Lagondaforum » Engine » Carburetion and Fuel » V12 4 carburetter: SU D4s v D5s

h14	May 18, 2016 10:30 am
Subject: V12 4 carburetter: SU D4s v D5s My V12 was originally a 4 carb car, as evidenced by the float chamber posts, but converted to 2 carb when sold in 1946 due to petrol rationing etc. I've always assumed it had 4 D5s because of the float chambers, and that perhaps just the Le Mans V12s had D4s...but subsequently I've discovered that original private customer 4 carb cars were fitted with D4s. So in my innocence I bought a pair of D5s, and when Mel rebuilt the engine, he rebuilt my D5s and fitted those. The manifolds, if it helps, are Alan Brown supplied, and I'm sure the throat diameter matched the D5s. Mel's main (reprehensible!) business was converting ruinous V12s into Le Mans (4 carb) racers, so I would have thought he would have stated the D5s were unsuitable. On the other hand, he never started the engine, neither have I, 6 years later. Mel is retired now so no contact. Does anyone have experience of 4 carb set-ups using D5s? What needles and jets are used? Would I be better sourcing D4s? What is the casting number on the D4 body (I recall on the D5s it's D1949), and will the D4s have the same flange stud spacings? Laurence

h14

May 18, 2016 10:30 am

Subject: V12 4 carburetter: SU D4s v D5s

My V12 was originally a 4 carb car, as evidenced by the float chamber posts, but converted to 2 carb when sold in 1946 due to petrol rationing etc. I've always assumed it had 4 D5s because of the float chambers, and that perhaps just the Le Mans V12s had D4s...but subsequently I've discovered that original private customer 4 carb cars were fitted with D4s.

So in my innocence I bought a pair of D5s, and when Mel rebuilt the engine, he rebuilt my D5s and fitted those. The manifolds, if it helps, are Alan Brown supplied, and I'm sure the throat diameter matched the D5s. Mel's main (reprehensible!) business was converting ruinous V12s into Le Mans (4 carb) racers, so I would have thought he would have stated the D5s were unsuitable. On the other hand, he never started the engine, neither have I, 6 years later. Mel is retired now so no contact.

Does anyone have experience of 4 carb set-ups using D5s? What needles and jets are used?

Would I be better sourcing D4s? What is the casting number on the D4 body (I recall on the D5s it's D1949), and will the D4s have the same flange stud spacings?

Laurence

h14	May 24, 2016 3:00 pm
Subject: Re: V12 4 carburetter: SU D4s v D5s Any observations, anyone?

h14

May 24, 2016 3:00 pm

Subject: Re: V12 4 carburetter: SU D4s v D5s

Any observations, anyone?

Julian	May 30, 2016 11:17 am
Subject: Re: V12 4 carburetter: SU D4s v D5s Hi Laurence, If your car has 4 x D5s then it will work fine, the engine will work on D4s or D5s just as easily as SUs are mirraculous at addapting themselves and let's face it, a 4.5 litre V12 can easily cope with 4 smallish carbs like that, just look at a 1960s 3 litre v12 Ferarri with 4 dual Webers. it has no more power tha a well tuned Lagonda and won't rev any higher either in std form than a 'Mell' tuned V12 Rest easy, it will work a treat. I would suggest going to a well established rolling road with an experienced operator and someone equipped with needles etc and spend a day getting it working really well, it can be some of the wisest money you will ever spend. Best regards, Julian

Julian

May 30, 2016 11:17 am

Subject: Re: V12 4 carburetter: SU D4s v D5s

Hi Laurence,

If your car has 4 x D5s then it will work fine, the engine will work on D4s or D5s just as easily as SUs are mirraculous at addapting themselves and let's face it, a 4.5 litre V12 can easily cope with 4 smallish carbs like that, just look at a 1960s 3 litre v12 Ferarri with 4 dual Webers. it has no more power tha a well tuned Lagonda and won't rev any higher either in std form than a 'Mell' tuned V12
Rest easy, it will work a treat.

I would suggest going to a well established rolling road with an experienced operator and someone equipped with needles etc and spend a day getting it working really well, it can be some of the wisest money you will ever spend.

Best regards,
Julian

h14	Jun 28, 2016 11:02 am
Subject: Re: V12 4 carburetter: SU D4s v D5s Hi Julian, Many thanks for your reply. Just located my SU Carb. Workshop Manual. Their listing shows the D5 is 1 5/8" throttle diameter, whereas the D4 is 1 3/8". A D4L is also listed, which is 1 1/2", but I assume it's the D4 on factory 4-carb. set-ups. So; the D4 is a fair bit smaller than the D5, not the other way round. Does what you say still hold true? Will petrol consumption be materially different between 4 D4s and 4 D5s? Interestingly, the manual states "Max. Capacity in B.H.P." For the D4: 55, and D5: 75. Unfortunately, no explanation or guidance is given. That indicates max bhp for a 2 carb V12 is 150 (oh dear), and for a 4 D4 version 220, which I think matches Lagonda's claimed output. Perhaps I can look forward to 300bhp! Laurence

h14

Jun 28, 2016 11:02 am

Subject: Re: V12 4 carburetter: SU D4s v D5s

Hi Julian,
Many thanks for your reply. Just located my SU Carb. Workshop Manual. Their listing shows the D5 is 1 5/8" throttle diameter, whereas the D4 is 1 3/8". A D4L is also listed, which is 1 1/2", but I assume it's the D4 on factory 4-carb. set-ups.

So; the D4 is a fair bit smaller than the D5, not the other way round. Does what you say still hold true? Will petrol consumption be materially different between 4 D4s and 4 D5s?

Interestingly, the manual states "Max. Capacity in B.H.P." For the D4: 55, and D5: 75. Unfortunately, no explanation or guidance is given. That indicates max bhp for a 2 carb V12 is 150 (oh dear), and for a 4 D4 version 220, which I think matches Lagonda's claimed output. Perhaps I can look forward to 300bhp!

Laurence

Julian	Jun 28, 2016 6:38 pm
Subject: Re: V12 4 carburetter: SU D4s v D5s Hi Laurence, Yes it still holds true, and 4 X D4s will easilly allow 260+ bhp and the D5s will easily allow 300 'IF' your engine is up to it. I have seen a few of my 5.9 Litre conversions giving a true 300bhp from 4 x D5s and pulling from idle upwards. You will not be able to tell the difenence low down between a well setup set of D4s or D5s don't worry a thing. Just make sure the rest of it is good also. Regards, Julian

Julian

Jun 28, 2016 6:38 pm

Subject: Re: V12 4 carburetter: SU D4s v D5s

Hi Laurence,

Yes it still holds true,
and 4 X D4s will easilly allow 260+ bhp and the D5s will easily allow 300 'IF' your engine is up to it.
I have seen a few of my 5.9 Litre conversions giving a true 300bhp from 4 x D5s and pulling from idle upwards.
You will not be able to tell the difenence low down between a well setup set of D4s or D5s don't worry a thing. Just make sure the rest of it is good also.

Regards,
Julian