DX6A Distributor advance curve - 401

[Geoff Dowdle]

John,
I don't totally agree with Julian's comments .
I believe the only way to find out the advance curve needed for a particular engine is to dyno tune it on a "rolling road " . Assuming the mixture is correct through the range you plot your advance curve by adjusting the advance to get maximum BHP at different rev ranges .
Starting at 1000 rpm in 500 rpm increments up to 5000rpm
Then build that advance into the distributor or electronic unit .

In my experience with many Bristol sixes over 40 years I've found the following curve to work well .
Static/idle 10 -12 BTDC
3000 rpm 36 BTDC
5000 rpm 43 BTDC
Most 85 series engines are now running 8.5 comp pistons with material shaved off head & block giving 9.0 plus .
Cams are often upgraded to later 405 type
In Australia most owners run their cars on 98 octane Unleaded fuel .
All these variables will effect the amount of advance needed.

If you want your engine to have the correct advance curve the only accurate way is to dyne tune it .

Standard 400 & 401 distributors had 18 - 19 deg advance (that is 36 -38 deg at the crankshaft plus the static gives between 43 - 45 at max rpm)

When I last dyno tuned my 400 with a reasonably standard spec engine , the operator wanted to start at 5 BTDC (it would barely run ) , he did not want to exceed more than 30 deg BTDC . It kept making BHP up to 43 deg BTDC at 4700 rpm . The carbies are jetted correctly with a mobile gas analyser , it does not over head , idles and performs very nicely.

I'd be interested to hear of others experiences .

Geoff