Fuel System

I spent a lot of time deciding on the best location for the AFM for the simplest and most efficient plumbing and settled on the passenger side of the engine bay, similar to how it is mounted in the Porsche 931. The Porche 931 AFM is downdraft as opposed to updraft and is ideal for a turbo application when mounted above the turbo because the outlet can be aimed directly at the inlet of the turbo. This reduces lag and improves throttle response. I had to make 3 brackets and clearance the wheel well to mount the AFM here.

The biggest problem to solve was how I was going to run fuel to the other side of the engine bay. CIS uses bango fittings that are difficult to find, metric threads which aren't compatible with commonly available hardware, and injector lines are small enough diameter that making your own with scavenged hardware is difficult (and expensive). To get fuel to the distributor, I cut the stock steel lines, bubble flared them and ran high pressure injection hose with standard hose clamps. I then scavenged the standard CIS hardware to make complete my custom lines on the distributor side.

Volvo 240 Turbo WUR Rebuild and Modification

Fuel System Upgrades

In an attempt to correct my lean condition I started by installing a fat hoss 911 turbo fuel pump (the Bosch 984 motor sport pump) which is capable of fueling up to 450 hp in a turbo application. The best part about it is that it was cheaper than the stock pump and just bolts right in. I then cranked system pressure up to 90 psi and still had a lean condition above 5500 rpm. My next step was to buy the fabled Mercedes Benz "gold" injectors (part number 0 437 502 047) . They are the same size as the 2.0L m10 injectors, which is exactly 12mm longer than the stock 1.8L injector. This means it protrudes into the intake port an additional 12mm. I installed the injector in a spare motor I have lying around to see how bad it would be. To correct this problem, I cut an old injector open to see how much meat was there, and it was obvious there was plenty. The pic shows the stock 1.8 injector, a VW rabbit injector cut in half (same length as the 2.0L injector), and of course the new MB injector. I took measurements, drew up a print and had a new groove cut in at the machine shop, so now the new injectors will fit the same as the shorter 1.8L injector. Unfortunately, the MB injectors coupled with high system pressure still didn't resolve the problem.

The last potential weak link was my air flow meter. When I ported the AFM I mistakenly put a radius on the very bottom edge of the bowl and I thought this must be the cause a my lean condition since I thought I had eliminated all other possibilities. So I bought an unmolested 931 AFM and FD from a board member on 924.org, and after cleaning and inspection I found that even though the castings are the same, the AFM I had been running was NOT off a 931.

I've now determined that it was off of a PVR v6 (the same v6 used in the Delorean), which is why the 6 cyl casting of the volvo 240 turbo FD was able to bolt to it. When put next to the 931 AFM, it was apparent the bowl shapes of the 2 AFM's are VERY different. The sensor plate of the 931 AFM is 76mm compared to 80mm of the Volvo 260 AFM, and the "enrichment" curve at the bottom of the bowl comes in much sooner on the 924 turbo. Here are the 2 bowls side by side, the 931 on the left and the Volvo 260 on the right:

I installed the "new" AFM and FD and the car initially ran terrible. I was getting richer than 10:1 AFR at part throttle. I realized that the Volvo WUR and the way it is plumbed with boost & vacuum lines was completely wrong for the 931 system. At 19" Hg vacuum (at idle), my control pressure would go from 53 psi to 47. Then at part throttle my control pressure was dropping from 47 psi to 30 psi and it would go pig rich. The stock 924 turbo control pressure is ~53 psi, which is what my WUR is set at with no vacuum/boost, so I decided to unplug it to see what happens. What a difference! I now hit 12:1 AFR and it holds that AFR all the way to 7000 rpm.

To recap, the major lessons I've learned here:

1) The Volvo 240 turbo and Porsche 924 turbo FD's/AFM's are NOT interchangeable. The Volvo has the 6 cylinder bolt pattern and the porsche uses the 4 cyl. FD bolt pattern.

2) Make sure you use an AFM that was originally designed for a turbo application. Bowl shapes are different and they WILL have an effect on fuel curve. In my case I've been trying to make the AFM from an NA V6 work on my 4 cyl. turbo the whole time under the false assumption that it was from a 924 turbo.

3) If you use the Volvo setup, you need to use the audi 5000 S or volvo WUR. If you use the Porsche setup, you should also use a Porsche WUR. You will save time in tuning in the long run. They are constructed differently, function differently and are plumbed different. I could go into enough detail on this to start a whole other thread (Which I plan to do).

As a result of all this I was able to turn the boost up to 14 psi and my AFR still holds steady at 12:1-11.9:1 all the way to 7000 rpm. The car is noticeably faster, and it's a glorious feeling to have it just keep pulling past redline instead of falling flat 1000 rpm before redline.

Here are some updated pics of the engine bay with the new fuel distributor. I had to run new injector fuel lines and re-bend one to fit over the center port on the FD. I also sacrificed an old volvo fuel line to make the one going to my pressure gage/WUR. Notice the main infeed fuel line to the fuel distributor is using a reshaped e21 fitting with new hose crimped on. Also notice a rest pressure of over 30 psi after the car had sat for over 30 minutes thanks to new accumulator, check valve and pressure regulator o-rings.

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