Fuel injection, rev limiting methods
 

Here's a question for you gurus:
Besides cutting fuel, what methods are employed to limit revs in fuel injection systems? I've heard that just cutting the fuel can lead to leaning out the mixture with bad consequences. Pointers to sources of information on this topic would be appreciated.
I'm in the very early stages of "hacking" the code in the Motronic DME in my '86 Carrera.
thanks,
Chris

Keywords: Motronic, DME, 8051, EFI

I believe that most modern forms of rev limiting work by cutting spark, not fuel. On my twin-plug MSD setup, for example, one set of plugs cuts out at 7,300 rpm and the second set at 7,500, giving a relatively soft limit.

Stephan

Thanks Stephan. Cutting one plug of a twin plug would seem to have it's own problems i.e. detonation. MSD must know what they are doing though.
-Chris

I consider the no-spark method to be the older of the two (early 911s, before fuel injection used to use rev limiting distributor rotors). I don't care for the idea of pouring fuel into cylinders and then not lighting it on fire.

Cutting fuel in a FI car results in a spark but no fuel to ignite. I like this idea better. Cutting fuel pressure in a FI car causes the engine to quit making power just as quickly as the no-spark solution. It is IMMEDIATE.

OK contestants, you all proceed to Round Two. :D
Here are some ideas for comment:
- If we cut the spark we get an immediate halt to combustion and unburned fuel goes out the tailpipe. Is this bad for cat?.
- If we cut the fuel, gas still intake tract gets sucked in and lit off.
Can the temporary lean mixture cause detonation?
- Wouldn't "hard" limiters like these be hard on the engine and
possibly upset traction/chassis if you're in a corner when it
happens.
- How's a "soft" rev limiter work?

TIA,
Chris

The ignition rotor on my 2.7 has a centrifical rev limiter built in.

Kurt Williams

Acording to the bosch fi manual for the DME, they cut back the pulse times to the injetors to conrol the rev limit. This book explians how the dme funntions.
Darren

Darren,
You you be more specific on the book you're talking about? It sounds like something I'd be interested in getting. All I have are the factory Porsche manuals and the Probst (sp?) book.
thanks,
Chris

Chris, thr name of thr book is called, Bosch fuel injection and engine mangment by Charles O Probst,SAE, I bought it from amazon.com for $19.95 i think, also I think wayne has it too.
Also there is a software program that runs off a pc to a eprom programer that I found on the net, that was made to program dme chips. I have it somewere, I will look for it.
Darren

Hi Chris

This might not help but on the CIS cars when the rev-limit is reached the power to the fuel pump is cut. The motor dies imidiatly (sp) if not sooner. It does not run on the fumes.
Dean

Thanks, I have that book. I guess I need to read it again! I know of two DME editing tools out there.
Bonneville Motor Works has one that they advertise on their web site (unchanged since 1998 I think) but they didn't return my email or calls (a few years ago) so I don't think they are interested in selling it to Do-It-Yourselfers anymore.
Promotive has one also - for $1000.

Do you think it was one of these?

Chris

Chris, the program I found was the Bonevelle motor works . Is there a site for the promotive software?
Darren

Darren,
Here is the link for Protomotive:
http://www.net-quest.com/~knighton/index.html
and their software:
http://www.net-quest.com/~knighton/software.htm

In the interest of completeness (as my professors used to say)
here is the link for Bonneville Motor Werks:
http://www.bonnevillemotorwerks.com/index.html
and their software:
http://www.bonnevillemotorwerks.com/dmeedit.html

The Protomotive one is DOS based and the Bonneville one is Windows based.
Chris

I agree that there are two ways; Cutting the spark and cutting the fuel. My understanding is that the preferred method nowadays is to (briefly) cut the spark. This is what was originally done for F1's traction control.

1) Cutting fuel results in a lean mixture and the absolute cut-off of the motor. Picture yourself making the big pass on a straight, holding the gear for max rev's ... and someone turns the motor off. The scenario gets even worse if you are in some sort of a corner.:eek: Nothing will happen until the engine rev's drop to the preset restart level. This in turn depends on how fast the car is coasting, if the clutch has been pushed in, the flywheel, etc. It also makes for a very bumpy ride as the motor kicks off and on again. I know because the Mazda I used to race had a fuel cut-off on the rev limiter.

2) Cutting spark is usually done on an intermittent basis, generally by randomly dropping sparks to different cylinders. (As opposed to dropping the spark to one plug in a twin plug head. I can only imagine this being a bad thing) The result feels like a car where the valves are floating, the engine starts to sound rough and basically stops pulling hard. Not a lot of fuel is pumped out the exhaust since the next time around that cylinder will fire and burn off a lot of the unburnt fuel.

No one is talking much about the newer systems in F1, but they don't make the engines sound anywhere near as rough as in years past. Given "Fly by wire", they may be playing around with throttle, intake trumpets, ignition timing, etc to basically limit the engine's HP at the rev. limit.

Thanks John. Excellent info!
-Chris

Ok now I am confused. I had to rebuild my 78 sc due to an over rev. This was NOT a mechanical over rev. Long story short is a buried the tach needle for a moment and bent 6 valves and damaged a cylinder.

When I pulled the distributor cap I saw my rotor had a rev limter which I assumed had failed. I planned on buying a new rotor so I would have a functioning rev limiter.

Someone mentioned that cis car cut the power to the fuel pump. Mine apparently didn't know this and continue to let me rev away. Where is the device located that cuts the power to the pump and how would I go about t/s the problem. How exactly does the rev limiter work in a 911sc

The car is a euro if it maters

There is no CIS cutoff on an SC.
There IS a fuel pump cutoff for a CIS 930.

The SC uses the distributor to limit revs.

A few comments about igniton rev limiters. I have found that most igniton limters that drop out the igniton, in time end beating the hell out of the rod bearings. The unloading and loading on the rods beat the bearings. Cuting off the fuel seems to be a better way. Everyone taks about running lean, but when you cut the fuel off,there is no fuel, just spark, you can't burn gas that is not in the chamber.
Darren

There are two distinctly different methods used on various SC engines, i.e. rev-limiter rotor, and fuel pump cut-off relay, and the spec books give all the specific details.

Soft rev-limiters of the electronic type cut spark to cykinders in stages, first step ... down two cylinders, second step ... down2 more cylinders, etc.

The only logical method of doing it with a DME system for the 911 would be to cut spark in three stages, always selecting opposing cylinders 1 & 4, 2 & 5, and 3 & 6 to preserve the smoothest running characteristics. If you wanted to make use of the injectors to cut fuel, it would be best to cut-off the injector bank one after stage two of the ignition cut ... when four cylinders aren't getting spark.

The 911 Bosch Motronic system fires the injectors in two banks simultaneously, so you really don't have much option ... all or three injectors. Rewiring the injector banks would optimize control and smoothness, if, for example, injectors 1, 4, & 5 were grouped, and 2, 3, & 6, likewise, you could cut injectors 1, 4, & 5 for fuel emmisions purposes simultaneously with stage two spark cuts, leaving only two running cylinders, but one injector output for cyl. #2 would be unburnt.

Darren; I'm confused -- how will shutting off the fuel not beat on the bearings as much as shutting off the spark???:confused: I'd think the result will be the same. If you ask me, if you are spending that much time on the rev limiter, you're doing something very wrong or you need different gears.

osidak, I admit that I don't know the details, but it sounds like the damage that you did was the result of an over-rev from either a missed shift of a botched downshift. Rev limiters won't help a motor with that situation. It's also not clear how often you had been using the rev limiter. I've heard that SC's don't like to be rev'd but I can't imagine that the motor wouldn't survive running at the redline or 500 over. I'm sure that Porsche margined themselves when they built the motor. It sounds like an interesting situation. but I don't understand why you feel that the rev limiter did you in?

Darren,
Killing the spark compeletely would seem to beat stuff up like you said.
Thinking out loud, when you cut off fuel, the injectors stop sending fuel of course but AFIK there is still a fuel "cloud" behind the valve and coming off the intake port walls that gets sucked in. That would result in a lean mixture if only for a little while.

Chris

I have never hit the rev limiter in my p car to clarify. I come from a very wide range of engine building history. And I will refer to MSD boxes for droping out the spark. These boxes limit the revs buy droping cylinders like warren had said in a few post ago, when you hit the limiter, the engines back fires from all the gases built up in the cylinders, imagine if the cylinder runs through its cylce a few times and then sparks, you have a few cycles of fuel in the chamber, creating a hydralic conditon, putting stress on the bearings.
You cut the fuel and it stops making power, you drop rpms, the fuel comes back on , and it fires as normal.
The lean conditon might be true for a split second, but you would have to run it lean for more than a few seconds to burn a valve.Maybe I am talking a bunch of crap, but just sharing some of my experiences.
Darren

Seems to me that a rev limiter that shuts off the fuel is no different than just lifting off at high RPM.
The cylinders are still sucking, the plugs still sparking, but the gas volume is way down.

But the air volume is still way up.
-Chris

To answer a question a few posts ago. The rev limiter on my 81SC is located under the hood behind the master cyl. It is about 2"x1"x1" black box with maybe a 5 wire connector on it. It works in conjunction with the tach and it opens the circut to the fuel pump.
Dean

Darren;
I've got an MSD with a 7000 RPM rev limit in my car and I've hit it a couple of times. (By the way, this is less then the 7200 RPM factory limiter in the distributor) It doesn't backfire, it just starts to run a little rough - like I said, it feels like the valves are floating and stops pulling. The MSD Soft-Touch rev limiter is far more gentle then the factory limiter which essentially turns the motor off.
I also used to race a Mazda 323 (SCCA-ITB) which had the factory fuel shut-off rev limiter. I only had to hit that once in a race to know that I didn't want to do that again because you can definitely feel the car "bouncing" against the rev limiter.

Having used and experienced all 3 versions, the Soft-Touch spark cutting strategy is the method that I prefer.

Many thanks to all of you!
-Chris