How to make your motor run right...

[matt]

For some strange reason, this point never seems to sink in, although I've posted about it about a thousand times.

Our motors (US motors) use 3.5bar fuel pressure. Changing the fuel pressure from this setting (up or down) will NOT effect your motor positively. In fact, if you add in enough fuel to make any difference, you are probably going to make your motor run like crap and cost you a ton of power.

So... if you have an adjustable fuel pressure regulator, either sell it or set it to 3.5 bar. The issue here is that by adding more fuel globally (idle, part throttle, and full throttle) you will confuse the ECU into thinking you have a mechanical issue with the motor and it will attempt to pull the additional fuel back out based on oxygen sensor feedback. Once you have changed the fuel pressure enough to actually change the air fuel ratio, that means you have maxed out the adaptation on the lean side... the computer has pulled out as much fuel as it possibly can.

On the other hand, using the software, we can add fuel at WOT and high part throttle where it will not be adapted back out, while leaving your cruise stoich so that your gas mileage is good cruising around town.

Here are a few of my posts from the archives on adaptation and why it's your friend.

If you have a working O2 sensor, it won't stay lean for long. It will adapt back to stoich. So, I would say you have an issue with either your narrow band O2 sensor or your meter is wrong. No/bad O2 sensor can cause all sorts of running issues. More importantly, it will prevent your motor from fixing its own issues by learning around problems (like vacuum leaks or non-stock fuel pressure)

Don't think of these ECUs as having "open loop" or "closed loop" modes. It doesn't work that way. At WOT, the ECU is ignoring the O2 sensor, yes... but it is STILL USING the adaptation data it obtained at idle and part throttle. Adaptation data is collected at idle and light load but applied globally. And it is not always richer, in fact I would say that most of the time the O2 sensor is actually adding fuel globally.

So... "Open loop" AKA no O2 sensor does NOT mean richer than usual. Richer than usual does NOT mean more power (leaner is usually more power).

Adaptation is a big deal. It can be the difference between a car that runs well and a car that doesn't. The ECU runs its "stock" mixture all the time (provided the MAF is working, and it has backup tables if the MAF fails) but it uses the O2 sensor feedback to fine tune the mix all the time, also.

The purpose of adaptation is to let the motor learn around issues that come up like vacuum leaks, fuel pressure or injector problems, and the programming of the chip not exactly matching the performance of the motor. So, it does the learning at part throttle and idle (when the target is ALWAYS 14.5-14.7) then it applies the adaptation globally (because WOT fueling will be effected by vac leaks or fuel system issues also).

In his case, the adaptation is not pulling fuel out... it's just not adding it back in. But I don't think that's the real issue. M50 motors will idle happily up to 17:1, they just fall on their faces under load if they stay that lean.

Bottom line...

Stop using adjustable fuel pressure regulators on DOHC motors.

[Ray Smoodiver]

I'm probably going to listen to Stu Mc on this one.

[techno550]

I'm probably going to listen to Stu Mc on this one.

who? If he said the same thing, he's right. if he said anything else, he's wrong.

[SpecM]

I'm probably going to listen to Stu Mc on this one.

-What does he say?

^Stuart McHenry

[nando]

for the record, this applies to M20s with motronic 1.3 as well. The only reason I kept my adjustable FPR is for the pretty gauge. :p

[backtrail69]

I run a stock 3 bar FPR on my turbo car... I may get a 3.5 or 4 bar someday if I get closer to maxing the 60# injectors, but from the current power output... I doubt it.

[PiercedE30]

It's also somewhat difficult to find an aftermarket FPR that is non-adjustable that will accept anything from -4 to -8.

[matt]

Whats wrong with the stock one? Nobody here needs anything other than the stock fuel system. Until your're pushing 600whp, you don't need anything other than the stock fuel system (with an upgraded pump over 350whp or so).

Most aftermarket FPRs are fine... but if you're using a stock ECU, set the base pressure at 51psi or 3.5 bar.

[shopkeep]

I know you are directing this one at me matt! lol. Thanks for this info though..i didn't really know the effects of running more pressure than needed.

I knew originally that I should have it at 3.5. I just wasn't too concerned about adjusting it right away, I just dinked around with the regulator a little and didn't bother to set it correctly. I wasn't even in the process of testing the car out around the yard at that point.

The reason I am still using the adjustable one however is that something seems to be wrong with the stock FPR.

[matt]

You're not the only one... if you were, I wouldn't have posted this. And I wasn't going to out you either...

What makes you think you have an issue with your stock fuel pressure regulator?

[Stu]

Matt,

I'm not refuting your knowledge of ECU's and tuning on these motors, because I know your knowledge base is vast, but I've had nothing but great results from using an adjustable FPR on my 3.0L M50.

The motor runs very well at any RPM/throttle angle, makes a lot of power, and consistently gets 30-32 MPG at ~70mph (without hypermiling + no cruise control to use), and has done so for the past 4.5yrs/90,000 miles I've been running the setup.

Unless my motors running situation changes drastically, I'm in no hurry to remove what I've got on it now.

[2002maniac]

Matt,

I'm not refuting your knowledge of ECU's and tuning on these motors, because I know your knowledge base is vast, but I've had nothing but great results from using an adjustable FPR on my 3.0L M50.

The motor runs very well at any RPM/throttle angle, makes a lot of power, and consistently gets 30-32 MPG at ~70mph (without hypermiling + no cruise control to use), and has done so for the past 4.5yrs/90,000 miles I've been running the setup.

Unless my motors running situation changes drastically, I'm in no hurry to remove what I've got on it now.

oh snap.

[nando]

Matt,

I'm not refuting your knowledge of ECU's and tuning on these motors, because I know your knowledge base is vast, but I've had nothing but great results from using an adjustable FPR on my 3.0L M50.

The motor runs very well at any RPM/throttle angle, makes a lot of power, and consistently gets 30-32 MPG at ~70mph (without hypermiling + no cruise control to use), and has done so for the past 4.5yrs/90,000 miles I've been running the setup.

Unless my motors running situation changes drastically, I'm in no hurry to remove what I've got on it now.

when you first started your motor you increased the FP right? higher fuel pressure won't neccesarily hurt anything - and it will probably help the engine start with a "safe" AFR until Motronic catches up, but eventually Motronic is just going to change the fuel tables back to the targets programmed from the factory, regardless of what you set your FP to. higher FP has other benefits of course (fuel atomization, etc).

Matt's point was some people are increasing the FP beyond the point that Motronic is capable of adapting to - basically, they are clipping the maximum range of the VE table, forcing it to run much richer than it was designed for. In that case the real solution is a tune specific for your engine, rather than forcing the engine to run too rich throughout the whole map, which will cause the poor idle, fuel consumption, etc. that he spoke of.

I doubt that is what you did Stu - you probably increased FP to 3.8-4.0 bar and left it alone. Motronic is probably still capable of dealing with that. But if you left your FP at 3.5 bar, it would have eventually arrived at the same VE table it's using now anyway.

The other danger is the fuel injectors you are using - at higher pressures, the opening times will need to be smaller. Since Motronic 1.3 BMW has used high impedance injectors, which have fairly long opening times (1.0-1.2ms). I don't know what kind of PW stock M50 injectors run at idle, but if you increased pressure high enough you could end up with injector pulsewidths too close to their opening time. Their operation becomes non-linear at that point, much like running them at 100% duty cycle - this will result in very poor driveability, unstable idle, and all sorts of other problems.

This is usually a problem with much larger injectors, and I doubt it would affect stock M50/S50 injectors, but it's something to keep in mind. I run 42# injectors on my M20 with 3 bar of pressure - idle PW is about 2ms, and it works fine. Opening time is 1.13ms so I'm still safe, but if my idle drops to 1.2-1.3ms, my AFRs jump all over from 19:1-12:1 and it's impossible to tune. If I wanted longer PW at idle, I could drop fuel pressure - something I've thought about experimenting with, actually.

[matt]

Matt,

I'm not refuting your knowledge of ECU's and tuning on these motors, because I know your knowledge base is vast, but I've had nothing but great results from using an adjustable FPR on my 3.0L M50.

The motor runs very well at any RPM/throttle angle, makes a lot of power, and consistently gets 30-32 MPG at ~70mph (without hypermiling + no cruise control to use), and has done so for the past 4.5yrs/90,000 miles I've been running the setup.

Unless my motors running situation changes drastically, I'm in no hurry to remove what I've got on it now.

How do you know it wouldn't run just as well under all circumstances with stock fuel pressure?

It may run crappy at first, but when it adapts back to where you are now, it should be exactly the same. Unless you're past the adaptation limits of the ECU, in which case you have a chip problem (or some other mechanical issue, like a vacuum leak), not a fuel pressure problem.

And Nando is on point with his post also. I don't really have anything else to add.

[Stu]

How do you know it wouldn't run just as well under all circumstances with stock fuel pressure?

It may run crappy at first, but when it adapts back to where you are now, it should be exactly the same. Unless you're past the adaptation limits of the ECU, in which case you have a chip problem (or some other mechanical issue, like a vacuum leak), not a fuel pressure problem.

And Nando is on point with his post also. I don't really have anything else to add.

That's sort of what I was getting at with my previous post--I don't know how it would run under those circumstances, but with the stellar results I'm getting right now, why change what has been a great setup? Excellent drivability, very potent at WOT, and great fuel economy to boot.

I'm not disagreeing with you, nor am I saying that my setup is better than the stock 52psi regulator...just sharing my results and why I'm in no rush to change anything.