Side Draft Carb Sizing

[Manta Rallier]

We're looking to get a pair of Weber or Dellortos side draft carbs for our rally engine. I have been pretty well set on 45's but someone tickled my brain about 40's amd I see from some older literature (a Haynes Weber carb manual) that 40's were the setup in the 70's for 1.9L engines. We have run 45's before with good results (torque band starting at about 3700 rpm and going to about 8k rpm) but am wondering if the 40's would be adequate. We will be running a very strong Isky cam (480lift/300 dur), probably some porting and larger valves on the head, and will up the dispacement to 2 or more liters.

Any input and thoughts and related experience are welcomed.
Thanks,
Mark B.
PS: Any jet sizes, emulsion tubes used, choke sizes, etc., are welcomed too. If you list these, please describe the eninge set-up too.

[baz]

mark i have moved this to the side draft carb forum
have a root around here there are 63 threads on jetting SDD and TSD carbs setups but the best and only way to do it right is on a rolling road with a bunch of jets and tubes to swop till it works best
hth

[RallyBob]

Mark, I'm going to be using 45 DCOE's w/ 36 chokes in my rally engine.

It's nothing too radical, 2.0 litre forged flat-tops with an extensively ported and unshrouded big-valve 1.6 litre head giving 10.8:1 true compression. Cam is .451" lift and 242 @ .050" duration with a fairly tight 106 degree lobe separation angle for strong mid-range grunt. Roller rockers, stud girdle, etc. Custom header with 1.625" primaries, leading to a 2.25" exhaust. The intake is an English-cast Magnoletsi knock-off, flows very well in stock form, and it's been extensively reworked. I've built a bunch of similar engine before, and it should make about 160-165 @ the flywheel. Best power from 3500-7500, but it will pull cleanly from about 2200 and will rev over 8k 'if needed'. I have a close ratio Quaife 4-speed gearbox, and a 4.75 final drive.

You can run 40's if you want, but believe it or not, with large venturies on the smaller carbs, you'll give up some torque down low compared to the 45's. Not enough signal strength due to the larger venturi/throttle plate ratio, so part-throttle transitions will not be as strong either.

Bob

[Manta Rallier]

Originally Posted by baz mark i have moved this to the side draft carb forum have a root around here there are 63 threads on jetting SDD and TSD carbs setups but the best and only way to do it right is on a rolling road with a bunch of jets and tubes to swop till it works best hth

Thanks Baz, Yep, I've been through it before and still have a box of jets, tubes and drills. 'Cept our rolling road was 'rolling on the road'. (Car will be licensed and insured as in rally form when we do this so don't panic. And we live in a rural area.)

Regards,
Mark B.

[Manta Rallier]

Originally Posted by RallyBob You can run 40's if you want, but believe it or not, with large venturies on the smaller carbs, you'll give up some torque down low compared to the 45's. Not enough signal strength due to the larger venturi/throttle plate ratio, so part-throttle transitions will not be as strong either. Bob

Thanks, Bob. OK, I'll stick with pan A and the 45's. Interesting reason for the larger carbs.

Regards!
Mark B.