Myth? Lean mixtures will cause engine damage - Page 4 - Fuelly Forums

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Old 08-11-2008, 05:11 AM   #31
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Originally Posted by bobski View Post
Uh... What? Warm air intakes work because they reduce the density of the air being pulled into the engine. Lower density means there's less oxygen (and nitrogen for that matter) per volume of air, so less fuel needs to be injected to reach a stoichiometric ratio. Manifold vacuum is unaffected by the temperature of the air.

No, the volume of air/fuel mixture being pulled into the cylinder during each intake stroke is reduced. The mixture is diluted by the exhaust gasses. Less air and fuel being burned means less heat released in the cylinder, lower combustion temperature and therefor reduced NOx emissions.
I think dkjones was saying the same thing on the vacuum issue- just in a different way. If you use WAI vs. no WAI, on the same stretch of road at the same outside air temp, you would need to open the throttle more to maintain speed with WAI (since, like you said, the air/fuel takes up more space). Opening the throttle plate slightly more would mean slightly less vacuum.

I agree that using EGR means less fuel used, but I think that EGR does NOT affect the air/fuel ratio, because EGR is neither air nor fuel.
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Old 08-11-2008, 06:46 PM   #32
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If you monitor your AFR with a wideband O2 gauge you'll notice that AFR spikes rich when EGR comes on. You're displacing air with exhaust. Exhaust doesn't burn.

I said the stock ignition system on the Honda Civic and Acura Integra (as well as across most of the rest of the Honda/Acura line-up) is mediocre. I didn't say it was bad. If I were to compare it to similar manufacturers I'd give it a C. I've had 3 Civics and an Acura Integra and the ONLY part that has ever failed me has been the ignitor, the coil, or the entire distributor assembly. Your opinion is the same thing I see repeated across all the popular Honda forums. That doesn't make it a fact.

The reasons are inherent to the design. 1) The size of the distributor limits the size of the coil. One coil has to charge and fire four cylinders every rotation. Honda designs their engines to rev relatively high. As RPMs increase, dwell time (time for the coil to charge) decreases. As a result, the small coil peters out at the top end and you get weak spark.

2) Inside the distributor housing is not the best place for a coil to stay cool.

3) You have a high voltage/current coil in the same housing as the ignitor and the low voltage cam/crank sensors. Honda actually had to design a work-around of sorts so the ECU keeps plugging away even when it loses the cam signal. You find this out when you replace the stock ECU with a standalone and you can't get it to sync. Replace the distributor with a brand new OEM piece and the engine fires right up. Better yet, convert to distributor-less ignition and you'll increase the reliability of your Honda significantly.

When I looked into the various options I had for converting to distributor-less ignition, the D17A coil on plug was an obvious consideration. Unfortunately I had read numerous posts/articles that the stock D17A coils were quite weak for anything above stock power. It's not that Honda isn't capable of making a good ignition. In fact, I ended up using Honda CBR coils. But just like any other manufacturer, Honda has cost considerations when making their cars. Everyone wants to save money.

Converting your ignition doesn't give you a power increase. It can only ALLOW you to make more power. If you rev the engine high, the weak cam signal causes your ignition timing to wander. If you increase the compression or boost the engine it actually blows out the weak spark. The vast majority of Honda "tuners" wont benefit from beefing up their ignition. I did it for reliability, emissions, mpg, smoother idle, and because it made sense for my application (if you have a standalone you might as well make the most of it).
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Old 08-27-2008, 08:03 AM   #33
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Oh yeah, and lean mixtures can cause you to burn up an exhaust valve. As you lean out from stoich your cylinder charge burns slower and as you get leaner and leaner the charge isn't done burning until the piston is well into the power stroke. When you lean to a certain point, EGT skyrockets because the charge is still burning as it leaves the cylinder which is the point at which you do engine damage.

Usually, you feel when it starts doing this because the engine loses power as sharply as the EGT rises because of the wasted energy.
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Old 08-27-2008, 08:42 AM   #34
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Originally Posted by dkjones96 View Post
When you lean to a certain point, EGT skyrockets because the charge is still burning as it leaves the cylinder which is the point at which you do engine damage.
dkjones,

What you are saying makes some logical sense, but I wasn't able to find any real world evidence/graphs that showed that EGT's go up as the mix gets leaner. I'm open minded enough to look at any that anyone else could find.

One of the better graphs that I found was this one (below), its shows EGT's going down as the mix gets leaner. Yes, it does show that some EGT's are maxed out as you go a little lean, but they go down as the mix gets leaner and leaner. This graph might support the idea that guys running a little lean by using an EFIE or O2 sensor extender would have the most to worry about- not those of us that are going super lean.




Think of all of the partly clogged fuel injectors out there on MPFI engines spraying 70-80% of what they should be spraying resulting in super lean mixes on one or two cylinders. It seems like we should be seeing lots of burned valves/melted pistons these days if lean mixtures were the culprits.

IMHO, I think that it's detonation kills pistons/valves etc. The connection between detonation running lean is that detonation can result from running lean at low rpms under high load conditions- thankfully knock sensors do a good job of helping to control detonation by helping the ECU to know when to retard ignition timing.
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Old 08-27-2008, 09:43 AM   #35
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Actually, you'd know it if you were damagingly lean, the engine starts to feel like it is misfiring and/or just really low on power. The misfiring could be from not enough ignition energy or the mixture at the point of spark isn't enough to spark, but we are talking REALLY leaning out the engine.

I wouldn't think knock would be a problem when you are going lean, the readiness of the mixture to even ignite at all is lower and when it does ignite it's slower which is why you need a timing advancement when going leaner. I'd personally think you would have a bigger problem with knock around 12.5:1 where maximum power is to be had and the mixture is not only happy to light but once it does it burns very quickly.
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