|
Originally posted by Collente
Advancing Timing to cure High EGT's? Wow that's the first time I've heard of that, would you mind explaining in detail the concept of that Matt. Thanks, Nick
Ignition timing has a LOT to do with exhaust gas temperatures. This is why tuning to an EGT gauge alone, is not the best way to tune.
When in boost, the Air Fuel ratio should remain somewhat of a constant, where typically, most power will be made from 12.0:1, give or take a bit richer/leaner, depending on how much boost you are running. Low boost can get away with slightly leaner, higher boost should be a bit richer.
Ignition timing and fuel octane go hand in hand, as they both need to be adjusted together. Higher octane fuel needs more timing, because of the way it burns, a bit slower for peak cylinder pressures to occur during combustion. Ideally, we want most torque/work applied to the crankshaft at around 14-15 degrees after top dead center. In order to acomplish this, we have to run the proper amount of timing, with a certain boost pressure, and a certain fuel octane. Take note that fuel octane is what really lets us make boosted power! There is only so much boost you can run, with so much timing, with a given fuel octane,and engine compression ratio. Its just the law of physics...
When running boost, and you dont have enough timing to keep most work done on the crank at the 14-15 degrees ATDC, combustion is happening too late, and more heat and lost energy goes out the exhaust valves. Depending on how retarded the timing is, you can have the actual flame burning out the exhaust, which would be hitting your EGT probe. This creates high EGT's. In order to keep more heat in the cylinder, and to ignite the A/F mixture at the "right time", you have to run the appropriate amount of timing.
Just to show how this all works, a case-in-point:
Since I run the Megasquirt, i have complete control of my fuel and ignition, along with a AEM wideband o2 sensor. When i tuned 10 psi, I set the A/F ratio to approx 12.4:1 This is a good A/F at this boost pressure for making good power output, not too lean, not too rich. I set the timing advance to around 22-23 degrees, and on a long 4th gear pull up to redline, EGT's were no more than 1550 degrees and steady, not climbing any more. The engine felt great, and it was making some good clean power, as it is tuned well for that boost pressure.
Now, i switched back to the stock ECU for controlling the ignition, and from my datalogs, ignition timing is roughly around 14-17 degrees under *any* boost pressure. Remember that the stock ECU cannot read boost at all, so it doesnt know whats going on, except that it thinks its at WOT. Now, my EGT's would SKYROCKET up to 1600 degrees at the SAME air/fuel, same boost, same everything except the timing. I could not even get close to running out 4th gear without having to let off the gas. This goes to show that ignition timing advance was too retarded, and combustion was happening too late, with too much heat going out the exhaust.
A too lean A/F can also cause high EGT's, so you have to be careful as well. Tuning is one big balancing act, and you have to get it all right if you want to make safe, good power for your given boost pressure. This is what sucks about using the stock ECU for ignition timing control..Just by lowering the compression ratio by almost a full point, and then running 93 octane fuel, that right there means we need more timing advance at ALL engine pressures/rpm. I'm running about 4-5 more degrees of timing thoughout my spark map on the megasquirt, compared to the stock ECU's timing maps.
Fuel octane lets us make power. Period. Yes, timing needs to be reduced as boost pressure increases, but only to an extent can you get away with this. There is a certain point where you will "run out" of sufficient fuel octane. Some may think that well, if i need 22 degrees of timing at 10 psi, then i can run 25 psi with only like 10 degrees of timing!NOT so! Lets say we do this, and upping the timing any more than 10 degrees causes knock, so we cant go higher, but even at a rich A/F EGTS are through the roof..we have run out of fuel octane to run that boost presure. If you have to pull timing below 15 degrees on these engines under boost, you dont have enough fuel octane for that boost pressure. There just comes a point where if you run too much boost, that more timing causes knock, and less causes too high EGT, then you need more octane.
Lower compression ratio pistons, high octane fuel, proper amounts of timing, and lots of boost is how you make lots of power. Making lots of power on pump gas is tricky. This is where other things come into play to run more boost can help, using pump gas. Water injection, colder intake temperatures, and more efficient turbo's can all help to make more power on pump gas, but ONLY to an extent.
Lets run 114 octane fuel, 28 psi boost, and 22 degrees of timing on a S20g, and i'll bet it makes damn near 500HP. No way could you do that on pump gas, at least not without a much lower compression ratio than our typical "rebuild" of 8.8:1(which really isnt that "low" for boosted eninges). Ok, that was long.
13.5 @108 MPH-2.2 60ft(stupid FWD!)
S16G @ 18 PSI/FMIC/Running on MegaSquirt II (Now with sequential
fuel injection)
My webpage: http://eclipsed4evr.home.comcast.net
-1998 Mitsubishi Eclipse RS-T- "Toy"
-1992 Plymouth Laser Turbo AWD(SOLD)
-2000 Honda CR-V(daily)
|
Search
Printer-friendly copy
1 User in Chat
