Spark Plug Readings
Example number 1
Lets
analyze this plug, sorry the numbered points aren't clear but it's 1-4 you can
figure it out.
Number1 Is a timing indicator, you'll see a definite colour 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 colour 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 centre of the 90 bend on the ground strap. This
plug shows too much timing for the combustion chamber efficiency or octane
level.
Number 2 The tip of the ground strap is loaded with OIL deposits, fuel deposits are usually flat black in colour 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.
Number 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
colour 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
Number 4 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.
Looking at the colour of the porcelain this carb has a passing grade at the mid
range and not to bad on the idle circuits although the oil leaking into the
chamber makes it tough to really get a good read.
Example Number 2
This plug is
showing 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 rich at idle.
The white porcelain is showing a lean condition at WOT, it's not too far
advanced as the total timing mark or colour 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 the
timing on this SB Mopar is about 18-20 initial and 34 total.
The heat range of the plug isn’t clear as the picture isn’t good enough
Changes:
You would need to try and lean it out just a touch at idle and up the jets by 2
points to richen up the WOT circuit.
That slightly lighter colour at the tip of the ground strap indicates too much
gap, nothing serious but next time you change plugs 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. 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 colour on the ground strap and its 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 it’s probably a Small
Block Chevy, 3 years old with 38* total timing and an Edelbrock Carb......
J
In the second image a green arrow is used 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 colour 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 discoloured 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.
This is what you don't want...
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...
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
Ignition timing and A/F ratio's are critical to achieving optimum performance between going to far and falling off the edge into detonation or 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 an 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 loads at the dyno shop but 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.
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.
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 really
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 grey colour 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 carburettor and can be tuned
with very minimal affect on the WOT position A/F Ratio.
Centre 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 colour 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 coloured ring right at the base of that porcelain which I
refer to as a Fuel Ring. The colour will vary depending on the type of fuel
you’re burning, Unleaded Pump gas will leave very little colour because of all
the additives such as injector cleaners and methanol. A 108 or 110 octane fuel
will leave a tan colour while the C17 and higher octane fuels will leave an
almost grey fuel ring. If you study the ring you will see that it starts to
show colour 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
colour. The ring will develop from each side and form a full circle of correct
colour 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 an indication of the plug not being
tight enough allowing air to be 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 grey colour 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 causes 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.
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 colour mark on ground strap: If you’re showing good colour 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 colour line but the porcelain is showing
no colour at the base then your plug is too hot, drop down a range.
Try using 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.
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