Mechanical Retard - How'd They Do It?

Yep I thought this thread was for me

Actually some bikes used to have a bob weight set up that opened up through centrifugal force at higher revs.

 

Ever wonder why an engine that revs to 6,000RPMs will generally be at full advance by maybe 2,200RPMs? It is due to the principal I described. Because of that same principal, in some cases retarding the ignition is needed to stay closer to ideal. There are engines that benifit from some aditional advance at high RPMs. What is ideal depends on various factors like combustion chamber design, breathing efficiency at a given RPM, mixture strength, rod length to stroke ratio, etc.

 

Here is an accessory from the 50s that let you change the timing from the drivers seat. I never hook it up. I just think it's a cool old trinket. They are rare so they didn't sell too well.

 

I think you mean to say the SB Mopar doesnt NEED as much advance. Large amounts of advance is not an advantage in and of itself, it is a way to compensate for poor chamber design and plug position...

It can be a little tricky to get the squish tight enough in a mopar, that will affect the octane tolerance, but that is a separate issue from plug position. The plug position in chevy heads, particularly in power packs and camel bumps is TERRIBLE. But we are wandering way off topic.

 

I have tried to run very healthy street motors (probably more truthful to describe them as race motors that are driven on the street a bit) with no curve, I have had them break starter noses when they "cough" at cranking speeds. Its common to install a separate ignition switch, so you can spin them with no spark, then flip the ignition on once they are cranking. Same deal as the handlebar control on the Indians

 

IF the vacuum advance cannister is connected to FULL manifold vacuum source, when you go to WOT the vacuum drops to '0' and there is no vacuum advance added. At that point the vacuum advance cannister actuall has retarded the ignition timing by elliminating any vacuum advance. The same is true when the vacuum advance cannister is connected to a PORTED source, because again, when going to WOT the vacuum drops to '0'. As the engine RPM's increase the mechanical (centrifugal) advance will increase until it reaches it's maximum at a pre-determined amount. As the vehicle aproaches cruise speed or the throttle is backed off some, the vacuum returns and the vacuum advance cannister increases the ignition timing. This is the normal function of the vacuum advance cannister. It is constantly advance or retarding the ignition timing depending on vacuum signal from the engine.

For racing, vacuum advance isn't necessary and the mechanical advance is often 'locked'.

 

When I was a Motor Pool Mech for the Army we did not change points and such, just whole distributors. A 302 GMC distributer appears identical to a Reo Gold Comet distributer. But they rotate opposite. One day I needed a distributer for a Reo but only had a GMC. So I stuck it in. Really killed the top end.

 

Heres' a mechanical retard on my small block, it has a magneto and retards timing with air pressure thru the linear actuator. The magneto when clamped can rotate, the dual action actuator gets air pressure via solenoids to lock it fully advanced at launch and the air pressure reverses at an adjustable rpm to retard the timing. The amount of travel is adjusted by degreed shims for how much retard.
the car is just sitting in the shop and the motor is to come out next week and the hoses to the cylinder (linear actuator) have been disconnected. Sorry about the shop dust, been messy lately.
Get the idea? This type of technology is/was more common in the faster drag cars as they didn't have batteries (well, they'd have drill batteries for tach power etc) and devices were are air powered and controlled. ie, mechanical timers, air solenoids etc, stuff like this was found up under those big mysterious black boxes with hoses going in and out of at the front of funny cars etc.

 

this is very interesting

I have a few magnetos for the early Olds motors
and was spinning them on my distributor machine
two af them had mechanical advance and behaved nicely
a third was rock steady until the rpm got up a ways
and then it started showing a few degrees retard
I figured this was a mechanical problem.
what I read here makes me think it may have been set up that way

 

Ive seen it done under the cap. It is so simple its stupid. But its a secret for racing. Ask any good distributer guy to hook you up.

 

That naturally tends to happen with inductive devices.

 

Slick.

 

so it is likely a malfunction?

 

I worked a a v drive boat shop years ago and some of them had a dist that had a lot of advance at low rpms and pulled back timing as rpm climbed.This was before MSD were big.Also some of the stinger and Accel early eletronics retarded timing at hi rpm,but I believe it was not designed to it just did.We learned to time them at full advance.

 

I could be wrong on this, but I think the fancy technical term for it is hysteris? Really good thread on the subject on Speedtalk some time back.

 

Not necessarily a malfunction. If the mag is in good repair and the magnets are still strong, it is just a side effect of the unit's design. Mags have a reputation for weak output at low RPMs. The more output your mag has at low RPMs, the more it will tend to do as you described at high RPMs.

 

Yes indeed that is all true.
Quite a few medium performance economy engines are like that, usually those that have fairly restrictive cylinder heads designed for low end torque.
As engine revs rise there can be a huge increase in combustion chamber turbulence (swirl and tumble) which rapidly speeds up combustion.
That effect can more than compensate for any further increase in engine revs beyond the torque peak. So as you say, some engines may be all in at 2,200 rpm.

High turbulence and very fast combustion rates can also come from adding forced induction to a stock engine, in which case a pressure retard is sometimes needed.

So yes, the advance curve sometimes has to go backwards, but I have never yet seen a centrifugal retard system, only electronic retard, or pressure retard.

 

Im wondering if a person were to reshape these parts if they could get the mechanical advance to behave differently?........hmmmm



Louie

Edit....but then again that may not be what you would want based on rpm acceleration. ie. first gear vs. high gear

 

Just get an aftermarket electronic engine management computer, either with full EFI, or just use the ignition part of it if the rules for your racing class say carb only.
Many of these have an "acceleration constant" setting, a fixed number programmed into the thing that can be changed.
So basically you set the whole lot up on an engine dyno, and set the ignition timing map at full load constant speed running.
Then do some sweep testing at a fairly fast rate of engine acceleration, tweaking only the acceleration constant for best results.
It should then be as good as it is ever going to be absolutely everywhere in any gear.

 

Looking at that picture, the whole mechanism could be flipped over so when the counterweights "throw out" they rotate the cam from the other side [ retarding ]
As for RPM, it is a matter of selecting the correct springs for the counterweight.
Early 60's pommy Fords used a 2 step advanced curve [ because of the crap Lucas starter motors ] basically it was 2 springs, 1 lightweight and tight and a heavier spring that was loose.

Early offshore boat racers used a centrifugal retard system to save the engine from grenading when they skipped over the waves
Apparently rev limiters didn't work too good because they would cut the ignition and the engine would "fuel up" and explode

 

Well, kinda sorta, but probably not in the way you are thinking, With big cams you would re-shape them to reduce the amount of mechanical advance in the dist. so you can run more initial advance which will help the idle. Altering the wieghts will also affect the rate of advance, but its easier in most cases to do the same thing by swapping the springs.

 

that's the whole reason I opened the post--scanning the titles made me stop and chuckle.

 

It reminds me of the "deth wobbel" thread....... http://www.jalopyjournal.com/forum/showthread.php?t=470139&highlight=death+wobbel

 

We used to retard the timing in high gear on the Junior Stockers. It was worth a part of a MPH and some ET too.
We'd use a Corvette dual point.

Let's see...I think we would run on the primary set, normal dwell,1st gear in a PG, then ground the second set through a toggle switch after the shift.
That way,when the 1st set opened, the trailing set would still be closed till a later point ,thus firing the coil later. You just adjust the second set for the amount of drop you wanted...
Maybe, anyway..It was 40 years ago!