Total advance on XK engine

Posting a reply on another thread reminds me to report back to the folks who kindly weighed in on this issue.
Although the Mk10 is (again) incapacitated, this time by power steering issues, it was back on the road for a time with the rebuilt distributor. That distributor did have a lot of problems before the work. With the rebuilt dizzy, the car was running very well, starting instantly, good power. I had a somewhat lengthy discussion with Jeff of Advanced Distributors, and here are some caveats that I learned from him:

  • Original curves were for fuels of the era, now completely different because of the many different additives (50 choices according to Jeff), which vary in mix from brand-to-brand.
  • It is now much more difficult to read plugs for mixture, because the additives turn the plugs brown. You may think you are not too lean because the plugs aren’t white, but can be fooled. Another good reason to use an oxygen sensor and a/f meter imo.
  • Modern fuels require different curves - as you’d expect, Jeff doesn’t disclose his curves, but they all start with a lot of advance. He recommended starting with 16 degrees (dynamic with timing light) on the 4.2 Mk10. He said all modern cars have lots of initial advance, even 20 degrees.
  • He said not to worry about total advance - it can be 40+ without problems.
  • He’s not big on electronic distributors because of the power losses he’s consistently found on dyno runs. Also, there can be significant electronic interference within the engine compartment, which he likens to a Faraday box. My rebuild has points.

So, I’m just repeating what someone with experience told me, so don’t beat me up if you disagree about the distributors. I have to take the word of the guy who has rebuilt 20,000 of them.

I’d have to see the data on that: quite the opposite, when I put Allisons, nee’ Cranes, on the few race engines that I had on the dyno, I consistently saw better numbers, especially at the high end.

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He made no specific comment about Allison, but said the average power loss with Pertronics was 7hp, occasionally much more. He considered, but declined to carry 123 after testing them, and I believe that was because he was finding too much electronic delay in response. He also believes that in the real world, the electronic distributors suffer from too much interference from other items such as alternators in the engine compartment. Like I already said, this is all above my pay grade.

My bottom line as someone not interested in race performance is to leave the curves to those who are doing a lot of dyno work, as long as we can access that at a reasonable price - not a problem right now. We really don’t have a good way to duplicate that experience without huge time investment in seat-of-the-pants tuning.

Modern cars also have knock sensors and electronic engine management systems that constantly optimize the timing.

Are there any racers that use points-type ignitions (other than where those are mandated by rules?)

I guess OEM auto manufacturers didn’t/don’t know what they’re doing, using electronic distributors to meet precise emissions regulations and fuel economy standards. They apparently never discovered the Faraday effect on their distributors in hundreds of millions of miles of road and track testing.

Dave

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My understanding is the original Pertronix Ignitor units suffered from the switching delay issue (which is a common issue in all of the older solid state designs from any manufacturer that still relied on the distributor mechanical system for advance) but the current Pertronix Ignitor II has a compensation circuit that advances the switch timing based on speed to compensate. But for an engine that no one revs above 5500 rpm I think his concerns are more academic than actual as the amount of effective advance lost to switching time of a transistor would be very small indeed.

I’m guessing his dismissal of 123 Ignition was simple anti electronic bias that all those old school distributor guys seem to have:)

And I would say with a 123+, if there is a measurable loss of advance due to electronic switching delay (which I do not believe is the case), then measure where the delay begins and add additional advance in to compensate. Takes 2 minutes.

Not to mention, think of all the horsepower losses F1 cars are suffering, due to not using a superior system like points and condensors.

I personally am glad the things are dead and gone!

:grimacing:

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I think if I were starting with a blank sheet of paper, I would concoct a crank-trigger electronic control. Eliminate the internal slop in distributor and its drive (minor in a Jaguar). Any time you have a mechanical system controlled by springs you have resonant harmonics that will affect the linearity of the response.

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I think most listers that have fitted (Rays ?) EDIS system rave about its smooth idle and good performance, and that most new vehicles have a distributor-less ECM controlled spark system

I admit to still having points in my 420G, but its got carbs as well

I probably should fit the electronic module I have to a spare dizzy and insert that to see if its better, but I also tend to fix things when they need fixing, and although mine is up for checking again sometime soon, I dont put many miles on it, and its not broken

Skepticism is a healthy thing, not just related to old engines. I’ll say again that I don’t have the real-world experience to confirm or refute anything said here, but I will make a few points.

We are talking about aftermarket add-ons to an archaic system of engine controls, i.e. carburetors and distributors, long since replaced by something better. A Pertronix unit or other add-on isn’t the same as an OEM designed system in a modern fuel injected computer controlled car.

I can’t speak about racers in general, but I do know that Hap Waldrop of Acme speed shop, who is a well-know expert in the BMC motors and who rebuild the cylinder head for my 1275 souped-up Morris truck, won’t use anything but points in his race engines. He specifically eschews electronic ignitions because of power losses. Ask him.

As a car guy, I always want to understand why something doesn’t work right, and try to logic my way through problems, as does everyone responding to this thread - it’s what gives us great satisfaction when we fix something, and great frustration when we can’t. When I’m outside my element, I’ll try the forums, but I ultimately rely on the expertise of professionals who make their living at what they do. Sometimes, it’s a pleasure to pay someone to fix something I’m struggling with. One reason I use Jeff at Advanced Distributors is the strong recommendation I got from Joe Curto, who I consider at the top of the game in SU carburetion. These are people that will talk to you. You would certainly be better off asking them than me.

Mechanical advance is all about maintaining peak combustion pressure at the optimum crank position.

Since ignition to peak pressure is a unit of time. The higher you rev the sooner this timed event must occur to maintain that crank position event.

It has zero to do with cam, carbs, or what you ate for breakfast.

For street use, a vacuum advance regulates the load on the engine.

On a V8, advance of cruise at 60 mph, 54* is not uncommon.

Whatever the factory used when designed is usually best.
Anything else is smog related.
However, race cars do not use vacuum, and most pin it at 36* .
Street cars require vacuum cans to be more “street friendly.”