Thank you. That was very helpful. The reason for the use of HD8 ‘s is I already have 3. ( need 4). I have a great selection of needles. And have raced for decades using them. I feel I can quickly adapt them to using E85 and pull some extra horsepower without much effort.
For example the stock carb manifold has a dog leg to pick up heat from the water manifold on the head. Elimination of that dog leg is worth 35 horsepower. Since the carbureted V12 is only 20 horsepower less than the fuel injected engine with no other change I’m 15 horsepower ahead.
As a race car devoid of the weight of extra seats, bumpers, air conditioning, and other luxuries the car will be significantly lighter. A Manual transmission is at least 106 pounds less, my goal will be to sit on the race track at a little less than 3000 pounds. with more than 400 horsepower. Or around 13 pounds per horsepower. ( stock its over 17 pounds per horsepower )
With a dog ring racing 5 speed non overdrive gear box I can use camshafts that will be very peaky
(more horsepower in a very narrow band) yet never be “off cam” In short, make more power.

 

Makes sense if you only need one. I quite like the HIF carbs, simply because instead of the float being in a seperate chamber at the side, it's integral in the carb body. Make a more compact carb, and the float valves seem less likely to stick than in the HD series. Same tunability of the HD series, but I think it does a slightly better job of keeping the air fuel ratio on target.

 

Height wise how do they compare? It would be nice if they were shorter in order to fit the carbs without the dog leg I need to lower the engine now.

 

About the same height. Looking down from the top, the HIF7 (1-7/8 bore) is basically square: 4" deep and 4" wide.

 

I just measured my HD8’s they are 5&1/2 from the top to the centerline of the throttle shaft what is that measurement

 

I just measured my HD8’s they are 5&1/2 from the top to the centerline of the throttle shaft what is that measurement on HIF7’s ?

 

From top of the damper to centerline of the throttle shaft is 4-3/4"

 

Saves 3/4 inch. Hmmm I wonder what the learning curve is compared to HD8’s. And there is the issue of needles.

What are they standard equipment on?

 

Some Rolls Royce, and XJ6 Series 2. If you're using them on a V12, I think they are probably too large if you're using 4 carbs. The HIF44 is smaller (44mm bore), and I think that is what SNG Barratt uses on their SU conversion kit for E type V12's. If you look at Burlen Fuel Systems in the UK, they are the OE maker of SU carbs and have all the needles, gaskets rebuild kits etc.

They are easy to tune, instructions come with the rebuild kits, and Haynes prints an SU carb manual. It's very similar, set the jet height and mixture screw is on the side. Float adjustment is much easier and usually only done once when the carb is assembled.

 

The 4 stock Strombergs are 1&3/4 on a V12 but group 44 raced their XKE with 4 HD8’s. (2” )

You are absolutely correct for the street. Racing? You’ll need to set the idle up and choke the heck out of them on startup. Maybe even a little either on cold mornings.
No, you cannot run the 2 inch without removing the drop leg. And that gets into lowering the engine stuff
I haven’t opened up a HIF7 to see if anything will be affected by the alcohol in E85. The nice old fashioned brass floats make the HD 8 work

Cork and certain kinds of rubber react poorly to alcohol even ethanol. Luckily they don’t use methanol because that is horrible to aluminum brass and most gasket material. You’d think it was acid instead of alcohol.

 

I saw dyno charts that TWR did when looking at developing the Jaguar Sport range of engines. They built a 6.1 and then did incremental changes to see what effect it had on HP. They ran standard 63mm throttle plates and got almost 400hp. They then replaced them with 70mm throttles and there was no difference in power up to 6000 rpm.

So doing the math, using 63mm throttle plates, a throttle plate area of 1022 sq mm/litre can support an output of at least 65 hp/litre (i.e. a 6.1 producing 400hp) Using a pair of HIF44's (44mm or about 1.75") on a 5.3 the area per litre is 1147 sq mm. 2" would be 1530 sq mm/litre

So I think that a pair of 2" carbs is more than is necessary, as we know a smaller throttle plate area is not an impediment to producing power.

 

You cannot compare carbs to fuel injection. Carbs need the Venturi effect to operate That limits the size of carburetors in order to get them to Idle. Fuel injection simply measures air flow and pumps the required fuel through the injector. It’s easy to over size them without much consequence.

You can back door it, which I think you were attempting to do. But then comparing a stock engine to a race motor changes things significantly.
let me see if I can clarify.
The stock fuel injection flows something in the order of 1200 CFM. ( I’ve got the actual numbers someplace but I’m too tired to find it) that’s for 262 horsepower 5.3 V12
The plenum determines the actual limitation of what size throttle body you have. Going larger on plenum changes the RPM peak. The formula for that is something called the Helmholtz ( I hope I spelled that correctly) while you can go to a bigger throttle body. You don’t get the maximum benefit without an attending increase in plenum size. And then that affects the port size and maximum valve size etc. Hot rodding is the art of breaking the rules and still achieving more.
The classic example is Group 44 made 450 horsepower with 4 HD 8’s and won the national championship in their XKE. Using a stock appearing intake manifold. Huffaker also did something similar but they used Strombergs.

 

Not here to thread jack, if anything I'm helping. Not all xj12's came with a posi, and some posi diffs fail the spin one wheel by hand test, that is all. I'm searching the forums looking to see if there is any xjs posi unit that will fit in a x305 diff.

 

I’ve just bought a Dana LSD from an XJS, but I’m not sure it is? I took the cover off and it looks like an open diff to me? Has anyone got a picture of the inside of the Dana 44? Is the above picture a Dana or is it a Salisbury diff?

 

That s a Salisbury diff.
If you want info about the DANA, post again as I have one I am sorting out at the moment.