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Legacy777 wrote:I'm pretty sure the ECU uses ignition for the rev limiter. I bounced off of it once when I still had the auto. Unless the TCU had a separate rev limiter.

Regarding the MSD Dis 2's. They are definitely worth it. Even on the n/a EJ22's. It shouldn't affect your emanage at all. It just goes in line between the ignitor and coil pack.

You bounced off what, the rev limiter? Everyone does...not sure what you're trying to say here.

Yes...that's what I was saying....bounced off the rev limiter in my automatic. That's actually pretty rare, considering the TCU shifts automatically. Dave's the only other person I've heard of doing that on a first gen AT legacy.

Anyway, the point I was trying to make was that the way it bounced off the rev limiter would indicate the rev limiter is based on ignition and NOT fuel.

But how is that proof for ignition controlled rev limiter?

In what "way" did it bounce off to show it is ignition based?

Have you ever taken a rental car out and left the gear selector in 1st or 2nd, and then dumped the accelerator? It bounces off the rev lmiter a whole bunch of times. The ECU is taking spark away from the cylinders to maintain engine RPM in the "safe" zone.
It's a very sudden loss of power.

My car experienced a very similar sudden loss of power, except mine didn't bounce off the rev limiter a whole bunch of times because the TCU shifted the trans.

Also, the loss of power was a lot like hitting the speed limiter on newer cars.

I have no definitive proof saying it was the ignition. However all my experiences with hitting the rev limiter on other cars leads me to believe the one on my car was ignition.

You can take my word for it, or not...

If you really want to know. Here's what you need to do. Find your fuel pump relay, or the lead coming from the fuel pump relay to the ECU. Tie a little light into that. When the fuel pump is on the light will be on. This will give you the info on the pump side, and will let you know if the ECU cut's power to it.

Also, use Vikash's scan tool, so you can watch the injector duty information.

Find yourself another body to watch the data, and go run your car to rev limiter. If the light goes out, or you see a huge drop in injector duty, it's fuel cut. If the light doesn't go out, or injector duty doesn't drop too much it's ignition. (note injector will drop some when the ignition cuts out, but I would figure fuel cut would be a much more pronounced affect on injector duty because the ECU is actively pulling injector duty)

Lastly, another reason I think most modern cars, including ours utilize ignition for the rev limiter is because it's far more precise control then fuel.

Legacy777 wrote:Yes...that's what I was saying....bounced off the rev limiter in my automatic. That's actually pretty rare, considering the TCU shifts automatically. Dave's the only other person I've heard of doing that on a first gen AT legacy.

Anyway, the point I was trying to make was that the way it bounced off the rev limiter would indicate the rev limiter is based on ignition and NOT fuel.

I've done it. It's not that exciting, except when you hear the clutch packs struggling to shift. Then it's scary. O_o

Legacy777 wrote:Have you ever taken a rental car out and left the gear selector in 1st or 2nd, and then dumped the accelerator? It bounces off the rev lmiter a whole bunch of times. The ECU is taking spark away from the cylinders to maintain engine RPM in the "safe" zone.
It's a very sudden loss of power.

Couldn't the ECU be taking fuel away from the cylinders? Wouldn't the loss of power be the same?

My car experienced a very similar sudden loss of power, except mine didn't bounce off the rev limiter a whole bunch of times because the TCU shifted the trans.

Also, the loss of power was a lot like hitting the speed limiter on newer cars.

I have no definitive proof saying it was the ignition. However all my experiences with hitting the rev limiter on other cars leads me to believe the one on my car was ignition.

How do you know the limiters on the other cars aren't injector controlled?

You can take my word for it, or not...

If you really want to know. Here's what you need to do. Find your fuel pump relay, or the lead coming from the fuel pump relay to the ECU. Tie a little light into that. When the fuel pump is on the light will be on. This will give you the info on the pump side, and will let you know if the ECU cut's power to it.

Also, use Vikash's scan tool, so you can watch the injector duty information.

I'd be very afraid if the fuel pump controlled the amount of fuel going to the engine. If the fuel pump cut whenever the ECU said "no fuel", whats the use of injectors? The injectors snap shut when told to give no more fuel (snap shut meaning, they are given no more power, and therefore do not open).

Find yourself another body to watch the data, and go run your car to rev limiter. If the light goes out, or you see a huge drop in injector duty, it's fuel cut. If the light doesn't go out, or injector duty doesn't drop too much it's ignition. (note injector will drop some when the ignition cuts out, but I would figure fuel cut would be a much more pronounced affect on injector duty because the ECU is actively pulling injector duty)

Lastly, another reason I think most modern cars, including ours utilize ignition for the rev limiter is because it's far more precise control then fuel.

Thats actually a good idea, I took a look through my eManage logs and can't find any where I hit the rev limiter, so thats null for now. I don't think dumping fuel and not igniting it with spark is a more "precise" way of controlling rev limit. I would believe that cutting fuel, what makes the engine work, would be the better way. Having a whole lot of unburnt fuel in the cylinder would be a lot messier. However, I would believe that it could be spark AND fuel, but I don't think Subaru would go to great lengths to do both instead of having the option to just do one.
I'm more inclined to agree with Vikash, but I need to do more research.

I doubt it works by cutting the fuel pump; an engine can run for a short time with the pump off, especially if load is low.

I don't think you can necessarily draw any conclusions from how it feels; whether it's fuel or spark that's taken away, you're still just feeling a misfire.

Your suggestion of checking pulse width with the scan tool is a good one and can resolve the question.

Ignition cut can be more precise than fuel cut, but that only really matters if you're trying to make a soft limiter.

Couldn't the ECU be taking fuel away from the cylinders? Wouldn't the loss of power be the same?

Have you been in a car, or experienced a car hitting the rev limiter where it just keeps bouncing off the rev limiter? Specifically where the engine sounds like it's sputtering, and engine rpm stays fairly constant?

Explain to me how you can do this with fuel control. i.e. make the engine react the way it does with fuel control.

Thats actually a good idea, I took a look through my eManage logs and can't find any where I hit the rev limiter, so thats null for now. I don't think dumping fuel and not igniting it with spark is a more "precise" way of controlling rev limit. I would believe that cutting fuel, what makes the engine work, would be the better way. Having a whole lot of unburnt fuel in the cylinder would be a lot messier. However, I would believe that it could be spark AND fuel, but I don't think Subaru would go to great lengths to do both instead of having the option to just do one.
I'm more inclined to agree with Vikash, but I need to do more research.

Regarding the cutting fuel for rev limit control. The crank angle sensor reads engine rpm. Let's say it reads that the engine is going, or has gone above it's redline. It needs to reduce speed. Injector duty cylce is reduced. However, how does the ECU know how much to reduce injector duty cycle? What if you were accelerating down a hill, and the engine had a lot of momentum? What if you were going up a hill with a full load of stuff in the car? Using solely fuel as a rev limiter, how is it going to know how much fuel to cut? All it can do is cut a rather large amount, so rpms drop. I suppose you could say that it can use the calculated load in the ECU to determine this, and vary the amount of fuel cut. But to be quite honest, that's rather complicated and messy.

With an ignition rev limiter, you know exactly where the crank is every rotation, and if it's speed is over the rev limit, it can immediately start taking away spark to reduce engine speed, and has a much faster feedback loop since everything is electronic. Compared to injectors which are electro-mechanical.

I think we might be thinking similarly, but I might not have explained things as well.

Personally I believe the rev limiter, or engine speed is controlled via the ignition system. Read over the first few pages of section 2-7 in the fsm. It states nothing about the fuel system controlling engine speed. I do believe the fuel system reacts in kind to what the ignition system is doing. So you're not going to be dumping tons of unburnable fuel into the cylinder.

Most newer ignition based rev limiters utilize a "soft rev limiter", and remove spark from ever other pulse or something similer, rather then completely shutting the spark off.

Cutting spark is also safer than cutting fuel and causing a lean condition especially after a heat inducing run to the rpm limit. As far as personal experience goes, when I hit mine and backed off I got some popping in the exhaust which may've been from an ignition cut (but I get it under other conditions also).

Steve

Since I originally started this thread, I have dynoed, and it 6k where I shift now.

This is where I believe you have it wrong.

Think about when you hit boost cut. We all have, and some of us have put the fix in to elimate the problem. That is a fuel cut.

Now, if it was going to create a lean condition, why would they do it while under high boost? Because if there's NO fuel, there is nothing to create a lean condition, am I wrong? The engine will not have a uncontrollable or explosive combustion event, because there is nothing to combust. And there is no extra heat from behing lean because there is nothing exploding. The engine succeeds in cutting boost and resumes combusting. Couldn't the same thing be happening with the rev limiter? They are the same thing to me, don't the RPMs drop when you hit boost fuel cut?

There is, indeed, something to combust.

Oxygen.

I would not want my car cutting fuel to limit the RPM. More than likely, the engine was being wound up at WOT if this happens. You don't accidentally get to redline (I've never bounced off my rev limiter, I don't get much above 6k because the midrange on these cars is way better anyway)

Anyway, with the high compression inside the cylinders on boost, if fuel was cut and ignition was still active, guess what might ignite. (see above) You want some run-away detonation, take away only fuel at high boost. In my opinion, that's not something that the Subaru engineers I'm so proud of would have done here. I'm willing to bet they cut ignition and not fuel to achieve RPM limits.

I'm still a bit shocked, though, that they actually cut fuel for the boost limit. Your wastegate line blows off, your boost raises uncontrollably, and what do they do in response? Cut the fuel?? O_o

I mean... how can they be absofuckinglutely sure that cutting fuel will result in zero fuel molecules making it into the engine next cycle? Just one can respond to a spark, and if the oxygen ignites its all over.

Take away spark, though, and all that could happen is a happy backfire and maybe a diesel cycle or two.

This is silly to debate. Someone go out and look with the scan tool.

Chris - Oxygen doesn't burn. Oxygen and fuel burn. If only a molecule of fuel is present, only a couple of molecules of oxygen will burn. Hardly enough to create the heat that is the danger of lean conditions, no?

Um, a diesel cycle or two? Not that I think that would happen if you're using the correct octane, but if you'd rather have detonation through compression than to cut fuel to control rev limit...

Many, many cars use fuel to control rev limit. Or else, what use would piggybacks be that can increase rev limit by running injetors past where the ECU tells them?

Oh and if you could combust oxygen without fuel, I think we'd all be dead.

This is freakin stupid!

Everyone believe what they want to.....

For what its worth, the UTEC can raise or effectively remove the rev limiter on the WRX.

My friend Jay (Vikash, you met him) has his set somewhere "around" 7k. But speaking to several tuners, you can simply raise the limit to beyond anything a subaru engine would be able to reach.

well, how rich does lean have to be before it can detonate?

I dunno.

As is my understanding, there are multiple ways in which an inappropriately lean mixture can lead to detonation... One is by causing high cylinder temperatures, and the other is by causing a heterogeneous mixture in the combustion chamber. It seems like the latter is more likely than the former if you're bouncing off fuel cut.

Interesting. What's the critical temperature, 1700, 1800 degrees F?

I don't know what critical temperature means exactly... The autoignition temperature of gasoline is a little less than 500 degrees F. But I don't know how that helps us understand...