Hi guys- I'm planning another project and need to know- what is the BIGGEST single barrel side draft carb? Some of you may remember I bought a bloody huge blower a few months ago, which will suit either a sbv8 or jag v12 perfectly- and I want a suck through system with a monster single barrel side draft, but I don't recall ever seeing one big enough.
I'm guessing the problem is with such a big barrel you would struggle to get any atomisation due to the huge changes in velocity- unless there is an SU (variable venturi) type carb big enough? An SU would be perfect for low revving high torque IMO...
Want something more old skool than Webers, apparently SU made 2.5" carbs but don't know what they would be called, should be H10 but anything with more than 1 number is normally metric.
Even at 2.5" I think I would still need more than 1 carb, in low tune a blown Jag V12 should push 400bhp, which will be double what I need
The biggest SU of recent years is the 2" H8, I think that prewar there were some slighly larger versions that were bored out to 2 1/8" or possibly 2 1/4" for blower Bentleys which used two side by side.
Two standard 2" SU's will flow enought for approx 280 BHP unblown, or sucked as the case may be
If you can find out what flow you require for a given power output then the correct size of carb can be sought.
Hi Tom
not thought of mechanical injection? then you can use a single big fat venturi (say 3.5 or 4") and dress it up to look like a big carb, not the cheepest solution but at £600 odd for the kit in the US plus shipping and duty it will be in the ball park of a Big bentley SU. You could hide a pilot throttle body, say 1", somewhere that would give you a more progressive throttle at idle. otherwise a bank of 4 X 2" all in a row would look pretty cool, all feeding into 1 big pipe into the blower just a bit tricky to set up!
Best regards
Mike
I very briefly thought about it Mike, but this one I'd like to do on a budget, 2 SU's would look great, I'd struggle to get 3 or 4 in, I did a design idea sketch last night, will play with it a bit more and scan a copy in as I think you will like it (even if it is V12)
Hi Tom
a bit more googling reveals that "custom" SUs were made up to 3" by a company called Norman, could be for rolls, bentley type use, also a possable use would bave been tanks/ armoured cars. I have had a bit of a look and a solex 40 NNIP keeps comming up in connection with the 6 and a bit litre alvis/rolls straight six fitted to various armoured cars in the 50s and 60s. I don't know much avout the carb apart from it was a fixed ventrui twin curcute thing (very crude!) but for an engine of that size I presume it had a big hole down the middle!
You could always make your own SU in 4" size they are not really very complicated, I know of model engineers making ones in 1/4" and 1/2" size for various model engine projects.
Best regards
Mike
If you are limmited for space there were a few twin choke SUs made, basically a pair of SUs in one body casting with all two throttles on one shaft, you again could do something like that with 3 or 4 sets of SU guts in a body you made up yourself, say for 2" internals the pistons can virtually touch making it quite compact.
Best regards
Mike
That would be a cool project- machining a 3" SU but I think it is too unlikely to work well, so 2x 2" H8 SU's would look good side by side with gauzed ram pipes. The draw from the blower will keep velocity up and good atomisation- and plenty of torque as I should be able to fit an intercooler in there too- though with an intercooler and suck through set up do you not risk fuel settling in the pipework?
Hi Tom,
I don't think making your own big SU would be that dificult, for a 4" basically take a 2" apart measure it and double all the dimensions, for the piston cover use a 6" air tank (divers or a nitrous bottle) top half, machine the inside parralel and make the piston match the body and make the dashpot/ piston guide as a screw in bit to where the regulator would go in. The actual body/venturi is really just three boring operations to get the basic venturi then angle the front and rear of the step and drill for the Jet tube. The needles and jets would be much easier to do than for smaller sizes as the tapers occour over twice the distance as a 2" a simple doubling job will get you in the ball park, then it is a bit of emery to fine tune on the needles from there on in. Remember they were designed in the early 1920s and are briliantly simple in design.
Best regards
Mike
It would be wicked but way too big, here are the quick rough area calcs for different sizes, I'm not sure how to guess flow from area but with FI it wont be as anal as with an NA engine...
I think I should source a pair of 2"+ SU's as a starting point. So when the weather turns good and auto jumble weather begins it will be a treasure hunt!
Hi Tom
I still think it would be far cooler to build your own 3 or 4" Su, just think of the bragging rights! A pair of 2" SUs infront of a blower should see you to the far side of 450bhp (I am pretty sure janspeed used 2" Sus on their turbo kits for a while) But building your own . . . .
Mike
and need to know- what is the BIGGEST single barrel side draft carb? Some of you may remember I bought a bloody huge blower a few months ago, which will suit either a sbv8 or jag v12 perfectly- and I want a suck through system with a monster single barrel side draft, but I don't recall ever seeing one big enough.
