I'm building a mildly tuned 302 GMC inline six for my '54 GMC 1/2 ton pickup. Bumped compression, mild cam, and split exhaust. I've got a handful of pretty good 2" bore SU carburetors and I'd like to run three of them on the motor. AN SU guide that I've seen indicated that 3 of the 2" bore SU's would be about right. I have a Howard intake manifold setup for Holley AA-1's and I'm wondering if I should build a similar "long box" style manifold for SU's? Or would it be better to individually mount the SU's on three separate intake tubes? The carbs need to be raised above the intake ports a little to clear the steering column. Using the elbows that are on the Howard, then building a new plenum to sit on top of the elbows that would accept the side draft SU's would take care of the clearance issues. Or I could fab separate manifolds angled up a bit, of tubing, for each carb. This isn't my area of expertise, so I'd appreciate some input from people that know the pro's and cons. Thanks for the help, if I'm posting this in the wrong forum, just let me know and I'll move it.
Individual runners with an equalising tube between would be better and not hard to fabricate. They can be a bit tricky to tune for the street but once it's right and you don't fiddle with them they would be fine. I run 1 3/4'' on my avatar but I'm either idling or flat out but it's still driveable around the pits. My engine is only 138 cu.in. People tell me that's too much carbys for this engine but S.Us work off vacuum, they don't pump the fuel in so the engine will only use the fuel it can handle. Most on here will tell you to use something other than the S.U.s but I say 'just suck it and see'.
You need an equalizing tube between carbs, very easy to fab. SU tuning guide, I find SUs easy to tune https://www.mgexp.com/phile/1/492119/SU_idiot_guide.PDF Dan
You might want to start a conversation with HAMBer Cody Walls. He has done the three SU carbs to a Chevy 6 cylinder. I’m sure there is a build thread on the car ( 59 Chevy Wagon ) and the engine.
Just a couple thoughts. You might look at a Jag 4.2 engine that used the 3 2” units. It’s a little smaller, but probably as close as you’ll get. They were pretty good in their day. And jimmy six on here has run a lot of different combos (including 12 port heads) and might some input. Neat idea.
Man, thanks a lot for all the great responses they really help me think this through. I forgot to mention I'm also going to add small block chevy valves to the head. Should breathe pretty well. I'll look for that chevy wagon thread and do some on the SU idiot guide. Thanks again, really appreciate the help.
Back in the 1960's/70's triple SU's were the bees knees on GM's Holden 6's which were up to 202 cubes ex factory, also on Fords 221 Falcon 6 and Mopars 225 Slant 6......1&3/4" SU's were the standard street carby size but 2" SU's were used on really hot street cars or drag engines....from memory all used individual runners and an equalisation tube between the runners, they also seemed to be mounted on a slightly higher, angled point rather than straight horizontal.....having said all that I only ever helped a mate who ran triple 1 & 3/4" SU's on his 186 Holden engined car........seemed to run enough to scare us.........lol.....tho' we were young & silly 50 yrs ago....lol....andyd
Would a "tunnel-ram" type mani work? (with a plenum underneath/beside the SU's) Jus think'in out loud...... 6sally6
Skinners Union is the best carb ever made. I have 2 on my avatar and one on my 1946 Harley since 1975
As mentioned earlier, SU's were a very common carb here in Australia. We were predominately brought up with British 4 cylinder vehicles and Aussie inline 6 cylinders so the SU was almost an exclusive in the early days. Lots of aftermarket manifolds were available mostly for 1-1/4" thru 1-3/4" and some were 2" versions. I learned how to tune SU's when I fitted 3 x 1-1/4" SU's to my 138" (grey) six cylinder street car and then I used 3 x 1-3/4" on methanol in a 160" Holden 6 cylinder drag car in the early 70's, eventually upgraded to 3 x 2" and went quicker again. I also found it important to use ram tubes as they helped overcome fuel standoff. SU's mostly come with a 30 degree float bowls and some are horizontal bowls, they can be interchanged if they are from the same era carb. You can easily fabricate a steel manifold by shaping round tube (or flat sheet) to suit half of the head port profile tapering out to half of the carb diameter, make in two sections and weld them together. Cut steel flanges to suit the head and carbs and then glue it all together ..... that's how I made my first version manifold. Yes, balance tubes help idle quality, 1/2" ID rubber tube stubs between the banks are adequate. This was my 3 x 2" x 30 degree version -
Where is that car now? I use 1 3/4" on the grey now but my spare engine ran better times with the 1 1/4". The 1 1/4" have longer ram tubes and sit on a Yella Terra head whereas the 1 3/4" sit on a special speedway head with larger combustion chambers.
The Skinners Union design has been around since just after Noah landed the Ark; so there can be absolutely no argument that the S.U. design has had the longest production run of any carburetor design. As to being the best ever: statements like that tend to cause dissenting replys. I will mention that the design (pretty much an S.U. patent rip-off) was used as production on at least a couple of early US automobiles (Hudson, Essex) from 1917 through 1923. After 1923, the design, at least to my knowledge, has not been produced in the USA. However, variants of the design are still being produced. The two companies of which I am familiar with that used the design tried another air valve design from 1924 through 1932~1933, when they discontinued the air valve design, and switched to a more conventional plain-tube design. I personally like the S. U. metering rod design, and would like the carbs a lot more if the British had used better materials in their manufacture, so they didn't warp so frequently. And yes, I have done a LOT of them, so posting from experience. EDIT: Sharpone - Joe is the dude to whom we send those who call us about S.U.'s. Jon
Agree, Jon. Brit engines were short-lived, but often the SUs were worn out before the engine. Finding good cores to rebuild is problematic. One is lucky if he's only going through three to find one good one. Back in the day, I'd always look in Volvos first; they seemed to not be as trashed. jack vines
Jack - we found about 3 out of 4 domes required machining. And when the customer had no spares, and said "fix it", well, we fixed it. Pistons were better, maybe 1 out of 3. Still better than the English Solex carbs. Jon
Angled up can help with the float bowl angle. It also helps when there is no room between the head and the Holden inner guards. Lots more room above the engine on a humpy Relieving the grey motor chamber helps flow, but can drop compression. Is the chamber volume (cc) the same on both? Those bare aluminium domes are just crying out for polishing to a mirror finish... the hot buffing process does ugly things to eccentricity. Had more than a few that failed the drop test. Cheers, Harv
Another viable option to finding good SUs to build is the Hitachi knock-offs used on 240Z Datsun sports cars. While looking nearly identical, nothing interchanges with the SUs. My champ car uses two of them, equivalent to the HS6s.
That was a 1976 photo at Castlereagh. I sold the motor, transmission etc to different people then sold the roller off in the 1990's. Ironically, I bought the roller back in 2005 and it's been in storage at a friends place ever since. The SU's need to have the dashpots dampened/controlled to maintain some vacuum and good fuel vaporization. Using Methanol makes the tuning easier because of its wider tuning range and cooling effect. Those 2" ones were off a Jaguar using an Armour's manifold.
The Jack Wilkinson speedway head has larger combustion chambers than the Yella Terra. The first time I ran the Wilkinson head with the 1 1/4s it wouldn't pull the skin off a rice pudding but I ran consistantly shithouse at Bodangora and won the event. I changed back to the Yella Terra for the next event and the difference was like chalk and cheese . I think the Wilkinson head was enlarged to accept domed pistons and methanol for speedway. I've since had it shaved a fair bit to pick up the compression.
I don't suppose you're interested in selling it. I've got a 202 with all the fruit (roller rockers, Starfire rods and forged pistons) built by Wilkie Engines in Newcastle and never fired in anger sitting under the bench.
Once again, thanks for all the additional information. I think I'll have a go at making separate intake manifolds. I'm going to mock things up and see what really needs to be done to clear the steering column, then lay things out and searching for possible manifolds to modify, or at maybe some carb flanges to use. Thanks again, and always open to new suggestions.
I’ve got a set of McGee injection on eBay at present to suit a red. Yours at mates rates if you want it. Have pump, pills and pulleys to suit if needed. cheers, Harv
H and HS SU come with different angle float bowls depending on application that’s why you see some horizontal and others at different angles.
The carb king is King of rebuild kits. Buy his stuff before he stops making each and every piece in the kit. I wonder how he makes the needle and the seat
Food for thought – here's a fabricated tunnel ram for a GMC 302. This one is for DCOE Webers on a 12 port cross flow head, but you could potentially build something like this with a common plenum for a stock head.
No, not really ..... it has more sentimental value. Plus it's not a legal chassis any more because of the current tube diameter requirements.
Forgive my ignorance, but what are the four spring assemblies for on the manifold? They look like a home-made relief valve assembly. Relief valves are often used on suck-through blower assemblies as a more friendly alternative to a burst panel. The description though sounds like an NA setup, which normally don't need relieving. Cheers, Harv
No problem. I know how you feel. If I stop racing my HAMBster I'll probably hang it from the shed roof rather than sell it. I had such a blast running it. Give me a yell if you do want to part with it. The cage would be the main thing to update. I only want to run a six at regional tracks.
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