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Race Fuel and Octane Choice


SK400
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3 hours ago, Dave W said:

From the photos, you do not see any detonation on the two plugs. Which might mean the condition is for a very short period. Tip coloring indicates slightly less timing needed, back off 2 degrees. The single plug show very rich at idle and lean at WOT.  The best indicator for WOT is at the base of the ceramic.  You need a clean throttle chop, while de-clutching for a good plug read.

Dave W

Totally agree - Use a magnify glass to look at the ceramic the silver specs are very small. 

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I have no wisdom to add, other than sharing a few photos of my experience with insufficient octane.  This happened after about three (maybe two) laps at NJMP.  I had filled my fuel containers on the way to the track, and have wondered if the attendant didn’t fill them with Premium as requested.

 

 

7A8AFD6A-1CE0-46BB-A415-AFBB6F9D968D.jpeg

DF1FC87D-1FC5-4ADE-9F34-2CF0C418B610.jpeg

Edited by bball7754
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Thanks Dave 

Steve B from SBD is surprised and does not think this is the result of auto ignition.  He states that he runs 13:1 on 98 RON / 93 octane with no real issues. On the other end, NGK had a closer look at Simons plugs and suggests that it is auto ignition or hot spots in the chamber.  We will probably try a combination of mods e.g. grade 9 NGK plugs B9EFS that have a more protected ceramic than the grade 8 competition ones we are running and perhaps a bit of octane boost and yes, fuel mapping tuning.   Perhaps one change at the time and see.   I do agree Dave that WOT condition seems a bit lean but again Steve suggests that the spark plug “lean looking” color is quite normal when ones drives these race engines flat out.   

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I may be off base, but looking at the single plug photo again, could there be two heat marks on the ground strap. One pass the bend "advanced timing mark on the strap and a 2nd mark closer towards the center "correct timing".  Any possibility that you have an actual timing change occurring at very high RPM?  An external ignition pick-up that vibrates or goes into resonance?

Dave W

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It appears you are using extended nose plugs. They are great for not fouling in street engines, but that might be your hot spot. You might consider regular plugs. The ceramic and center electrode do not protrude as far and the tip will not get as hot. 

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This is Star Trek stuff.  How are you able to see all of this from the pictures Dave?  From now on, you shall be known as Dr Plug.  

 

Yikes on the engine blow up Bball. Really nasty and very unfortunate mistake from the attendant.  Sorry to see this. Makes our stomach turn. 
 

Carl - yes agree on the latest plug I’ve been using. They are the competition ones and these extend further into the chamber and have a longer ceramic nose.  I plan to change back to B type NGK plugs that are shorter, with a protected ceramic nose, and perhaps go for an even colder plug, B9EFS.  
 

I’m also scheduling a road dyno tuning session and will have SBD connect remotely to fine tune the map, advance etc.   Let’s see. I will keep the group posted. 

thanks all. 

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I’m just an old fat that raced a looong time ago when you used carbs and no exhaust-temp or A/F sensors.

It’s not hard to figure out that if you are burning the ground strap off the plugs you have detonation. I’ve been thinking there is another possible contributing factor. You may have such an efficient combustion chamber that the spark lag time is shorten at high RPM. Some race motors need the ignition advance reduced on top end because the flame front travels much faster at high RPM. That might show the two marks on the ground strap, if the advance curve is flat, say after 2,000 RPM. If that is the case you would see the correct timing at mid-range but have too much on top end. In either case you still need to reduce timing.

     Also, the A/F ratio and/or cylinder distribution is probably contributing to the detonation since it is only two cylinders??? I would think the dyno tuning would have an exhaust sniffer to determine WOT mixture.

   After the tune, I would still pull the plugs at the race track just to verify the state of tune. $hit happens, that you only see at the track! 

You need to look at the base of the plug right where the ceramic and steel meet. You need to see a narrow 1/8” wide dark grey ring at the base. A wider ring is rich and a small or no ring is lean, and the timing mark on the ground strap should near the bend, NOT past the bend.   

     One thing I do is save the last set of plugs [in cylinder order] just to be able compare them, to look for any A/F mixture or timing changes after race day.

   

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