SU Carburetors on a slant 4

General Discussions about The Vauxhall Viva Owners Club, do you require something from the club we dont offer? What the club is doing for you. NOT for slagging people off, all such comments will be deleted.
Forum rules
Please pay attention to the ‘protocol’ in this forum. This is a friendly club, and we really don’t wish to get into any personal or abusive dialogs. Any such e-mails may be removed, and the person responsible may have their Forum membership revoked

SU Carburetors on a slant 4

Postby jamezz666 » Mon May 04, 2020 2:34 am

Hello everyone

So I tried to find information, on fitting SU Carbs to a slant 4 and I found some information, regarding to using 2" twin SU's. I found some for sale, which I purchased for £100 delivered and after doing a bit more research, I feel like I may of bought the wrong carbs. I purchased twin HS8 rover p6 SU's, but after doing some reading, it looks like I should've bought HIF44 carbs instead?

I'm not upto date with carb swaps, but can anyone help or give me advise on what I should be looking for with an su carb swap and to what carbs?

The engine in my GT is from victor vx490 2.3. Would 2" HS8 carbs be over the top for a standard engine? Or would I need 1.75" SU's?

What benefits would I see from SU's and is there many people on here who have done the swap?

Regards
James
User avatar
jamezz666
GT Viva
GT Viva
 
Posts: 335
Joined: Tue Jul 28, 2015 3:24 am
Location: worcestershire

Re: SU Carburetors on a slant 4

Postby jpsmit » Mon May 04, 2020 2:58 am

Looking at the attached chart I think you might be right 2" is a little on the large side, honestly, even 1 3/4"

I once read an SU book and IIRC you should err on the small side for best performance - so, maybe even 1 1/2". I know nothing of SU's on Vauxhalls but run a single HS4 (1 1/2") on my 1500cc Midget.

User avatar
jpsmit
Baby Bertha
Baby Bertha
 
Posts: 1009
Joined: Fri Jun 03, 2011 4:00 am
Location: Toronto

Re: SU Carburetors on a slant 4

Postby Mike Attew » Mon May 04, 2020 7:54 am

Hi James, I don't know if you are on Facebook but presently I have been messing about with an HIF38 on a 1256. The SU is far better as it doesn't have a diagphram, to set the mix there is a screw on the side which moves the jet up and down and the jet has a bimetal strip which alters the jet height to the temperature of petrol to do with density. The fuel chamber is a complete unit so no leaks.
The HIF38 IS 1 1/2" and will bolt straight on, the HIF44 is 1 3/4" which should bolt straight on though I am not sure. The HIF38 uses 0.090" jet which I am trying with a AAA needle, the HIF44 uses 0.100" size jet which is the same size as a Stromberg.
My first impression on one run is it seemed sluggish but I have been talking to David Maxwell,his idea is the throttle springs which are very heavy so this week I am going to look at this. A couple of other ideas are to use ATF instead of 3 in 1 in the pot which may allow quicker movement of the piston on acceleration , also I think but am trying to find out if the HIF44 jet and needle will fit the HIF38. If so and there is no improvement with what I have I will try the jet with something like a BDL needle.
If you go the HIF44 route I suppose the manifold will have to be opened out from the 1 1/2" if you have CD150 carbs?

I have been doing this just to keep me occupied, also I much prefer SU's. Hope this may help.

Mike
Mike
Mike Attew
GT Viva
GT Viva
 
Posts: 297
Joined: Tue Mar 13, 2018 11:51 am
Location: Norfolk

Re: SU Carburetors on a slant 4

Postby 1972nail » Mon May 04, 2020 9:45 am

The standard twin carbs on a 2300 are 1.75" and with even slight tuning they are considered restrictive. Certainly the HIF 44's will be suitable and I know of one rally engine that produced 158 bhp on a pair but then maximum torque was the aim and 2" weren't tried. The 2" SU's were the preferred option over Dellorto 48's by Chris Coburn on his DTV rally conversions in the early 70's and produced in the region of 175bhp.

I would guess that 1 3/4" SU's would be a good conversion on a standard engine but the 2" ones should be considered once any tuning, even mild is carried out.

If you are considering further modifications I would stick with the 2".
Editor of the VVOC Magazine ReVival

"When you have a Firenza like mine, who needs any Minis?" copyright David Maxwell 1979.
User avatar
1972nail
Old Nail
Old Nail
 
Posts: 3603
Joined: Sun Aug 07, 2011 4:16 pm
Location: Ballymoney Co Antrim - some say....the World's best place to live!

Re: SU Carburetors on a slant 4

Postby jamezz666 » Mon May 04, 2020 11:41 am

Thanks for the replies, i'm on facebook and that was going too be the next place to ask, after asking here.
I always liked the idea of getting hold of a 4 branch manifold, but that would be something to get in the future.

I'll be focusing on carbs for now, just i'm not in the know when it comes to fueling.
I was told by a friend today, that if i was to put 2" su's on the car, id have too open up the inlet manifold to match the 2" carbs, otherwise there will be a lip and cause problems?
I'm at the point where i feel like i should've asked, before just buying a pair of 2" :lol:
What cars used twin 1 3/4" su's? as i may just look for a pair of them instead.

I did read on Facebook somewhere, that 2" su's has shown to lose torque lower down in the revs, with a 2.3 and the 1 3/4" su's gained more torque lower down the rpm range, than the strombergs.

Regards
James
User avatar
jamezz666
GT Viva
GT Viva
 
Posts: 335
Joined: Tue Jul 28, 2015 3:24 am
Location: worcestershire

Re: SU Carburetors on a slant 4

Postby 1972nail » Mon May 04, 2020 12:56 pm

In theory 2" SU's have the potential to produce less torque that 1 3/4" carbs as a smaller choke area will give higher gas speeds and better fuel atomisation at a given engine speed and throttle opening, however an SU is a variable choke, constant depression carburettor which varies the choke size according to engine demand by lifting the plunger and needle.

So in practice a slightly larger carb size in SU or Stromberg carbs doesn't make much difference to torque. But it does affect low speed driveability because small movements of the pedal will increase the air flow by a much larger amount as the throttle begins to open from closed. This can be difficult to manage on a standard linear linkage and will seem that the engine gets bogged down as you progressively open the throttle. Experimenting with a variable linkage should improve the feel as you move off.

But back to reality. Rover fitted the 2" SU carbs to a 2200cc 4 cylinder engine without any complaints. I assume those are the carbs you have got. You are fitting them to a 2300cc 4 cylinder engine, so personally wouldn't be too worried about them being too big.

Yes, you will have to open out the manifold a bit to prevent a step but that will be easily achieved with a half round file. Remember it's only 3 mm all round so it will not be a big amount of work. If you leave the lip there will be a place for partially atomised fuel to get trapped and become a puddle behind the lip causing a weak mixture.
Editor of the VVOC Magazine ReVival

"When you have a Firenza like mine, who needs any Minis?" copyright David Maxwell 1979.
User avatar
1972nail
Old Nail
Old Nail
 
Posts: 3603
Joined: Sun Aug 07, 2011 4:16 pm
Location: Ballymoney Co Antrim - some say....the World's best place to live!

Re: SU Carburetors on a slant 4

Postby jpsmit » Mon May 04, 2020 1:40 pm

jamezz666 wrote:I was told by a friend today, that if i was to put 2" su's on the car, id have too open up the inlet manifold to match the 2" carbs, otherwise there will be a lip and cause problems?
I'm at the point where i feel like i should've asked, before just buying a pair of 2" :lol:
What cars used twin 1 3/4" su's? as i may just look for a pair of them instead.
Regards
James


James, looks like TR6's and TR4's and MGC's all used HS6 carbs in some applications.

You might eventually need to port the openings but, I suspect you could experiment if you could find some to bolt on - proof of concept as it were. I would note that the HI44 carbs (apparently) have metric throats as opposed to the HS carbs which remain Imperial.
User avatar
jpsmit
Baby Bertha
Baby Bertha
 
Posts: 1009
Joined: Fri Jun 03, 2011 4:00 am
Location: Toronto


Return to General Discussion

Who is online

Users browsing this forum: No registered users and 0 guests

cron