Spark Plug Questions

[JTSpeedDemon]

Ok, so my 3 HP flathead has a 2 inch Autolite 295 non-resistor spark plug in it, and a well working points system, and it has very, very strong spark.
My tiller flathead I'm working on has the original stock spark plug and a questionable points system. It has weak/intermittent spark.
And my HiPo flathead has the Magnetron system and a ~1 yr. old OEM 796112S spark plug. It has a somewhat weak spark to my eye but it runs.
BTW all of these are properly gapped and cleaned.
Why does the 3 HP flathead have such strong spark?
Are the Autolite plugs that much better than the OEM plugs?
Or do the properly working points systems simply spark stronger?

Also, what the deal about resistor spark plugs? What do they do?

 

[65ShelbyClone]

1.) Why does the 3 HP flathead have such strong spark?
2.) Are the Autolite plugs that much better than the OEM plugs?
3.) Or do the properly working points systems simply spark stronger?

4.)Also, what the deal about resistor spark plugs? What do they do?

1.) The ignition system is in better condition, has better parts, or is better adjusted.
2.) No.
3.) Properly working ignition systems in general have strong spark.
4.) They suppress radio frequency interference that can cause noise on radios, TVs, and can cause all sorts of problems with digital electronics.

 

[itsid]

first a brighter spark isn't necessarily "stronger" in terms of energy (joules..)
next: discharge triggering has little to no influence on spark energy either.
that's 99.9% coil,
only exception:
a failing trigger can prevent (propper) discharge or charging of the coil (faulty points trigger)

next: resistors, yes a resistor does make a difference..
namely RF noise (ever heard a weird humm in an old car radio? THAT)
the spark itself isn't any weaker with one than it'd be without
(comparing otherwise identical plugs on the same engine)
You can easily try that yourself.
Find an sparkplug boot with preresistor installed (5k usually)
and install that on the 3horse doesn't make a difference in what the spark looks like I bet.
(if the boot isn't faulty now that is )

IDK autolite plugs.. so I can't tell what quality they're comparable to,
but there are good and bad plugs available of course..
I doubt OEM plugs are bad however.
doesn't make much sense if the mfg lessens the overall experience with his engines
to save a few pennies IMHO.

Anyways.. main cause for a weak spark is the coil,
sometimes it's caused by the magnet getting weaker as well
or a mis-set coil gap.
(both resulting in a weaker magnetic field and thus lower amount of charge in the coil)

weak points, a faulty capacitor or a misaligned points system itself can cause weak sparks
as well, either not allowing the coil to fully charge or eating up too much energy on triggering.
that's why electronic ignition is more common these days

'sid

 

[JTSpeedDemon]

Ok, so I just did some testing between the OEM plug and the Autolite 295.
There was no tuning changes between plugs, and both are set to 0.030" spec gap.
I did determine that the Autolite 295 produced visibly stronger spark than the 10ish month old OEM plug I had in the HiPo.
While the OEM plug has clocked up some hours and has some carbon deposits on it, I cleaned both prior to testing, although not perfectly.
The summary of the results:

The tiller needs ignition system work
The HiPo's Magnetron system is functioning perfectly
The 3 HP's points system is functioning perfectly
And the very low hours Autolite 295 plug outperforms the older OEM plug in multiple engines.

When I ran the HiPo with both the OEM and the Autolite plugs in separate tests, I listened closely for differences in smoothness, etc.
The Autolite plug made the engine run noticeably smoother than the OEM plug, and it was more willing to start.

Granted, the results are offset by the fact that the OEM plug was older and used more, but if I were to buy another spark plug, I'd get the Autolite 295.

Sometime in the future I'll do some more scientific tests with 2 brand new plugs, but I was able to figure all this out in about 20 minutes.

 

[itsid]

same temp on both plugs?
since THAT indeed does make a noteworthy difference...

'sid

 

[JTSpeedDemon]

I swapped them multiple times as the engine got warm, and the Autolite was better every time.
But of course the OEM plug was older and dirtier, but the Autolite did make a difference.
Whenever I have money I'll buy a brand new 295 and a brand new OEM and do some more tests.
I also took the Hothead for a ride with the Autolite installed, and it was noticeably less finicky tuning wise and didn't want to die all the time.

 

[itsid]

no not engine temp.. spark plug temperature

what some idiot advertisersing sells you as "performance plug" or worse "racing plug" or alike
is usually a "hot plug"
So a hotter than OEM spark plug.

And those do spark brighter
(and no that's usually bad if you don't need one.. that's why OEM uses a colder one )

'sid

 

[JTSpeedDemon]

After some interwebz sleuthing, I found out that the OEM plug has a heat range index(whatever it's called) of 9.
The Autolite is a 7.

---------- Post added at 08:38 AM ---------- Previous post was at 08:31 AM ----------

So the Autolite is indeed hotter than the OEM plug, as it has a longer insulator.
I did read however on Engine Builder Magazine that hotter plugs are better for low RPM, which is what I'm aiming for. And colder plugs are the opposite, better for high RPMs.
I know that hotter plugs can result in detonation, but I'm already running 93 octane fuel, so I don't think it should be a problem.
Thoughts?

 

[Flyinhillbilly]

With Briggs and Stratton compression ratios and 93 I doubt you’ll have a problem.
Look up how to read spark plugs and study the pictures carefully. You’ll be amazed at what you spark plug can tell you.

 

[JTSpeedDemon]

Mine is also the hi comp head though, would that change much?
In case you didn't know it's the 10 Ci head.
BTW can anyone give me an estimate of the compression ratio on this thing?

 

[J.S.@SMS]

I thought that It was that hotter plugs are for lower compression engines, while colder plugs are for higher compression engines. But I may be wrong. I have used a plug that, apparently, was a racing plug in my stock compression minibike. It ended up bogging around the 3k range, and didn't do much over 20. I put the stock plug back in, and it's been able to do 50+ since then. So I'd say that hotter or colder plugs do have an effect on performance.

 

[JTSpeedDemon]

Oh yeah, and in case you were wondering 'sid, the Autolite 295 is recommended in my Haynes manual for this engine. That's why I got one.

 

[itsid]

not wondering, just saying you shouldn't wonder about a differntly looking spark if plug temperature isn'T identical..
hence I asked you to install a preresistor boot if you want comparision.

'sid