S3 with triple SU's

[Mountain King]

Bit of a mad question, but....

Did Jaguar ever consider fitting triple SU's to the V12? obviously the manifold would have to have been completely redesigned and tuning 6 carbs would be interesting, but with the carb set up already completed from it's use on the earlier models I would have thought that some costs which were trying to be saved at the time may have been possible?

just a left field query....

[MarkE]

An interesting left field query! I've certainly never seen that setup in drawing form. The main reason for the Strombergs, I believe, was that thay can run very lean and give a resonable emmissions value, whereas the SUs can't. But then, folks have in the past put 4 SUs onto the V12 and claimed greater performance AND fuel economy!

6 SUs on each side would look rather spectacular!

[Woolfi]

Original Jaguar planned to mount the SU 2" SU carbs on the EV12. The width of the part, to which the carburettors are mounted, is 2 inch.
Why mounting 6 SU's to this motor ? Why mounting 4 SU's to this motor ? The 4,2 XK motor has more power with 3 SU's than with two 1,75" Strombergs. How much horsepower would this motor have, when two SU's would be mounted on the XK-motor ?
The big difference in power between 3 SU's and 2 Stromberg mabybe is, because the big displacement of one cylinder of 700 ccm can breath better through a mouth of 2 inch, than a mouth of 1,75 inch. There would be only a small difference if 2 or 3 cylinders ar using one carberettors, because they are breathing one after the other. Therefore I think, that 2 SU's would produce only few power less, than 3 SU's an the XK-motor.
The V12 ha a displacement of only 445 ccm of every cylinder. Therefore a less wide carburettor like the 1,75" Stromberg can deliever in relation the same amount of air to each cylinder like the 2" SU to the bigger cylinder of the 4,2 L XK motor.
A bigger produces more power. But the bigger the carburettor is becomming, the smaler the profit in horsepower is.
If you "calculate" the power of a 1000 ccm BMW Boxer motorcycle , you see, that the motor has 70 horsepower (70 hp per 1000ccm, a 4.2 XK has 50 hp per 1000 ccm) but only 40mm Bing carburettors. 40 mm is roundabout 1,57 inch. Why has BMW not choosen bigger carburettors ? I think, that such a small carberettos of 1,57 inch is big enough for 70 hp per 1000ccm. I know , that there is a gain in horsewower of swinging gas, if every cylinder has his own carburettor. Like with 3 doubble Weber's for 6 cylinder.
Conclusion: I dont think, that the power of a V12 with 6 SU's is much bigger than with 4 strombergs. I think, that the the V12 with 4 SU's maybe has max. 20 horsepower more. Or maybe less than 20. Who knows ?
If somebody has done such a conversion AND made a dyno-test with a bigger "profit" in power, let me know. I am allways searching for some more ponies for my EV12 with a 6 liter motor from a XJ81 limousine and 4 Strombergs.
I have seen pictures of idiot-tuning in the internet. Somebody mounted 4 bigger SU's and is still using the small trumpets into the airfilter-boxes. The trumpets have only 32 mm diameter in the front. I have mounted a straight pipe of 58 mm inner diameter AND have made the needles thinner. Sometimes I think, that a V12 motor, which has ben equipped with 4 SU's, has more power, mostly because of a self produced bigger air-filter box inlet. The diameter of one times 32 mm is muuuuuuch smaler than two diameters of 44 mm (1,75").
Excuse my simple english, but I hope you can understand.
Regards Wolfgang Gatza

[Mark Gordon]

Your English is much better than my German, so no apologies are necessary. You make some very interesting observations regarding the relationship between cylinder size, carburetor intake size and the fact that not all cylinders are taking air in at any one time. As you say, I would like to see some dyno testing of the various setups to see how much effect that switching the carbs makes.

Mark

[Woolfi]

I found math formulas in a tuning book of a experienced motor-tuner. These formulas for a powerfull carburettor dimension are the same for one up to four cylinders. For one carburettor for 6 cylinders there is a correction factor.
If you have three cylinders feeded from one carburettor, like on the V12, 720 degrees divided through 3 = 240 degrees. Therefore only one cylinder is sucking at the same time. When he is "ready with eating", after a short brake, the next starts to suck. Therefore there is no need for a bigger carb, because of three instead of one cylinder.
If you are searching to find some more "ponies" in your motor, you must be shure, that the things you are doing, are succesfull. The test is on the street compared to the same car without the changes or on dyno before and after. Power-talking of car owners, car mechanics , people which are selling tuning parts and other people often is wrong. If you want to be succesfull you have to separate truth from rumors. The best book in the world about tuning still is David Vizards yellow bible "Tuning the A-Series engine".
I tried some hours to find dyno sheets for the EV12 with "Strombies" vs. SU's. Also in jaglovers.org I could't find anything. No "hard" facts, only rumors.
Regards Wolfgang Gatza

[Mountain King]

Thanks Woolfi!
very informative, I hadn't looked at it that way, i carb feeding 3 cylinders - 240 degrees of rotation which is more time than the valve is open.

Thanks a lot!

[Woolfi]

With this calculator you will be able to see the advantage of a swap from Stromberg to 4 2" SU's.

http://www.secondstrike.com/Technical/CarbCalc.asp

Does anybody see an advantage in flow ?

Kind regards Wolfgang Gatza

[Aldeburgh]

Try talking to the guys at CMC. they have done quite a bit of work on figuring out the correct needles for SU carbs on the V12 engine. they can probably give you the dyno-run comparators for the original Strombergs Vs the commonplace SU conversions

[Woolfi]

http://s828.photobucket.com/albums/zz20 ... i/Jaguar2/

In these 2 pictures you can see, how the type of "connecting" carburettors to a V12 is changing the torque-curve and how the length of the "sucking-pipe" is changing the area of maximum torque.
- An EV12 with four Stromberg carbs is acting like 4 thre-cylinder motors. This generates a high torque in the lower area of the rev-range.
- An EV12 with an original injection system is acting like two six-inline motors. The max torque is "lower (!) than with a thre-cylinder arangement and the highest torque is in a higher area of the rev-range.
Regards Wolfgang Gatza