Summary

"A soft rev limiter vs a hard rev limiter and possible damage?" Okay, So, I think, probably, the bigger thing that you're, maybe getting to here is the way a rev limiter, limiter is integrated. so, generally, there's two ways of an ECU applying a rev limiter, and these are either through fuel cutting, or through ignition cutting.

00:00 Chris has asked, "A soft rev limiter vs a hard rev limiter and possible damage?" Okay, So, I think, probably, the bigger thing that you're, maybe getting to here is the way a rev limiter, limiter is integrated.
00:17 so, generally, there's two ways of an ECU applying a rev limiter, and these are either through fuel cutting, or through ignition cutting.
00:26 Now, on top of that, we do, as you have mentioned, have a soft rev limiter and a hard rev limiter.
00:32 So, how this works is, we'll have a point in the engine rev range where the ECU will begin what's called soft cutting, soft limiting, soft rev limiting.
00:41 So, that's where it starts to begin the rev limiting process.
00:44 So it might just be applying a very small amount of cutting to either fuel or ignition depending how it's set up, at that point.
00:50 Now, if you are only just coming on to the rev limiter, so you're just coming up to it, and touching it, it's probably not going to take very much ignition or fuel cut in order to control the RPM and stop it rising.
01:04 If, on the other hand, you've got an engine that's producing a lot of power, and you're really aggressively hit that rev limiter under wide open throttle, and full power, the soft cut will probably not be enough to control the engine RPM, and it will continue to rise.
01:18 And this is where the hard cut comes in.
01:20 So this may be, perhaps, 100 or 200 RPM higher than the soft cut, and at this point, all of the fuel or all of the ignition is completely switched off, to make sure that the RPM can't overshoot any further.
01:33 So that's the, the difference between soft limiting and hard limiting.
01:36 However, what I touched on there, was the difference between fuel and ignition cutting.
01:41 ignition cutting can be quite a dangerous option particularly on engines where there are known valve train issues.
01:50 So, a really classic example of this is the Nissan SR20-DET; if we're using, ignition cutting there, for our rev limiter, what's going to happen is, when the ignition is cut, we're going to end up with a full cylinder full of unburnt fuel and air, pass through the cylinder, because it's not igniting it inside the cylinder.
02:11 And that can end up igniting in the exhaust manifold.
02:14 Now, what happens there, is this can be a sufficient to, create a large pressure spike inside the exhaust manifold.
02:22 Which, in turn, can be enough to pop the exhaust valves back open off their seat, and in turn this can also upset the valve train, it can pop rockers off, and generally achieve all sorts of nasty stuff that we really don't want happening.
02:35 So, definately, the safer option in most instances is to use fuel cutting.
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