Blueprinting a distributor would involve making sure it works exactly as
designed. Meaning no shot bushings, fatigued advance springs, worn or out
of spec lobes, worn breaker plate pivots, proper shaft end-play, and
dead-on advance curves. (maybe a few other things I didn't think of right
Centrifugal-only doesn't loose advance when you mash the pedal to the floor
and the engine looses vacuum. Of course there is the issue about whether
to hook up vacuum advances to ported vacuum or manifold vacuum. They are
two completely different vacuum sources. Generally, centrif-only
distributors operate in a much tighter advance curve requiring much more
initial timing and the advance comes in more quickly (at least on
aftermarket dizzys). Some people want this for flat-out performance
reasons (never for driveability reasons). Vacuum advance units are
generally more suitable for street use, operate in a much wider advance
range, and the centrifugal advance curve is changed to meet engine load
demands by the vacuum advance mechanism - it actually rotates the position
of the points relative to the lobes.
The real performance advantage of an electronic distributor is that there
is no variation in spark timing coming from variations in point lobes -
since there are no points. Is this better than a point distributor? Only
if there is something wrong with the point type distributor. Furthermore,
aftermarket out-of-the-box advance curves have not been tailored for your
specific application and may be completely wrong.
As far as high performance coils go, having one on a V-8 can be important
at high RPM, due to the time necessary for the coil to build up charge for
the next spark (rise time). With 8 cylinders, the sparks come twice as
fast as a 4 cylinder. As long as a coil has sufficient voltage to fire the
fuel mixture, a stock coil on a 4 cylinder should be good to twice the rpm
it would on an 8. Just don't expect it to make a difference in the way
your car runs. Ya have to understand that to me, the coil is a non-issue
on a 4 cylinder. The aftermarket part shouldn't hurt anything unless it's
incompatible with the car's ignition system - but much of the posts have
been about compatibility - so I say why do it in the first place?
As far as the issue of distributor points floating, the same applies -
sparks occur at half the rate of an 8 cylinder. Unless you're twisting
your 4 cylinder over 9000 RPM, the stock points shouldn't bounce.
I didn't say that 0.045 was a useful limit. I was implying that above
0.045 can cause breakdown of ignition system components and introduce
certain driveability problems. I gap mine to the gap the spark plug,
engine, and ignition system was designed to run at and look elsewhere for
gains in performance or driveability.
SUs should be able to supply a proper mixture throughout the RPM and load
range if they are properly set up.
The question about "why then did the new cars go to wider gaps (ie 0.045)"
is a really (really, really) good one. Made me go hmmmmm for a while,
grasping at mental straws to support my position - but I thought of one
after a while. Here it is. I think it is probably because with tight
emissions the engine is run leaner than they used to, and this may indeed
require more spark intensity to properly light the fuel mixture (in
comparison to the richer mixtures used in earlier years). I have to adnmit
I'm not quite sure if I'm coming to this conclusion simply because I don't
want to back off on what I have said, but I don't remember 50s and 60s cars
missing or running poorly because their plug gaps were too narrow - so yes,
I suspect modern plug gaps don't really apply to a non-emissions engine and
the increase in plug gaps was purely for emissions concerns. I might even
take the next logical step and say that I suspect development for
electronic ignition systems in general was prompted purely by the need to
meet emissions standards on leaner-burning engines that were more demanding
on the ignition system - the "timing" of their development (mid-70's and
later) was right for this - pun intended! :-)
C'mon Treavor, you know you like this kind of discussion. The stuff above
is how I understand it to be, not a set of facts. I would be genuinely
interested in your opinion or anyone else's. Let's get technical.
Where's an engineer when you need one :-)
At 10:57 PM 6/24/98 -0400, you wrote:
>Sounds good to me. Thanks for bringing us back down to earth. What does
>'blueprinting our distributors...' mean? I'm also keen to know about the
>advantages of centrifugal-only vs vacuum advance. I've often wondered about
>it. I agree, this is the venue for such a discussion. And you're right, I
>have a limited understanding of the effects of a larger spark. Your post
>that 0.045" may be the useful limit is good to know. Thanks!
>But, honestly I don't to see how going to a different triggering mechanism
>(i.e., Hall effect vs original points) constitutes a voodoo system. As for
>the Sport coil, since I was replacing my old one and could get a Sport coil
>for the same price, why not? Also, and please correct me, but don't SUs run
>a little rich at idle or low rpm? I do a lot of stop and go driving during
>the week (but not on the weekend!) and I thought widening the plug gap
>might help get a cleaner burn. As far as I can tell, my mixture is correct
>as per Haynes, though I'm going to be spending some time with the local
>guru of the AHCA chapter to check my work. Again, you're right, I don't
>know how much wider is enough, or if any wider than 0.025 is useful at all.
>Maybe I'm hiding a mixture problem. OTOH, is the 0.025 spec a compromise
>based on plug and points technology of the 50s? Why do modern cars use
>0.035 to 0.045? Is it an interplay of modern factors that just don't apply
>to our cars? I know all of this must seem awfully naive to you; Hope it's
>>At 01:54 PM 6/24/98 -0500, Les Myer wrote:
>>There is a lot more performance to be gained by precise and correct
>>ignition timing than by extra spark intensity. Why don't we discuss
>>blueprinting our distributors, vacuum vs centrifugal-only advance stock and
>>aftermarket setups, proper heat ranges for spark plugs, and determining
>>advance curves for modified engines? Or do we all just throw in a
>>hodge-podge of aftermarket electronic ignition system parts and run them
>>straight out of the box?
>>Sorry, but this thread sounds like of a lot of ignition system vodoo, IMHO.
>> Of course, everyone has the right to do anything they want to their cars
>>or discuss anything they want in this forum. I'm just not buying it.
>Jeffrey H. Boatright, PhD
>Editor-in-Chief, Molecular Vision
>"Seeing the Future in a Very Tiny Way"