O-Gauge Steamer
NAXJA Forum User
- Location
- Colorado Springs, Colorado
OK, here is the place for discussion on limiters.
Rev Limiters
- Thread starter O-Gauge Steamer
- Start date
cruiser54
NAXJA Forum User
- Location
- Prescott, Az
LOL. Great idea.
- Location
- Torrance, CA
A fuel cut based rev limiter has been in use for decades on force induction engines and I even hit fuel cut on my Starion at close to 3 bar. According to the log 37.5 PSI is where the ECU cut fuel and logged 0 knock events as a result. So why didn't my engine "Lean out" and detonate?
I know why, but I want to see if anyone else knows why.
I know why, but I want to see if anyone else knows why.
Last edited:
Alexia
NAXJA Forum User
- Location
- Huntsville, AL
The fuel cut so there was no more fuel to detonate seems like the obvious answer. Yes, an injector could have just started squirting during a fuel cut, but hopefully the PCM is not dumb enough to cut fuel in the middle of a squirt. Fuel cuts after a complete squirt with that cylinder firing fine and the rest just get air to do nothing with. Assuming the PCM is smart like that then it makes sense to cut fuel only and finish any injections it was in the middle of doing.
cruiser54
NAXJA Forum User
- Location
- Prescott, Az
Makes perfect sense to me..
- Location
- Torrance, CA
Even if it cuts fuel in the middle of an injection cycle, one lean combustion, even if it raises the cylinder temps, the next stroke is going to have no fuel injected so even then there is no chance of knock. :repair:
O-Gauge Steamer
NAXJA Forum User
- Location
- Colorado Springs, Colorado
Of course it did not detonate. No fuel, no burn. It is intuitively obvious to the average idiot that if the fuel is removed, the engine stops firing.
Period.
What I need is an aftermarket limiter. As of now, I am using the AEM F/IC8's "overboost" control function to cut the fuel. Functionally, this is the same as sidewaysstarion's ECU. See the overpressure, cut the fuel.
What is actually needed is a cut off that is determined by the engine speed as I have no intention of running my compressor past it's "safe to operate in" zone. I would like to be able to cut the fuel at or about 4,856 rpm.
Problem, for me, is that all of them I have looked at cut the ignition. IMO, about the worst thing you can do as it floods unburnt fuel into the exhaust. Once the ignition comes back, you get an explosion in the exhaust. Nevermind the fact that the Cat is going to be damaged by being exposed to raw fuel. The entire system is stressed.
I have tested the functionality of the F/IC8 control circuit and it works exactly as advertised. Which is an issue for me as the boost is full on way before the engine redline as it is.
If anyone out there has had direct experience with a fuel based limiter, please post up the product name.
Back when Carburetors were King, the only way to rev limit was to cut the ignition. Back then, it was considered OK to light a fire in the exhaust as there were no other choices. Cutting the fuel pump off would not work as the Carb would run on the fuel in the float bowl. And, in the overwhelming majority of cases, the fuel pump was mechanical driven off of the camshaft by an eccentric lobe. Unless it had been replaced by an electric.
Insofar as that goes, my Brother installed spark plugs in the tail pipes of one of his Hot Rods and would dump fuel into the exhaust just to burn it off for the flame effect. Talk about backing off a Tailgater...
You install the plug about 10 before the outlet and the fuel jet another 2" further upstream from the plug. Trick is getting the fuel right which is why you need replaceable jets...
Not advocating anything here, just talking Hot Rod History.
- Location
- Torrance, CA
Reflash the ECU to have a 4,856 rpm limiter, problem solved. 
O-Gauge Steamer
NAXJA Forum User
- Location
- Colorado Springs, Colorado
^^Good point. I should look into it.
I do understand that there are now more than one place to get the work done. The downside is that you have to ship the PCM to the shop to get it done. As I understand it, the reflash can not be done through the OBDII port.
Too bad. I would buy the electronics needed if they were available. I have several variables I would like to diddle with as it turns out.
Given that the transmission (AW4) has a WOT shift point of between 4500 and 4800 rpm (really need to look that one up...) cutting the red line down should not adversely affect the transmission.
Maybe someone should start a thread on PCM reflashing. There must be more than one of us that needs to get it done. Plus, that would prevent this thread from drifting off topic.
As threads have a habit of doing...
I do understand that there are now more than one place to get the work done. The downside is that you have to ship the PCM to the shop to get it done. As I understand it, the reflash can not be done through the OBDII port.
Too bad. I would buy the electronics needed if they were available. I have several variables I would like to diddle with as it turns out.
Given that the transmission (AW4) has a WOT shift point of between 4500 and 4800 rpm (really need to look that one up...) cutting the red line down should not adversely affect the transmission.
Maybe someone should start a thread on PCM reflashing. There must be more than one of us that needs to get it done. Plus, that would prevent this thread from drifting off topic.
As threads have a habit of doing...
- Location
- Torrance, CA
I've seen his work... I'm less than impressed. He does all of his tuning through the SCT tuning package which is very lacking in what it has access to.
CobraMarty
NAXJA Forum User
- Location
- PM me any questions
This.Reflash the ECU to have a 4,856 rpm limiter, problem solved.
Seems like an easy solution to a simple problem.
MoparManiac
NAXJA Forum User
- Location
- Amherst, NH
This all makes sense and I have no argument, but cutting fuel just doesn't sit right with me :laugh:
O-Gauge Steamer
NAXJA Forum User
- Location
- Colorado Springs, Colorado
Given that we run a dry manifold (i.e. no fuel in the air stream) cutting the fuel is actually the preferred method.
Back in the mists if time when I was working on Aircraft (the EC-121 Super Constellation series) The way the engines were shut down was to move the mixture levers to fuel off then, after the props had completely stopped turning, shut off the magnetos killing the ignition. This was to ensure all fuel was out of the engine.
The only time cutting the fuel can create an issue is if the manifold is wet. It will most certainly detonate then.
The Factory PCM uses fuel cutoff as the rev limiter as does every other Factory PCM/ECU out there. For starters, emission control regulations prohibit the discharge of raw fuel into the environment and it is a pretty good way to run a catalytic converter.
Useless Trivia that you can feel free to skip...
Starting the engines (R-3350 18 cylinder dual row Radial Engine, two speed internal supercharger with three exhaust driven power recovery turbines aka a "turbo-compound") of the Connie involved turning the engine, with the fuel and ignition off, for 6 blades of rotation, both magnetos were then switched on and the mixture lever advanced until the engine caught and ran. Once the engine was running, the mags would be tested for operation by shutting one off at a time and noting the rpm drop. Dual mags and dual plugs.
Why the 6 blades of rotation? To make sure all of the cylinders were clear of oil. The bottom cylinders had a habit of collecting oil and hydrolocking. It is quite a sight to see a cylinder blow off of an engine... 6 blades is 2 revolutions as the prop is a roughly 3:1 reduction from crank.
If this isn't nearly useless trivia, what is?
Here is another.
Radial engines always have an odd number of cylinders. Be it 3, 5, 7 or, 9 in number.
The 18 cylinder R-3350 (3,700hp) is a two row of 9 with the rear bank rotated to allow air flow. The R-4360 is a four row, 28 cylinder radial, each bank rotated for air flow. I believe it was the largest ever built. In various configurations, it was able to develop up to 4,300hp with the use of twin turbochargers force feeding the internal supercharger... Just think, 56 spark plugs to change during a tune up. 600 hours run time between engine replacement.
The number after the "R"? It is the displacement of the engine in cubic inches.
Back in the mists if time when I was working on Aircraft (the EC-121 Super Constellation series) The way the engines were shut down was to move the mixture levers to fuel off then, after the props had completely stopped turning, shut off the magnetos killing the ignition. This was to ensure all fuel was out of the engine.
The only time cutting the fuel can create an issue is if the manifold is wet. It will most certainly detonate then.
The Factory PCM uses fuel cutoff as the rev limiter as does every other Factory PCM/ECU out there. For starters, emission control regulations prohibit the discharge of raw fuel into the environment and it is a pretty good way to run a catalytic converter.
Useless Trivia that you can feel free to skip...
Starting the engines (R-3350 18 cylinder dual row Radial Engine, two speed internal supercharger with three exhaust driven power recovery turbines aka a "turbo-compound") of the Connie involved turning the engine, with the fuel and ignition off, for 6 blades of rotation, both magnetos were then switched on and the mixture lever advanced until the engine caught and ran. Once the engine was running, the mags would be tested for operation by shutting one off at a time and noting the rpm drop. Dual mags and dual plugs.
Why the 6 blades of rotation? To make sure all of the cylinders were clear of oil. The bottom cylinders had a habit of collecting oil and hydrolocking. It is quite a sight to see a cylinder blow off of an engine... 6 blades is 2 revolutions as the prop is a roughly 3:1 reduction from crank.
If this isn't nearly useless trivia, what is?
Here is another.
Radial engines always have an odd number of cylinders. Be it 3, 5, 7 or, 9 in number.
The 18 cylinder R-3350 (3,700hp) is a two row of 9 with the rear bank rotated to allow air flow. The R-4360 is a four row, 28 cylinder radial, each bank rotated for air flow. I believe it was the largest ever built. In various configurations, it was able to develop up to 4,300hp with the use of twin turbochargers force feeding the internal supercharger... Just think, 56 spark plugs to change during a tune up. 600 hours run time between engine replacement.
The number after the "R"? It is the displacement of the engine in cubic inches.
MoparManiac
NAXJA Forum User
- Location
- Amherst, NH
That's probably why it doesn't sit well with me then. I run a six pack-TBI setup so my manifold is wet.
O-Gauge Steamer
NAXJA Forum User
- Location
- Colorado Springs, Colorado
^^And there you are.
Running a wet manifold adds a layer of complexity to the issue. Most folk cut the ignition and live with the subsequent explosion as there really isn't an alternative.
Pertronix, MSD, MicroDynamics, Mallory, Auto Meter, and even Crane Cams make them.. The Crane Cams system uses, if memory serves, a soft limit to help reduce the back fire issue.
Could be wrong...
Running a wet manifold adds a layer of complexity to the issue. Most folk cut the ignition and live with the subsequent explosion as there really isn't an alternative.
Pertronix, MSD, MicroDynamics, Mallory, Auto Meter, and even Crane Cams make them.. The Crane Cams system uses, if memory serves, a soft limit to help reduce the back fire issue.
Could be wrong...
