Spark Plug Reading -

[Travel'n Man]

Let us know your tips and tricks to reading a properly air/fuel mix spark plug. Be sure to state your cars manufacturing date, engine type, and what type of fuel supply you are running. I would have to guess that carb'd cars vs fuel injected cars would have different looking plugs.

I am really looking for a perfect color'd plugs with '72 carbs - show us what you've got and how you read them.

http://www.wallaceracing.com/plug-reading-lm.html

http://www.4secondsflat.com/Spark_plug_reading.html

Edited November 3, 2010 by Travel'n Man

[FastWoman]

I would think plugs have always and will always read the same way in internal combustion engines, irrespective of gas grade, vehicle/engine type, etc. I read my lawnmower plugs the same way as I read the plugs on our new truck -- and the same way as I read them on the Z. The engine (head and block) doesn't know or care how fuel was delivered to it. There's an ideal mixture that results in maximum power and minimum emissions.

The only differences I can imagine are that cars with cats are calibrated a bit leaner, and cars with EFI are able to teeter on that "just right" mark a bit better. Carbed cars typically run a bit richer because they aren't as accurate at metering, and they err on the safer side -- rich.

But the plugs should read the same way.

[Gary in NJ]

I am really looking for a perfect color'd plugs with '72 carbs - show us what you've got and how you read them.

Dry with a tan to mocha coloration on the insulator. Look for even wear on the electrode. Old plugs don't tell a true story. Put fresh plugs in the engine and drive 25-50 miles. Review them every so often for to see if and how they change (say 200 miles and 500 miles) but beyond 500 to 1,000 miles they become a archeological dig.

[ozconnection]

I would think plugs have always and will always read the same way in internal combustion engines, irrespective of gas grade, vehicle/engine type, etc. I read my lawnmower plugs the same way as I read the plugs on our new truck -- and the same way as I read them on the Z. The engine (head and block) doesn't know or care how fuel was delivered to it. There's an ideal mixture that results in maximum power and minimum emissions.

The only differences I can imagine are that cars with cats are calibrated a bit leaner, and cars with EFI are able to teeter on that "just right" mark a bit better. Carbed cars typically run a bit richer because they aren't as accurate at metering, and they err on the safer side -- rich.

But the plugs should read the same way.

Wow, I don't think I agree with any of this!

In a perfect world, where every engine was the same, there was no such thing as two stroke, all grades of gasoline were the same and all engines were efi or carbed, we all lived with the same temperatures all year round then what you've said may be more accurate. (I'm sure there are even more variables.....I'm only listing what comes to mind quickly)

Even the plugs themseves will look somewhat different due to their designs, ie heat range, fine wire, projected tip and manufacturer etc.

"Maximum power and minimum emissions" are two opposite ends of the performance spectrum and biasing your tuning efforts towards one or the other will dramatically alter the way the plug will colour.

If carb cars ran "safer" then it would be in the best interests of auto manufacturers to continue using carbs. However, we don't.

Carbs can be surprisingly accurate at metering fuel. It's the fact that the intake manifold is a wet flow design that has to warm up sufficiently in colder weather to help vaporize the fuel that requires the mixtures to be richer. A good manifold design with sufficient heating will work very well indeed. EFI doesn't have this design 'limitation' and can reduce emissions immediately after start up. Also, carbs don't measure inlet air temp directly, so no recalibration can be made with reference to this variable.

So can plugs colour differently on hot and cold days?.....or.....does the air/fuel ratio change on hot and cold days?

The thermostat used in the engine will vary the operating temperature of the engine and will effect the air/fuel mixtures entering the engine too. I recently tested my Holley 4 barrel equiped L28 with several thermostats, all different temperatures and found that my fuel ratios were changing based on what thermostat I had in the car. A colder one required slightly richer jetting, which allowed for a degree or two more timing which can also have an effect on the temperature of the burn in the cylinders.

A P90 head will burn the fuel differently to an N42 which will burn differently to an E31.

Can you see the problem here?

Back to the original poster, there was an article I read some years ago that suggested you rev the engine in 3rd or 4th and cut the ignition and declutch the transmission, pull over and pull the plug to read it. Pulling a plug from a hot engine is not recommended as it can strip the softer aluminium threads in the head, but it's dangerous to do this anywhere in suburbia too. If you let it idle at all, you don't get an accurate reading of the mixtures at WOT.

My solution: I got myself a wide band oxy sensor and never had to touch my plugs again until its time to change them.

[Gary in NJ]

there was an article I read some years ago that suggested you rev the engine in 3rd or 4th and cut the ignition and declutch the transmission, pull over and pull the plug to read it.

That is the method I use on my dirtbikes. It just isn't practical in a car. First and foremost, unlike a dirtbike, we typically don't drive our cars at WOT. We throttle them and therefore we need to set the air:fuel mixture in the range that we normally operate. Also, on a single cylinder dirtbike that revs at 10,000 to 15,000 rpm, one minute of operating gets you a lot of combustion cycles from which to read.

The method of spark reading I provided above will give you a fair representation based on your typical driving style.

Edited November 4, 2010 by Gary in NJ

[doradox]

Wow, I don't think I agree with any of this!

Carbs can be surprisingly accurate at metering fuel. It's the fact that the intake manifold is a wet flow design that has to warm up sufficiently in colder weather to help vaporize the fuel that requires the mixtures to be richer. A good manifold design with sufficient heating will work very well indeed. EFI doesn't have this design 'limitation' and can reduce emissions immediately after start up. Also, carbs don't measure inlet air temp directly, so no recalibration can be made with reference to this variable.

So can plugs colour differently on hot and cold days?.....or.....does the air/fuel ratio change on hot and cold days?

The thermostat used in the engine will vary the operating temperature of the engine and will effect the air/fuel mixtures entering the engine too. I recently tested my Holley 4 barrel equiped L28 with several thermostats, all different temperatures and found that my fuel ratios were changing based on what thermostat I had in the car. A colder one required slightly richer jetting, which allowed for a degree or two more timing which can also have an effect on the temperature of the burn in the cylinders.

A P90 head will burn the fuel differently to an N42 which will burn differently to an E31.

Can you see the problem here?

No, I see a plug reading as an indicator of the total fuel delivery system. A good plug reading means the system is working, regardless of what that system is.

That being said, reading plugs is a ballpark measurement of the average mixture for the range of conditions (engine load, environment, etc.) just prior to the reading. If you are sweating the details then you need a better measurement system like, as you mentioned, an O2 sensor.

Steve