Single SU Carburettor?
Posted By : Ben Ferguson, 21-Nov-2004, 11:49pm
Hi there,
Yep it should have twin 1 1/4 inch SU's really, I imagine yours has either a single stromberg 150 carb or a single 1 1/2 SU at the moment, both of which will restrict its performance at higher revs. If swapping to twin carbs though I'd recommend twin 1.5" SU's from a spitfire 1500. They can be almost if not as economical as the smaller 1.25"'s when driving under "normal" conditions but give a usefull bit of extra ooomph at higher revs, and also give scope for further mods, with fancy cylinder heads / cams etc
Both pairs of carbs are similarly priced too,
Good luck in the tidying up!
Ben
Yep it should have twin 1 1/4 inch SU's really, I imagine yours has either a single stromberg 150 carb or a single 1 1/2 SU at the moment, both of which will restrict its performance at higher revs. If swapping to twin carbs though I'd recommend twin 1.5" SU's from a spitfire 1500. They can be almost if not as economical as the smaller 1.25"'s when driving under "normal" conditions but give a usefull bit of extra ooomph at higher revs, and also give scope for further mods, with fancy cylinder heads / cams etc
Both pairs of carbs are similarly priced too,
Good luck in the tidying up!
Ben
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Single SU Carburettor?
Posted By : Nigel Gibbins, 22-Nov-2004, 06:10pm
Those higher revs (where the power may be restricted) are around and above 5000 rpm and if you aren't going to push your engine that far then that 1.5" SU will be perfectly fine.
I have heared that a single, slightly bigger carb can (when set up properly) actually produce more power than a twin set up, so you may not be as badly off as you first think.
The theory on that is - a carb in a normal twin setup supplies only two cylinders and they fire in order 2-1 or 3-4. So the twin carbs works for half the engine revolution and is idling for the other half. This means the fuel/air mixture has to be accelerated into the first cylinder and will back up when the second cylinder closes. This results in the two cylinders getting different charges and so will affect the final power output.
The single carb is constantly in demand and the fuel/air mixture is constantly moving and so getting the and equal charge is much easier and a smoother power delivery is the result... and if it's the right carb more power is available.
You say your money is tight, so I recommend you leave it alone until it becomes a proper issue. Sounds to me like you have bigger fish to fry anyway
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Single SU Carburettor?
Posted By : Mark Telkman, 23-Nov-2004, 12:12pm
I've had the chance to play today, setting the timing, tuning the carb (nice and easy with just the one) etc... Nothing is drastically wrong with it at the moment, but I can't help feeling sorry for it every time I open the bonnet and imagining how amazingly lovely and shiny it could be with a grand or so spent on it. Not money I've got, mind...
Anyway, the number on the cylinder block doesn't match that on the V5... However, it is an FH number, so unless its had some drastic swapping of parts between engines, I think its safe to say its a MkIV engine.
The number on the tag for the carb is AUD516, but I couldn't find any sites with info on the model numbers to help identify the carb. Here are a couple of pics of the carb in question (what a lovely air cleaner box):
Single SU Carburettor?-
Single SU Carburettor?
Posted By : Mark Telkman, 23-Nov-2004, 12:13pm
One from the other side. The inlet manifold looks very much like one from an American MkIV/1500, but I don't know how similar the single carb manifolds from other models would be...
Single SU Carburettor?-
Single SU Carburettor?
Posted By : James Carruthers, 23-Nov-2004, 12:26pm
Looks to me like a HS4
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Single SU Carburettor?
Posted By : Chris Taylor, 23-Nov-2004, 06:05pm
The carb and manifold also look very much like a Toledo/Dolomite 1300. (Probably the same item as American Spitfire, except perhaps for adaptors for brake servo, emission control pipework etc). This is actually a better flowing manifold than the 1300fwd and Herald 13/60 type (with Stromberg carb) even though it LOOKS more angular etc.
This begs the question as to what head you have on. The block seems to be Spitfire, but does the head match the block or the carb? Not sure how you tell from the outside, but no doubt some more knowledgeable member can identify parts numbers and casting identifiers.
Assuming the engine is an early FH (how far is the number from the engine shown on your V5?) a late Dolomite etc head MAY be better in that it may have usefully larger inlet valves, even though the compression ratio will 8.5:1 instead of the Spitfire's 9:1. The ideal (preferably on a small crank block, but never mind) is a late LARGE valve head with 9:1 compression ratio, and twin 1.5" SUs, a mark 3 camshaft and uprated exhaust manifold. 80BHP available just by raiding the parts bin.
BUT the old adage says that if it ain't broke don't fix it! If the car runs fine, and you are happy with it, just don't open the bonnet too often among other Spitfire owners, unless you can quote a significantly improved mpg figure! Ideally you should have a distributor that matches the HEAD, but if that is a large valve 8.5:1 head, the only standard distributor would probably be a Lucas one WITHOUT tacho drive. If the engine responds eagerly and doesn't have incurable pinking leave well alone. (Of course, if the head is a Mk4 spitfire one, you may still have the original and correct distributor).
As others have already said, the difference in performance will not be great in everyday driving (especially if you DO have the larger valve head), and the general state of health of the engine and maintenance of ignition, carb, rockers and timing will make MUCH more of a difference than a second carb.-
Single SU Carburettor?
Posted By : Mark Telkman, 23-Nov-2004, 07:08pm
I've got lots of work to do before I would get around to replacing the carbs (if I ever do), but I thought I might as well try and figure out what's going on in the mean time.
The V5 claims the engine to be FH52xxxHE (High compression, emission controlled? - Not sure on that one) whilst the cylinder block number is 20,000 before that, also an FH/HE. By the early engine, do you mean pre FH25,000? I'm not sure what other changes they may have made over the years.
Its got a Delco distributor on there at the moment, which I thought was the standard for a MkIV, but with so much mention of the Lucas distributors around the place, I might be wrong on this one.
If anyone reckons they'll be able to work out what its made up of, I'm more than happy to find any numbers or take any pictures that might help you identify things.-
Single SU Carburettor?
Posted By : Chris Taylor, 26-Nov-2004, 01:13pm
By the looks of it, you have a standard Mk 4 block. Speculation of course, but as both the previous (ie on the V5) and current engines are Mk4, is it reasonable to assume that the head is also a Mk4?
If so, the distributor is correct (all Mk4s were Delco, at least in the UK). The only thing that MIGHT be slightly out is the carb needle IF it is from a lower compression ratio engine originally. That said, if it runs OK, then there isn't a problem!-
Single SU Carburettor?
Posted By : Grant Brooks, 13-Dec-2004, 11:40am
Sorry, I'm a bit late to this thread but better late than never.
I heard somewhere (in an edition of the courier I think) that a single carb will give a torque advantage over the standard twin set up (as well as better fuel economy). I understand that there will be a limitation on top end performance but as I seldom take my car over 4500rpm the single carb would seem better suited to my driving style. Any thoughts on this?
I also happen to have a project spit with knackered carbs and just happen to have a single carb manifold lying about, so for ease/cost etc was thinking of going this route, so the next question is - which carb? 1.5? bigger?
thanks,
Grant
'80 1500 daily driver
'77 1500 pile of bits-
Single SU Carburettor?
Posted By : James Carruthers, 13-Dec-2004, 12:40pm
Grant,
In theory it should work better - it's a plenum type effect. On a twin carb system the total "suck" the engine makes on the carbs is half. Even worse on the standard inlet manifolds as the carb has to deal with two firings right after each other. Theory is that the carb doesnt have enough time to recover - so the second firing on the single carb doesnt quite work as well. With having a single carb the engine sucks in at a constant speed the whole time... and therefore when you set up the carb you're just setting it up for that single "shot" each time.
A list I worked out... based on area of intake...
single HS2 1.23
single HS4 1.77
single HS6 2.41
twin HS2 2.45
single HS8 3.14
twin HS4 3.53
triple HS2 3.68
twin HS6 4.81
triple HS4 5.30
twin HS8 6.28
triple HS6 7.22
triple HS8 9.42
i.e. a single HS8 is the same as a twin HS4... (kind of obvious that!) so a HS8 would work best in theory? HS6s are ten a penny on eBay though - so maybe start there and see how you do. Unless you're going to rolling road it you'll never know anyway...
James
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Single SU Carburettor?
Posted By : Chris Taylor, 17-Dec-2004, 05:43pm
While a (slightly) bigger single carb may have some advantages over twin carbs, there is no point in going much bigger than the carb size the inlet manifold was designed for (probably 1.5")
There is also an issue (unless you go down the rolling road tuning option) of identifying a suitable needle and spring combination. Taking the manifold, carb and needle of a (Dolomite?) 1500 is a good starting point, although the compression ratio and flow characteristics of a Spitfire head may be slightly different. (Which exhaust manifold to use can also have an effect).
Personally if you have the right bits for the Spitfire, I'd keep it standard (ie twin SUs). Spitfire's aren't unduly thirsty (by the standards of their day!) and being properly set up (ie to manufacturer's spec) and maintained will make a bigger difference to fuel consumption than anything else. There are other things you can do to improve fuel consumption, such as electronic ignition, Kenlowe fan, keeping the hood up (yes! less drag, but less fun also).-
Single SU Carburettor?
Posted By : David Rumens, 17-Dec-2004, 07:48pm
I have tried 1.5 and 1.75 single SU carbs on both the 13/60 and Toledo/Dolomite/1500 manifolds. In the case of the 1.75 SU I bored out the manifolds to match the carb. The 1.5 took an ABF needle with a red spring. The 1.75 took a BAF with a yellow spring. These gave the best results. Interestingly the 13/60 manfold gave the best overal results in MPG, torque and top end power.
However, when I compared the MPG, torque and top end power with the normal twin 1.5 SU's I found these could be made to give the best overal results.
Apart for the ease of setting up and may be a lighter throttle pedal I found no advantage using the single carb on the 1500 Spitfite/ Dolomite engine.
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Single SU Carburettor?
Posted By : Philip Rosenberg, 22-Nov-2004, 11:30am
There should be no problem picking up a manifold, air filter housing and a pair of (maybe in need of a refurb) carbs along with all the copper pipe
that link the two from a scrappers like spitfire graveyard in sheffield (I keep recomending this place, but that's cos I think it's the most useful
parts source ever).
The swap itself is easy, just a handfull of nuts to take off the carbs and manifold. Then it's just a case of the condition of the carbs and setting them up (or calling in a pro to set them up for about &163;40)
Phil
The swap itself is easy, just a handfull of nuts to take off the carbs and manifold. Then it's just a case of the condition of the carbs and setting them up (or calling in a pro to set them up for about &163;40)
Phil
Single SU Carburettor?
Posted By : Mark Stradling, 22-Nov-2004, 11:33am
You might like to check your engine number here (http://www.canleyclassics.com/infodatabase.asp?art...).
I think some of the 1300 saloons had just the single carb, and it's quite possible some previous owner has just just chucked a swap engine in with it's single carb manifold.
I think some of the 1300 saloons had just the single carb, and it's quite possible some previous owner has just just chucked a swap engine in with it's single carb manifold.
Single SU Carburettor?
Posted By : William Davies, 22-Nov-2004, 04:26pm
Hi Mark,
Sounds like you've got a Toledo or Dolomite 1300 engine in there as the single HS4 would be standard on those models - as a sanity check the engine is likely to have a prefix DG or DH.
To be honest you're not going to notice a great deal of difference between single and twin carb installations, not unless you're keeping you're revs above 4500rpm continuously, which wouldn't be such a great idea with an unmodified late 1300 engine. If you're going for performance, or you're eager to get the car back to standard spec then that's a different matter,
Cheers,
Bill.
Sounds like you've got a Toledo or Dolomite 1300 engine in there as the single HS4 would be standard on those models - as a sanity check the engine is likely to have a prefix DG or DH.
To be honest you're not going to notice a great deal of difference between single and twin carb installations, not unless you're keeping you're revs above 4500rpm continuously, which wouldn't be such a great idea with an unmodified late 1300 engine. If you're going for performance, or you're eager to get the car back to standard spec then that's a different matter,
Cheers,
Bill.
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Single SU Carburettor?
Posted By : Mark Telkman, 22-Nov-2004, 06:36pm
I still haven't had the chance to look it over properly, but it certainly looks like a MkIV Spit 1300 engine from the outside from what I've seen of them, though I've no idea how similar a Dolly engine would be. The engine prefix on the V5 is an FH, but thats hardly a reliable indication of the engine within... I'll have to check and see what it is.-
Single SU Carburettor?
Posted By : Mark Stradling, 23-Nov-2004, 10:25am
How similar is a Dolly engine... more or less identical from the outside!
As for the V5, it's all too common for people to update their V5 (or V5C now) when they change the engine... which reminds me, that engine Jigsaw put in my car 6 months ago... Doh!
To follow up the mighty Nig's comments on the agvantages of a well set up single carb over a badly set up pair... It's very hard to have a single carb out of balance, but I only have to look a a balanced pair of SU's and they seem to un-balance them selves!
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