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MG MGB Technical - Help: Vacuum or non-vacuum dizzy?

Hi guys,

my car originally came with a 2.1 big bore engine, breathing on a 45DCOE and a 43D4 vacuumless dizzy (with electronic iginition). I'm planing on swapping the 45DCOE for a pair of HS6 SUs, as I'm interested in bottom to mid range torque rather than just copious amounts of top-end horses. I also have a 45D4 that came "free" with the car.

What i'm interested in is:
1) Would a 45D4 provide more bottom end?
2) Would the 43D4 need reprofiling to focus on the bottom end, and if so, would the 45D4 be easier?

cheers,

Grant :-)
G Hudson

Vacuum advance give improved cruising economy, and a slight improvement in part-throttle acceleration at a given throttle opening. At wide throttle openings vacuum advance has no effect.

Vacuumless distributors are used in competition because cruising economy is hardy an issue, and they are much easier to tune - basically you just have to adjust the curve to give the highest torque short of pinking through the rev range, all at full throttle i.e. only two dimensions. With vacuum advance you are adding a third variable which makes things much more complex as you then have to do many more runs at various throttle opeings to make sure the additional advance from the vacuum isn't causing pinking.

Horsepower is just a measure of 'work done' which is why it keeps going up with revs pretty-much to valve-bounce. It is torque that throws you up the road, that usually occurs much lower down, and you want to keep the revs around that figure through the gears for maximum acceleration.
Paul Hunt

Changing from a choke size of 45mm (1.77in) to 1.75in doesn't seem to be much of a change if you're looking for more gas speed to improve low end torque. I swapped 1.3/4 for 1.1/2 SU's, albeit on a 1868 engine, but it made a significant difference from 1500 to 3000rpm, not sure though if 1.1/2's would cope with 2.1 engine.
On the timing it sounds as though you need a little more advance earlier in the rev range without ending up with max adv too early, so a curve with lesser gradient. If you have access to an advance timming light you can plot the curves for both distributors to see which is better and whether slight mods to the adv springs could help. Alternatively you could get a 123/GB4-R-V distributor from Peter Burgess (who is not retiring thank heavens) and have 16 lovely curves to play with.

ps personally I believe vac adv is preferable on road going cars.

good luck
Graham


G Cherry

Hi guys,

thanks for your replies. I'll propbably give the 45D4 a go and get the car properly tuned up by, as it will be used on the road. Once that's done and I've save a few pennies, the 123 dies seem very interesting.

Cheers,

Grant :-)
G Hudson

Bear in mind the 123 replicates the original curves, which are largely irrelevant for todays fuels, so much so that at least one supplier recommends the *non*-MGB application for MGBs!
Paul Hunt

Choke size alone does not define the speed of the air over the jets in an SU. The whole point of the design is that it gives constant depression at the jet via the piston, which blanks off the choke to achieve this. Depression and airspeed being linked by Bernoulies equation.Paul is 100% correct about torque, the peak is where the engine is breathing the best as it is a function of BMEP and swept volume only. So this is where you will get the best specific fuel consumption. A race car is intended to answer a different question.
Stan Best

This thread was discussed between 13/07/2009 and 17/07/2009

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