Quote: Originally Posted by Isthatatank
on my car i run a 50 trim with st0(Stock computer) 650cc injectors a return line at 42 psi, and a walbro fuel pump running just under 20 psi
how is timing affected by my mods? and how much do you think it is running? ive read that some st0 cars run about 25 degrees of timing, after tuning a few DSMs ive noticed thats quiet a bit of timing anythots ?
First off, I'll copy your mods below for future reference in case you change your sig and I'm trimming out items not related directly related to power/tuning:
AGP T3T4 50 trim .63, Turbonetics Evo wga, AGP fuel return, SAFC2, 650cc RC's, 255walbro, JoePMBC@18psi, 60mm T/b, Griffen big FMIC, PUMP GAS@17PSI 10.9-11.1A/F 364WHP 348WTQ
Looking at that list the first thing I notice is your S-AFC, but we'll get to that in a minute. The stock (S0) PCM as well as the Mopar "Stage" computers on the SRT-4 run quite a bit more timing advance than what you'll usually find on many other turbocharged vehicles. The stock computer for example will run around the lower to mid 20°'s advance in the mid-range during a WOT pull and start to advance it even more towards redline, ending up around 29-30° on a stock car. That's quite a bit of timing advance but it works reasonably well with the small stock turbo.
Talking about a stock car, Chrysler added and designed a few things that make running that much timing advance possible. The compression ratio is relatively low, it requires premium fuel, the intercooler works well, the PCM has a good knock detection system, etc. The turbo also isn't moving huge quantities of air so the overall cylinder pressures aren't too high (again remember that when talking about different turbochargers you need to think in overall airflow and not boost since boost is just a measure of pressure or restriction to flow).
Before going off too far it might be good to talk about what timing is and how it affects power. In an engine you have pistons moving up and down in their bore that are connected to the crankshaft by connecting rods. Their movement rotates/spins the crankshaft which in turn eventually drives the wheels. The pistons gets forced downwards by an ignition of the air/fuel mixture that comes into the combustion chamber when the intake valve opens. The piston comes up and compresses that mixture and the ignition system fires the spark plug to light it. And in our case a turbocharger is forcing more air into the engine so you can in turn burn more fuel. That creates a bigger 'explosion' that creates more force pushing down on the piston which creates more twisting force (torque) at the crankshaft.
So timing is the actual point in time when the air/fuel mixture gets ignited. That reference point is measured in degrees rotation of the crankshaft, with the piston at top dead center (TDC) being 0° (out of 360° total that's in a complete circle). TDC is the exact point between when the piston is still coming up and when it starts going back down. Here's where timing advance comes into play.
When you go to fire the spark plug to ignite the fuel it doesn't all immediately go from not burning to fully burning. If it did that you would have detonation, or uncontrolled spontaneous ignition/burning of the mixture. It takes a little bit of time for the flame front that's created to spread out and fill the combustion chamber. So what happens is the spark occurs and starts to light the air/fuel mixture as the piston is still coming up. This is measured in
degrees of
timing advance before top dead center (BTDC). Remembering the 360° to a circle that the crankshaft has to turn when making one revolution, if you have 15° timing advance, the ignition is lighting the mixture off 15° before the piston has come all the way to the top (or 345° into the rotation). The flame starts to spread out and burn (with the gasses expanding and increasing in temperature) after a certain amount of time and it eventually starts pushing down on the piston which forces the crank to continue spinning. If it happens correctly the mixture is lit and the flame front that's created travels throughout the combustion chamber (the area machined into the cylinder head and between the top of the piston) and starts to push down on the piston with maximum force slightly after top dead center (ATDC).
If you light the mixture too soon (too much timing advance) it can start pushing down on the piston before it has a chance to reach the top of its stroke. That would be the equivalent of trying to stop the crankshaft and lower end rotating assembly and make it turn the opposite direction. What normally happens with too much advance is that the mixture spontaneously combusts at once which creates a large spike in cylinder pressure that's called detonation. It causes the engine to vibrate which you can hear as 'knocking' if it's loud enough. If you light the mixture too late (less timing advance, which would happen if timing is retarded/pulled), the flame front in the combustion chamber doesn't reach its peak pressure until the piston is well past AFTD and it doesn't transmit as much of that force into mechanical energy. You end up with a lot of that heat energy being wasted out the exhaust, which is why EGT's go up when you run less timing (such as when it get pulled/retarded due to something like knock).
If you were to look at the timing curve on a naturally aspirated engine you'll probably see the timing being advanced as rpm's go up. This is because the fuel generally burns at one fixed rate of speed, but the piston is moving faster. To keep that optimal power where the combustion forces are pushing down right after TDC you need to light the mixture off sooner (more advance) since there's less time for the flame to spread when the piston speed increases.
A forced induction application is usually a little different. Out of boost it may behave similarly to a N/A engine (which is why you'll see a lot of advance cruising with the engine in vacuum), but when you come into boost the cylinder pressures start to increase. The higher the pressure, the greater the temperatures in the chamber. The greater the temperatures, the greater the chance for detonation. In order to limit the chance of that happening you need to reduce timing.
The SRT-4 uses a DIS (distributorless/direct ignition system) where the PCM/computer controls when the timing events occur. It's programmed for the air flow ranges of the stock turbocharger and also considers things like charge temp, MAP pressure/vacuum, barometric pressure, coolant temp, etc. when determining advance. When something like the charge or coolant temps go up it knows there's a greater chance the air/fuel mixture will want to ignite. So if we're doing a wide open throttle (WOT) run through the gears, at the higher loads in the lower part of the rev range where detonation is more likely to occur it reduces timing. But with the stock turbo dropping boost up top, the decent stock intercooler, and premium fuel it can advance timing a little to make better power without running into detonation.
Now let's say you have a big turbo. Most of them start flowing a greater quantity of air on top which leads to a higher cylinder pressure (higher cylinder pressure is the equivalent of increasing the compression ratio), which also increases temperatures. The more you compress a gas the hotter it gets. You're trying to burn more fuel as well to go along with that extra air and then get a bigger "bang". Because of this and other factors you usually don't want that much timing advance, especially on the top end when the big turbo is just starting to get spooled up. One of a few things is going to occur. First, if you're running a high enough octane fuel it might be able to take advantage of either some or that entire amount of timing advance a stock turbo car runs, also depending on how much boost/airflow is being produced. The higher the octane fuel, the slower it burns so you can ignite it earlier to take advantage of maximum power without getting detonation.
Second is that you will get some mild detonation but the PCM reacts fast enough when it hears it via the knock sensor. It will then pull/retard the amount of timing advance its running. If it's just mild detonation you might not have a big problem. Now let's say you tricked the PCM by altering one or more of its inputs such as the MAP signal. What's going to happen if the PCM
thinks that it's operating under one condition and isn't expecting knock due to its programming when in fact you've just tricked it? You could potentially get bad detonation because it doesn't react fast enough and it damages the engine (such as fracturing the top ring or ring land of the piston that normally seals the piston to the wall of the block). And on an engine with high specific output even small detonation can be very hard on things like the rod and main bearings, and we haven't talked about the oil pump, etc.
A third scenario is someone could be running water or alky injection to limit the chance of detonation and take advantage of the timing. Depending on the mixture being injected it can either add fuel (alcohol to richen the mixture in the combustion chamber) and/or help take heat out of the combustion chamber (works similar to an evaporative cooler where it absorbs some of that heat in the chamber in order to change the liquid to a gaseous state). If it was running lean and subsequently hotter the alky can help, and if it's just too hot the water helps to a certain extent. But if you use it to suppress detonation and it suddenly fails at high boost the PCM may not be able to correct (reduce) timing and catch the detonation in time if a component in the water injection system fails.
Those are a few ways some of the big turbo cars are running. It would be great to have a lot of timing advance like the stock turbo can use but in reality it's hard to control detonation with the increased airflow, so you should be running less advance, especially on pump gas and higher boost. And if
you aren't doing it the PCM probably is (because of detonation/knock). Things like a bigger intercooler to help keep charge temps down and adding fuel can help, but only to a point. The maximum timing you can run will depend on mechanical properties, so if you don't want to further lower the static compression ratio you end up running less timing in order to push a lot of boost on a big turbo.
Finally back to the S-AFC or any piggyback/MAP clamp for that matter. An optimal tuning solution for the piggyback would be to adjust fuel first, such as changing fuel pressure, and then use the piggyback to fine tune. The bad thing about how it works is that if you end up having to remove or take out excess fuel to get an A/F you want the PCM is going to see less boost. When it thinks there's less boost it will try to running more timing advance because it expects that there's less air going into the engine and less chance for it to knock/detonate. So if you have a big turbo you might be starting out with too much timing due to the stock or "Stage" PCM's and then getting even more advance because the PCM isn't even seeing the full boost it was designed to read.
In your case RC rates their injectors at the industry standard 43.5 psi unlike Mopar which is at ~58 psi. So at 42 psi you're getting nearly the full 650 cc/min of fuel. If you have a rising rate fuel pressure regulator, at 20 psi of boost you'll have 62 psi fuel pressure and the RC's will be flowing nearly 775 cc/min. If that's enough fuel for the peak power and airflow requirements while staying maybe slightly rich with a margin of safety it might work fine. But as the boost and subsequent fuel pressure is ramping up together, if it runs rich at any point and you have to take a lot of fuel out by using the piggyback you'll get more timing advance. And if knock occurs then you've pushed it too far. You're also trying to idle with much bigger injectors and the PCM will have to alter long and short term fuel trims to correct for it (which it can also alter at other rpm range/MAP voltage levels).
On pump gas I'd rather run more boost with less timing advance to make power rather than a lot of timing and slightly less boost. The more timing/less boost method may work as a lot of SRT-4 owners with aftermarket turbo's have experienced, but it's far from being the safest way to get the same power. There are certain limits to the fuel octane and even reducing timing can't overcome them if the cylinder pressures get too high (e.g. a lot of boost on a big turbo). It amazes me how well these cars hold up at times with some of the tuning methods in use. On my scan tool I've seen 400+ whp cars at high boost that are running over 30° timing advance on top. About the only thing that's most likely holding the engine together was C16 race gas at those power levels. And because of that timing your "tune" has very little margin for error. A lot of cars hold up, but there's a lot more you don't hear about that have experienced failures trying to push stock/modified components too far.
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