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Converted from auto to 4 speed


sredmyer

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I have just completed (well almost) the conversion of my Silver SS from a floor shift automatic to a 4 speed.  I need a bit of help to get the last bit working. 

  1. I am not sure where the neutral safety goes and how it is wired?
  2. How are the reverse lights wired up?

Any help greatly appreciated.

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The reverse lights work off the reverse lock out rod. When you put the car in reverse the rod turns the column. The factory lock out. Or you could wire in a switch on the trans.

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The reverse lights work off the reverse lock out rod. When you put the car in reverse the rod turns the column. The factory lock out. Or you could wire in a switch on the trans.

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The reverse lights work off the reverse lock out rod. When you put the car in reverse the rod turns the column. The factory lock out. Or you could wire in a switch on the trans.

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The reverse lights work off the reverse lock out rod. When you put the car in reverse the rod turns the column. The factory lock out. Or you could wire in a switch on the trans.

Is there a switch I need to add to the column?  I have the linkage for the factory lock out installed and you must put the car in reverse to get the key out but if understand it right, all that does is rotate the column collar to the "Park" position.  Seems there would need to be some sort of switch on the column (that likely would not be there in an automatic column) to actually make the reverse lights come on.

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Your floor shift automatic had all this at the floor shifter. If I remember correctly, you have to unplug the floor shift harness at the main harness under the dash and get the switch for an automatic "on the column" which is mounted on the column itself. It's a relatively easy conversion....plug and play.

 

Steve

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Well it seems I have one more problem with this conversion...the tach appears to be wrong.  Based on discussions with others and my own research online, I have determined that the RPMs at 70MPH should be 2880 but my tach is reading 3300. 

 

Here are the details...

  1. Rear end ratio 3.31
    1. This was verified by spinning one wheel 10 times (other wheel blocked) which resulted in 16.5 revolutions of the driveshaft.  Using the formula of driveshaft rotations x 2 / wheel rotations we get (16.5 * 2) / 10 = 3.30 rounded to the nearest know ratio is 3.31
  2. Tire size of 255/60/15 yields a tire height of 27”
  3. The Muncie M20 transmission gear ratio is 1:1 in 4th gear

 

Given those values, we can calculate the engine RPM at 70 MPH as follows…

336 x MPH x Gear Ratio / wheel height  = RPM

Or

336 * 70 * 3.31 / 27 =  2883

 

As mentioned above when driving down the road at 70 MPH (as verified by the WAZE GPS app on my phone) my factory tach was reading 3300 RPM.

One other bit of, possibly relevant information is that the car has been converted over to HEI ignition.

 

Any ideas as to what is going on?  Why is the tach reading wrong?

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I can see your speedometer being off but the tach is based off the engine rpms. It's a one wire to the distributor as well...... And this is the factory tach too. Many timing lights have a tach option in them (I know my Snap-On does), do you know anyone with one of those? Or you could hook up a temporary aftermarket tach, it's only a couple of wires.

 

I realize this doesn't really answer your question but they are suggestions to help you verify what you have. I find the factory gauges to be sort of useless. I have the factory tach but also have a hidden shift light that has a tach built in from MSD. It's what I depend on to shift when racing.

 

Steve

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My factory tach is off also. It reads about 350 RPM higher at 65 MPH than another Sun tach I temporarily hooked up. I guess the factory gauges are close but can't be relied on as accurate. By the way I have the setup as yours 3.31 M20 4speed only mine is a small block. The tires I have are 235/70/15 28" tire height.

John S

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Steve, the formuas look good, but actual tire slippage is not accounted for.  At 70 MPH, depending upon tire traction, there would be greater than the math tire rotations to be at the actual speed.  This should not be 10% tho, 3 to 5 % max at 70 for slippage is what I would expect.  The RPM/timing light or an additional tach would verify the degree of error in the dash tach.  

 

john

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