Stromberg or SU??

[TR4A1965]

I am just about to rebuild the engine on my TR4A. It is fitted with Strombergs. A few people have said I would be far better off with SU's??

 

I guess that would mean changing the inlet manifold etc?

 

What's your view on the different carbs and the conversion?

 

 

Many thanks

 

Paul.

 

[Sparky Brum]

Paul,

 

I have a 6 that I've been running for three years on it's original Stromberg's. I had it converted to SU's on Friday and can honestly say it's one of the best conversions I've had done. The car seems to run a lot better and is more responsive in the lower gears. I fitted heat shields and K&N filters and used the existing manifold (not sure how it'll work on your 4 though). As far as SU's v Stromberg's there's loads of views on the forum either for or against, my personal view is go for it, you won't be disappointed!

 

Sparky.

[Chris Hubball]

Hi Paul

 

Go for SUs far better than Strombergs but I would say that as I worked for SU for 32 years. Seriously they are easier to tune and maintain no diaphragms to perish or pin hole, and parts supply is good. You will be able to us your existing manifold the Moss catalogue will help identify the parts needed.

 

Cheers Chris.

[TR4A1965]

Thanks guys, Glad to hear your experiences are good with SUs. Thanks for the tip on the Moss catalogue. It's going to be expensive:-))It always is:-)

 

Best

 

Paul.

[peejay4A]

If you can source an original SU linkage then I strongly advise you to do so. The bell crank plate on the reproductions isn't quite the right shape. From this you will have deduced that whilst the manifold for Stromberg CD175s and HS6 SUs is the same, the throttle linkage is different.

Edited June 2, 2013 by peejay4A

[ChrisR-4A]

Hi Paul, I have had a pair of nos Strombergs with K&Ns on my 4A for 5 years as at the time new/ recon SUs were not available and I can say that if they are in good condition they are fine, I have no plans to change them.

However if you have to buy new carbs then from a spares point of view if nothing else go for SUs, make sure you also have all the correct linkages.

 

When buying as always shop around prices can vary a lot for the same items.

Edited June 2, 2013 by potts4a

[Badfrog]

Hi Paul,

 

Don't tickle me on this topic. I am a great fan of SU's for 4 reasons:

1) simple and reliable

2) tweakable to the end of times (great winter time hobby).

3) NO nitrile membrane

4) great mileage when tweaked the right way.

 

Only caveats: Avoid K&N filters and be very wary of parts numbers and specs depending on the HS6 units you buy in the beginning: AUC 209 and 284 have different parts and should not be made a medley. Read this for a start:

http://www.tr-register.co.uk/forums/index.php?/topic/41153-the-counter-intuitive-use-of-su-needles/

and then search the forum for more info (plenty of it).

 

Have fun,

 

Badfrog

[ctc77965o]

I have SUs on my TR and Stromberg 175CDs on a jag. I would say that the strom is a better design, its a more delicate device, but when properly setup I think it exceeds the HS6. The SU is a great tinkerers carb, difficult to break but a rather clunky design.

So my advice would be to stick with what u have. AFAIK manifolds are same for both carbs, as is most of linkage.

[Alec Pringle]

Hi Paul,

 

SUs and Strombergs utilise the same manifold but with minor detail differences in linkages.

 

There's nothing wrong with Strombergs, and on a standard engine it makes little if any difference which is fitted, always assuming of course that the carbs are in in equally good condition. Often enough folks enthuse about their replacement SUs - forgetting that they have replaced tired old Strombergs with newly rebuilt SUs, so of course the latter will perform better.

 

I'd suggest that Standard Triumph's choice of carburation in period had more to do with motor industry politics than anything else, there is no particular engineering or performance reason to prefer one carb over t'other.

 

A seriously 'upgraded' engine is a different matter, the SUs offer greater and perhaps more convenient scope for tuning.

 

If I was rebuilding a standardish engine and had a reasonable pair of Strombergs I'd be having them rebuilt by Andrew Turner, I certainly wouldn't be lashing out my hard-earned on a pair of SUs which will offer no real benefit.

 

Cheers

 

Alec

[peejay4A]

as is most of linkage.

No, it's quite different. See my post above.

[TR4A1965]

Thanks all.

 

I'll let you know how we get on.

 

Best.

 

Paul.

[trevor]

Hi just reading this thread and wondered why you don't like K&N's Badfrog ?

 

[Ashley James]

I agree with Alec and think Strombergs are probably better carbs, it's just we're all used to SUs. If I had Strombergs I'd use them, but Im tight!

[Badfrog]

Hi just reading this thread and wondered why you don't like K&N's Badfrog ?

 

 

Hi Trevor,

 

I've had the habit of tinkering with my car, with an unequal success to be honest. I especially target carburetion as it has rapid rewards on the road (or not).

Classic engines fall into categories: those which lack in-breathing (TR) and those which lack out-breathing (Volvo B18). I give here the examples I know best but it is a general condition.

 

A TR lacks inlet performance. There's not much you can do with valves, due to the design of the head and it's quite expensive for a small benefit. So you're left with accessories: a better camshaft, rolling rockers, uprated SU (Strombergs you can't do much about) and air filters.

First, you must check your air filters for the right flow (SU require 215 cfm) and the quickness in flow-through response to sudden changes of speed. Most air filters are OK except the very thin chromed pancakes types. Rule of thumb: the dimensions of the original TR4A paper units are optimal. I use these.

I bought K&N filters some years ago before knowing any better. They are smaller than the stock units but still of a decent size, plus they look good (me stupid). I was not too happy with the acceleration response and I observed some trouble during long steady high speed driving (freeway style). Like the filters restrained air flow speed. Could be that or could be anything, as there are so many factors involved. Then I read Paul Tegler's paper on K&N filters and realized I had exactly the symptoms he described. See there:

http://www.teglerizer.com/triumphstuff/index.html and especially :

http://www.teglerizer.com/mgstuff/a_stumble_at_cruise.htm

That's when I went for "loose" filters: the original TR3-4 mushrooms and the original TR4A large paper units. Both function equally well and I use them indistinctly. Generally the paper units are on during mushrooms refurb and painting.

 

I hope this helps.

 

Badfrog

 

PS: To be honest, my only criticism about Strombergs is I find them ugly.

Edited June 3, 2013 by Badfrog

[trevor]

Hi badfrog,

Wow thats a lot to take in , I don't suffer any hesitancy at speed , but I will check the exact K&N's I have .

Many thanks,

Trevor

[Superaquarama]

I'd suggest that Standard Triumph's choice of carburation in period had more to do with motor industry politics than anything else, there is no particular engineering or performance reason to prefer one carb over t'other.

 

 

I always thought the change to Strombergs was made due to US emission regs., Alec, but wouldn't swear to it. Having said that, our early 4A is a US import and has SUs.

[graeme]

I always thought the change to Strombergs was made due to US emission regs., Alec, but wouldn't swear to it. Having said that, our early 4A is a US import and has SUs.

 

My understanding is that when Skinner Union (SU) became part of rivals BMC, then ST funded the development of the Stromberg carb for use in place of SUs, which ST didn't want to buy from BMC. Triumph reverted to SU, probably late '65 but I am not sure what triggered this.

 

I am sure someone will be along to clarify.

 

Cheers

 

Graeme

Edited June 9, 2013 by graeme

[Alec Pringle]

SU became part of Morris back in 1926, long before BMC.

 

Standard Triumph had, I assume, no great wish to be beholden exclusively to BMC . . . . SUs were not cheap to companies outside BMC . . . . SUs were prone to icing in cold climates . . . . US emission controls were on the horizon.

 

All good reasons to look for an alternative carburettor source, quite apart from the fact of most British motor component suppliers being prone to industrial action interrupting supplies.

 

Cheers

 

Alec

[graeme]

SU became part of Morris back in 1926, long before BMC.

 

Standard Triumph had, I assume, no great wish to be beholden exclusively to BMC . . . . SUs were not cheap to companies outside BMC . . . . SUs were prone to icing in cold climates . . . . US emission controls were on the horizon.

 

All good reasons to look for an alternative carburettor source, quite apart from the fact of most British motor component suppliers being prone to industrial action interrupting supplies.

 

Cheers

 

Alec

 

Fair enough Alec. I was guessing a bit.

 

Any idea why Triumph started using SUs again from the mid sixties?

 

Cheers

 

Graeme

[Alec Pringle]

Hi Graeme,

 

politics I think - and price !

 

Triumph and Zenith-Stormberg had made the point that SU didn't have a monopoly on sportier carbs . . . . and there was nothing new about Standard Triumph utilising the products of both companies on different models.

 

Cheers

 

Alec

[Peter Cobbold]

 

Hi Trevor,

 

I've had the habit of tinkering with my car, with an unequal success to be honest. I especially target carburetion as it has rapid rewards on the road (or not).

Classic engines fall into categories: those which lack in-breathing (TR) and those which lack out-breathing (Volvo B18). I give here the examples I know best but it is a general condition.

 

A TR lacks inlet performance. There's not much you can do with valves, due to the design of the head and it's quite expensive for a small benefit. So you're left with accessories: a better camshaft, rolling rockers, uprated SU (Strombergs you can't do much about) and air filters.

First, you must check your air filters for the right flow (SU require 215 cfm) and the quickness in flow-through response to sudden changes of speed. Most air filters are OK except the very thin chromed pancakes types. Rule of thumb: the dimensions of the original TR4A paper units are optimal. I use these.

I bought K&N filters some years ago before knowing any better. They are smaller than the stock units but still of a decent size, plus they look good (me stupid). I was not too happy with the acceleration response and I observed some trouble during long steady high speed driving (freeway style). Like the filters restrained air flow speed. Could be that or could be anything, as there are so many factors involved. Then I read Paul Tegler's paper on K&N filters and realized I had exactly the symptoms he described. See there:

http://www.teglerizer.com/triumphstuff/index.html and especially :

http://www.teglerizer.com/mgstuff/a_stumble_at_cruise.htm

That's when I went for "loose" filters: the original TR3-4 mushrooms and the original TR4A large paper units. Both function equally well and I use them indistinctly. Generally the paper units are on during mushrooms refurb and painting.

 

I hope this helps.

 

Badfrog

 

PS: To be honest, my only criticism about Strombergs is I find them ugly.

That's a lot of air for one SU. I use rule of thumb that 100hp needs 70cfm air. So one SU - in a pair supplying 200hp total - needs 70cfm air.

 

That Tegler stumble at cruise I doubt very much has anything to do with filtering. The engine only makes about 20hp at cruise, needing about 10hp per SU or 7cfm. That's about 10-15% of max air flow at wot and max rpm. His claim that the stock plenum and filter gives the "...proper air pressure differential across the jet bridge" is highly dubious, that is not how an SU is designed to function. If his claim were true he will have a huge problem at wot at higher rpm in the form of enormous power loss due to a big pressure drop imapiring air intake. Note there's no mention off that duct tape effect on wot perfomance!

So why did his filter swap from K&N back to stock work?

I suspect when he swapped filters he inadvertenlty blocked the air intake on the flange that feeds atmospheric pressure under the piston. With that blocked the piston will be very sluggish to move if at all, and the pressures' distributions across the bridge will be destroyed, no wonder mpg went up. Simple cure, drill a hole in the filter base to let filtered air at atmospheric pressure under the piston.

Peter

 

That 'atmospheric vent passage' is in this general position #19 :

 

Edited June 10, 2013 by Peter Cobbold

[ChrisR-4A]

Triumph did also use Strombergs on later models like the Stag.?

 

Another point, when I purchased a pair of K&Ns from Rimmers at the international a few years ago at a very good price which were susposed to be for my 4A I found when I went to offer up the gaskets that the K&NS did not have the vent holes you mention above. I just drilled my own to match and they work fine.

Edited June 10, 2013 by potts4a