Distributor advance curves - Why are they so different

[Paul Hollingworth]

My friend has recently had his engine rebuilt after boring to +0.06", its still running in, but its not performing as well as he expected. Its a bitzer really but is closest in specification to 18V581/582/583 with the larger inlet valve head and HS4 carbs with No 5 needles. It aslo has a fast road exhaust stystem and manifold.
His block comes from a 1975 18V engine and because of that has a 45D4 didtributor with 41610 advance curve. If we look at that advance curve (see below)we see that its very lazy compared to the one which should go with his set up which is 25D4_41288. Why is there such a massive difference. Even acounting for changes in fuel specifications this feels wrong. If we look at the Moss catalogue we see that they reccommend advance curve 41155 for the 18GG and early 18V engines. This curve looks much more sensible, a little less advance in the mid speed range, no doubt to safeguard against pre-ignition. I wonder if anyone elase has experience that could help us. What are other people running ?

[Vic Butler]

Morning Paul.
I have a 1977 BGT. The original gutless equipment was removed a long time ago when I was lucky to buy a pair of genuine British Leyland HS6 carburettors on the manifold. I then acquired a 43 D4 distributor from Brown and Gammons which was unusual as it had a vacuum advance fitted. With a head from an Austin 1800 which had been gas flowed and ported it went well as the vacuum advance take off point was where it should be, on the carburettor on the air filter side of the throttle disc not on the inlet manifold as originally.
About 3 years ago the cylinder head developed a crack so I got a big valve18V head and took it to a company, Slark Race Engineering in Amesbury Wiltshire. Brian Slark used to be Downton Engineering's ace cylinder head man and he built me a bespoke Downton style head with MGC inlet valves as one of the valve seats was damaged and it was a "Downton Dodge."
The distributor had been sent to the Distributor Doctor for a rebuild which was done to suit the state of tune of the engine. It goes extremely well and is well mannered in traffic, a Downton trait.
I suggest you contact the Distributor Doctor and get a distributor built to match your engine. Just tell him what modifications have been made.
. Vic Butler

[Vic Butler]

PS.
Forgot to mention the exhaust. It's a stainless steel 2inch bore with a stainless manifold and one silencer at the rear.Lumenition Optronic ignition too
. Vic Butler

[Vic Butler]

Afternoon Paul.
Had a look in the workshop manual and the 41610 distributor was fitted to rubber bumper cars that were built to ECE 15 emissions. The take off point for the vacuum advance is on the inlet manifold where there is a constant vacuum. Presumably the take off point is now on a carburettor which would explain the lack of performance as the vacuum advance unit on the distributor is totally wrong as is the centrifugal advance. Probably best to get the distributor rebuilt by the Distributor Doctor as I suggested previously.
. Vic Butler O

[Paul Hollingworth]

I should have acknowledged Paul Hunt's Bee and Vee website for the curves. I've added the static to the centrifugal advance to get the total dynamic advance. Still waiting to hear from anyone running Moss part number 12H2772, the 25D4 with 41155 advance curve especially on and early seventies 18V engine with the larger inlet valve.

[Peter Cresswell]

A simple way of looking at why advance curves are different is to consider why they are needed in the first place!
Combustion of the fuel air mixture in the combustion chamber is not instantaneous, and the fire spreads (the flame front) from the tip of the spark plug across the combustion chamber. The purpose of the distributor timing is to start this at such a point in the cycle of the engine that maximum energy is available to push the piston down at a time when it is going to produce the maximum turning force on the crankshaft. This occurs at a point after the piston has passed Top Dead Centre (TDC), so the flame has to be initiated at some point before TDC. As the fire always burns at the same speed, but Internal Combustion Engines in cars are variable speed devices, then as the engine revs rise the combustion has to be initiated earlier to ensure maximum turning force is still available at the same point.
The need for the different curves for different ages of the B series engines is largely due to the need to meet emission legislation, which increasingly affected the MGB engine as they were sold in sunny California - an early adopter of emission legislation. This brought the change from the 25D4 to 45D4 and the associated less aggressive advance curve. In Peter Burgess' book he describes the effect of this very well - I won't copy or try paraphrase it here as it will take (me!) too long so I suggest you read it for yourselves. It is a very useful book for understanding the MGB engine so should be on owners bookshelves anyway!
I don't know if the 25D4 distributor fits a later block but if it does that seems to be the way to go.