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KwikFiVo

M10B18 Swap Cam/Distributor Question

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(edited)

Hi Everyone,

 

Due to a recent engine failure, I am looking for a way to get my car back on the road on a budget.  I found an M10B18 locally (from a 1985 318i).  I have read on here about the cam gear/distributor differences between the M10B20 that I currently have (1975) and the M10B18.

 

What would be the best way to get my 123 Distributor to work with this later motor?

 

It looks like the cam turns the same direction, but the cam gear is cut differently.  I doubt my distributor gear will match the cam.  The rotation does not matter to the 123, but the gear seems like it may be an issue.

 

I am open to putting a re-grind from IE in the motor to help with performance.  I am curious if I could use the cam from an M10B20 so it provides the correct cam gear.

 

Any input would be greatly appreciated!!

 

Thank you,

 

Bob

 

Edited by KwikFiVo

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Contact 123 directly. They can supply a new drive gear for your existing 123. The guys at Terry Sayther’s shop just did this on a customer’s car. Took a small amount of tweaking to get good mesh between the gears, but nothing dramatic. 

 

Ed

 

 

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9 minutes ago, KwikFiVo said:

The rotation does not matter to the 123

I'd bet that it does matter.  The rotor's position relative to the post in the cap changes with the firing advance.  The software/firmware would need to know the rotation is opposite or the rotor position on full advance would be way in the wrong direction.

No one here has discussed where the rotor position really is at TDC so it may be compensated for the advance for the rotation that the distr is made for (i.e. is it after the post so with advance it is not too far away ahead of the post or is it right on the post and moves away in the direction of advance).

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11 minutes ago, jimk said:

I'd bet that it does matter.  The rotor's position relative to the post in the cap changes with the firing advance.  The software/firmware would need to know the rotation is opposite or the rotor position on full advance would be way in the wrong direction.

No one here has discussed where the rotor position really is at TDC so it may be compensated for the advance for the rotation that the distr is made for (i.e. is it after the post so with advance it is not too far away ahead of the post or is it right on the post and moves away in the direction of advance).

Got it.  I may have made an incorrect inference from the 123 manual.  It references the CW/CCW rotation when setting up the distributor, and it does not seem to discuss it anywhere else, so I thought it was insignificant.  Probably another good question for 123.

 

Thank you,

 

Bob

 

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Rotor rotational direction didn’t matter on Terry’s customer car. They fit the new gear, insured the correct timing order with the cap and wires and off she went. 

 

Ed

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I wonder what 123 uses as a 'base' trigger time,

since electronic spark 'advance' is really just 'less delay'.

Most EFI uses 90 degrees btdc- but that's crank indexed.

Sounds like 123 just picks an arbitrary point, then does math from there.

CW/CCW would then only matter for defining what 'advance' is, versus retard.

 

Jim brings up a good point, tho- it's pretty easy to set up 

an aftermarket ignition with a stock distributor such that the

spark happens before the tip of the rotor's under the turret for 

the plug wire.  The wider aftermarket VW rotor's a quick fix for that.

123 must do something, as a purely electronic fix for the CW/CCW issue

would mean that spark could potentially be happening in 2 different

'bands'.  That would take a pretty wide rotor tip...

 

 

t

 

 

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Hi Everyone,

 

Quick update:  I spoke with someone at IE and they confirmed that the only difference between the early and late M10 cams was the distributor drive gear, and it would be fine for me to use the cam from an early M10 in a late M10 (along with my 123 Distributor intended for the early M10).

 

While I am in there I am going to upgrade to either a 284 or 292 cam, it should help the 32/36, 123, and header combination.

 

Once I get it together I will let everyone know how it goes.

 

Thank you,

 

Bob

 

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On 7/14/2019 at 9:45 AM, TobyB said:

The wider aftermarket VW rotor's a quick fix for that.

 

... not on a 123 though.  In this other thread, Ed (the 123 guy) said the 123 rotor has a step on the inside, so the typical Bosch rotors don't fit right and wobble loose.  It'd've been nice if he'd given a part number...

 

Here's a quote from that thread,

 

The 123ignition rotor has a slight step in the bore and when used on the 123ignition distributor it is a positive fit.

Step.JPG.57bbf0eb5b9822179f355c6ae7a57e73.JPG

The aftermarket rotors have a tendency to work loose. In my opinion because the step allows for wiggle room at the top and the metal piece inside the rotor cannot compensate for that difference. See attached picture. I have always encouraged the replacement rotors to be the oem rotor that came with the 123.

 

 

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