advanced timing key

[m8magicandmystery]

Ok...im interested in the responses in regards to a ported bigbore with a slightly higher compression head at 2000-5000 ft with a 2.5 degree advanced timing key to advance the spark running/recommended 91 octane..

What are we achieving..
What are we preventing..
Is it even required...

 

[m8magicandmystery]

ttt

 

[nomasittinright]

On my d&d 900 I have a 3° timing key with 12.8 compression and i need to run 94 octane at 1300 feet. Without the key in I can get away with only 91 and there is a noticeable difference between the two. If you want to run good fuel I would go for it. What compression are you running

 

[m8magicandmystery]

i do have the 2.5 timing key in it with 13.5 compression, so are you indicating that to use higher octane fuel a person needs the key..??..why do some bigbores run 91 and still need the key..like mine..??..thats why im wondering if i need the key..?? what all the benefits from the key or disadvantages if any..?? .what elevation do you ride at..??

 

[Mjunkie]

i do have the 2.5 timing key in it with 13.5 compression, so are you indicating that to use higher octane fuel a person needs the key..??..why do some bigbores run 91 and still need the key..like mine..??..thats why im wondering if i need the key..?? what all the benefits from the key or disadvantages if any..?? .what elevation do you ride at..??

On my dragoncat motor I ran for several years I had a 4 degree key on, and was told it was worth about 25hp. The idea is that the fuel has a longer time to burn completely and thus more push on the piston. Also if you think that the spark plug is basically in the center of the head and when it sparks and ignites the fuel then the flame front travels to the edges of the piston. In a big bore situation it has a longer distance to travel than in a stock motor because the piston is bigger and therefore longer distance, so advancing the timing gives it more time to travel. The only drawback's that I experienced was of course having to run a higher octane fuel because it is more likely to detonate having more time to burn in there (I'm somewhat guessing as to the reason why here), and also that with the advanced timing I couldn't hold a steady throttle in the midrange for very long without overheating (the egt's would rise quickly). Other than those 2 reasons I didn't think it was a drawback at all. More power is ALWAYS better!!!

 

[m8magicandmystery]

The only drawback's that I experienced was of course having to run a higher octane fuel because it is more likely to detonate having more time to burn in there

also that with the advanced timing I couldn't hold a steady throttle in the midrange for very long without overheating (the egt's would rise quickly). /QUOTE]





Two issues i like that you brought up..i did have the det sensor go off running 91 the first time in three years on the ONLY TANKFULL i ran out of my 60 point (6 octane) additive..so that leads me to say the motor is not intended for 91 like it was built for..

and your midrange heat issue i think is partly addressed in the doo forum by BIGJOHN..which i just read..he says go away from controlled valves to the STM..and have the valve open earlier..he indicated that the mid temp heat is the bigbore trying to exhaust through too small a port if the valve aint open so have it open earlier with the adjustable stm...which i have..and you can run a leaner mixture in the range that was previously trouble..so im gonna try that

i am going to send my jugs/pistons/heads/and stm valves to Linderman and he is going to recheck port timing/squish etc etc and put my stuff on his test engine..so i think we may discover a few things..if he can;t find it im told nobody can..lol..although i think RKT is a safe bet as well...

but back to the topic..i only am accessible to 91..but removing the key isn;t the answer...i would have thought that having the advance spark would ignite the burn earlier and ending it a bit sooner just at the right time..to PREVENT DETONATION of having the burn happening too long after TDC..??

 

[nomasittinright]

http://www.cpcracing.com/site/256083/page/509138

There is alot of good information here timing is section 6 I learned alot from this and talking to dale at d&d. I melted my motor the first time I rode it because I wasn't aware of the mid range temps and holding it too long cruising down the trail. At 1300 feet elevation I needed 94 but at 6000 feet and up I only needed 91. If you don't have egt's I would definitely get some

 

[m8magicandmystery]

i hve egts...i think at my elevation i may have to go for a less compression ratio...Linderman will figure it out for me...maybe increase the 900 to a 925..lol..

 

[Mjunkie]

To comment on the spark starting sooner and therefore being able to end sooner, remember that on a big bore you have added fuel over the stock programmed fuel settings and timing settings, and so having more fuel takes longer to burn, so I don't think it will end sooner. Plus like I said before, the burn has a greater distance to travel over the bigger piston crown surface. That would explain why a longer burn time or advancing the timing could improve efficiency and power.