Summary

Any comment on running a richer mixture at higher RPM versus lower RPM? So when we are running in power enrichment, when we are running at wide open throttle, generally what I will do is run a slightly leaner air/fuel ratio at lower RPM and then at higher RPM I'll taper a little bit richer. This is pretty typical and the reason for this is that if we're at sustained high RPM, we've essentially got more combustion events happening per second.

00:00 - Any comment on running a richer mixture at higher RPM versus lower RPM? So when we are running in power enrichment, when we are running at wide open throttle, generally what I will do is run a slightly leaner air/fuel ratio at lower RPM and then at higher RPM I'll taper a little bit richer.
00:17 This is pretty typical and the reason for this is that if we're at sustained high RPM, we've essentially got more combustion events happening per second.
00:26 The engine's simply spinning faster so we're creating more heat in the combustion chamber, we've got less time for the engine to basically get rid of that heat.
00:34 So if we're at sustained high RPM, we really want to manage that combustion chamber temperature and it can be smarter to taper our air/fuel ratio a little bit richer.
00:45 And I'll bring this back to something that is really important with our understanding air/fuel ratio course.
00:51 And we'll bring this back to how that affects our air/fuel ratio targets in a second.
00:56 So a lot of tuners, I would say 98% of tuners out there basically go into every tuning job with an idea in their head of exactly what air/fuel ratio their engine is going to end up running and they don't test, they just simply achieve that air/fuel ratio, send the engine out the door.
01:12 Now that's not to say that that is a dangerous way of tuning but it can potentially be leaving some power and torque on the table.
01:19 So just let's talk about how that works.
01:21 Let's say we've got a tuner who is really really familiar with tuning Honda B series engines.
01:27 Now a naturally aspirated Honda B18C, that's going to run really really nicely on pump gas at wide open throttle with an across the board 13.2:1 air/fuel ratio.
01:37 Leaner won't make any more power, richer won't make any more power and the engine will not suffer from knock on a good quality pump fuel.
01:45 Let's say that tuner then tunes their first GM 6 litre LS2, they've never seen one before so of course they apply what they've learned on the Honda B18C, they tune for 13.2:1.
01:56 Now if that engine is stock standard we will be able to tune it at 13.2:1 but what we're going to find is that particularly at higher RPM, maybe above about 4000, 4500 RPM, the engine is going to become very sensitive or prone to suffering from knock or detonation.
02:14 Now that's an abnormal type of combustion that's very damaging so we need to stay away from that.
02:18 To do that, we would need to retard the timing.
02:21 So we retard the timing, the knock goes away and the engine will be safe.
02:25 It's not to say that the 13.2:1 air/fuel ratio will damage it, it won't but we'll need to retard the timing in order to prevent knock.
02:32 Now if instead of just blindly applying 13.2:1, the tuner actually experimented, what we would find is that by targeting closer to maybe 12.6, 12.7:1 at high RPM, the additional fuel from that richer mixture will cool the combustion charge temperature, allowing more timing to be used and the net result will be the richer mixture would be able to make more power.
02:54 That's why it's so important to test rather than blindly applying the same air/fuel ratio to every engine we tune.
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