MGB Register Forum

I have to disagree Vic. When operating at wide open throttle there is little vacuum anywhere in the induction system. Granted you will probably see some differences on part load between the position just downstream of the throttle and the manifold tapping. After all the function of the vacuum is to advance the timing ('cause you can) on part load for economy purposes. Its hard to imagine the interaction of the butterfly with the airflow and what that does to the amount of vacuum.
I find myself in agreement in with pages 71 and 72 of Peter Burgesses book "How to power tune the MGB 4 cylinder engine". Its available from Amazon at a reasonable price. Perhaps he doesn't say the 45D distributor is 'junk' but he does describe in as 'wrong' when talking about both the mechanical and vacuum advance mechanisms.

Paul, Peter Burgess refers to that particular version of the 45D4, which is peculiar to the late rubber bumper MGB's. As the vacuum take off is on the inlet manifold where there is a constant vacuum, unlike the air filter side of the carburettor throttle disc, that particular 45D4 had to be calibrated completely differently and as stated if used in the conventional position the timing would be wrong as would be the case if a 25D4 were used. Because of this compromise rubber bumper MGB'S were gutless. It's not the fault of the 45D4 in general just the 41610 version.
I have a 45D4. on my 1975 Land Rover which has a 2.5 petrol engine from a later one and I obtained a spare 45D4 from a Land Rover breaker and had it rebuilt to standard spec by the Distributor Doctor. The vehicle runs extremely well with this distributor so 45D4's aren't junk.
I have had my copy of Peter Burgess' book long before Amazon was ever heard of in this country and I would never buy anything from them on principle.
. Vic Butler

Interestingly, although I have commented on the 45D distributor elsewhere on here, I recently bought a new Aldon distributor with modified advance curve suitable for the Piper 285 cam. This turned out to be a 45D type, so I queried with them where I should take the vacuum from and the initial answer was 'wherever it is now'! When I pointed out my MGB ran a 25D distributor that took the vacuum from before the butterfly there was a bit of silence then after some discussion they suggested from the manifold as per cars with a 45D. There is nothing wrong with the 45D distributor per se, but BL used it to manage the emissions on the MGB. Many people have found that changing the 45D to a 25D and replumbing the vacuum produced a more 'revvy' engine. But hopefully with the modified curve of the Aldon 45D it will out perform a standard 25D with the modified cam when the vacuum is taken off the manifold.
Incidentally, I have often thought Don Loughlin (Don in Aldon) tried out some of his distributor ideas on my racing MG TC back in 1973 or so. During a 4 hour rolling road session he kept taking the distributor out (it started as a standard TC item) to play around with different bob weights. Power went up from around 75bhp at the wheels on the first run to around 95 after 4 hours! It also turned the car into a race winner as all the horses were Shires rather than Shetlands! Although the distributor wasn't the only thing he played with - exhaust length and jetting (45DCOE) contributed - the big gains came from his distributor work.

Peter, the 45D4 was fitted to other vehicles as well as the rubber bumper B's but the MGB'S were the only cars of that time to have the vacuum take off on the inlet manifold where there is a constant vacuum. Because of this peculiar set up the distributor had to be calibrated to suit and it was a compromise which drastically hindered performance. Series 3 petrol Land Rovers were also fitted with a 45D4 but their vacuum take off is on the air filter side of the throttle disc so the 45D4 here was calibrated according. The later 2.5 petrol engine retained the 45D4 but modified with sliding points.
My Stage 2 BGT has the vacuum take off in the proper place and has a distributor calibrated to suit the state of tune. The distributor, a Brown and Gammons modified 43D4 with vacuum advance, was rebuilt by the Distributor Doctor.
This particular version was sold by Brown and Gammons back in the 1980's as a performance distributor for rubber bumper B's
. Vic Butler