Lotus Elan

Stromberg To Su Conversion

PostPost by: steveww » Sat Mar 20, 2004 10:52 pm

There has been so much chat about Webers I thought it was time for a change.

I have heard mention, mainly by the Europa chaps, of swapping out the Strombergs for SU carbs. either for 1.75" or 2". Anyone out there know of any info on this? Apparently the HS6 bolts directly on. I would think that the 2" carb along with a cam would give a good power gain.
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PostPost by: bvt » Sun Mar 21, 2004 9:05 am

Why stay with carbs if you're gonna change why not injection?
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PostPost by: steveww » Sun Mar 21, 2004 1:23 pm

Injection is not retro. If I wanted a car with fuel injection, ecu and all that other computer stuff I would have kept my Porsche. I got an old classic car because I wanted carbs, dizzy and other simple stuff so that I could do most of the work myself. As much as I enjoy the driving I also enjoy the mechanical work. It is not that I can not work with injection systems but I work in IT and have enough of computers during the week.
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PostPost by: simonknee » Sun Mar 21, 2004 7:03 pm

1. Wouldn't fit under an Elan bonnet.
2. Surely SUs are a bit pants (or am I being a snob, I consider I graduated up to Webers from the lowly SUs my old MGBGT had)
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PostPost by: DrEntropy » Sun Mar 21, 2004 7:31 pm

IIRC, the Strombergs ~just~ clear under the bonnet. SU's are likely to be too tall. A friend managed to make an intake for the Stromberg head to accept 40mm Webers, though looking at the Stromberg head, I can't see a really good way to make the Weber conversion efficient, short of whacking off the siamesed runners and grafting on some sort of Weber adapted ones... I'd go find a Weber'd Big Valve head and do the task that way. That would be the easiest, albiet pricey.
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PostPost by: steveww » Sun Mar 21, 2004 10:16 pm

I knew it, I just knew it. Mention carbs and someone always mentions webers ;)

I already have a spare weber head and a pair of 40s sitting around just waiting for the rest of the engine. I see Burton are now casting new blocks in ally - yum nice :D

I just want to see what I could do with the Stromberg head whilst I build up the new engine.

BTW: I do have an old article than reports how to fit the SU HS6 carbs and the do fit <just> under the lid. The HS6 is 1.75" the same as the Strombergs so perhaps not much to be gained here?

There is a lot of talk about how bad the Strombergs are but so far I have not had any problems with them. What problems, if any, have anyone else had? However I think the Strombergs must give less trouble than the Webers due to the endless threads on Weber jetting, flat spots etc... ;)
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PostPost by: bill_s708800 » Mon Mar 22, 2004 10:17 am

I swapped my Strombergs for 1.75 S.U's following the article in Lotus reMarque. Needed new studs but otherwise they fit straight in, even use the old air box.
I found no improvement so I've just gone back to Strombergs. I expect that if I was competent to tune them by selecting needle etc they might have made an improvement.

Good luck,

Bill
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PostPost by: DrEntropy » Mon Mar 22, 2004 12:07 pm

There is a lot of talk about how bad the Strombergs are but so far I have not had any problems with them. What problems, if any, have anyone else had? However I think the Strombergs must give less trouble than the Webers due to the endless threads on Weber jetting, flat spots etc...


Actually the Stromberg units aren't "bad" per se... just not up to the CFM of Weber. The diaphragm is the only real 'wear item' they have as a negative. SU's are as reliable as a house brick in comparison.

The Weber threads are by-and-large a study at getting as much as possible from them. Webers generally are a "set 'em and ferget 'em" piece of gear, but as designed, they are nearly ~infinitely~ tuneable. Variables built in all the way up to the 400CFM full chat! Quite a thing for the period, and still lots of fun (and challenge) for peeps like us to play at optimising.

And besides: if we didn't thrive on challenge we'd be driving a shiny-new Toyota instead of a 30+ year old Lotus, right?
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PostPost by: Elanman99 » Mon Mar 22, 2004 9:12 pm

There is a picture in the photo section of the LotusElan Yahoo group page showing an SU (2") that I used on my Elan whilst I rebuilt the Webber carbs.

I think the link below need to be entered as one line. If it does not work the picture title to look for is 111-1178.jpg

<a href='http://photos.groups.yahoo.com/group/lotuselan/vwp?.dir=/&.dnm=111-1178.JPG.jpg&.src=gr&.view=l&.done=http%3a//photos.groups.yahoo.com/group/lotuselan/lst%3f%26.dir=/%26.src=gr%26.view=l' target='_blank'>http://photos.groups.yahoo.com/group/lotus...rc=gr%26.view=l</a>

My bonnet has the bulge so had plenty of clearance.

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PostPost by: steveww » Tue Mar 23, 2004 9:33 am

Wow that is quite some contraption :o

I guess it did fit under the lid?

I was thinking that 2 SU 2" on the Stromberg head would help the breathing a bit. However the limitation may be in the head and not the carbs.

Some quick calculations:

40DCOE with 30mm choke gives a total area of 2827 mm2
33mm choke = 3421 mm2

Stromberg 1.75" gives a total area of 3103 mm2

SU 2" gives a total area of 4053 mm2

Of course it is a bit more complicated than that but it shows that the breathing through the Strombergs is not too limited. BTW we are talking about the non-emission set up here.

Looking at the standard Stromberg install I can see many areas of improvement to the plumbing. Some ram pipes and a better air box / filter would be a good start.
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PostPost by: jopalm » Fri Mar 26, 2004 3:52 am

40DCOE with 30mm choke gives a total area of 2827 mm2
33mm choke = 3421 mm2

Stromberg 1.75" gives a total area of 3103 mm2

SU 2" gives a total area of 4053 mm2


FWIW, If you're computing cross sectional area at the throttle plate for the Strom & SU then for comparison purposes, you should do the same for the Weber.

40DCOE gives a total area of 5026 at the throttle plates.

Other alternative would be to compute the (maximum) venturi area of the Strom & SU. Not something I want to tackle, though it might be published somewhere...

Regards,
-John
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PostPost by: ardee_selby » Wed Aug 17, 2011 6:59 pm

Seen this?

http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... 1423.l2649

Anyone else tried it?

Cheers - Richard
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PostPost by: stugilmour » Thu Aug 18, 2011 6:20 am

I have SU's on mine Richard, although they are slightly different than the one's in the listing. They are pretty much direct bolt on I believe, although I did not do the mod myself. I had to lower the engine a bit to clear the Plus 2 bonnet.
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PostPost by: elangtv2000 » Thu Aug 18, 2011 1:09 pm

SUs can be made to fit by milling off some of the top of the dashpot - the threaded portion that the damper rod screws into. It usually needs to be reduced by about half, and the link provided by ardee_selby shows those SUs were so modified.

SUs, either new or rebuilt, are more widely available than top condition used Strombergs many times, which may account for the popularity of the swap. A buddy of mine had some "professionally" rebuilt Z-S carbs that while nice and shiny with new rubber bits, simply had too much wear between the air valve and body, and too much wear at the jets, so he went the modified SU route and gained some HP, and the engine is smoother-running now.

Zenith Strombergs get quite a bashing by many, but I've driven many Weber-head Elans and several Stromberg-head Elans, including my own S4 on Strombergs and S3 on Webers. I have found that all things being equal and with the carbs in good condition and not worn (air valve to body wear is a problem, and can't easily be fixed by swapping air valves, as the body and valve are manufactured as a fitted pair), Stromberg Twinks are quite powerful, and more throttle-responsive than Weber versions.

Cheers,
Greg
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