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Old 02-10-21, 11:39 AM
Geoff Dowdle Geoff Dowdle is online now
Join Date: Jul 2008
Location: Sydney, Australia
Posts: 360

David ,
Your plan of putting the 403 on a rolling road dyno is really the only way to accurately design the best adance curve for a Bristol. That plus checking the mixture through the rev range is also very important .
Over the years I have had three of my Bristol engines in 400's on a rolling road. Two had similar curves(plus a 406) and the last 400 engine needed 8 deg less at 4500 rpm.
The engine of an Arnolt Bristol of a friend needed even less again, this is a high compression sports engine running on 98 octane fuel.

I suggest you might start with a curve of
Static/Idle 12 deg BTDC
3000 rpm 36
5000 42

This is what my first two 400 engines (85A & 100) were graphed at.

The information I have on the original 403 Lucas Distributors are....
Part# 40345A , Advance Curve 520
Advance Starts 650 - 900 Engine RPM & Deg
16 - 20 deg @ 2900 Engine RPM & Deg
34 - 38 deg @ 4000 Engine RPM & Deg

So with a 12 deg Static would give
28 - 32 deg @ 2900
46 - 50 deg @ 4000
That is assuming the distributor is in excellent working order and using the low octane poor quality fuel of the mid 50's .

I look forward to hearing what curve you decide on .
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