PDA

View Full Version : Replacement Plug Wires Question



RedDog
09-29-2009, 12:17 PM
I want to replace my plug wires. Should I just buy the Mercruiser OEM part or go with something from a performance shop?

see - http://www.summitracing.com/parts/MSD-3548 - Would this be a good option?

How do I know I am getting the right set - don't they have to be longer that what is used in an auto application?

Any recommendations?

1998 Merc 502 Mag - standard Merc exhaust manifolds / risers

mrfixxall
09-29-2009, 12:44 PM
I want to replace my plug wires. Should I just buy the Mercruiser OEM part or go with something from a performance shop?
see - http://www.summitracing.com/parts/MSD-3548 - Would this be a good option?
How do I know I am getting the right set - don't they have to be longer that what is used in an auto application?
Any recommendations?
1998 Merc 502 Mag - standard Merc exhaust manifolds / risers

the ones you posted will probably work, me, i like to get the custom set which you have to cut to legnth and install the boots and crimp on the ends but that requires a special crimping tool for the msd terminals which you can get from msd or jeggs..mds usually supplys one with the plug wire kit that you need to put into a vise to crimp the ends on to he wires.

this is the style i have.http://www.summitracing.com/parts/MSD-35051/?image=large

cutwater
09-29-2009, 12:47 PM
How do I know I am getting the right set?

When you find them in green :wink:

Ghost
09-29-2009, 01:04 PM
Not knowing better, I always thought plug wires were wires, and thus that their resistance should be near zero. Since then I am led to understand that they are actually not wires proper, and have surprisingly high resistance, like ten to fifteen thousand ohms per foot.

My questions are both general, but pertinent to the thread topic.

1. Assuming a range, something like the 10k-15k ohms/foot, how important is it to match replacement plug leads to the orignal equipment? (Put another way, if my motor came with 10k ohm/foot leads, what will be the consequence of replacing with 15k ohms/foot?)

2. What is the impact of the varying lengths of the leads on the resistance? (Some of those cables have to be close to 2 feet or more longer than others--if they were all 10k/foot, there would be big differences in the longer and shorter ones.) Are there any circumstances where engine builders match them all in length, to keep the resistance equal? Or, do they actually manufacture different lengths that have different cable inside, such that a matched set of leads has equal resistances despite the varying lengths? (He11, now that I think about it, I have a couple sets right here, I think I will take a break and run down to the shop to test them...)

MOP
09-29-2009, 04:16 PM
The OEM wires work fine but for the same $$ you can get a better quality set, just don't go for the over sizes can be a pain.

Phil

Forrest
09-29-2009, 04:31 PM
I agree with MrFix in building you own set since usually the pre-terminated lengths tend to be a little short for marine use. The pre-terminated "automotive" lengths assume that you will be routing the plug wires over the valve covers, where as in a marine application, you will need to route the wires under the exhaust manifolds.


Also, to answer some of Ghost's questions (no, you do not want near-zero ohm ignition leads) there is a great FAQ on spark plug cables at the Magnecor Ignition Wire (http://www.magnecor.com/magnecor1/main.htm) web site:

http://www.magnecor.com/magnecor1/truth.htm

http://www.magnecor.com/magnecor1/frequent.htm

Dr. David Fleming
09-29-2009, 06:45 PM
Graphite core wires were developed back in the 1960's to eliminate interference in the radio reception of AM/FM car radios. Marine radio is the same I am sure. Anyone know about onboard computors and GPS? Most automotive ignition is a high voltage capacative discharge which means with all that voltage any sparkplug wire that is not premium in quality will break down and short out the plug ignition. Any one know if marine ignition is stepped down from automotive because of the water enviornment.

Mercury knew what they were doing when they put those wires on your engine. Be sure you remove them by grasping the boot over the plug and pulling it off. Never pull on the wire shaft as it will break the graphite core in the wire and the resistance will be infinite. This broken graphite will cause the audable snapping sound on the radio which can be heard while the engine is running.

The old school option to graphite wires was to use copper core wire and use a resistor sparkplug this also eliminated the radio interference. Everyone has hated the fragility of graphite wires but they seem to be industry standard everywhere. I would think it hard to buy better than Mercury.

One neet aftermarket option for Harley Davidson motorcycles is a neon tube insert in the plug wire that glows when it fires - makes the engine look kool! Never seen this on a boat.

Dr. d

Planetwarmer
09-29-2009, 11:27 PM
Graphite core wires were developed back in the 1960's to eliminate interference in the radio reception of AM/FM car radios. Marine radio is the same I am sure. Anyone know about onboard computors and GPS? Most automotive ignition is a high voltage capacative discharge which means with all that voltage any sparkplug wire that is not premium in quality will break down and short out the plug ignition. Any one know if marine ignition is stepped down from automotive because of the water enviornment.

Mercury knew what they were doing when they put those wires on your engine. Be sure you remove them by grasping the boot over the plug and pulling it off. Never pull on the wire shaft as it will break the graphite core in the wire and the resistance will be infinite. This broken graphite will cause the audable snapping sound on the radio which can be heard while the engine is running.

The old school option to graphite wires was to use copper core wire and use a resistor sparkplug this also eliminated the radio interference. Everyone has hated the fragility of graphite wires but they seem to be industry standard everywhere. I would think it hard to buy better than Mercury.

One neet aftermarket option for Harley Davidson motorcycles is a neon tube insert in the plug wire that glows when it fires - makes the engine look kool! Never seen this on a boat.

Dr. d

Plus it would let you know if you were getting spark to all cylinders!

MOP
09-30-2009, 07:37 AM
Being an X boat mech I have a set of what I call sparkies they are a standard ignition checking tool. I made a setup about 50 years back using GE90 bulbs, I made up a socket array then using telephone wire wrapped around the spark plug leads managed to steal enough induced current to fire the bulbs. My friends said I should have patented the setup, you could see instantly when a plug miss fired. Somewhere in my stash I still have some GE90's might be fun to build another setup.