Lotus Elan

SU carbs on S4 SE

PostPost by: TonyWalker » Tue Sep 17, 2019 1:20 pm

Hi everyone
A little while ago I bought a 1971 Elan S4.
It was fitted with twin SU carbs. This seems unusual and obviously was not standard. I have the old Strombergs.
I was looking for some advice really .... leave the SU's or rebuild and fit the old Strombergs (although will they need a different manifold?).
Thanks Tony
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PostPost by: USA64 » Tue Sep 17, 2019 2:20 pm

"Carburetor is a French word meaning -leave it alone-." If the car runs well I wouldn't touch them. If the car doesn't run well it's probably not the carbs. Personally I think SUs are the more elegant design. However, I'm told by those with more experience that Strombergs are easier to service because they have replaceable rubber where the SUs have metal on metal.
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PostPost by: steve lyle » Tue Sep 17, 2019 6:38 pm

USA64 wrote:"Carburetor is a French word meaning -leave it alone-." If the car runs well I wouldn't touch them. If the car doesn't run well it's probably not the carbs. Personally I think SUs are the more elegant design. However, I'm told by those with more experience that Strombergs are easier to service because they have replaceable rubber where the SUs have metal on metal.


It's a good thing Strombergs are easier to service, because their rubber diaphragms require it. SU's don't require diaphragm replacement, because they don't have one. Instead they use metal pistons and cylinders that are precisely manufactured to provide the correct seal. SU's are more expensive as a result, because they cost more to manufacture, which is probably why Lotus didn't use them.

But - DO NOT SWAP SU PISTONS BETWEEN CARBS! They are a matched set.

Your SU's look like HIF's, which are all but bulletproof. Earlier SU's might eventually wear out throttle shafts, but HIF's use a different design there and generally don't wear out shafts. If they're balanced, and the mixture and idle is set correctly (all done with the same technique you'd use for Strombergs) they really are about as close to no-maintenance as you'll get.

My MGB has HIF SU's, my Elan has Strombergs.
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PostPost by: 1owner69Elan » Tue Sep 17, 2019 6:51 pm

"Mister E" in an earlier thread had SU's on his Elan:

viewtopic.php?f=19&t=34218&start=45

elansus.jpg and


Seemed to be happy with them.
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PostPost by: Elanman99 » Tue Sep 17, 2019 7:11 pm

68 Elan S4 DHC. Built in a weekend from a kit (just like the advert said)
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PostPost by: TonyWalker » Tue Sep 17, 2019 7:34 pm

Many thanks for the feedback and advice! Seems like I should keep the twin SU's!
I was also wondering if I could change the standard air filter arrangement for a cone filter at the front and whether this will suit or even benefit the SU's? Or otherwise filters directly on the carbs as in my previous MGB (from many years ago!)? The reason I ask is because the existing pan shaped filter assembly is broken and I have read on this Forum that the cone filters can be beneficial. Cheers and thanks for the discussion....
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PostPost by: Andy8421 » Tue Sep 17, 2019 7:50 pm

Your car will go best with cool, clean air. Having the air intake in the engine bay behind the radiator isn't ideal, so a cone filter in front of the radiator will pick up the coolest air and may also get a small pressure boost.

SUs need to have a ram pipe fitted for best performance, so fit ram pipes inside the air box. Leave a decent gap between the mouth of the ram pipe and the face of the air box, and you should be good to go.

Good luck.
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PostPost by: TonyWalker » Tue Sep 17, 2019 8:00 pm

Thanks for the useful info!
Andy 8321, I see that you are also in Surrey, UK! Do you go to any local meets. The ones in the Club Lotus are a little far away for after work for me....
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PostPost by: Quart Meg Miles » Thu Sep 19, 2019 11:49 am

Andy8421 wrote:Your car will go best with cool, clean air. Having the air intake in the engine bay behind the radiator isn't ideal, so a cone filter in front of the radiator will pick up the coolest air and may also get a small pressure boost.

SUs need to have a ram pipe fitted for best performance, so fit ram pipes inside the air box. Leave a decent gap between the mouth of the ram pipe and the face of the air box, and you should be good to go.

Good luck.

You can put right angle bends in the ram pipes to accomodate them but in my limited experience (MGA engine) you need a way to balance the air pressure in the piston by inluding the holes above the air intake (I believe). When mine activated, at about 4000 rpm, the car misfired and wouldn't run and I didn't have time to sort it (in 1966). Perhaps Andy can advise
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PostPost by: Andy8421 » Thu Sep 19, 2019 2:53 pm

TonyWalker wrote:Thanks for the useful info!
Andy 8321, I see that you are also in Surrey, UK! Do you go to any local meets. The ones in the Club Lotus are a little far away for after work for me....


I am on the Surrey / West Sussex border. Great for Goodwood, not so good for Lotus Drivers Club which meets (or perhaps 'met', I haven't been a member for a long time) at the Bolney Stage which is about as far from me as it could be without being in East Sussex. There are quite a few Loti around here, but generally we don't seem a very sociable bunch. If you do come across a meeting nearer to home, please let me know.

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PostPost by: Andy8421 » Thu Sep 19, 2019 3:06 pm

Quart Meg Miles wrote:
Andy8421 wrote:Your car will go best with cool, clean air. Having the air intake in the engine bay behind the radiator isn't ideal, so a cone filter in front of the radiator will pick up the coolest air and may also get a small pressure boost.

SUs need to have a ram pipe fitted for best performance, so fit ram pipes inside the air box. Leave a decent gap between the mouth of the ram pipe and the face of the air box, and you should be good to go.

Good luck.

You can put right angle bends in the ram pipes to accomodate them but in my limited experience (MGA engine) you need a way to balance the air pressure in the piston by inluding the holes above the air intake (I believe). When mine activated, at about 4000 rpm, the car misfired and wouldn't run and I didn't have time to sort it (in 1966). Perhaps Andy can advise


The ram pipes don't need to be long, its more about avoiding the sharp lip on the carb entry than having a tuned length. If you look closely, the picture above of 'Mister E' has very short ram pipes fitted and could be easily accommodated in an airbox. The holes above the main air intake on an SU have to be at the same pressure as the intake - they form part of the 'constant depression' system of the SU design, and they should vent into the airbox. They are visible on the rampipes on Mister E above.
The amount of pressure pickup by having the filter in front of the rad is apparently very small, but may go some way to overcome the losses in the trunking and filter. Bog standard SUs don't have a pressure balance to the space above the float chamber (unlike Weber) so if there was too much pressure increase in the airbox, there is the risk of upsetting the mixture or in an extreme case blowing bubbles in the float chamber. I recall there was an SU that had this facility, but I can't find a picture. It is unlikely to be a problem in the Elan.
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PostPost by: Mick6186 » Thu Sep 19, 2019 9:12 pm

I had twin 1.5 inch SUs on a tuned 1340MK 1 mini Cooper S. I switched to a single 45 Weber.
With the SUs the power delivery was smooth & progressive, the Weber power delivery had flat spots and the car wasn't nice to drive in traffic. It certainly didn't feel any more powerful. Stick with the SUs.
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PostPost by: Matt Elan » Fri Sep 20, 2019 9:44 am

FYI the Stromberg carb was designed by Standard Triumph in the late 50s/early 60s as by then SU was owned by BMH / BMC and Standard couldn't get a good price on them. They touted the design around various component manufacturers and Zenith took the design on. The design had to be cheap to make and circumvent the various patents SU held for CV carbs, hence there had to be various changes to the SU design, the most significant being the rubber membrane.
Apparently they were able to meet the US emissions regs easier than the SUs, hence their use on Federal cars, and of course they were significantly cheaper than a pair of Webers so they were fitted to UK cars until Weber came up with a better deal for Lotus!
I believe the manifold bolt spacing is the same on the Stromburg and the SU so no need to change the manifold....
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PostPost by: TonyWalker » Fri Sep 20, 2019 10:02 am

I really appreciate all the posts and information - extremely helpful!!
I think I will stick with the twin SUs; it's good to hear that there are other Elans out there running on twin SUs. I will check inside the airbox to see if there are any ram pipes fitted.
Tony
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PostPost by: The Veg » Sun Sep 22, 2019 2:35 am

Interesting Matt, thanks for that. I wonder how the Bing CV related to that story, since it's fairly similar to the Stromberg CD.
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