ATD Advance Timing Curve?

[nkt267]

As usual it was at it's best just before it let go..John

 

[Bill Thomas]

It looks like you had what we call "A Nip Up " Cheers Bill.

 

[redbloke1956]

33 is an excellent setting to choose with modern fuels and compression of around 7.5:1, should result in a smooth running motor. BUT I recall you are running 9:1 pistons, so even 33 may be a tad high. Others with similar compression to your motor are running with full advance around 28 to 29 degrees.

When on the road, with the engine under load, such as when pulling hard up a hill if you detect any sign of pinging/detonation then reduce the advance by 2 degrees (down to 31) and road test again. Keep doing this (reducing the advance) till the pinging has abated. Detonation or pinging is a sure way to destroy the piston and also induce premature failure of the big end bearings. It also means, that with the detonation happening early, the motor is NOT producing its best power.

Hi Martyn, can you please explain why a higher Comp Ratio requires less advance?

Regards
Kevin

 

[Martyn Goodwin]

Hi Martyn, can you please explain why a higher Comp Ratio requires less advance?

Regards
Kevin

It seems to be a common requirement and is also stated as a requirement by PEI in "Tuning For Speed". I am sure Max can provide a incisive and correct reply.

For me, logic suggests as the air/fuel mixture becomes more and more compresses (by increased compression ratio) the force produced by its explosive burning must also increase and as we are dealing with a fixed volume of charge, it implies that the burn rate is faster. If I am correct then we need to reduce the advance as we increase compression ratio to ensure we get the energy of combustion happening at the best time to provide optimum power.

M

 

[Kansas Bad Man]

To best explain, I will start with a mean. The mean is say 10:1 compression ratio and a 90 octane fuel rating number. For this combination ,with an air cooled engine , you would expect the engine to achieve its best fuel expansion point by starting the ignition at 35 degrees BTDC. If you were to increase the compression ratio to 11:1 you would have to do one of two things, increase the octane fuel rating number to, say 100, this would slow the burn rate by 5 degree of crankshaft rotation allowing the expansion point of the fuel burn to be equivalent to 35 degrees BTDC , or retard the ignition to 30degree advance BTDC. The afore ,is the best by far, as you are taking advantage of the H.P. gain ,by increasing the compression ratio. The expansion of the fuel taking place at the best crankshaft position but is not giving up fuel burn expansion to a larger volume but to a smaller volume , this gives more effective push of expansion on the piston crown. Keep in mind that the jetting at half throttle, running at say 3000 rpm, the fuel /air ratio is 14 to 1 which is good.. Take that same jetting to 6000 rpm and the fuel/ air should be much richer say 11.5 to 1 , so when tuning with a plug chop, make sure the needle is in its bottom grove, this will insure the needle is out of the picture, when tuning top end fuel/ air ratio. Then tune the needle position for mid range.

 

[Kansas Bad Man]

I forgot to mention this point, which should make things a bit clearer, when the engine is under load at higher RPM the piston is harder to push down. The fuel air ratio is greater due to the greater throttle opening, this extra heat of expansion is no longer following the path of least resistance " pushing the piston with a lesser ease ", the heat has to go some where, hence the combustion chamber becomes hotter then the optimum temperature of 350 degrees. The hot combustion chamber can aid in detonation at higher RPM. A richer mixture can cure this however.