrs†(He has one sweet 240, IMHO. Off the subject) Russell said that the SU carburetors won’t cut it at high RPM’s in a modified engine.

Now he runs triple Webers, I have had experience with them, duels on an old 510, but I’m looking for that stock look with less maintenance.

From what I have found on the net, I think that the big SU’s could support about 220 horsepower. (Link) Not that my motor will put that out, but I’m hoping to exceed the >170 HP that this graph states 1 ¾ SU’s can handle.

Oddly, I can’t seem to find much information on this swap. (Hybridz link) Would I need to modify the flanges on my stock N36 intake manifold? What about the linkage, and would the stock (I’m sure modified) air filter choke off any gains?

Any Information would be helpful.

Thanks, Jim

[SHO-Z]

I really think that you will not gain a lot without changing the intake manifold. I would look at triple SUs like the E type Jags had. But that is just my opinion. The stock manifold was designed to flow the max stock carb flow. The large carb could only flow what the down stream manifold will allow.

[260pos]

I thought of that, I figure I’ll just take the old rotary file and hog the runners to match. It all has to get down to the size of the intake ports on the head. I feel that the triple su setup would decrease the velocity too much and lose some throttle response, that’s just a SWAG, I could be wrong…

[Dtsnlvrs]

I have seen a pair of HS8 Jag SUs modified to work on the N36 manifold, but I cannot recall the specifics of doing it, but it has been done. In your case, I say we get a pair of "stock" SUs running right on your current engine and go from there.

[Tony D]

HIF8's?

Do they make such an animal? They make an HIF6, which is similar to the 73/74 "Flat Top" carbs, except the nozzle height is adjustable with a screw on the side (much easier than the HS style nut under the body setup), with an auxillary air circuit for idle only. This makes for a carburettor that idles with proper A/F Ratio, but once you crack off the idle circuit, you run on needles that are much richer. Best of both worlds...

 

There are people making over 220 HP with the Stock Hitachi SU's, the manifolds flow suprisingly well. With CV Carburettors, the power level is really determined by jet diameter (the 2" Jag Units use a .125" jet I think...) so whatever Pounds Per Hour you can flow through that jet, it the Horsepower capability of the carburettors.

 

For all the talk of Triple SU's, every setup I ahve seen on L-Engines have been failures. Mostly because they use 46mm Hitachi Stockers. Way too much flow capability for the suction piston to lift and fully uncover the jet and give full fuel. For proper sizing, you really need 38mm SU's like on the SSS setup. Now you take some Fairlady Z L20A SU's, and stick three of those babies on a 3.0L Engine, you should have proper flow capabilities and jetting out of the box, with the ability of easily supplying 150 rear wheel horsepower easily, if not 180...

 

Lot of work for little reward, and dubious functionality over a set of properly prepped twins---those 38mm units will ultimately only flow about what the 46mm Twins will flow...

[260pos]

I have seen a pair of HS8 Jag SUs modified to work on the N36 manifold, but I cannot recall the specifics of doing it, but it has been done. In your case, I say we get a pair of "stock" SUs running right on your current engine and go from there.

 

Definitely get the Hitachi's going first! As I have mentioned in previous posts I’m building a “paper†car first. I’ve build to many vehicles that just weren’t thought out well enough before I started modifying them. I’m going to make a plan… Mwah hah hah!!

[randy 77zt]

rebello bores the stock su's-i have been at thunderhill running against a z with rebello stroker and su's-it ran even with my stock 81 turbo powered 77.i would guess to say that 2 bored out su would flow more than 500 cfm.if i was going to run carbs on a z it would be su.

[260pos]

HIF8's?

Do they make such an animal?

 

There are people making over 220 HP with the Stock Hitachi SU's, the manifolds flow suprisingly well. With CV Carburettors, the power level is really determined by jet diameter (the 2" Jag Units use a .125" jet I think...) so whatever Pounds Per Hour you can flow through that jet, it the Horsepower capability of the carburettors.

 

For all the talk of Triple SU's, ...

...Lot of work for little reward, and dubious functionality over a set of properly prepped twins---those 38mm units will ultimately only flow about what the 46mm Twins will flow...

 

As far as the world of Google, there aren’t any HIF8’s. I know Ztherapy can hog the Hitachi’s out to 46mm. But the 2†SU is almost 51mm, which would give you quite a bit of flow. I imagine at least 575 – 625 cfm, quite a bit less than the 1086 cfm triple Weber 40’s will give you. But keeping that all important velocity up and less trouble than going to triple SU’s that most likely wouldn’t give you much more power.

Thanks for the reply Tony, you have lots of insight on the little Z-monsters, we all appreciate it.

[260pos]

rebello bores the stock su's-i have been at thunderhill running against a z with rebello stroker and su's-it ran even with my stock 81 turbo powered 77.i would guess to say that 2 bored out su would flow more than 500 cfm.if i was going to run carbs on a z it would be su.

 

I just checked out Rebello’s Big bores are $850.00 a piece plus parts. They say they flow 365 cfm, my SWAG of the 2†SU’s cfm must be way off, or Rebello really bores these beauties’s out! (Speed is money, how fast do you want to go?)

Thanks for the insight Randy.

-Jim

[Tony D]

Yes, the HIF 6 is popular in Europe due to the adjunct idle circuit. It really makes tuning the power range of the carb easier when you aren't running pig rich at idle.

 

The CFM rating on Webers is a bit confusing to most people. It all depends on where your main venturis are, saying this setup has "XXXX" CFM is really only a paper guess, as that number will change mainly on the main venturi size. And that venturi size dictates more where the power they will ultimately produce...in combination with the cam.

 

Throw in ITB's and all of a sudden you chuck all the rules out the window. A set of 45mm ITB's can be a drivable streetable combination and make just as much HP as a set of 55 MM Webers, which would never be a streetable combination.

 

The downfall of a carburettor is it, by design, must restrict the airflow in order to work! If there is no restriction through a carburettor, there is no pressure differential, and therefore no fuel being metered out to the engine!

 

Take those same SU's from a stocker, bore them straight through after removing the suction dome, and you have two honking bores capable of flowing massive amounts of air compared with the standard SU.

 

Injection, gentlemen....injection! That keeps things looking stock to the casual observer, but the flow increases to the engine are far beyond what the same bodies would be capable of in carburetted form!

 

FWIW, was that CFM ratingfor the Rebello units per carburettor? They are vague on that point. Ultimately on the top end, the vacuum in the intake manifold will determine ultimate pumping efficiency of the engine, and therefore potential horsepower. Given 14.7 psia, and 9" Hg Abs on the manifold side, at 70 degrees, a 1.36" orifice should flow 365cfm....but that's a straight tube. See where I'm going with the logic? The stock EFI T/B flows almost that much, but does not require a 9" HG Differential in the manifold to function relating to atomization of the fuel...

[WEBEZEEed]

260pos....... you have an E mail:wink:

[260pos]

260pos....... you have an E mail:wink:

 

 

Sure PM sent

-Jim

[2126]

I just checked out Rebello’s Big bores are $850.00 a piece plus parts. They say they flow 365 cfm, my SWAG of the 2†SU’s cfm must be way off, or Rebello really bores these beauties’s out! (Speed is money, how fast do you want to go?)

Thanks for the insight Randy.

-Jim

 

Just a correction on the cost of the Rebello 50mm SU's....that $850.00 a pair, not each! How do I know this, I purchased a pair. These babies are little jewelery. The profile on the needles are custom ground, as none are available for a 50mm unit.

 

Tom

[johnc]

I have a pair of HS8 SUs that were part of a Vintage Racing project plan that got cancelled. They are from a Rover 2000 TC. I'll sell the pair for $250 if one of you guys are interested.

[Z-Gad]

A local old race team also bored the SU's, and used the Jag butterflies in them to give the large diameter...

.

[JMortensen]

I think Dan Baldwin ran 2" SUs before he went to SK triples. Maybe he'll comment...

[Tony D]

I have a pair of HS8 SUs that were part of a Vintage Racing project plan that got cancelled. They are from a Rover 2000 TC. I'll sell the pair for $250 if one of you guys are interested.

 

 

Reeeeeeeeeeallly....

 

I may have to stop by when next in town...

[OlderThanMe]

so everyone makes such a big deal about the 50mm SUs...I measured mine just to see where I was at and measured 47mm...is 3mm such a big difference? I don't see how that could flow SOO much better than regular SUs with air horns. That would aproximately be like boring each intake runner on a triple weber manifold 1mm...not THAT much of a difference...

The other things like new needles and a rebuild probably do more for performance.

I guess I am just being a pessimist here...oh well.

[260pos]

Olderthan, Well… 3mm is not really 3mm, by the specs Hitachi’s are 46mm, and the difference (dia x pi) is almost 12.7 mm squared or 8½% bigger. So you can figure at least 8% more flow considering Tony D’s observation that a carburetor HAS to restrict airflow to draw fuel. Rebello is boasting a 46% CFM gain in there advertising, but that is using an unrealistic 25” of vacuum. Fluid dynamics confuse the hell out of me! But by the link in my original post, 220 Hp should be a supported with a 2” 50mm carburetor. The most Nissan ever pulled out of the L26 was 162 hp gross, well within the working range of the 46mm carbs. If GM can get 71 hp per liter out of a pushrod crate engine with a 10.5:1 CR, we should be able to easily get 195 hp from a L28 engine, as a smaller engine is more efficient. **Some applications may not be emission legal; check state and local ordinances** At that power, the bigger SU’s should be just the ticket, without the complications of side draught tripples.

[viperredls1z]

I have a set of Rebello bored 50mm SU's that I'm not useing that I would sell for $500, they put out 280 HP on Rebello's dyno, on a pump gas stroker.