DISTRIBUTOR - What the A-Series Ignition needs
Large-bore engines are less fussy about exacting ignition requirements than small-bores. The 1100-type being the most difficult to get right, with very particular requirements mid-range when tuned and worlds apart from large-bore curves. This should be remembered when swapping distributors from one engine to another. An 1100 dizzy in a 1275 will at run OK, but a 1275 in an 1100 can be courting disaster. To try and give detailed advance requirements for the myriad of engine specifications used in tuning the A-Series range is impossible.
To all intents and purposes each and every engine should have a dizzy modified to suit it. In essence this isn't realistic either. The main area for cause of concern for the performance or high CR A-Series is at low engine speed where optimum ignition advance values demand high octane levels to avoid detonation. This is generally up to around 3,000 rpm.
Applying this to current low-octane fuel availability for street use means creating an advance curve with less than optimum values up to this point to circumvent engine-destroying detonation. The cost is loss of low-end torque.
From here on up, the A-Series can utilize optimum advance values for maximum power so a maximized advance curve can be employed. Having said that, I have built and dynoed some engines that must be running at very high VE, as ignition had to be slightly retarded, or held in check, at around 5,000 to 5,500 rpm - usually more typical of forced induction applications.
Race engines are a different kettle of fish. Where cams with 300-degree duration or more are used, there's going to be little performance below 3,000 rpm to speak of. Consequently the ignition advance requirement is no more than that to get it started easily, and allow it to run. Peak power for the A-Series has been generally quoted as needing around 36 degrees total advance. Personally I've never found any more than 31-32 degrees necessary where good, high-octane (100+) fuel is used.
When running on 'pump' fuel (95/97 octane) I usually see 27-29 degrees as needed, but as much as 32 on odd occasions on engines I haven't built. One reason for this is a good illustration of why dynoing engines and getting advance curves sorted is pretty important - VE.
Engines that are running efficiently (good VE and combustion characteristics) will need less total advance than one not getting a complete lung-full. It applies to street engines as much as race engines.
Street engines will need the advance to be gradual, whereas a race engine generally has the advance increase quite rapidly so little or no more advance occurs after about 5,500 rpm. Note that this is a very general over-view - not a hard and fast statement!
Considering the above, the dizzy will need somewhere in the region of 10 to 15 degrees of advance built into it. That's dizzy degrees incidentally - not crankshaft degrees where you check the timing. Don't forget the crank is going twice the speed - so that relates to 20 to 30 degrees of timing when checked at the crank pulley.
Dizzy - Distributor
CR - Compression Ratio
VE - Volumetric Efficiency
Article Date: Sep 15, 2000