Octane, carbon deposits and ignition timing's effect on exhaust gas temperature

[John Gervais]

In my pursuit of a better-running, cleaner engine, I've pulled a few key sentences from these articles. 

My engine has developed some heavy carbon deposits and when the engine is under load when cruising down the highway, the center exhaust tunnel gets extremely hot - pretty much impossible to touch.

So, although my carb appears to be adjusted correctly (CO values confirmed at my local VAT station) and the mini appears to drive 'fine', there's an ever-worstening carbon crust forming on the piston tops and likely the valve stems as well.

What I've found embedded in these articles:

From the article on commercial cleaning of carbon deposits:

• From an emissions standpoint, the same environmental concerns that drove the development of unleaded fuels, higher energy ignition systems and electronic fuel injection also greatly reduced carbon deposits. Just three decades ago these deposits could accurately be described as massive. Further reduction in carbon deposits were realized later by the addition of various chemicals to create detergent fuels, which help keep excessive carbon deposits from adhering to hot metal surfaces like intake valves and fuel injectors.
  
  However, carbon waste deposits have reappeared with a vengeance in recent years. Since the EPA first established the minimum additive performance standards in 1995, most gasoline marketers have actually reduced the concentration level of detergent additives in their gasolines by up to 50%!
  
  Fuel octane and the quality or type of fuel used in an engine can also be an area of concern. Driveability Index (DI) is a measure of gasoline's total volatility, or tendency to vaporize completely. A high DI number is less volatile than a low number.
  
  Premium grade gasoline is rated at a higher DI (less volatile) than regular or midgrade gasoline.
  
  Since fuels with a higher DI number or octane burn more slowly, higher compression ratio engines typically use higher octane fuels to avoid heat-induced preignition.
  
  Conversely, when using a high octane (less volatile) fuel than an engine was designed for, fuel will burn too slowly, resulting in incomplete combustion, increased carbon deposits and driveability concerns such as increased cold start, warm-up sags, hesitations and stalling at moderate ambient temperatures.
  
  

And the other article on ignition timing's effects on exhaust temperature:

• Peak cylinder pressure and therefore temperature of combustion and exhaust gases are affected by spark timing.  Timing which provides maximal peak cylinder pressure, also provides maximal brake torque (MBT) (Haywood, 1988). ...  Usually in production engines ignition timing is retarded from MBT.  Retarded timing lowers burning temperature and increases temperature of exhaust gases.  Lower burning temperature can decrease NOx emissions.  Increased exhaust gas temperature can reduce Hydrocarbon emissions (Haywood, 1988).
  
  Temperature of the exhaust gases depends on an average temperature in the combustion chamber and timing of combustion in the engine cycle.  Decrease of EGT is attributed to higher ignition timing advance.  Higher ignition advance increases maximal pressure and peak temperature in combustion chamber (Haywood, 1988).  This effect does not directly reflect on EGT.  As with higher timing advance combustion starts earlier in the engine cycle, larger part of oxidation reaction chain takes place inside the cylinder, which causes decrease of EGT.  Ethanol has higher heat of evaporation comparing to gasoline (Turner et al., 2011). 
  
  Retarding of the ignition timing moves the start of ignition later in engine cycle, exhaust gas temperature is higher, and the amount of unburned ethanol is reduced in emissions.
  

So - I'm going to advance my ignition timing a tad and next time I fill up, it'll not be the high octane Shell V-Power, but regular unleaded.  Let's see what happens -

[94touring]

Do you have a wideband to see how efficiently you're burning fuel at cruise?  Is your max timing between say 30 and 34°? 

[MiniDave]

If your cylinders, valves and tops of the pistons are heavily encrusted with carbon like you think they are, then your compression ratio is now higher too, reducing the octane could result in detonation ("pinking") which is a very bad thing. Bumping the timing will also increase the chance of this if you have heavy buildup......

The only real cure is to do what we used to do back in the 60's - pull the head and take a wire brush in a drill to the carbon and clean it off.

But for sure check the timing and set it like Dan says at between 30-34* BTDC @3-4K rpm, vacuum line disconnected.

[John Gervais]

Yep - in fact, tmsmith recently sent a couple of replacement lambda sensors that I could use to see if mine was working properly.

I'd have to put the timing light back on it to see where the timing is at 5K, but if I remember correctly, it's currently around 31° max (9.5CR, modest 997 Cooper camshaft).

Y'all set timing at 3-4K even though the distributor keeps advancing 'til 5K (2500)?

The green graph was pulled from The Mini Forum (UK) -

[94touring]

What air fuel ratios are you seeing?

[John Gervais]

The gauge fluctuates a bit, but cruising at 70 mph(ish) shows around 13.8 - 14.5 AFR.

By the way - the Aldon Yellow that I'm using has no vacuum advance and is essentially new, direct from Aldon.  I've got under 1000 miles on it.  I had an old, genuine Lucas Aldon Yellow distributor re-shafted/re-bushed (http://www.distributordoctor.com/) and sent it to Aldon for re-calibration/re-curving to 'Yellow' specs.

[94touring]

You shouldn't be having any issues then.  8.gif

[John Gervais]

I'll check the timing again tomorrow - you're using between 3K - 4K (~32° @ 3500 engine rpm)?

I think I may have used 5K.

[94touring]

I'm using curve 8.

[John Gervais]

Hmmm - the 'yellow' seems close to the blue #6 at the bottom end and the #7 at the top end.

[94touring]

On the dyno I was timed a bit high...36 or 38 or something, and there was no loss in power dialing it back down to 32.  I'll be switching to mapped ignition when I go turbo to pull advance out under boost.  I'll create a map that mimics what I currently run for anything in vacuum and pull it back to 22-24° under full boost. 

[94touring]

Hmmm - the 'yellow' seems close to the blue #6 at the bottom end and the #7 at the top end.

My car comes on better with more advance sooner.  I can lug around in 3rd or 4th in low rpms and it takes off.