Spitfire MK3 manifold

[Martin mon]

Hi everyone,

I have a spitfire MK3 engine fitted in my herald which is currently on a Stromberg carb with herald 13/60 inlet manifold. In my spares I have a pair of twin 1 & 1/4 SU carbs on a MK3 manifold but the water heater pipe needs replacing. I see the club can do the repair which is great however on inspection the inlet manifold the threaded casting on the underside has snapped at some point (painted over).

Two questions 1) is it really  worth switching to swin SU's 2) is it worth repairing the manifold or seeing if anyone is kind enough to sell me one in the club. I have attached pictures.

Thanks in advance for your help 👍

[Colin Lindsay]

Do you know what camshaft you're running? If it's only a block that has been rebuilt with Herald components including a Herald cam then replacing the carbs alone won't really make much difference and may even cause running problems; if it's the Spitfire Mk3 cam then you should see an improvement if the carb is uprated. (I'm assuming of course that the original Spitfire Mk3 and Herald 13/60 camshafts are interchangeable for this engine and don't differ in bearings etc)

Hopefully some of the Spitfire owners will be able to advise on what cams are currently available!

Put an ad in the 'Parts Wanted' section and we'll keep an eye out for a manifold.

 

[Martin mon]

Hi Colin thanks for the reply it's much appreciated. The engine block has HE at the end of the numbers and the. Beginning is FE indicating a US export engine?? I have not done the rebuild which was done by previous owner but it definitely feels nippier than the 13/60 my dad had even on the Stromberg. I am just a bit worried about the chip out of the old manifold block ha ha ha

[Peter Truman]

Ref the manifold damage, the pedestals on the inlet manifold are used to attach the inlet manifold to the original cast iron exhaust manifold I assume to help support the inlet manifold? and/or transfer heat to the inlet manifold, now if you install an exhaust header you won't have these functions as there's no lugs on the steel pipes so I think you can get away with the damage without any impact on the cars working. Others may have more info!

When modifying the daughters Mk2 Spit with a 1500 engine I had to cut the pedestals shorter so the inlet manifold cleared the twin siamese cast exhaust pipe could be used, there were other mods in hand too. 

[Clive]

That damage is not an issue. In all teh cars I have owned, many have had either similar damage or the attachment bolts missing. 

As to performance, John Thomason (who was the most excellent late spitfire Registrar for the club for many years) raced a standard 1500 car in a multimarque series, which he won, beating alfas and all sorts. He used teh car as everyday transport to, doing many many thousands of miles and driving to/from the racetracks. To hopefully save fuel, he saved teh twin HS4 carbs for the racing, but fitted a single HS4 carb/manifold for the everyday driving. Both set up on a rolling road. 

His results were a bit surprising. The twin carbs gave a little more power, only a few bhp. And te single carb was a little more economical, but a coupe of mpg. 

2nd bit of evidence is that John Kipping spent ages trying carb combinations. He settled on a single 1 3/4" carb on 1500 engines over twin 1 1/2" as being best. 

All that suggests that a single larger carb may not be as good looking, but is probably at least as good as slightly smaller twin carbs. And the single is easier to maintain/set up. A slightly richer needle may be required for the fuelling, and in your case the correct distributer advance curve to match the cam and head/CR. 

Also it is worth checking the condition of the carb spindles. If the bodies are worn, that is an expensive/difficult fix. And the type of HS2 carbs you have are far more likely to have worn bodies as they use the earlier return springs that pull the spindle against the body (later type uses a concentric spring around the shafts that greatly reduces the wear)

Just my 2p.

[chrishawley]

While pondering your question I came across a report which might help with the 'is it worth it?' question.

The study, early seventies, made systematic variations to the four cylinder engine and measured the outcome under controlled conditions. One variation was to interchange a Stromberg carb with a pair of HS2s. The finding was a 10% uplift in power at 4000rpm and above with the SUs. But below 4000 there was no difference at all.

And that accords with my own experience of once having swapped a Spitfire Mk3 engine with all ancillaries into a 13/60. Gave just a little bit more 'perk' upper end but nothing transformative.

For myself the deciding factor would be the condition of the SUs. If they were in tip top condition (or I could readily afford to make them that way) then I'd do the swap because there's nothing to lose and a bit to gain. But a worn pair might never tune up properly so, overall, any theoretical advantages would be negated. 

 

[Colin Lindsay]

Did you ever get sorted with this one, Martin? I've just noticed this bit:

On 25/07/2021 at 14:36, Martin mon said:

 I see the club can do the repair 

Where did you see that bit? I've just found pinholes in the 13/60 manifold water pipe so am looking for options.

Colin

 

[Martin mon]

Hi Colin,

I went to the spitfire graveyard and bought another one with water pipes ok still not fitted carbs yet.  Do you still need a herald manifold I have a nice one in my spares collection if it keeps you on the road? 

[Colin Lindsay]

Bought one from Chic Doig and then was offered one locally so collected that last tonight - will check it over this morning. Thanks for the kind offer!

Next thing will probably be an exhaust manifold... the studs have been replaced with incredibly rusty bolts and it may need work at a later date, so I'll start searching now.

Did the club offer a repair service on manifold pipes, but no longer?