Timing and Octane

Well the past few days have been interesting.
I managed to fill up with 100 octane in Germany where its cheaper than 95 octane in the UK.
This allowed timing to be increased 3 deg with what seemed like a small increase in power.

I had been wondering why, if lower timing was better, why not just run lower octane and lower timing. Well after some reading and investigations the answer appears that if you run too low with timing the combustion will still be happening when the piston has past its best position for combustion pressure. (Around 15deg ATDC) So the ideal octane and timing is the one which puts the most pressure around 15 adtc and is all spent before the exhaust valve opens.

From others comments and feedback this optimal timing appears to be around 24-28 deg, now there will be variables such as combustion chamber design of aftermarket heads etc which could move this a little one way or the other.

My timing with 95 octane was around 18 deg. With 98 Im back to around 20 deg. Im hoping with the changes detailed here to be able to creep towards 24 deg with no observed knock.

So Im now officially running 98 octane and am embarking down a track to optimise compression and timing settings. My compression with Dart heads is right on the line, so extra margin for error will be good and hopefully allow more power from optimised timing.

A quick read of this excellent FAQ will reveal more for those interested. Particularly section 7.

So in order to see how this goes I have just ordered a set of 8 x NGK BKR6E-11 plugs which are a gasketed equivalent to the TR6 plugs. They are standard copper. But if they work well Ill change to Iridium.

Anyway my quest for reliable compression and timing will be as follows;

• Fit OTRCAI cold air intake to reduce IAT temps (on its way, hopefully here in the next week or two)
• Fit lower temperature Thermostat 170*? (currently researching this, David Vizard has a nice article covering all this here)
• Fit colder spark plugs as mentioned above

Other changes such as combustion surface polishing etc are out of scope for the time being, as the engine is already assembled. I could also retard the cam and flycut the pistons (dual benefit of allowing more lift and dropping compression) but wont for the time being.

The maximum benefit looks like it will be realised by the use of a lower temperature thermostat. Here the idea is to reduce heat soak to the intake air and closely related to this, the combustion chamber pre ignition temperatures. Thus inhibiting the spontaneous combustion that goes with knock. See the chart below from the David Vizard Popular Hotrod link above. My DCR is 8.3 or close to, according to this calcuator here. (Use 3.91 for gasket diameter, 3.9 for bore, 3.62 for stroke, 6.1 for rod length, 0.055 for stock mls gasket on stock ls1)

So based on 98 octane (98 + 87)/2 = 92.5 I need to be looking at a 170* thermostat and regulate engine temps around there. Users of the popular 160* thermostats report general operating temps in the mid 170's so. I think a 160 might be the go. I can regulate things somewhat with fan operation. Just need to order the thermostat now...

Other than that, I spent a good few days logging and one lap of the Nurbergring. Which is situated in the nice little town of Nurgerg in SW Deutchland. I logged the lap, but sadly cronic understeer from shoddy worn cheap chinese tyres prevented to much speed. I will rearm with better tyres and schedule another visit. It was great fun. Next time Im going to book a lap in the BMW M5 Taxi that takes people around. Apparently the drivers are excellent and will do a very very fast lap including some good sideways action.

I also attended the F-Body meet in Meerbusch, a good crowd from throughout Northern Europe were in attendance. Mainly older 3rd gen's but a few LSx vehicles were around. I took the organiser for a blast with a few of his friends. He said power was very smooth and fast. Which reminds me, its soon time to redo my 0-60 times.