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Old 04-14-2007, 09:26 AM   #1
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Default Hitting rev limit with WI...

Ok, read over the DO kit instructions.....seems simple enough. My only worry now is the "caution" notice in the adobe instructions to hitting the rev limiter (fuel cutoff). Has anyone hit the rev limiter with the WI on? Is this something to worry about if I set the pressure switch to say 4psi?
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Old 04-14-2007, 09:32 AM   #2
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nope nothing to worry about when you hit the limiter the boost goes down and the pump turns off even on quick decel ie sudden braking during boost ramp up or while at max boost the pump response is pretty quick and no harm should come to your motor they put that there to cover there *** most likely not that it would happen. And even if it did worst plausible scenario i can see is stalling the car and doing a caost restart or just twisting the key to fire it back up. So on worrys man
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Old 04-16-2007, 11:29 AM   #3
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I'd never considered this before, but I can see the logic in it.

I don't know exactly how the rev limiter in the Miata is configured, but consider the following hypothetical:
The design of the Rev Limiter is such that when activated, fuel is cut off but ignition continues to operate. You are running an injection mixture which contains a very high proportion of methanol, such that the mixture can support combustion in the absense of gasoline.
As you hit the rev limiter, the gasoline supply is shut off, but the methanol supply continues. Since the ignition is still active, the engine begins to run completely off the methanol. Unfortunately the relatively small amount of methanol present results in an extremely lean A/FR, and you soon break a piston. You then become pissed off at Chance because he didn't warn you not to run on the rev limiter while under methanol injection.

In actual practice, I have hit the limiter many times while running roughly a 30% by volume meth-to-water ratio, and my engine seems none the worse for it.
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Old 04-16-2007, 11:46 AM   #4
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I was once told it was spark and fuel that is cut. I thought it was only spark, but I guess I was wrong.

I can't see it being fuel only, since that could lead to some really lean conditions.
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Old 04-16-2007, 02:36 PM   #5
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If they cut spark only, but left fuel on, then raw fuel would flow into the exhaust system, through the cat, etc. during a rev limit condition. This would result in a backfire or explosion when the spark came back on.

Spark and fuel seems plausible. Actually, if anybody here has an EMU with their CAS lines connected (mine aren't) we could pretty easily figure this out just by datalogging a rev-limit event.

Quote:
Originally Posted by jayc72 View Post
I was once told it was spark and fuel that is cut. I thought it was only spark, but I guess I was wrong.

I can't see it being fuel only, since that could lead to some really lean conditions.
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Old 04-16-2007, 02:40 PM   #6
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With megasquirt you have a choice of rev limiter type. Fuel&Spark, Fuel only, Spark only.
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Old 04-17-2007, 08:19 AM   #7
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If you had pure meth in their I would watch your azz with a revlimiter...or more you should watch your ringlands
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Old 05-12-2007, 04:08 PM   #8
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Spark-only rev limiters must sound sick.
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Old 06-15-2007, 02:24 AM   #9
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Quote:
Originally Posted by Savington View Post
Spark-only rev limiters must sound sick.
They do. If you ever get a chance to watch a D1GP show live, check out the Bee*R skyline. It's ******* amazing.
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Old 06-15-2007, 04:08 AM   #10
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Quote:
Originally Posted by jayc72 View Post
I was once told it was spark and fuel that is cut. I thought it was only spark, but I guess I was wrong.

I can't see it being fuel only, since that could lead to some really lean conditions.
I've always thought it was only fuel that was cut. The spark continues but since there is no fuel nothing burns. Even if there is a tiny bit of fuel (so I have been told) it is not enough to give what you would think of as a "lean" condition (i.e. no damage is done).
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Old 06-16-2007, 05:58 AM   #11
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Quote:
Originally Posted by rmcelwee View Post
I've always thought it was only fuel that was cut. The spark continues but since there is no fuel nothing burns. Even if there is a tiny bit of fuel (so I have been told) it is not enough to give what you would think of as a "lean" condition (i.e. no damage is done).

Looking at some EMU datalogs i agree, my AFR's go past 20 but the ignition seems to remain constant.



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Old 06-16-2007, 12:36 PM   #12
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Hit the rev limit today in 3rd....no probs. I have the 6-12psi pressure switch though, so I'm starting to spray at 6psi.
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Old 07-31-2007, 01:38 PM   #13
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So the simple answer seems to be: if you're using Megasquirt or another standalone - select Spark&Fuel for rev limit just to be safe. For others, be cautious but not overly so?
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