Plug Reading - Gas Engines - 4 stroke
Updated 2-13-05
The summer has been very busy, I'll get back with more info here after we're done racing.
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Don
Order the Book! Tuning Guide 101
Lets analyze this plug, sorry the numbered points aren't clear but
it's 1-4 you can figure it out.
#1 Is a timing indicator, you'll see a definite color change on the
ground strap, it doesn't show well here but you can still see it right
about at the arrow. Too much timing and the color change will be very
close to the threaded body of the plug, too little and it'll be closer to
the tip. Ideally we want it right in the apex or center of the 90
bend on the ground strap. This plug shows too much timing for the
combustion chamber efficiency or octane level.
#4 arrow shows another indicator of timing, you'll usually see a brown
ring right at the tip of the porcelain area it should be a sharp and defined
ring about .020 wide. Wider indicates not enough timing and any smaller
, or only 1/2 way around or nonexistent as in this image is the second indication
of too much timing in the motor.
#2 The tip of the ground strap is loaded with OIL deposits, fuel deposits
are usually flat black in color and almost like a fine powdery deposit,
this motor is leaking oil into the combustion chamber, bad valve guides,
leaking valve covers allowing oil to seep through the plug threads, whatever
it needs to be fixed.
#3 The threaded portion of the plug gives you the heat range, look at
the threads you'll see that a few toward the tip are a dull burnt looking
color the rest are black and shiny. You want about 2 threads showing
the heat on the end of the plug and the rest of the threads to be shiny,
this plug is impossible to read because of the oil mess. If you using
a longer reach plug than this one 2.5 to 3 threads is optimum.
To increase the number of burnt threads increase the heat range of the
plug, if you have 4-5-6 threads burnt you need to get a colder plug.
Looking at the color of the porcelain I'd give this carb a passing grade
at the mid range and not to bad on the idle circuits although that dam
oil leak makes it tough to really get a good read. I'll get some better
shots for lesson 2.
I'm going to keep working on this page and try and get some real good
shots of various plugs, we'll study each one and find the good and bad
tell tales of each.
I'll also start pulling some plugs out of my race car at various settings
and get pictures so we can see how jetting, timing and heat range affect
the plugs in the same engine under the same conditions.
This will turn into a Spark Plug bible when I'm done but it will take
several months to get all the pictures and details posted.
End of lesson #1 May 26,2004
June 4, 2004
Example #2
The plug is showing me by the deposits on the tip of the electrode and
also the deposits right on the edge of the threaded body.
that it's slightly fat at idle.
The white porcelain is showing a lean condition at WOT, it's not too
far advanced as the total timing mark or color change is right in the apex
of the ground strap curve. The Idle timing is shown by the triangular
hazing up on the flat of the ground strap and without even looking at the
distributor specs I can tell you that the timing on this SB Mopar
is about 18-20 initial and 34 total.
I would need a better picture of the threads to determine the heat range.
Changes:
I would try and lean it out just a touch at idle and up the jets by 2
points to fatten up the WOT circuit.
That slightly lighter color at the tip of the ground strap indicates
too much gap, nothing serious but next time you change plugs I'd go to
about a .036 gap from the current .040. Too much resistance caused
by too wide of a plug gap can cause excessive heat on the tip which will
shorten the life of the plug and really give you no benefits. I believe
excessive plug gaps are not required on most Muscle and bracket cars, once
you get into real big compression and major power you would open up the
gap and replace plugs 2-3-4 times a year.
Let's look at Image 1.......
Note the change of color on the ground strap and it's position, this shows
a timing issue. Unfortunately the reader who sent this image in didn't
say what engine he was working on but by the condition of the plug I would
guess a Small Block Chevy, 3 years old with 38* total timing and an Edelbrock
Carb......I'm sure he will respond :-)
In the second image I used a green arrow to show the area where we want
the timing mark to be on this engine and of course you can see that the motor
has too much timing.
Image 3 shows only one thread heated on the plug and rest are oil soaked
and wet this normally indicates that the plug is too cold, the oil indicates
a valve cover leak and a New set of Cometic Aramid fiber gaskets would
cure it.
The color on the plug indicates a decent idle A/F ratio and a good ignition
system probably an MSD or some other type of aftermarket ignition. The hot
spark is keeping the porcelain clean on the top area anyhow. Now if
we look down into the bottom of the plug as seen in the above image where
the porcelain reaches below the steel body we see black deposits indicating
a rich condition at mid range to WOT. I would guess this engine is
a little lazy in mid range and could get better mileage.
Changes:
Back off the total timing but keep whatever the initial is as it appears
pretty good, I would shorten the total by 4* and do another check.
Fix the oil leak
Go up one heat range and after about 50 miles check to see if you have
2-3 threads heated up and discolored for correct heat range. Heating up
the plug will help reduce the carbon build up and further tuning with the
A/F ratios can be done
Before you can really tell what to do with the carb the ignition must
be right as well as the correct heat range on the plug to get true readings.
...and check your Idler arm it appears to have some slack in it and your
left rear tire needs 3# of air.
This is what you don't want...
This is Bob Riggles Hurst Hemi Under Glass motor after a pass...a little too
much timing and or not enough fuel. Note the spark plugs completely melted.
The piston detonated itself into fragments and beat the combustion chamber
to a pulp...his day was done...
The hole in the piston in the second picture is not a window to inspect the wrist pin, although it seems to work well....
Where's the ground strap?
WOT Race Tuning
Installing a thermo coupler sensor (Available on our Stewart Warner Page)
in the combustion chamber is about the only real way to know what is
happening inside your engine the alternative is to learn how to accurately
read your spark plugs. Ignition
timing and A/F ratio's are critical to achieving optimum performance with
going to far and falling off the edge into detonation while still not being
so rich as to not reach maximum power potential.
The best way to tune for WOT is by doing a shut down at the end of the
track and doing a proper WOT reading on the plugs, use caution, coast to
a safe spot away from return lane traffic. Or tow the car back to the
pits where you can read all 8 plugs for combustion chamber efficiency.
Accurate measurement of the combustion chamber temperature is the only
way to achieve the perfect tune up. A combination of a O2 Sensor with
a data logging feature combined with accurate and rapid responding EGT Gauges
will achieve the quickest and most accurate track tuning. Dyno's are
OK for a baseline setting, NO DYNO will ever replace a good track tuner with the right tools.
I look at it this way....you spend $400-$600 at the dyno shop all conditions
are determined by the dyno operator and climatic conditions of the ambient
air of the particular day of the test. You now take your engine and
bolt it into a car with a transmission, engine compartment heat, different
fuel, fuel pressure variances, G forces, tire slip, wheel stands, elevation
etc. All your money did was set a baseline which can easily be done
in your own shop with a screwdriver....$1.99.
Spend that money on a WEGO 5 wire, wide band, data logging O2 sensor,
it'll last for years and give you accurate combustion chamber efficiency
from the burn-out box to the freeway. Team that up with a Stewart Warner
EGT (Exhaust Gas temp gauge) and it won't matter if it's 105* or 55* outside,
300' or 5,000' elevation you'll be able to make decisive changes with no
guess work or risk to your investment.
In My Opinion ...Dyno's are a waste of money and fall into the category of "worthless information".
If your interested in a WEGO or EGT system call I'd be happy to discuss how they work with you.
The Air/Fuel Map
Look at the porcelain and divide it into 3 areas, top, middle and bottom in about 1/3rds.
Top Area: This area is your idle circuit and for the racer real has
very little meaning. However if your car is a street driven vehicle
then we need to attempt to get this portion to a dark gray color by tuning
the AFR, Idle Air Bleeds and the four corner Idle adjustments until we get
a clean crisp idle and total combustion of the fuel at idle. Keep in
mind that the idle circuits are totally isolated from the rest of the carburetor
and can be tuned with very minimal affect on the WOT position A/F Ratio.
Center Area: This is the area that will show the combustion chamber
condition at part throttle or cruise with the power valve and secondary venturi's
closed. Usually about 30-50 MPH depending on the gear and weight of
the car.
Bottom Area: This is where the racer needs to concentrate all
his efforts, using a 5x or 10x scope you need to look right down to the bottom
of the porcelain where it meets the metal housing of the plug. This
is the area that will leave the color that you need to study and tune to.
The other 2/3rds of the porcelain will be burned clean by the high
heat generated by WOT and high RPM, this is not the time to read for cruise
or Idle characteristics.
We're looking for a colored ring right at the base of that porcelain which
I refer to as a Fuel Ring. The color will vary depending on the type
of fuel your burning, Unleaded Pump gas will leave very little color because
of all the additives such as injector cleaners and methanol. Most of
us run a 108 or 110 in our bracket cars and these fuels will leave a tan
color while the C17 and higher octane fuels will leave an almost gray fuel
ring. If you study the ring you will see that it starts to show color
just below the base of the ground strap, this is caused by the shielding
affect of the strap leaving that portion cooler thus showing the most color.
The ring will develop from each side and form a full circle of correct
color as you get closer and closer to the correct tune up. If you look
down into the bottom of the porcelain and you detect a thin oily deposit
you are seeing oil from the combustion chamber caused by either worn or improperly
seated rings, you may also detect this with a new engine that hasn't fully
seated the rings yet, just don't confuse this with the "Fuel Ring".
Plug Characteristics
Shiny or Glazed Porcelain: This occurs for two reasons, excessive idling or rich idle condition
leaving heavy soot deposits. These deposits will burn and create excessive
porcelain temps causing the glass to melt usually resulting in a down track
popping through the exhaust..
You will usually see splotchy deposits on the plugs after the run, either
green or yellow in color and of course the tell tale shiny glaze. The
porcelain should have a chalky or dull finish.
Base of threads sooty: This is usually a indication of the plug
not being tight enough allowing it allow air to pulled in to the chamber and
fuel being pushed out, change the plug, tighten to spec. This area
should have a nice dark to medium gray color without heavy deposits.
Black or brown specs: This is an indication of detonation usually
caused by too hot of a plug, drop one or two heat ranges and it should go
away. The plug gets so hot that it will start to detonate the fuel
before the ignition fires, this cause a double flame front and reduces the
efficiency of the combustion process as these two flame fronts battle each
other in the combustion chamber. The black spots are the result of
the fuel deposits being burned onto the porcelain by the double flame front.
Once you cool down the plug and remove the secondary ignition source
you can re-set your timing to produce the correct combustion chamber temp
for optimum power.
I've heard 100's of so called tuners and engine builders tell people that
this is normal and nothing to worry about......WRONG. Do not underestimate
the damage that this condition can do to your engine, drop the plug heat
range.
Shiny little diamond like deposits: It's too late you've already
started to blow the pistons out of it. Start over from scratch, fatten
the carb, back-up the timing and cool down the plugs. Now start a proper
tuning procedure and hopefully you caught it before too much damage was done.
No color mark on ground strap: If your showing good color on
the porcelain but the ground strap fails to show an indicator then your plug
is too cold, go up 1 heat range.
If the ground strap is showing a good color line but the porcelain is showing
no color at the base then your plug is too hot, drop down a range.
I will use maybe 3 different ranges on one bank, make a pass and from those
three different plugs determine which one is correct for the engine.
Variable heat readings: On a small block motor most have adjoining
exhaust ports, these will always be your hottest cylinders and in some cases
where there has been excessive port work and thinning of the metal between
the ports it may be necessary to run a plug one step cooler in those cylinders,
use a plug indexing washer to back the timing down in those cylinders slightly,
or if you have an adjustable ignition system you may want to electronically
back the timing down in the hot cylinders.
Always tune to MPH and not ET: There are too many variables
on the ET side of the time slip, tire slip, lane choice, driver error, tire
pressure etc. If you use the MPH most of these variables are eliminated,
but the driver still has to do his job, once a baseline is established don't
change anything forget about your RT, do everything the same every pass and
the end result will be a very quick and extremely consistent car.
Tire Pressure: This is off the topic but I want you to think about this critical aspect of Bracket racing and consistency.
I always see guys adjusting their tire pressures in the staging lanes just before a pass ...why?
1st pass: Think about this.....your tire temp is 50* you set it at
8#, you do a big burnout and the tire temp goes to 135* the pressure picks
up to 9.5#.
2nd Pass: So you go back to the lanes and you wait for an hour, the
sun comes out and your tire temp is now at 75*. Just before you make
your pass you check the tire and it's at 8.5#s so you drop it to 8#, into
the box and tire heats to 135* but because the tire was already considerably
warmer than the first pass the pressure goes only to 9# and the car runs
quicker in the 60'.
3rd pass: Now it's noon and the ambient air is at 90*, you check your
tire and its at 8.25 so you deflate it even more, but this time because of
the starting temp of the tire after your burnout the tire only grows to 8.5#,
again the 60' improves, you break out and your looking for the trailer and
the beer stand.
Try this, set the tire according to ambient air so if it's 50* in the morning
you know that the tire will grow pressure as soon as you get into the burnout
box so set it low, maybe 7.5#. As soon as you get back to the staging
lanes set the tire to 8.5 Hot and leave it alone. If you check before
your next run you'll see that it's dropped pressure considerably but when
you get through with your burnout and the tire is back to 135* your pressure
will be the same on every pass. Have your crew chief use an infrared
gun and determine how long your burnout needs to be to reach the desired
temp, count down your burnout's and do them exactly the same. Adjust
your tire pressure immediately after each run, you'll be surprised to see
that once you get it down right you won't be doing anything to the tires
after the second test and tune pass.
The theory being that your making your pass with a tire temp of 135*, so why set your tire pressure when it's at 70*?
Got a plug question
Send me a good picture like this and I'll analyze it for lesson
#4
3 pictures would be best, one straight down, one of the ground strap and
one of the threads.
Please send this info with your pictures:
Engine type
Compression Ratio
Fuel Octane
Total and initial timing
Street driven or all race
Plug Picture chart