Greg said:
>
> Dave,
> Now I hate to be the one to break this to you, but you may not
> get the extra power you're looking for with this conversion. I am
> assuming that you plan to use HS-2's.
> What you should do, (listen to me, hah), is buy a manifold
> directly from England, which will enable you to simply bolt on HS-4's
> from any MGB. Get the appropriate needles along with the manifold. This
> way, you'll have twice the carburettion that you currently have, and your
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> conversion will be effective.
You will have the SAME amount of carburettion, but a *lot* better manifold.
The reason behind dual (or triple) S.U's, DCOE's, etc. isn't to get more
carb, it's to get a good, straight, shot from the carb to the port.
You should not look at it as X amount of carb on the engine. You need to
look at it as "this cylinder sees X amount of carb when it sucks".
For the sake of clarity, let's say that a cylinder on intake will suck evenly
for 180 degrees of crank rotation. On a four cylinder engine, the intake
strokes are phased 180 degrees apart. So, on a four banger, only one cylinder
at a time is sucking on the carb(s). On a single carb application, the carb
will feed #2, then #1, then #3, and finally #4. In order. On a dual carb
setup, the front carb will feed #2, then #1, then shut down while the back
carb feeds #3 then #4. In the case of dual DCOE's (effectively four singles),
each barrel while wake up, feed it's one cylinder, then shutdown for the other
75% of the time. (Obviously generic references. Minis don't have front/rear;
Kents would be 1,2,4,3)
The case is pretty much the same for an inline six with at least two carbs.
While the intake stroke phasing is now 120 degrees, two carbs and the firing
order team up to put the effective phases for each carb 240 degrees apart.
Playing with this, you'll see that going from two to three carbs on a six will
not gain you anything in carburettion. Any gains would have to be found in
the manifold. (assuming like carbs)
Front 1 x 3 x 2 x
Rear x 5 x 6 x 4
Front 1 x x x 2 x
Center x x 3 x x 4
Rear x 5 x 6 x x
Things get interesting when you start stacking intake pulses closer together,
such as in an eight cylinder. I've never tried to hot-rod one of these, so
I haven't put enough thought into it to comment. :>
But cylinders do not suck evenly though 180 degrees. The duration is usually
much longer, which will cause some overlap between cylinders. But the suction
level isn't even either. It is by far the strongest during the 80-150 degree
area. At this point there is no overlap, and it is stronger than the combined
suction of two cylinders during the overlap. During overlap, one is just
starting while the other is almost finished. Max valve lift during max
piston velocity win.
Just for fun, off the top of my head...
Car/ carbs (mm) cylinder carb area relative index
Engine size seen by cyl. (area / CC)
========= ========== ======== ============ ==============
Midget 1275 2 x 32 (SU) 318cc 804 mm^2 2.56
Austin 1300 1 x 38 (SU) 318 1134 3.56
Spitfidget 1500 1 x 38 (CD) 375 1134 3.02
MGB ->74 2 x 38 (SU) 450 1134 2.52
MGB <-75 1 x 44 (CD) 450 1520 3.38
TR6 2 x 44 (CD) 417 1520 3.65
AH 100/6 2 x 44 (SU) 500(?) 1520 3.04
AH 3000 (62) 3 x 38 (SU) 500 1134 2.27
AH 3000 2 x 51 (SU) 500 2027 4.05
Lotus TC 2 x 40 (DCOE) 400 1256 3.14
Alfa 2L 2 x 40 (DCOE) 500 1256 2.51
There! All typed into a nice table.... and pretty useless. Late model B's
have much more carb per cylinder CC than the earlier cars. The table says
so. Then why are they so damned slow? In a word, manifolds! The cylinder
has the potential to see 34% more carb than the early cars. But it's looking
at this carb through two 90 degree *sharp* bends (not to mention exhaust
and air filter). Net change is a 25% power loss.
The bottom line is that there is enough to be gained in manifold flow to
justify the expense and complexity of mulitple carbs. The Spitfire that
started all of this *may* run a little better with twin HS2 1.25 SU's. The
stock 1500 manifold is pretty bad. It would be much happier keeping
the same amount of carburettion, but with a good straight-shot manifold.
This requires two carbs; one pointed at each cylinder pair. Or a very well
designed single carb manifold.
Randy
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