Twin SUs vs single Stromberg

[rougeherald]

Is there any information on the difference in performance of twin SUs vs a single stromberg carb, both on 1300 herald engine. The twin SUs are off mk4 spitfire. 
we are just wondering what the gain might be, fuel economy would be down I guess but would you see much gain in BHP? 
 

thanks in advance 

Aaron 

[AndyTV8]

i think switching to twin SU's (with the appropriate needles) and an extractor exhaust used to be expected to gain 10-12bhp in the SAH booklets - I'm sure someone with a better memory may be along shortly.

.....Andy

[sulzerman]

I had both on my herald. Acceleration was far better with the SU's and in theory economy should be about the same.

Hope this helps.

[Pete Lewis]

i would think you need to use a spitty cam to really  gain from twin SU   there were a lot of tests done years back 

(probably in the Kipping years )     and the outcome on a std  13/60   engine the big single strom was best all round

Pete

 

[Colin Lindsay]

8 hours ago, AndyTV8 said:

i think switching to twin SU's (with the appropriate needles) and an extractor exhaust used to be expected to gain 10-12bhp in the SAH booklets - I'm sure someone with a better memory may be along shortly.

.....Andy

My SAH catalogue doesn't mention twin carbs, (probably an earlier version?) but it does have the Solex conversion with extractor manifold which gave a stated 11 bhp boost. I've actually just realised I have one of the shaped pancake filters for it, must try to dig it out. This is using their combined four branch inlet / exhaust manifold but no mention of any camshaft upgrade.

It's been posted many times that you need the combination of camshaft and carbs to get the best out of twin carbs; I had an Alexander manifold on one of mine, plus I tried twin SUs on a Herald twin-carb manifold with no other alteration, but any increase in performance was negligible due to the camshaft being standard Herald. I reckon the best improvement I had on any basic 1200 was the single Stromberg conversion using an adaptor for the carb on the 1200 manifold. 

[rougeherald]

Thanks all I knew you would have pearls of wisdom on the topic, not sure camshaft change is on the cards so probably best sticking with the Stromberg instead, which after some tunning seems to be working well. 

Interesting you mention the alexander manifold @Colin Lindsay I know someone who has one, not in use.

Thanks again 

Aaron  

[Colin Lindsay]

Mine ran on twin Stromberg 125s, never really set up properly so it ran very rich. The original and not very sporty 1147 camshaft was probably the issue. It may have run better on the 1300 but I didn't have that manifold.

[ahebron]

I recall (and it might be wrong) hearing that twin carbs are best at wide open throttle, other than that the single is best.

[Nigel Clark]

Have a look at this thread on the TR Register forum:

https://www.tr-register.co.uk/forums/index.php?/topic/71107-carburettors/

I am not convinced that twin carbs offer real world benefits on engines with four cylinders or less. The twin carbs may give more power with wide open throttle at high rpm, but how often to we drive like that on the public highway?? 

Certainly not me officer! 

Nigel

PS: Twin carbs may well be more efficient on straight sixes, due to the length on the engine and number or inlet ports.