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Pedal Smasher
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Discussion Starter #1 (Edited)
Sometimes I learn things when I'm just surfing the net. It doesn't surprise me that this idea has been done but I didn't know it existed until today when browsing stuff for classic 911's. You can have a rev limiting distributor using old school tech. The rotor has a counterweight built into the contact between the coil and the spark plug. Centrifugal force will eventually overpower a spring and cut out the ignition. They make these with various limits in 100 RPM increments. I wonder if anyone has tried to install one in an Opel? I'm sure some of you guys knew about these rotors.



 

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Pedal Smasher
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Discussion Starter #3
I'm surprised these aren't more common place once they were created. I don't think these were ever used on American engines but they were somewhat common on German engines.
 

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Opel Rallier since 1977
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I would guess that you would get a lot of exhaust backfires when these operate.... there would be unburned fuel going out the exhaust.
 

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Pedal Smasher
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Discussion Starter #5 (Edited)
Momentarily, ya that would happen. It shouldn't happen often however, if you weren't trying to hit redline regularly. Even when I owned my Mustang which had a 7,000 RPM redline, I rarely ever tried to push it that high.

I think this is useful however to protect the engine if you mess up a down shift. I know the PerTronix Ignitor 3 has a rev limiter built into it but I've always liked old school ways of doing things before electronic methods took over, simply because it took more creativity.
 

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My first Opel had one set at 5400 rpms. I got another one which was set to 6200 which was more in line with the powerband of my engine.

As I recall these came in Porsche 914’s as OEM.
 

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My first Opel had one set at 5400 rpms. I got another one which was set to 6200 which was more in line with the powerband of my engine.

As I recall these came in Porsche 914’s as OEM.
Lots of them on eBay...

 

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Pedal Smasher
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Discussion Starter #8
There are a few German cars that had them but the 914 is where I learned about it. Bob, were they OEM on your past Opels or done by an owner?
 

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There are a few German cars that had them but the 914 is where I learned about it. Bob, were they OEM on your past Opels or done by an owner?
Done by the PO, who was one of the original owners of C&R
 

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I would guess that you would get a lot of exhaust backfires when these operate.... there would be unburned fuel going out the exhaust.
Surprisingly not. They were pretty smooth at the cutout. It almost felt like you were running into a soft ‘valve float’ situation. No popping and banging.
 

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Opel Rallier since 1977
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That is interesting.. maybe it was not a hard cutoff of spark but a gradually opening gap that just weakened the spark and caused a power loss rather than a complete cutout? I have cut off the ignition on a couple of engines and then quickly re-connected it (all by mistake) and got some good 'bangs'!
 

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Pedal Smasher
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Discussion Starter #12 (Edited)
I have a feeling that due to the compact and mechanical nature of this design, the high voltages would arc across the gap besides going to ground. There would be a considerable loss of spark, kinda like fouled plugs, and it would become difficult for the engine to still accelerate. I could be wrong of course, but this would be a gentle rather than abrupt loss of power. At the point of disconnect, the gap would be very small.

I wonder if it was different for a missed down shift? You went into the wrong gear, RPMs go way up and the rotor has a large gap. No ignition at all, all the fuel goes into the exhaust. I'm sure backfires would be more common in this scenario.
 

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Sometimes I learn things when I'm just surfing the net. It doesn't surprise me that this idea has been done but I didn't know it existed until today when browsing stuff for classic 911's. You can have a rev limiting distributor using old school tech. The rotor has a counterweight built into the contact between the coil and the spark plug. Centrifugal force will eventually overpower a spring and cut out the ignition. They make these with various limits in 100 RPM increments. I wonder if anyone has tried to install one in an Opel? I'm sure some of you guys knew about these rotors.



I have used them and they work great. Mine was for a Porsche and had a cut off about 7200.
 

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I used this type of rotor once and noticed that they have a large hysteresis, meaning that once the rev limiter engages, the rpms have to drop way low, maybe to 4000 before it disengages. That's fine if one never intends to use it and only has it as insurance for emergencies. However, it is very frustrating if one pushes it a little too far during acceleration and then has to wait a while until the engine comes back on..
Thomas
 

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Pedal Smasher
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Discussion Starter #19
I used this type of rotor once and noticed that they have a large hysteresis, meaning that once the rev limiter engages, the rpms have to drop way low, maybe to 4000 before it disengages. That's fine if one never intends to use it and only has it as insurance for emergencies. However, it is very frustrating if one pushes it a little too far during acceleration and then has to wait a while until the engine comes back on..
Thomas
Thomas, that's interesting. I've been thinking of how that could be possible if the mechanism was working perfectly fine. Springs will always seek equilibrium so I wouldn't expect there to be a large hysteresis unless the spring in this case is nonlinear. This would cause it to hold out and then suddenly give way, which would then require the centrifugal force to decrease a bit until the spring could retract. At least that is what I'm thinking.

For me at least, my interest is to have one as a safety measure. If I miss a downshift and the engine over-revs, then I'd know at least the ignition was cut. If my operating redline was 6,500 then it would make sense to have the rev limit at like 6,800.
 

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Hi Joe,
That's a good question. I apologize in advance if the answer gets a bit theoretical and long-winded and possibly irrelevant to the original discussion.
The rotor has a mass that can move in radial direction and is held by a spring. The mass is limited by an inner stop r0 and an outer stop r1. The centrifugal force is proportional to the radius and to the square of the rpms.
One could use an ideal spring whose force is also proportional to the radius. At low rpms the spring would hold the mass tightly against r0. Above the rev limit the centrifugal force overcomes the spring force and the mass can move to r1. The problem occurs at or near the rev limit. Since the spring and centrifugal forces nearly cancel, there is only a small net force moving the mass. And because both forces are proportinal to the radius, the near cancellation is true for all values of the radius. As a result, the mass is not tightly forced into one position, but depending on rpm slowly bounces between the two stops or even floats in the middle. This creates a lot of high voltage arcing and would quickly destroy the rotor.
What we want is a mass that quickly moves from r0 to r1 when the rev limit is reached and then shorten the HV to ground. This can be done with a weak spring and a large pre-load (meaning there is already a large spring force when the radius is zero). This leads to a run-away or trigger condition. Above the rev-limit when the mass moves to a larger radius the centrifugal force increases, but the spring force changes very little, because the pre-load dominates. This forces the mass progressively faster towards the outer stop r1 and largely shortens the switching time.
Say the radius of the outer stop where twice as large as the inner stop, r1=2*r0. Above the rev-limit, the mass has moved to the outer radius and makes the centrifugal force nearly twice as large and much larger than the (pre-load dominated) spring force. In order for the spring to be able to pull the mass back to r0, we have to reduce the centrifugal force by reducing the rpms to 71% of the rev-limit (centrifugal force scales with square of rpms). That's where the hysteresis comes from.

Thomas
 
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