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View Full Version : My Miatas rev-limiter is gone!! Overclocked ECU



ctrench
12-22-2013, 08:21 PM
Well i noticed today, while doing a 1-3 pull, that i have no rev limiter, or at least nothing up to 8k... i thought it was wierd, so afterward, i revved the motor up so that it would bounce off revlimiter and it never hit, it just kept going, so i let off before it went past 8k

i have a 93 with the stock ecu, and im not sure why its doing this, and was interested to see if anyone has heard of this.

MazRoadAdmin
12-22-2013, 11:02 PM
If that's the ECU you bought from me, it may be modded. In fact... it's probably the one out of my car, and it IS modded.

theothersawyer
12-22-2013, 11:46 PM
My brother in laws old miata did this too!

MazRoadAdmin
12-23-2013, 12:13 AM
If the ECU has been modded, the typical mod is a crystal replacement. This overclocks the ECU so that it runs faster and essentially "sees" the events that it is monitoring as being slower than they actually are.

This one is most likely modded from an 8 MHz to 9 MHz, an increase of 12.5%. The way it effects the car is that it raises the rev limit by 12.5%, and also stretches the timing and fuel maps. It doesn't really decrease fuel by 12.5%, though. What it does is delivers the fuel at XXXX actual rpm that it is programmed to deliver at an rpm 12.5% less.

So, the factory rev limit is actually 6900 rpm. The tach lies. It's quite optimistic, showing that you can wrap a stock 1.6 all the way to somewhere between 7200 and 7500... but the limit is actually 6900. (look at anybody's stock dyno plot... stops at 6900)

If I'm right, this ECU has a 9MHz crystal, which raises that rev limit by 12.5% to 7762 rpm. Now, if your tach is showing you at least 7200 for an actual 6900, it stands to reason that it will show over 8000 for an actual 7762. And I remember having that ECU in my car. I never EVER hit the rev limiter! It would zing all the way up to an indicated 8000... and dammit, that's enough! I never pushed it further than that. But, if you kept going, you'd either float valves, break something, or hit the rev limit. It IS there.

FYI, what it's doing to fuel and timing... say you're at 3000 rpm. The ECU sees that as 2625 rpm. So, it delivers timing advance and fuel pulsewidth as if you were at 2625. It sounds bad, but when you're cruising around town, you're in closed loop and O2 feedback is correcting the fuel mixture. The actual fuel map only comes into play when the engine is cold and when you're at WOT. When you're at WOT, a stock Miata is PIG RICH. Like 11.5:1 rich. I dynoed my '96 with a modded ECU (10%) and found that while it was leaner, it was still "safely rich" enough for NA. (13's and less)

Much more info about this mod here (http://forum.miata.net/vb/showthread.php?t=145076) on some other forum.

treeafodo
12-23-2013, 09:45 AM
^Pretty cool info.

Wow, an admin who is actually involved in a forum, AND knows his stuff. I like that. Most forums I'm on the admin is no where to be found.

etikoner
12-23-2013, 10:56 AM
I once shifted from third gear full throttle into second gear by accident and saw the tach over rev hard :(

What I'm trying to say is don't rev a stock engine too high, or it may cause damage lol..

kung fu jesus
12-23-2013, 11:27 AM
Loren, that post was amazeballs!

MazRoadAdmin
12-23-2013, 06:50 PM
Heh. I don't really follow this forum that much. Just happened to catch this thread at the top of the new post list.

But, thanks! :)

ctrench
12-23-2013, 08:59 PM
THANKS LOREN! that probably it, cause i took that ecu and gave my buddy my ecu so he could get around town, yeah it definetly scared me when it happened, i thought my nitrous f'ed something up.

but it still pulled hard all the way up to 8k, though it makes me skeptical about auto-xing the car now because i know before i hit limiter a couple times each auto-x

Phatmiata
01-16-2014, 01:57 PM
Loren, that post was amazeballs!


http://farm8.staticflickr.com/7152/6710329835_4b290550f9_o.jpg

I<3flippyheadlights
01-17-2014, 06:53 AM
You bored today, Randy?

SM16RMSM
01-17-2014, 08:35 AM
He must be.

BoBo
01-17-2014, 03:47 PM
Well i noticed today, while doing a 1-3 pull, that i have no rev limiter, or at least nothing up to 8k... i thought it was wierd, so afterward, i revved the motor up so that it would bounce off revlimiter and it never hit, it just kept going, so i let off before it went past 8k

i have a 93 with the stock ecu, and im not sure why its doing this, and was interested to see if anyone has heard of this.

Yeah, it's a crystal mod. My mr2 and my old miata came with one, After a few months I finally got a Megasquirt and a real tune. I can't wait to get one for my 96.

adam777
01-19-2014, 08:51 PM
Your car did not rev that high. The Miata tach is wildly inaccurate at high RPMs. If you were to attach an external tach and do the same thing again, you'd see the revs are actually where they should be. I have an overclocked ECU (for now, anyhow) with a 10% increase. It shows well past where it should be on the car, but hooked up to an external tack (at the dyno and also when tuning on a PC), it was, as I said, right where it should be. Don't sweat it and keep driving it.

ibid
01-20-2014, 12:58 PM
this is a 'trick' some cheater spec miata drivers do too for that extra edge (my ecu's have always been stock though)