Bigger injectors or raise the fuel pressure?

[Jeff N.]

Trying to decide the merits of each approach, working under the assumption that the modified engine is running a bit lean.

I can get the Bavarian Auto rising rate FPR. Stock fuel pressure at idle, faster rising rate with low vacuum. It can also raise the base FP above stock with a corresponding increase in FP based on low manifold vacuum. The adjustable FPR seems to offer the benefits of being "tunable" via base pressure adjustments, getting some (potential) benefit of better spray pattern & atomization via higher FP.

Optionally, I could switch from 19lb injectors (stock size) to 21lb injectors and get a similar flow upgrade (I think) from the slightly larger size.

Calculations suggest raising the FP by about 5 lbs is very similar to running 21lb injectors.

All this of course is on the presumption I'm running a bit lean in spots something I'm still working to verify.

Anyone have any thoughts on what might be the better approach?

thx!

Jeff
90 535i,
Metric Mechanic sport head w/cam

[Martin in Bellevue]

Why not get a mixture gauge to find out whether the lean condition exists? Then, make a decision based on known conditions.

[MikeV]

If you have a voltmeter, you can tap into the O2 sensor output and monitor A/F - just have a friend monitor the voltmeter while you drive through some high-stress situations (like, 4th gear too slow uphill). I don't have charts on me, but you can find plenty of O2 sensor output - A/F ratio maps on the internet. Stock O2 sensors are not real accurate for measuring a wide range of A/F measurements (so you can't really rely on using the reading to calculate how much increased flow you need), but it will tell you if you are running too lean or not.

Just curious, why do you think you're running too lean? Are you experiencing knock-related timing pull-back?

[Jeff N.]

Good questions Mike.

I plan to install an Autometer air/fuel meter to help monitor the mixture. I'm aware of the limitations of this vs. a full wideband setup but plan to start with this. I've thought about the voltmeter and tapping in but I wanted something a little more permanent.

There is no knock sensor on the M30 with Motronic 1.3 so the timing map is static based on what's in the chip.

I have an issue with mid range power in the car. Good power from idle to 3k or so. Flat in 3 to 3.8k range, picks up at 3.8 to 4k and strong to redline. Car has a ported head and cam, stock compression.

Theory that I'm exploring is that the car is starving for fuel in midrange area. Have a friend with a similar setup who had issues like this and raised the FP by 5 lbs and saw good improvement throughout the band. Why the midrange? Well...it appears this is about the area where the AFM flapper goes wide open and the DME switches from a load based fuel map to RPM based map. If the fuel map in that mid range is not matched to the engine requirements, I could be down on HP. The car comes on cam about 3.8 to 4k and goes hard from there. As the maps aren't really "linear", you can't assume that the fuel map increases with the RPM.

Car doesn't knock on a regular basis but I believe I have occasionally heard a little pinging once and a while. Might be gasoline related...or..fuel...or ???

Well..at least this is the theory at the moment.

[Jeff N.]

I seem to recall you upgrading to the 21lb version...

[Dave Rutkowski]

Jeff,

You definitely need to verify whether your engine is running lean. Measuring voltage from the stock O2 sensor isn't real accurate, because it's narrow band and is essentially on or off, no gray in between. Ideally, you want to get the engine running where you want it without the O2 sensor feedback loop active (make your big adjustments without it), so it can focus just on the minor tweaking. You don't want the feedback loop making gross adjustments. On my 635 track car I have a wide band O2 sensor with a readout, so I can see where it's running with some sensitivity.

The adjustable FPR has some advantages. You mentioned tuning. Another is that running higher fuel pressures should give you some benefit with better atomization of the fuel. That's what I've done on my 635, and I generally run about 5-10 psi higher than stock (I don't have a cat) - the euro engine really wants to run rich.

Good luck.
Dave Rutkowski
'90 535i
'81 635CSi

[Jeff N.]

Great input - thanks Dave. Who's FPR did you go with? Seems like BavAuto is the only US firm with one. In the UK, you can buy a "FSE Power valve" which may very well be the same as the BavAuto unit. Korman doesn't show anything on their website but I would suspect they have something.

Any other source? About $200 seems to be the going rate for the part.

I suspect I'll either get a wideband O2 unit or head to the dyno. BTW, who's wideband O2 setup did you select?

Jeff

[Dave Rutkowski]

I think I ended up getting the FPR from BMP; but the one I had to use for the early M90 euro motor is different from the M30 on the e34 due to a different fuel rail. I think it functions the same as the BavAuto one, and it was about $200.

On the O2 sensor, it's a Bosch unit, think it was like $350 for the sensor and display, if I remember right. If interested, I can hook you up with the outfit I got it from (they do a lot of kart and snowmobile race engine tuning, and use this stuff all the time.)

[Bruno]

Good questions Mike.

I plan to install an Autometer air/fuel meter to help monitor the mixture. I'm aware of the limitations of this vs. a full wideband setup but plan to start with this. I've thought about the voltmeter and tapping in but I wanted something a little more permanent.

I didn't see much changes with the MAF, But I also need an air/fuel meter gauge, along with a gauge panel...
I think they all kind of sucks and aren't very accurate. To do a perfect job would be to get a Wide band O2 sensor....

But like you, I will probably pick up an Autometer... I think it is the most accurate of the bunch.

[Bruno]

I seem to recall you upgrading to the 21lb version...

I am using the M3 pink 21lb injectors... Didn't see much change.