Spark Plug colour

[Roscobbc]

Thought I'd post these images of the spark plugs as they come out of my engine. They have been in the car for two or three years. I normally change them every two years - not on a mileage basis but because I live in a fairly built-up outer London suburb they are operating a lot of the time on idle and the transition to main jet. Being a fairly 'cammy' engine with a choppy idle the idle mixture on the race spec 950 DP carb does need to be on the rich side and the plugs will ultimately 'soot up'. Primary main jets are fine - these are jetted bang-on so no issues here. You'll see the electrode tips are a mid coffee colour and the carbon build-up lower down.

 

[teamzr1]

Would going 1 step hotter in plug help
and it seems the gap on the new plug is too wide ?

 

[Roscobbc]

45 thou' gap. I did go up a one step from the original plugs the builder installed - one or two of those would eventually go open circuit.

 

[Vetman]

My plugs tend to be whiter at the bulkhead and blacker towards the radiator. Running Holley 600 cfm and Performer Air Gap manifold. Any thoughts?

 

[Roscobbc]

My plugs tend to be whiter at the bulkhead and blacker towards the radiator. Running Holley 600 cfm and Performer Air Gap manifold. Any thoughts?

Sounds typical of uneven front to rear mixture distribution. Wouldn't expect that on a dual plane inlet.

 

[Forrest Gump]

One of my plugs. All are more or less the same, some slightly more brown than the grey of this plug. I think it looks a bit rich on idle, but otherwise okay. Any views are welcome. I‘ve never been able to lean out the idle to get the plugs any less sooty than this. 0.040 gap Autolite plug

 

[Roscobbc]

Can't speak for SBC's but BBC's as stock certainly have variations in each cylinders combustion. Seem to recall cylinder 6 and 8 are the most lilely to run lean (with stock heads)

 

[teamzr1]

How many miles on that plug ?

 

[Roscobbc]

My plugs tend to be whiter at the bulkhead and blacker towards the radiator. Running Holley 600 cfm and Performer Air Gap manifold. Any thoughts?

Been thinking further about this one......edelbrock inlet manifolds are generally one of the better aftermarket inlets available. What carb are you using? - is it one with '4 corner' idle adjustment screws? - if not that could explain the 'bias' in plug colour (white and running weaker at perhaps idle speeds). Think about it - stock type carb with idle jets and adjustment only on primaries would potentially allow the fuel/air mixture to be correct if adjusted observing colours of front 4 cylinders........but potentially weaker on rear 4 cylinders. So perhaps try making your idle adjustment using only rear 4 cylinders for observation.........or.......upgrading to a higher spec carb with 4 corner idle. Does the engine have a 'cam'?
What you you think Team ZR1?

 

[teamzr1]

It has always been that the 4 corner cylinders run hotter than the center ones
Starting with the C5 GM added coolant lines to those corners to help pull the heat out of them
Here is a shot of my 1999 LS1 when I was hacking on it, and you see how it was done
There is a rubber coolant hose from radiator that goes to throttlebody for de-icing and then Ts off to below it to the coolant lines
for 4 corner plumbing



The GM bean counters felt in 2001 to save the money and quit putting this in and then kicked their ass for doing it as the heat related problems returned
Dummies via cyberspace were telling 97-2000 C5 owners well GM did it so must be good and then owners later found they screwed up doing that

 

[Vetman]

My carb only has two idle screws at the front. I wondered if rear cylinders are hotter because the water passages in the heads are blocked by the inlet manifold. There is a modification possible to connect the rear passages external to the manifold. Would hotter cylinders result in whiter plugs?

 

[teamzr1]

Yes,
The hotter a cylinder is, and hotter air pulled in, the leaner the Air Fuel Ratio (AFR) will be
Helps of course in using some type of cold air box, using a lower coolant Tstat and keeping the fuel cooler

 

[Roscobbc]

It has always been that the 4 corner cylinders run hotter than the center ones
Starting with the C5 GM added coolant lines to those corners to help pull the heat out of them
Here is a shot of my 1999 LS1 when I was hacking on it, and you see how it was done
There is a rubber coolant hose from radiator that goes to throttlebody for de-icing and then Ts off to below it to the coolant lines
for 4 corner plumbing

View attachment 18139

The GM bean counters felt in 2001 to save the money and quit putting this in and then kicked their ass for doing it as the heat related problems returned
Dummies via cyberspace were telling 97-2000 C5 owners well GM did it so must be good and then owners later found they screwed up doing that

Yes - but later small blocks and LS's all had multi-port fuel injection so differences in fueling shouldn't be an issue like it can be with carbs. Differences in cooling and individual cylinder temperatures......guess that can still be a potential issue.
The idea of using the rear pair of redundant cylinder head cooling ports that are typically 'blanked-off' by the inlet manifold flanges on both SBC and BBC can still theoretically be utilised - this is sometime seen on race engines where the usual cast-in cross-over porting at the front of the engine (that also houses the thermostat and by-pass hose connection to the water pump) has been removed or not present (like on three piece stack injection manifolds) and the front and rear cylinder head coolant ports are all linked together - supposedly to improve cylinder head cooling - an older 60's idea?. The size of the tube/hosing used for linking-up thr front and rear crossovers is so small any additional cooling/balancing benefits must be minimal IMHO.

 

[teamzr1]

There is a heat layer on top of the coolant, the end cylinders would hot spot,
Even with the small coolant tubes is enough for coolant flow which take with it that heat layer
Would work just as well with engines with carbs.

Back in the 1970s I would better that cylinder charge by also injecting water/Methanol which reduces the charge good 40 degrees

Same as the UK and USA did for military planes in WWII