What is ignition advance and why do you need it?
Ignition advance is how long before a cylinder hits TDC the spark plug is fired. This is essential because gasoline does not burn instantaneously (obviously) and the flame front takes a little while to engulf the air/fuel charge.
If the plugs were fired right at top dead center, by the time the charge was combusted (if it ever would get fully combusted) the piston would be so far down the cylinder that no torque would be produced. Generally, one should target maximum cylinder pressure for about 10-20 degrees ATDC, like in this image:
Notice what happens with the cylinder pressure in the case of detonation. At TDC, the pressure is almost as high as the peak pressure of the high power pressure curve! Now imagine standing on top of a piston at TDC (work with me here) and jumping as hard on it as possible. It's not going anywhere, because the rod and crank are pointing straight up. Instead, BITS of the pistons and valves and other parts are going somewhere. This is why diesel engines have to be built so tough, to withstand this pressure.
That's why igniting too early is bad. Igniting too late however, and you'll produce jack squat for torque, so it's tough. There is a definite sweet spot.
For all the rpm range and load range, maximum efficiency (and thus power, as well as fuel economy) is made by targetting the cylinder pressure to be maximized in this range. However, different loads and rpms change when you have to start the combustion.
At low load (meaning high vacuum) engines need more advance because at low pressure, the flame speed of gasoline is slowed down (although I'm not entirely sure by how much). At high load (meaning when the engine's running near atmospheric pressures, or in boost on turbocharged cars), the flame speed is increased, especially at pressures higher than atmospheric. This is the reason why engines need less advance at high load (and why you should/need to retard ignition when you boost).
At low rpm (like idle), engines do not need much ignition advance, for instance at idle (low MAP/rpm) 10 degrees is generally sufficient (and is the industry standard, I believe). This again places the maximum cylinder pressure at just the right spot, and should provide enough torque to keep the engine going. If any of you have ever adjusted your distributor for more advance (as I'm sure most of you have, actually), and if you've noticed that the idle is a little smoother as well, or the idle speed has increased, even, that is because the cylinder pressure peak has been moved closer towards the sweet spot, and the engine is producing more torque at idle (mind you, without using more fuel! ignition timing is exceedingly important for making the most power and best fuel efficiency!)
As rpm increases however, fuel does not burn any quicker, or if it does it has a relatively negligible effect. However, the engine is spinning faster, so for the combustion pressure peak to hit the sweet spot, you need more advance! Take another look at the image I posted above however. Then squash the scale's x-axis, keeping the pressure curve centered above the sweet spot still. You'll notice in your mental image "hey, now that the engine's going faster but the combustion isn't, that curve for the cylinder pressure is starting to creep up along the TDC line!". This is why at higher rpm, you really have to watch yourself with ignition timing. Too much advance and you'll blow your engine up real quick at high load and rpm. It's also easy to have too little advance, which means you're losing out on torque.
Luckily, there has been extensive dyno tuning of BPs over the years, and there is quite a bit of information out there about optimal timing maps for our engine (and for non-BP guys, I'm sure there's information out there for you too).
Anyway, for a small engine like the BP, 28-32 degrees of maximum advance should be fine. Any more than this is unnecessary and probably will reduce your engine's efficiency. For boosted engines, ignition timing should be watched quite carefully, but there's almost no reason to go below 20 degrees of advance (probably 16 degrees ABSOLUTE minimum, although that's pretty conservative) even with a full BAR of boost or more. If you're having trouble with detonation at this amount of advance, retarding the spark or dumping more fuel in is NOT the answer. Most likely you have a hot spot on your engine, or in the combustion chambers.
This is the ignition timing map that I am currently running in my own car. It is based off of Flyin Miata's dyno-tuned Link setup for Miatas, with changes to the cranking areas of the map as well as setting a flat idle advance of 12 degrees. This timing map runs perfectly happily on my 12:1 compression engine. A slightly modified version of this map is being used in TheRielDeal's car (I believe) with MAP bins for boost. I think we set his car to drop to a minimum of 20 degrees advance and it's been running perfectly with no knock.
Ignition advance is how long before a cylinder hits TDC the spark plug is fired. This is essential because gasoline does not burn instantaneously (obviously) and the flame front takes a little while to engulf the air/fuel charge.
If the plugs were fired right at top dead center, by the time the charge was combusted (if it ever would get fully combusted) the piston would be so far down the cylinder that no torque would be produced. Generally, one should target maximum cylinder pressure for about 10-20 degrees ATDC, like in this image:
Notice what happens with the cylinder pressure in the case of detonation. At TDC, the pressure is almost as high as the peak pressure of the high power pressure curve! Now imagine standing on top of a piston at TDC (work with me here) and jumping as hard on it as possible. It's not going anywhere, because the rod and crank are pointing straight up. Instead, BITS of the pistons and valves and other parts are going somewhere. This is why diesel engines have to be built so tough, to withstand this pressure.
That's why igniting too early is bad. Igniting too late however, and you'll produce jack squat for torque, so it's tough. There is a definite sweet spot.
For all the rpm range and load range, maximum efficiency (and thus power, as well as fuel economy) is made by targetting the cylinder pressure to be maximized in this range. However, different loads and rpms change when you have to start the combustion.
At low load (meaning high vacuum) engines need more advance because at low pressure, the flame speed of gasoline is slowed down (although I'm not entirely sure by how much). At high load (meaning when the engine's running near atmospheric pressures, or in boost on turbocharged cars), the flame speed is increased, especially at pressures higher than atmospheric. This is the reason why engines need less advance at high load (and why you should/need to retard ignition when you boost).
At low rpm (like idle), engines do not need much ignition advance, for instance at idle (low MAP/rpm) 10 degrees is generally sufficient (and is the industry standard, I believe). This again places the maximum cylinder pressure at just the right spot, and should provide enough torque to keep the engine going. If any of you have ever adjusted your distributor for more advance (as I'm sure most of you have, actually), and if you've noticed that the idle is a little smoother as well, or the idle speed has increased, even, that is because the cylinder pressure peak has been moved closer towards the sweet spot, and the engine is producing more torque at idle (mind you, without using more fuel! ignition timing is exceedingly important for making the most power and best fuel efficiency!)
As rpm increases however, fuel does not burn any quicker, or if it does it has a relatively negligible effect. However, the engine is spinning faster, so for the combustion pressure peak to hit the sweet spot, you need more advance! Take another look at the image I posted above however. Then squash the scale's x-axis, keeping the pressure curve centered above the sweet spot still. You'll notice in your mental image "hey, now that the engine's going faster but the combustion isn't, that curve for the cylinder pressure is starting to creep up along the TDC line!". This is why at higher rpm, you really have to watch yourself with ignition timing. Too much advance and you'll blow your engine up real quick at high load and rpm. It's also easy to have too little advance, which means you're losing out on torque.
Luckily, there has been extensive dyno tuning of BPs over the years, and there is quite a bit of information out there about optimal timing maps for our engine (and for non-BP guys, I'm sure there's information out there for you too).
Anyway, for a small engine like the BP, 28-32 degrees of maximum advance should be fine. Any more than this is unnecessary and probably will reduce your engine's efficiency. For boosted engines, ignition timing should be watched quite carefully, but there's almost no reason to go below 20 degrees of advance (probably 16 degrees ABSOLUTE minimum, although that's pretty conservative) even with a full BAR of boost or more. If you're having trouble with detonation at this amount of advance, retarding the spark or dumping more fuel in is NOT the answer. Most likely you have a hot spot on your engine, or in the combustion chambers.
This is the ignition timing map that I am currently running in my own car. It is based off of Flyin Miata's dyno-tuned Link setup for Miatas, with changes to the cranking areas of the map as well as setting a flat idle advance of 12 degrees. This timing map runs perfectly happily on my 12:1 compression engine. A slightly modified version of this map is being used in TheRielDeal's car (I believe) with MAP bins for boost. I think we set his car to drop to a minimum of 20 degrees advance and it's been running perfectly with no knock.
Comment