Lean LJet 3.5 Liter & Adjustable Fuel Pressure Regultor

[Jan Gullett]

Lean L Jet 3.5 Liter & Adjustable Fuel Pressure Regulator

My original 3.0 liter engine was converted to a 3.5 liter with a sport cam and ported heads by metric mechanic. It has a 1981 USA L Jetronic ignition from a 528i. I have low restriction stainless exhaust. Over the years I have tried a number of fuel pressure regulators trying to get the best balance of high end power with low end OEM idle.

With the OEM set up, at maximum throttle from a standing start through third gear, my engine would lose its power band above 5,000 rpm and seemed to be running lean at high RPM. I found that the rising fuel rate fuel pressure regulator from Bavarian Auto with pressure slightly higher than normal helped the high end but then the low end seemed suboptimal. I had the version that fastened directly on the fuel rail. Eventually the Fuel Pressure Regulator broke at its flare fitting and began leaking fuel.

After some research I read the the OEM fuel pressure was 2.5 bar. I had been running 2.8 bar. After doing some research I found the Saab 3.0 bar unit that is bolt on compatible. This is item Bosch 0 280 160 256 and is common on many older Saabs. The power output with this set up was wonderful. But I can be a bit of a perfectionist, and it seemed to run a rich on idle particulary before the 02 sensor was warmed. After some research and calculation i found that flow is a function of the square of the system pressure and a 3.5 liter should be about 2.8 bar versus 2.5 OEM for the 528i all else being equal. I identified a Saab part fixed at 2.8 bar, but this was Euro only, uncommon and no longer seems to be available anywhere.

I began looking for adjustable fuel pressure regulators, but wanted an OEM look. Bosch has the ideal solution in a unit sold by its Motorsport division (FPR 15-50). But it was 300 euros plus shipping.

I decided to go with the Bosch unit 0280 160 001 as modified by Mallory for vacuum pressure adjustment and sold as MSD 2222. I sourced this for $99. But it does not fit directly to the fuel rail as it has a barb fuel inlet. So I needed a barb adapter to fit the fuel rail's 17mm barrel nut bubble flare fitting. After some research I determined the internal flare thread size was 14M x 1.5. From this I found a couple of adapters. I went with a Parker 68HB-6-M114. To make this work I needed to shave down the barb height a bit as it is over sized for the BMW fuel injection hose. I also honed the bore to an angle to better fit the bubble flare. I also found a two piece adapter at Autozone by searching for 14 x 1.5 fuel injection adapter. It looked less sturdy, but in retrospect would probably have been a quicker install.

I love this new setup. 2.8 bar at a fixed pressure indeed seems perfect. I have a great wide power band to the RPM limit. And the idle is OEM perfect. Bosch's fuel pressure regulators are highly regarded versus aftermarket units and the whole set up was $108, one half to one third the cost of other adjustable solutions and only slightly more than the OEM fuel pressure regulator.

 

[decoupe]

Thanks for sharing this very useful look at the balance of air, fuel and engine components in the pursuit of performance.

I too used the 528i throttle body, intake manifold, fuel rail (one piece pipe), refurbished and balanced 528i injectors and a new 528i fuel pressure regulator. All of this on a mid 80's 535i L block and head with some porting and a 282 cam two piece exhaust (OEM resonator and muffler). Similar to what you run except the MM head and asymmetric cam and low restriction exhaust.

My fuel, ignition and engine management displays the duty cycle for the injectors and at wide open throttle to redline with A/F average 13.0:1 this % never exceeds 75% which I interpret as there isn't enough air coming into the engine to justify either larger injectors, more fuel pressure or more volume. The 528i and the 533i share the same throttle body but not the 535i. Since you are using the 528i throttle body and injectors, I assume that it is the MM head and the exhaust you run that is the difference in air flow through the engine. Simply put your power train flows better.

Your cam and head work combined with the exhaust upgrades flow just enough more volume that the increase in fuel pressure makes a difference at higher rpm. You might even be close to the max output of the 528i injectors - have you considered increasing to larger 535i injectors - no doubt you would be getting into some mapping issues.

Changing to the MSD 2222 regulator would be a pretty cost effective way for me to get a little/some but ultimately I would have to increase the air flow to make it work. So head work (apparently), headers and exhaust, increased cubic capacity or forced induction. Wonder which is the most cost effective?

Doug

 

[Jan Gullett]

Airflow

I do not know the answer to your question, but if you are still exploring this, you might want to know that my 528i injection uses runners from the 320i. This also probably helps flow a bit.

 

[decoupe]

And I have a set of the 320i runners arriving any day. I'll install them before returning to the dyno and see if there is an improvement inherent in the 320i runners.

Doug