Ignition BN1

[johnea]

Hi Healeyfans,

I'm replacing the stock distributor on a BN1 that is fitted with high compression pistons. I decided to use a Mallory dual point unit. I've also got an advance curve change kit, with different advance springs as well. Now I need a little help: should I start with the 24 degree advance curve, as it comes from the factory or go straight to the max advance of 28 degree, wich is closer to the original dissy spec.?

 

[Dave Russell]

Assuming that your numbers are in crankshaft degrees & not distributor degrees:

I would use the 24 degree plus a 10 degree static advance setting.

This would be a total advance setting of 34 degrees at 4500 rpm which is where the final setting should be made. Vacuum advance disconnected.

If you used the 28 degree advance, the static advance would need to be 6 degrees to get the same total at 4500 rpm.

The original distributor & it's curve were not very accurate & not particularly good to duplicate.
D

 

[johnea]

Thanks Dave.
The original distributor has stamped 17 degrees on the centrifugal weights (34 crankshaft degrees)+ 6 degrees initial advance would give 40 degrees total advance. Why do you suggest not to go beyond 34 degr. max advance?
John.

 

[Dave Russell]

Hi John,
It's very easy to get into detonation at higher rpm full power. You likely won't hear it, but it's still happening & can cause a great deal of damage. Most engines fall in the range of 30 to 36 degrees safe total advance. More effecient combustion chambers can work well with as little as 30 degrees.

The BN1, BN2, are not safe above 34 to 35 degrees total advance under power.

The vacuum advance will increase this by 10 to 15 degrees, but only at light throttle settings.

As I said earlier, old distributors or even newer ones, can vary considerably from their stated/intended numbers. The only safe way is to check total advance at the actual rpm where the advance has maxed out.

There is very little power or economy to be gained by over advancing but much to lose.
D

 

[johnea]

Thanks for your information Dave!
John