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Author Topic:   Spark plug interference.
MilwaukeeWhaler posted 08-08-2001 04:06 PM ET (US)   Profile for MilwaukeeWhaler   Send Email to MilwaukeeWhaler  
I have tried to get a responce in the performance forum but to no avail. Can certain spark plug interfer with the transducers? MY dealer took the liberty to install new plugs for that reason and I swear I have lost performance. I was all set to install "split fire" plugs and then this.
LarrySherman posted 08-08-2001 04:24 PM ET (US)     Profile for LarrySherman  Send Email to LarrySherman     
Ignition noise can be a real nusance. In cars, one usually resorts to "resistor wires" to reduce EMI from plugs, thus reducing noise. Plugs can definatly change performance, however, using the wrong plug can also lead to hot charges, and nasty holes in your pistons. I would look up OEM factory recommendations, and ask dealer for any service notices for your engine.

On what basis did he change plug specification?

JFM posted 08-08-2001 04:30 PM ET (US)     Profile for JFM  Send Email to JFM     
Larry, I have had plug problems before. I have been using Champion marine stainless steel. They cost more but stopped the problems. Also, I have heard NGK makes good marine plugs that are even better than Champion. I am by no means an expert on this suject. Regards, Jay
MilwaukeeWhaler posted 08-08-2001 04:33 PM ET (US)     Profile for MilwaukeeWhaler  Send Email to MilwaukeeWhaler     
He sold me the boat in May. I had him install a fish finder before I took delivery. Two months later I had them install a speed and temp transducer. When I picked the boat up he had installed 3 new plugs and said there was some UHF interference with the Lowrance x65. I currently run top end 40-41 mph solo with light gas and stock prop. Maybe its only mental since I have an exact speed infront of me. Not that big of a deal just curious. It may be that I put in 89 octane instead of 93.
MilwaukeeWhaler posted 08-08-2001 04:36 PM ET (US)     Profile for MilwaukeeWhaler  Send Email to MilwaukeeWhaler     
The new plugs were supposed to be designed to limit the field of electric noise. (or something to that effect) 1995 Merc 90
LarrySherman posted 08-08-2001 04:39 PM ET (US)     Profile for LarrySherman  Send Email to LarrySherman     
Could it be drag from transducers? So he said he swapped the plugs to eliminate UHF interference? Do you notice any interference? For autos, ther are normally two OEM plugs avail. resistor, and resistorless. If you use resistorless plugs, then EMI is reduced by using resistor wires.

Please: anyone with the Lowrance X65 or experience with marine ignition technology, take over here, I'm in the deep end...

JFM posted 08-08-2001 04:45 PM ET (US)     Profile for JFM  Send Email to JFM     
The stainless plugs I use say on the pkg. that they reduce noise interference and increase preformance. Regards, Jay
LarrySherman posted 08-08-2001 04:55 PM ET (US)     Profile for LarrySherman  Send Email to LarrySherman     
I would put in the OEM recommended until you are sure these plugs won't hurt your engine.

Buhler? anyone?

whalernut posted 08-08-2001 05:08 PM ET (US)     Profile for whalernut  Send Email to whalernut     
I have used Champian, Autolite, NGK, and Denso. I find the Denso`s get the best fuel mileage, power, speed, and my 75` 85h.p. Johnson will troll for a long time without stalling at in-gear speed! I love those Denso plugs! Regards-Jack Graner.
lhg posted 08-08-2001 06:04 PM ET (US)     Profile for lhg    
You're being given bad advice by the Dealer.
Spark plugs themselves don't interfere with the sonar. Your sonar unit cables are probably in the same tunnel with the engine cables. That is your problem.

Lowrance recommends that the power lead for the unit be connected directly to the battery, not to the boat's power buss, and the transducer cables not run alongside the ignition cables from the controls. Same applies to the VHF radio.

dgp posted 08-08-2001 06:37 PM ET (US)     Profile for dgp  Send Email to dgp     
On most installations it's almost impossible to route x-ducer cable away from the instrument and control wires, especially on the Montauk. The wires to the tach are the worst contibutors to electronic "noise". Shielded cable is what should be run to the tach but that costs more money, so the boat builders don't supply.
I don't get any EMI/RFI problems on my Lowrance from the engine or instruments, but the noise from the Lowrance plays havoc with my handheld VHF. Lowrance supplied me with some clamp-on ferite blocks that might solve the noise problem. I'll try them this weekend and report back.
jimh posted 08-08-2001 10:52 PM ET (US)     Profile for jimh  Send Email to jimh     
When a spark occurs there is a broad spectrum of radio-frequency energy generated. Some of that energy is at the same frequency as the sonar receiver that is listening for echoes off the transducer. This is a cause of interference, usually seen in the sonar as hash on the fish finder display.

The sonar receiver is directly connected to the transducer via (what should be) shielded cable. Unfortunately, because of the high impedances involved at the transducer, the shielding is not as effective as one would like.

Because of poor shielding, the sonar receiver is suseptible to ingress of other signals along the cable or in the region of the transducer itself (which is close to the engine). So radio-frequency energy from the spark plug arcing could enter the sonar via this route.

Also, the sonar is powered by the same battery source that the engine is connected to, so common mode noise in the battery leads could couple noise and interference into the sonar receiver. This is another path.

Finally, if the sonar cables are closely coupled to cables from the engine, there could be inductive coupling between them, introducing noise into the sonar.

There are several approaches to solving this.

First, resistive plugs are often used. A resistor plug has a resistor in series with the electrode. This causes the arc to ramp up a bit slower. The slower the rise time on the arc the smaller the range of radio frequencies generated. Use of resistor plugs may affect the engine performance as well. Check that the engine permits using them as an alternative plug.

Second, common mode noise in the power lines can be reduced by connecting the sonar directly to the battery via its own power cables. The battery is extremely low impedance and tends to clamp or swamp out any induced noise voltages. Eliminating common power lines reduces the chance of noise entering the sonar receiver from other equipment's cables, too.

Third, separating the sonar receiver cable from other cables (which may have noise induced in them) reduces the coupling of radio frequency interference into the sonar from these other lines.

I have a Lowrance x65 and it shows not interference from my pair of Yamaha engines, but my Lowrance x48 gets plenty of interference from the very hot spark of the "Thunderbolt" ignition on my old Mercury 50-HP. That engine also tears up the VHF Marine radio, too.

I think the difference is the spark on the Yamaha engines is a bit tamer, and perhaps the shielding is better in the cowling.

I think the only cure for the noisy Merc is to go to resistor plugs.


MilwaukeeWhaler posted 08-09-2001 09:46 AM ET (US)     Profile for MilwaukeeWhaler  Send Email to MilwaukeeWhaler     
Thanks jim and everyone else for your "feedback" Ran last night and so far the sonar seems ok, but then again I didn't notice any issues before I took it it. Jim, you did a great job summing up what the dealer was trying to tell me.
jameso posted 08-09-2001 10:39 AM ET (US)     Profile for jameso  Send Email to jameso     
My 2cents worth. I just put a Zercom flasher on the 15, was not familiar with the unit gain settings ect,,,,If you want to know if you are having any interference problems motor out to a quiet spot study the readout, then shut down all power except the sonar. Now what? If there is no difference you are OK. If you have a difference restart, and adjust gain and suppression unti display shows the same as engine off. note this setting and use it as a starting point for future. Also most gain controls are logrithmic and not linear. This makes a BIG difference in the way the sonar will react. Thanks Jim Armstrong

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