Rev Limiter/ MEFI 3 computer question
10-05-2007, 07:00 AM
#21
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Thread Starter
The failure was due to extreme detonation. Caused by either a true lean condition or by the actual injector/s getting overheated and not being able to deliver enough fuel.
I am going from 38 lb/hr injectors to 63 lb/hr injectors so I will eliminate the possibility of over taxing the injectors. That is why I need to know about what controls the rev limiter on the MEFI3. I don't want this to happen again. I was definitely hard against the limiter on this motor as I was trying to get the other motor to rev up as much as possible. I let it sit at the rev limit because I always believed that the MEFI3 would pull out spark and not really hurt anything like it does on the old Thunderbolt ignition systems. Obviously I was wrong !!!
Thanks for the info on this thread. I will be calling soon to order software because I will need to retune when I get new shortblocks built.
Thanks,
Smitty
I am going from 38 lb/hr injectors to 63 lb/hr injectors so I will eliminate the possibility of over taxing the injectors. That is why I need to know about what controls the rev limiter on the MEFI3. I don't want this to happen again. I was definitely hard against the limiter on this motor as I was trying to get the other motor to rev up as much as possible. I let it sit at the rev limit because I always believed that the MEFI3 would pull out spark and not really hurt anything like it does on the old Thunderbolt ignition systems. Obviously I was wrong !!!
Thanks for the info on this thread. I will be calling soon to order software because I will need to retune when I get new shortblocks built.
Thanks,
Smitty
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Want your ECU tuned right?? Call Mark at Precision www.pmefi.com
10-06-2007, 06:11 PM
#22
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Location: Fresno, CA, 93722, USA
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Thanks Smitty, we are at:
www.mefiburn.com
or the link is also posted above by Jason Smith and Ben Bowman. Smitty, were you or your engine builder able to examine the piston you lost to see what the cause of the failure might have been?
Ben and Jason, thank you for the kind words. Yes that screen shot looks almost identical to yours, but believe it or not was on a MEFI 3 equipped 350 small block rolling on a dynojet, and not your Whipple supercharged 496's screaming along at 95mph!
Excellent call RichardCranium572! The x201A low order bit switch / flag would appear to have the capability of setting the controller to do spark only rev limiting. I do not know how this works in practice because I do not have a good log of it in action, and I do not know the mechanism or algorithm GM might have used to implement it. Some MSD spark setups have soft rev limiting where it depletes spark on a rotating basis so it is gentle on the engine. I will try and set up a test with a MEFI 3 in the next few weeks where I set the rev limits with and without this flag to see the effect on a running engine.
I have gone through the tune library that I have compiled so far, and for the MEFI 3's it appears the factory settings are to use fuel cut off as the mechanism for rev limiting. Most all appeared to be normally aspirated engines. I also had tunes that were boosted applications that were set for spark only rev limiting with that switch / flag set, and some boosted applications that were set for fuel cutoff. There didn't seem to be any consistency in which method was used.
Armed with this information I talked to a number of well known engine builders and tuners who use MEFI's extensively. Again, I didn't get a good consensus on whether one method was better than the other.
Cobra Marty, yes, you can manipulate the tables to do exactly that. Now which is the best method to implement this (or if it is a good idea at all after talking to a few of the builders) I can not tell you because I do not know how the algorithms in the MEFI3 controller make use of this data. By pulling a few degrees out (such as 8-10) or taking the timing all the way down to 0 or -20 degrees, will this cause the engine to backfire and do damage, or will it cause the engine to just laydown and put out no power? Again, I did not get a consensus among the engine builders and tuners I spoke with on this issue.
Ideally the MEFI 3 flag for spark only rev limiting would do some type of soft rev limiting like some of the MSD controllers do where it is gentle on the engine.
I do not have a definitive answer at this point on what is the best way to rev limit on the MEFI 3. I will try and gather some test data on a MEFI 3 in the next few weeks, I have to do a MEFI 3 tune in a car, so I will experiment. This will hopefully shed some more light on the subject.
Maybe Dustin Whipple or someone else with a lot of experience in this area can chime in....
www.mefiburn.com
or the link is also posted above by Jason Smith and Ben Bowman. Smitty, were you or your engine builder able to examine the piston you lost to see what the cause of the failure might have been?
Ben and Jason, thank you for the kind words. Yes that screen shot looks almost identical to yours, but believe it or not was on a MEFI 3 equipped 350 small block rolling on a dynojet, and not your Whipple supercharged 496's screaming along at 95mph!
Excellent call RichardCranium572! The x201A low order bit switch / flag would appear to have the capability of setting the controller to do spark only rev limiting. I do not know how this works in practice because I do not have a good log of it in action, and I do not know the mechanism or algorithm GM might have used to implement it. Some MSD spark setups have soft rev limiting where it depletes spark on a rotating basis so it is gentle on the engine. I will try and set up a test with a MEFI 3 in the next few weeks where I set the rev limits with and without this flag to see the effect on a running engine.
I have gone through the tune library that I have compiled so far, and for the MEFI 3's it appears the factory settings are to use fuel cut off as the mechanism for rev limiting. Most all appeared to be normally aspirated engines. I also had tunes that were boosted applications that were set for spark only rev limiting with that switch / flag set, and some boosted applications that were set for fuel cutoff. There didn't seem to be any consistency in which method was used.
Armed with this information I talked to a number of well known engine builders and tuners who use MEFI's extensively. Again, I didn't get a good consensus on whether one method was better than the other.
Cobra Marty, yes, you can manipulate the tables to do exactly that. Now which is the best method to implement this (or if it is a good idea at all after talking to a few of the builders) I can not tell you because I do not know how the algorithms in the MEFI3 controller make use of this data. By pulling a few degrees out (such as 8-10) or taking the timing all the way down to 0 or -20 degrees, will this cause the engine to backfire and do damage, or will it cause the engine to just laydown and put out no power? Again, I did not get a consensus among the engine builders and tuners I spoke with on this issue.
Ideally the MEFI 3 flag for spark only rev limiting would do some type of soft rev limiting like some of the MSD controllers do where it is gentle on the engine.
I do not have a definitive answer at this point on what is the best way to rev limit on the MEFI 3. I will try and gather some test data on a MEFI 3 in the next few weeks, I have to do a MEFI 3 tune in a car, so I will experiment. This will hopefully shed some more light on the subject.
Maybe Dustin Whipple or someone else with a lot of experience in this area can chime in....
One thing I would like to add, since most don't address this. For knock control, it's very important to richen the cylinder or motor during detonation, in most cases, this will help control it. But when you max out your injector, like so many other aftermarket SC systems that use FMU's, you have zero control of the injector @ WOT if the motor detonates. Therefore you need approx. 5% minimum, ideally 10% of injector range above your targeted HP.
Dustin
Gold Member
