Triple HD6 Carbs on MK2?

Saloons
1

Just brainstorming, but wonder if anyone put triple HD6 carbs on a MK2 instead od HD8s?

I have complete triple HD8 setup from MkX and many HD6 carbs as well as 420 compact intake.
I can go many directions and probably the best is Two HD8s. With that said I still fancy triple carbs.

But was wondering about using triple HD6 carbs?? Needles?

More details on my project is 3.8, 9:1 compression and straight port head. Manual transmission with overdrive. Full bare metal restoration with XJ40 seats. Kind of a custom restoration to my specs. This car isn’t numbers matching, so I don’t mind making changes or altering the engine bay.

2

Not an expert but on first consideration I can’t see the advantage of triple HD6’s over HD8’s.

The 3.8 E type uses HD8’s so you’re not going to be over carburetting it. Off the top of my head I can’t think of a Jaguar that used triple 6’s although they were fitted to some Aussie GMH holden 202 cubic inch straight sixes…

I have a 4.2 engine in my Mk2 and fitted a pair of HD8’s from an XJ6. These have 100 thou jets and I’ve needed to use the richest needles they make to get it enough fuel.

The great advantage of sticking to the twin SU’s is that you don’t need to move things around and do panel work to fit them as you would have to do with triples.

3

I think it can be fairly argued that for a street-going version, the HD8s actually were overcarbureting.

My 2 liter Rover has two HD8s on it, and I’d like to experiment w/HD6s.

I think the HD6s would work just fine, and any losses from their smaller size would only happen in very high RPMs, a place where these engines arent at all that happy, without serious work.

4

Hammill claims triple HD8 are overkill, and triple 1-3/4" carbs would give superior low & mid range torque. He gives some figures to back this up

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5

Then you must have HS8 of HIF7 because the HD8 in the XJ6 have the same jets and needles as the other models.
UM for the 4.2. UVV/UVX for the 2.8. Both .125

Hammill sees very little advantage (so, none really) in triples for regular engines. But why not try!

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Yes you’re quite correct; they’re HS8’s. Typo. I think the needles are BCA.

7

That’s interesting; Des is usually right.

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Its in either the engine building or carb book.

I thought an aficionado like yourself would have both ?

If not have someone buy them for Xmas :grinning:

He mentions air flow tests

I could get those books down and have a read, but I know he does discuss the issue

I have a pair of 1-3/4" SU carbs, which I think are off a Volvo, (but maybe a Holden)

They are set up originally to have a manual choke

the flange bolts are not the same as Jag, but the small parts could be used to concert an HD6 to manual choke operation

9

For a 3.8 triple HD6 would be ideal.
With Dual HD8 the middle 2 cylinders get the most fuel, for triple you get de best distribution of the fuel.
Triple HD8 would have no advantage on a 3.8, only for a 4.2.
There were several discussions about this is the past.

1 Like
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For me, it makes a lot of sense for air and fuel distribution and it’ll look good. The downsides are re-arranging things like the windscreen wiper motor and brake fluid reservoir, possible panel work, finding the right needles, and making space for an air filter that actually works.

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It’s the engine book, I haven’t read the other.
The middle ones would get more mixture but when do you notice that? My engine, a 4.2, idles smoothly and sometimes almost imperceptibly at 500 rpm, it would give a little more power maybe.
Triples take more effort to set up and obviously to install, I would avoid cutting up and rearranging things only to find that it just looks better?

12

I often find the inside plugs are darker than the outside plugs of 1 and 6 in a two carb set up. This suggest the possible advantage of the triple manifold. I like to go with the smallest carb that works well for the intended purpose.

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Plan is to convert to manual choke. The hisser is a fire hazard in my opinion.

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I have the engine book. Doesn’t help a lot with carbs - apart from referring you to the carb book.

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This is from Des Hammill’s book.

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Note modified engine, if yours is otherwise standard, there’s even less point in changing, apart from the bling factor.

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Along with the wisdom of Des Hammill, David Vizard, and with many years of dynamometer experience, I’ve been trying to tell people that for years.

But, they still buy into the old bullshit that, “more is better.”

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But when I turn my amp up to “11”, it’s definitely louder than amps that only go to “10”.

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Yes, Nigel…:stuck_out_tongue_winking_eye:

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I presume that is a tongue in cheek comment, but as an analogy, think of pouring fuel into a funnel. You can pour it in as fast as you like, but it’s still going to come out at the same rate. If your engine can only consume X fuel\air ratio, providing X+ won’t help anything.

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Good points. I want to build the MK2 of my dreams and really like the bling having 3 carbs.
The engine I’m building is basically a high compression 3.8 E-type engine. The head I’m using is an actual 3.8 E-type head complete with cams.

The other idea is go 4.2 as I have two engines, but I would rather use the 3.8. I thought this would be a interesting topic as I found nothing on 3 HD6 carbs on any Jag.