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Author Topic: Possible Reason for Mis-Firing 1949-51 Motors  (Read 402 times)

Offline Jay Friedman

  • Posts: 2536
Possible Reason for Mis-Firing 1949-51 Motors
« on: April 25, 2020, 06:10:35 PM »
I wrote this several years ago and thought it might be useful to repeat it.

1949-51 Cadillac cylinders and spark plug wires are numbered from front to back 1-3-5-7 on the driver’s (left) side and 2-4-6-8 on the passenger’s side of the motor.  Early ’49 Cad valve covers have metal conduits which route the spark plug wires from the distributor at the back of the motor to the spark plugs at either side of the V8 motor.  Similarly, late 1949 as well as 1950 and 1951 valve covers instead have metal loops through which the spark plug wires are routed.  I obtained a copy of the December 1950 Cadillac Serviceman, a monthly publication for Cadillac mechanics, in which there is a short article entitled "Poor Engine Operation May Be Due To Improper Wire Routing".  The article goes on to say that “poor engine operation may be due to improper routing of the No. 5 spark plug wire" and that this wire "should be rerouted to its correct position". 

The article then has a drawing showing the correct routing of the number 5 and the other spark plug wires through the metal loops on the driver's (left) side valve cover.  The drawing shows the number 5 spark plug wire going into the first metal loop at the back of the valve cover on the top, the number 1 under the 5, and below the 5 are the number 3 and then the number 7.  Continuing through the two metal loops on the side of the valve cover, the wires stay in the same order from top to bottom.  Finally, the article says correct routing through the metal loops is as important as "having them connected to the correct spark plug".  Although the article does not describe routing through the conduits on early ’49 motors, the principle is the same. 

Never having heard of this before and not understanding why it would make a difference, I nevertheless re-routed my '49's driver side spark plug wires as the article instructs.  I then took the car for a drive and it seemed to be running a bit better.

I asked about this on the forum and received responses from several club members: Art Gardner, Bob Schuman, Ron Sotardi and T. Lewis.  Art explained that the phenomenon is known as electrical inductance.  When current flows in a wire in a one direction, it creates a magnetic field perpendicular to the direction of the flow.  Due to the magnetic field, an adjacent wire lying in that magnetic field will experience an induced electric current in the opposite direction.  If two spark plug wires which are adjacent to each other are phased (timed) so that the induced reverse current in the second wire occurs at the almost the same instant the ignition current runs down that wire--meaning the two wires follow each other in the firing order--there is interference which can cause a slight miss.  The induced current in the second wire is not nearly as great as the regular ignition current, but it can diminish the ignition current. 

This does not happen in any other spark plug wires, since being out of phase enough (that is, they don’t follow each other in the firing order), they can receive an induced reverse current while lying next to each other in the routing without resulting in a misfire.

A 49-51 Cadillac’s firing order is 1-8-4-3-6-5-7-2.  So, looking at this sequence you can see that two spark plugs wires could have an inductance problem if they are both odd-numbered or both even-numbered (that is, they potentially could be routed next to each other on the same side of the motor), and if they follow each other in the firing order.  In this case the odd-numbered 7 follows the 5 in the firing order, which explains why Cadillac engineers decided that these two wires should be widely separated in the driver’s side loops or conduits. 

The article doesn't mention the passenger's side spark plug wires.  I wondered why since the even-numbered 4 follows the 8 in the firing order.  So, to be on the safe side I also re-routed my '49's passenger side spark plug wires so that these two are also widely separated in the passenger side metal loops.  Specifically, I put the number 8 spark plug wire going into the first metal loop at the back of the passenger's side valve cover on the top, the number 6 under the 8, and below the 6 are the number 2 and then the number 4.

On 49-51 Cadillacs, the wires are crowded together inside the metal loops or conduits, so that close proximity creates the induced voltage.  Art Gardner went onto say that inductance and possible mis-firing became a non-issue from 1952 when Cadillac started using rubber grommets to keep the spark plug wires separated from one another
1949 Cadillac 6107 Club Coupe
1932 Ford V8 Phaeton (restored, not a rod).  Sold
Decatur, Georgia
CLC # 3210, since 1984
"If it won't work, get a bigger hammer."

Offline Jeff Rose CLC #28373

  • Posts: 2324
  • CLC Number: 28373
  • Name: Jeff Rose
Re: Possible Reason for Mis-Firing 1949-51 Motors
« Reply #1 on: April 25, 2020, 09:56:24 PM »
Interesting read. Thanks.
For those of us who don't know, I would appreciate if someone would post a couple of pictures of the older style valve covers.
« Last Edit: April 25, 2020, 10:07:54 PM by Jeff Rose CLC #28373 »
Jeff Rosansky
CLC #28373
1970 Coupe DeVille (Big Red)
1955 Series 62 (Baby Blue)
Dad's new 1979 Coupe DeVille

Offline Jeff Maltby 4194

  • 49 fastback
  • Posts: 1987
  • Deceased Member.
Re: Possible Reason for Mis-Firing 1949-51 Motors
« Reply #2 on: April 25, 2020, 10:58:53 PM »
I sold 3 early sets over the years to hot rodders.
Jeffo 49er chapter

CLC 1985
Honda Gold Wing GL1500


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