FD-67 Just about done

[RogerH]

Concerning the Sw.boxes, I really don't know how to tell which ones have the limiter. I think it is safe to assume the boxes marked 6-cyl do not (at least when used on a 4-cyl.) and these were used on the 454 I/O's too - all the same part number. I agree with Sam that the CDI is an overall better unit and its packaging is better (Sw.Box & coil on same plate). Their power output is better, that's the key. Pretty good price and I think they give APBA members a discount.
Question to Sam: Are you using CDI's trigger or a Merc trigger in the distributor? The job of the Sw. box is to fully charge the coil before it needs to discharge to fire a cylinder. If the RPM's get too high the coil may not get fully charged and the spark falls off. Also, if the optical trigger fades (with age or grime) it will actually change the dwell time as it triggers the switchbox late. I don't think that is why you're seeing certain Merc boxes fade away. Don't know why.
I wouldn't mix and match coils with switchboxes. The switchbox and coil are engineered to work together. CDI has the advantage of a newer electrical design and better magnetics for use in the coil. The CD units fire hot a 0 RPM and thus help starting a lot. I personally like the Oddesy batteries as they pack a punch given their weight / size and hold a charge while not in use. Worth the money!!!
BTW these are a Type III not IV system, my mistake.

[Mark75H]

Roger a 6 cylinder switch box will not work correctly on a V-8 ... it will have the wrong tach signal

I haven't had any trigger failures other than bad insulation on wires so I'm still using Merc triggers

It is rumored that some of the 6 switch boxes were limited, but some people who should know say none were limited

The big difference in the original Merc switch boxes is the tach output signal

I agree, I would not take the little OMC coil off of a CDI box either ... it works fine. The Merc coil that works with the Thunderbolt distributor is actually a medium sized one. The small 50 hp ADI coils will not work with the Thunderbolt distributor for more than a few minutes before they burn up. The slightly larger Merc coil and the tiny OMC coils will work

I like the Oddesy batteries too. About 20 - 25 starts of my padded 6's on a charge

[John (Taylor) Gabrowski]

When I quite trying to develop a dirct charged Merc & Mariner 3 cylinder 2 carb 3 Holer I just set aside their ignitions when I got rid of the blocks. I never gave their ignitions a second thought until I started looking at a second Crescent. If a 6 cylinder Merc trigger box will work on a 4 banger Merc sans the tach circuit would not the 3 cylinder trigger boxes work on a Merc 3 cylinder too sans its tach output?

[Mark75H]

yes yes

[RogerH]

I agree that the tach ckt. is different in the 4-6 boxes and the I/O box, but the I/O box will work fine (sans Tach). Basically, that's why Larry and I run all of our Sw.Boxes at speed to verify they aren't limited. I haven't found anyone who knows how to tell unless you knew what year motor it came off.

Careful on the 3-cyl boxes. Remember that the switch box is triggered by either the distributor or flywheel magnets/coils. I think that all of the 3-cyl were flywheel triggered - not sure. In this case (Type IV I think) the switch box generates its own power from the stator and is triggered by another set of magnets / trigger coils under the flywheel. The 6's just use two of these Sw.Boxes (and a different F'wheel trigger.
I haven't had the best of luck with my 6-cyl. motors (stock) and this ignition. The Sw. boxes are EXPENSIVE and I was told that you shouldn't mix different revision code boxes on the same motor (2X expensive!!!) I always thought their low speed spark was weak. Does anyone have a good replacement - setup - suggestion for the 6's???

Maybe we ought to move this to a technical thread?

[Mark75H]

If you are going to speed test them, I'd recommend you do a 10 minute minimum run at speed

I've never heard anything about a mismatching problem with switching switch boxes on distributors and have had no problems that seem to relate to a mis-application. I don't know if that makes me brave or stupid. CDI only makes one replacement box for all the 4 & 6 boxes.

When the Merc boxes were new their low speed spark was impressive ... maybe SCARY is a better word. If you've had some with weak spark at any speed there was something wrong with them.

I use Merc triggers and CDI boxes on my 6's (all padded, one has 275psi cranking compression). They fire instantly and need little choking even on the chilliest days. This was not the case with old Merc boxes. I would not even trust a NOS Merc box ... they haven't been made in many years and could go bad just sitting on the shelf in my opinion.

I have a bunch of used Merc boxes that I would be happy to get $30 each for if anyone wants them.

The technical portion of this thread continues here: http://www.boatracingfacts.com/forum...ead.php?t=6242