I've been thinking..... (not enough to be useful, just enough to be dangerous) about carby setups for fours.

Quad throttle bodies (twin webbers or injection) are always going to be the best solution. But are twin SU's really an improvement on a single large 2 barrel carby?

Twin SU's are set up on cylnders 1&2 and 3&4. These pairs of cylinders fire directly after each other (Ive always reckond a more logical way of looking at 4 cyl firing order is 2-1-3-4). This means that each SU is supplying fuel to 2 cylinders in rapid sucession and then doing sweet FA for the rest of the cycle.

To get consistent flow through a twin setup you would have to have to have 1 carb supplying cyl 1 & 4 and the other supplying 2&3. This way the fuel/air pulses would be directly opposite each other in the cycle. This would make for one mongrel of an inlet manifold if you sorted it out with equal length runners......

Which brings me to a single large carby. Flow through the carby is more or less constant as it feeds all cylinders. The main problem with them is the different length manifold runners. Without too much hassle you should be able to make an equal length inlet manifold for a large single.

So what do you reckon? Would it give an actual advantage or would the inlet just end up with too many corners in it?

(or is there actually a benefit from the twin SU setup that I have overlooked)

Twin SU's are a huge improvement over a standard two barrlel carb. I have run a standard hitachi, weber 32/36 and then the twin SU's. For starter the Su's are much bigger. Both 38mm throats as well as not having any chokes like a conventional carb. When the butterfly is open there is not restriction at all. Also they are a sidedraft style carb which is alot better setup for a straigt engine with ports or the side. The fuel does not have to make any radical changes in direction therefor keeping a better air/fuel mixture. Also they are variable venturi carbs. There is an air/fuel ratio for all rpms making them very smooth to drive.

From my experience the stock 2barrel hitachi gives great drivability but shcking performance. Doesnt like to rev to 6000. The weber gives decent performance. Revs to over 6500 easy but then seems to starve abit resulting in slight power dropoff. Also the weber as shocking driveability. I basically had to build up a few revs before taking off to get it out of the flatspot. Once above 1500-2000rpm it was fine. Sure it could have been set tp a bit better than mine was but i didnt think it was worth it. The twin SU's are best by far. Drivability of the stock hitachi and top end performance is great rev to 7000 in my engine no hassles at all and feel like they would cope quite easily at higher rpm if the cam would let them. Also they get nearly as good fuel economy as the stock carb and better than the weber. Until you floor it. Then fuel just dissapears.

Much better carb in my opinion. They are the next best things to twin 40s for perfromance and give great drivabilty with very easy tuning, setup and they can be had alot cheaper than twin webers. Probably the best allround fast street carbs in my opinion.

Cheers
Daniel

That reminds me, the stock A12 manifold has equal length runners.

On another topic, with individual runners you can run carbs the same size. You don't end up with "too much carb". Another way to put it, a single Weber sidedraft has close to the same bore requirements as dual Weber sidedrafts. At least the weber book shows a chart about this, it recommends only going down in size a little bit when using more carbs. Anybody know why this is?

Id say it would be to do with the pulses. For instance a single 40 still shoots the same amount of air and fuel in each pulse as does twin 40s. I cant really think of how to explain it. The main benefit I can see from having twin over single is a more direct flow rather than any curves and stuff because an engine can only get so much air and fuel to be effiecient and make power N/a unless turbo or blown but thats a different matter.

ddgonzonal- you're right about the stock manifold..... I had just been reading another post about 1 and 4 leaning out and made the assumption that they were a different length. Should open the bonnet more often!

As for the other topic, I reckon that's just a slightly altnative look at the same problem.
What I am getting at;
in the simplified textbook 'model' engine, you only ever have one cylinder filling at any one time therefore if a carby is big enough to fill the cylinder, its big enough to do the job for four. In reality this is a load of crap because of cam timing..... But the overlap period occurs when the pistons are moving slowly (at the top and bottom of their strokes) so presumably there would be less 'demand' for fuel anyway. Having individual throats for each cylinder will allow for better sacvenging because the momentum of the charge continues into that cylinder rather than being directed to the next one, so that (I would have thought) is the main advantage of twin webbers.

What I am trying to work out is if there is a better way to set up a pair of SU's (or a big single webber)

Edit: Moving a bit fast for me- Freak, so you reckon its mainly just to so with the curves in the manifold (or lack of them)?

I know this is a pretty subjective question if you didnt dynotune, but if you had to put a number on it, how much more power/ torque do you reckon you got out of the SU's over the standard carb? (i.e. 5-10% or noticably more than that?)

I have never had it dyno tuned so ill just go on my own comparison. The Su's are about 15%better than the original hitachi due to the fact that it would run out of puff not much over 5500. It still made some power but you could tell it was starving and time to change gear. An I would say about 5% better than the downdraft weber agian becasue it let my cam work much better in the upper rev range. However I do not think that figure would hold up on a dyno. Its mainly due to the fact that it pulls better through the whole rev range and then still supplies enough to keep pulling up high where the weber didnt. Also the driveabilty is the biggest improvement. It is honestly like fuel injection (apart from cold weather starts) They are just so responsive. Tap the throttle and it revs with power right off the mark and no stumbling.

One of the main reasons the Su's are better is due to the straight flow. There are no bends what so ever. Open the butterfly and look down the manifold and you can see the valves. Thats how straight it is. The fuel which is heavier than air isnt running into a solid 90degree turn and affecting the mixture ratio nore the flow rates. Also remember with a a downdraft carb one cylinder sucks which draws air/fuel into the manifold. However the manifold has this big open space which takes away the flow characteristics. In other words it disturbs the flow resulting in less performance and economy.

(please note: if any of this does not make sense due to spelling mistakes or misplacing of words its because im half pis-sed waiting to go out)

You should drink more- makes better sense than ya normally do

I've noticed the same, it makes good torque down low but runs out of puff before things get interesting...... I was trying to figure if this was just because of the longer stroke of the A15. The previous owner had the engine built with a 'mild' cam so once I find out the specs on the cam and pistons (the engine builder was nice enough to stamp a job number on the head) I will get a better Idea of which way to go with the carb.

Also need to work out if my SU's are the right size and jets. They came with a factory air filter and L series manifold, which isnt much use to me (but for $60 the lot with linkages I figured that it was worth the risk) I couldnt measure the throat diameter properly till I got home and they are 35mm at the front not 38 which makes for 1'3/8 rather than 1'1/2...... Do SU's come in 1'3/8??? I thought they came in 1/4in size increments. Or do you measure them from the back????

They would be 1 1/2". Try measuring from the back where the butterfly is.

Thanks for your help Freak, now all I have to to is re-kit them and find or make a manifold...... do a brake upgrade,wheels, interior, exhaust, tonneau, coupe dash (if I can find one) stezza etc......

Getting back to where this post started tho..... I still like the idea of having the SU's set up on alternately firing cylinders i.e 1&4 and 2&3 to get more continuous flow through the carb...... I spose if it gave a genuine advantage it would be common practice.

Please excuse the noob-ness of this reply but what is an SU? What makes them so different to a webber or the like?