Mechanical Retard - How'd They Do It?

I have to imagine that at some point some one figured out that retarding timing at higher revs could be beneficial to increased performance. I also know that this can be done now a days with electric ignition systems.

My question is were there MECHANICAL advance/retard (not manual a la Model T or A column levers) distributers that were activated by centrifugal or vacuum devices? Were these used in racing, perhaps drag racing, maybe even early top fuel racing?

If used on the street, again in the early days where everything would be either vacuum or mechanical activated, could you present examples.

Thanks in advance, John

 

Maybe one exists somewhere, but I have been around a lot of different engines and have never seen a mechanical advance mechanism that does high RPM retard. Fords of the early '70s had a double sided vacuum unit that combined both advance and retard into the same unit. The performance of inductive ignition pick-ups varies with speed. High RPM retard could probably be designed into one of those.

 

Most racing magnetos are fixed timing. No advance curve, just set it where it works best for the rpm you run.

Racing typically keeps the engine in a narrow rpm range so there is no need to make it optimal outside that range.

Edit: I've also never heard that high-rpm retarded timing helps power except in supercharged applications or two-strokes.

 

I like your screen name Relic. Sorry but that's all I have to contribute.

 

So what sort of engine phenomenon rapidly speeds up combustion so very much at high rpm enough to need to retard the ignition ???

About the only thing that would possibly do that is massive combustion chamber pressure and turbulence that rises very rapidly with rpm, for example from a turbocharger or centrifugal blower.

And that is exactly where you find pressure retard fitted to the distributor. These also usually have vacuum advance fitted as well.

But a mechanical high rpm retard, why would you ever need that ?

 

They basically just stop advancing at some fixed RPM, which amounts to retard as the RPM continues to go up. The need for retard is more a function of cylinder filling (load, VE, however you want to look at it) than just RPM.

 

Grumpy used a dual point and switched points in high gear as he could not pull as much advance in high.

 

I've been a mechanical retard my whole life.......(sorry somebody had to do it.)

 

Ok, Finch. So explain...how'd you do it?

 

Hahaha, and being a vacuum retard really sucks................

 

In the Mid 50's and early 60's it was the thing to retard the timeing on the big end and it would increased the MPH a little. This was done with a choke cable and a few companys made a device to advance and retard but later years it went all electrical.

 

The primary reason; when air is compressed heat is generated. That's why air compressor pumps and tanks get hot. The same thing happens when mixture is compressed in the cylinder. Diesels rely on that principal to ignite fuel. At high RPMs the mixture is compressed more quickly than at lower RPMs. That means less time for heat to be absorbed by the surrounding metal. That is why a diesel engine won't start if it doesn't crank fast enough. The higher heat that results from compression the charge more quickly causes the mixture to burn faster. As a result, there is a point where less ignition advance is needed. For the same reason, it actually takes less ignition power to ingnite the charge at high RPMs than at lower RPMs.

 

That is all very true.
But as engine speed rises there is less time available for combustion.
The net effect is that generally we need more advance as rpm rise, which is why the majority of engines require some centrifugal advance.

 

Yeah, this is what I was thinking of also. Bill Jenkin and Dave Strickler used the dual point distributors on their 409 Stockers and "Strick" had a switch that he would flip in high gear that would ground out a single set of points which resulted in retarded timing and a slight HP gain. I've always been kinda curious as to how many degrees were taken out and what this was really worth in ET reduction.
On another note, the factory GM Transistor Distributor Ignitions had a built-in feature that would pull out 1-degree of timing for every 1,000 RPM.

 

Oh man, you beat me to that one!

 

Dead on. How much the actual retard was I dont know. might be able to dig it up somewhere.

 

I never saw it, but was told of a setup that used a shift light as a triger to ground one set of points in a dual point dist.. The car had two tachs as I remember the story.

 

Mechanical retard, that would be my buddy- we say he does "tune-downs"
He's never put a wrench to anything that didn't run worse when he got done, and bullheaded... He'll never ask a question, read directions, or crack open a manual, just calls and says it's running like crap- followed by "I dinked with it"

 

hey who ya callin a mechanical retard?

 

i work with 50 of them all day

 

My thought would be that the closer to top dead center you can ignite a fuel charge, the less the piston has to fight to to get past tdc and start it's way down the other side. I understand that all fuels take a certain amount of time to burn across the combustion chamber, and that as rpms go up that time becomes less and less, much like after 3000 rpms there is only time for one spark per ignition cycle instead of many sparks per ignition cycle (a la MSD). That is why with two spark plugs per cylinder more advance can be pulled out the dist, You get that fuel charge ignited quicker and more evenly. What it really boils down to is fuel octane and the ability to burn quickly without detonation. One wouldn't need as much advance if you could get the flame front across the combustion chamber faster (with no detonation). A quicker explosion.
Anyway, I thought that with a magneto you could lock it in at say 40* and have a centrifugal RETARD system that would retard as the rpms went up.
Again, thanks all that have contributed. To those comedians who added their humor ....
don't give up your day jobs.

John

 

Yes, ideally you want peak cylinder pressure to always occur about 20 ATDC.
At very low rpm you can light it off pretty close to TDC.
At extreme high rpm, you need to light the fire pretty damned early, particularly with a big bore engine.
So to start it and run it you don't need a lot of initial advance. But flat out you usually need a lot more which the centrifugal ADVANCE is designed to provide.

 

Interesting discussion. The small block Dodge places the spark plug closer to the cylinder/piston actual center than does the small block Chevrolet. The SB Dodge cannot tolerate the total advance that SB Chev engines can.

 

Hey, it may just be a little off-topic, but don't older-era Indian motorcycles have either an advance/retard control on the handlebars. Anybody care to elaborate to me what/how/when the rider would do with that? Thanks in advance! (Pun most certainly intended!)

 

On motorcycles, the handlebar distributor advance/retard control was usually used when the rider started the motorcycle. In other words, the rider would twist the handlebar grip to the distributor retard position so that when he kicked over the engine to start it, the engine would not kick back since the distributor was retarded a certain amount of degrees (say 10º). After the engine started, the rider would twist the handlebar grip control to advance the distributor to it normal advance level.

 

Interesting. So it's really just to get it started easier......that makes sense. I suppose if you're really bookin' along with said motorcycle in high gear and slightly twist that grip to retard timing a little, you'd even go a lil' faster?

 

Why do you think reducing cylinder pressure is going to make you go faster ?

 

Well gawd dam if I gotta explain it, it's because that's what was talked about on the first page of this thread, you were there, I seen you......high gear retards are obviously worth a little horsepower. GM designed it into their Transistor Ignition in the Sixties and racers have been grounding out a single set of points in dual point distributors in high gear to get the same effect. I'm not an Engineer and I don't know WHY they're doing it.......I just know they're doing it.

 

Ah ... but the question would be which set of points (leading or trailing) and why?

 

I can tell you exactly why...............

At a constant speed, or where the engine is under full load, and the rpm is not rising very fast, you need a certain amount of ignition advance so that peak cylinder pressure occurs just after TDC.
Now this might be like on a dyno, or flat out in top gear, or towing a trailer. So the piston takes a certain time to go from the ignition point, to where you want max cylinder pressure. That is where you set the ignition timing.

Now think about what happens in first gear.
The revs are rising at a phenomenal rate, everything is going faster and faster, including how long it takes for the piston to go from the ignition firing point to where you want maximum cylinder pressure.
So you can fire the ignition a bit earlier in first gear, knowing the piston is going to accelerate faster to TDC than it does in top gear, at the very same rpm.

So under very rapid engine acceleration, the engine needs more ignition advance than it does under slower rates of acceleration as you will find under more load in the higher gears.

But that is NOT retarding the ignition with increasing rpm as your earlier posts suggest, but just decreasing the amount of advance.
And that is a very different thing.
I believe it was Grumpy Jenkins that started all this, a very clever man indeed.
In fact on most modern EFI engine management computers you can set an acceleration constant in the ignition timing that does it all automatically. The software measures revolution by revolution engine acceleration and times the ignition such that peak cylinder pressure occurs at exactly the right time, no matter what gear you are in, how heavy the trailer, or how steep the hill.

It is a bit like shooting at a flying target.
The faster the bird, the more lead you need to give so bird and bullet arrive at the same place and the same time.
So first gear you aim your ignition for a bit more lead on the "target" than you need to give it in top gear.