Triple 3/4" SU's HD6

[DerekJ]

I want to upgrade to triple SU's on my BN4. Money wise I think this is good option and makes the this makes the best use of the existing carbs.

When I buy the extra carb does it matter if its a 'front' or 'rear' carb?

Are there stock pieces of equipment readily available to connect the three together, e.g fuel lines etc.?

I think I'll have to relocate the brake resevoir. Is that right?

Anything else I need to consider? I plan to install a cable throttle of the type sold by Cape Sports

Derek
www.healeysix.net /ubbthreads/images/graemlins/england.gif

 

[Dave Russell]

Hi Derek,
There would be several issues involved with your project.

1- The tricarb uses three HS4 1.5" bore carbs. The HD6 carb is 1.75" bore. The three HD6's would have 36% more throat size than the tricarb's three HS4 carbs. Your BN4 has 10% less displacement than the tricarb engines. This would make your proposed arrangement even more over carbureted. Roughly 45% more than the tricarb engines.

2- The tricarb requires three individual intake manifolds, different linkage, a separate balance pipe, different plumbing etc. The HD6 with a 1.75" bore would not match up to the HS4 1.5" manifold bores. The tricarb manifolds are not readily available. There would be a large expense in fuel lines, linkage, piping & other parts.

3- There would likely be a very large amount of work & expense to do this project for no performance gain or probably a decrease in performance. The tricarbs with their 1.5" bores are somewhat over carbureted, using 1.75" bore carbs would exagerate the situation. This extra bore area would be largely wasted since the slides, (pistons) would only partially open on the HD6's to accommodate the needed air flow for your engine.

Comparative displacement vs carb bore area:
BN4 = 550cc per sq. in. of carb bore
Tricarb = 549 cc per sq. in. of carb bore
BJ8 = 428 cc per sq. in. of carb bore
Your proposed = 367 cc per sq. in. of carb bore

Good luck,
D

 

[RVMAN]

Neat looking set-up, but if it was such a good deal healy would have stuck with it. Balance problems are also multiplied. Is it worth the extra work/expense? Bon chance!!(did I spell that right?)

 

[DerekJ]

Dave thanks for the feedback, Individual 1 3/4" intake manifolds are available. A friend has just done the same on an MGC. Its not necessary to find or modify a tri carb inlet manifold

Re another posted comment. One of the reasons the tri carb was introduced was for homologation purposes. Cars could then run triple webers in competition.(BTW wouldn't that be over carbed?) The balancing "problem" seems to vary massively depending on who you talk to. SU carbs once set up properly, usually just don't "go out of tune".

The Subsequent move back to 2 carbs; first in the BJ7 then in the BJ8 may also have been a cost issue. 2 were cheaper than 3. Balancing problems may have been a good excuse. This was BMC.



regards

Derek
www.healeysix.net /ubbthreads/images/graemlins/england.gif

regards

 

[Dave Russell]

Derek,
To reiterate the third point of my last post:

"The tricarbs with their 1.5" bores are somewhat over carbureted, using 1.75" bore carbs would exagerate the situation. This extra bore area would be largely wasted since the slides, (pistons) would only partially open on the HD6's to accommodate the needed air flow for your engine."

This is the advantage of the SU constant velocity type carb. If you put on a carb that is too large for the engine the carb will only open as far as needed. Although a waste of the larger carbs, it will still run fairly well. A highly modified race engine that was developing power from 4000 to 7000 rpm could well take advantage of your proposed carb setup.

If you want to do it "just because", it won't hurt anything & may look more "racy", performance is in the mind of the person doing the modification.

Have fun with the project,
D

 

[tahoe healey]

Su's do go out of tune more than you like. That's way they sell all the books and sync tools. Because I live above 6000 feet I need to re-adjust each time I tour off the mountain. Choke and mixture seem to be a tri-annual problem. Don't get me wrong, I love tinkering and trying to out think the darn things. My vote... two's company, three's a crowd.

 

[DerekJ]

Hi Dave,

I wasnt meaning I was ignoring your third point. I should have mentioned that the triple carb set up Im thinking of will be in conjunction with other mods to the engine that will already be increasing its power. (Not an overbore though) I was thinking therefore that the carb increase might be worthwhile.

re above: 2's company 3's a crowd. maybe, but all performance Healey's run with 3 carbs. BTW when I mentioned that SU's "don't just go out of tune" I was just quoting straight from their literature.

I haven't made a final decision re the triple carbs so all the input is really helpful

cheers

Derek
www.healeysix.net /ubbthreads/images/graemlins/england.gif

 

[Dave Russell]

Hi Derek,
Great web site, a beautiful car to be proud of. I personally would get 1.5" carbs & duplicate the tricarb setup to keep things as original as possible on the external part. The extra cost of three 1.5" carbs vs one additional 1.75" carb might not be a lot in the overall project. I'm sure that it will come out fine either way. Best to you,
D

 

[piman]

Hello Derek,
being in Paris, you should not have TH's problem with having to frequently re-tune the S.U.'s be they a dual or triple set up. They do not go out of tune; as a general rule whenever the car starts to go 'off song' the carburettors are generally blamed when the most likely cause is the ignition, especially if a points system is still in use.

Alec

 

[Keoke]

/ubbthreads/images/graemlins/savewave.gif
Nah TH, they don't go out of tune much its just when you come off that mountain that you live on that they need a little tweeking and oh yes them points don't help none either- /ubbthreads/images/graemlins/laugh.gif---Keoke

 

[DerekJ]

Dave,

Thanks for the compliments.

Based on the feedback I'm going to proceed in stages. The car will have Dennis Welch headers, a new cam (probably slightly hotter profile than a BJ8 , (although thats worth a "thread" in its own right!) and the head will be polished and ported. Then I'll see what power I've got and decide what if any, change to the carb set up I should make)

Derek
www.healeysix.net

 

[Dave Russell]

Hi Derek,
I love the Healey history on your web site. Very well done & the photos are outstanding. You must have reworked some of the old ones to get them so clean.
D

 

[DerekJ]

Alec,

I agree totally with your comments, SU's don't go out of tune. Ill give an example of mine. They were "tuned" i.e simply set for optimal driving, by Michel, a guy from St Germain who is well known on the classic race circuit in europe. That was 18 months ago, I haven't touched them since as I always mess them up when I do. In August I drove from Paris to St Moritz for the 2nd European Healey Meeting. St Moritz as you know is pretty high, I cant remeber exactly how high. Anyway I was there for a week, participated in the Stelvio and Gavia pass rallies, about 150 miles each, and reaching 9,000 feet. We were pressing a bit as well. I never touched my carbs and the car ran like a turbine. Maybe in reality it was not at optimum performance but not enough of a diference for me to notice. I never touched the carbs. They work great whether there's 2 or 3 of them.

If you're interested in seeing pictures of this event which was fantastically well organised by Vic Jacobs & Marco Trevisan, & their Swiss Healey club, they are on my website at
www.healeysix.net
D
cheers

Derek /ubbthreads/images/graemlins/england.gif