On "Microsquirting" the Porsche 914 - Part 3, The Design

The Design

Return to Part 2

Recall my design parameters so far:

• Use the stock intake manifold(s) and throttle body;
• Use stock injectors;
• Use stock fuel pump and pressure regulator;
• Replace D-Jet components only when it makes sense;
• Upgrade fuel injection only, ignition to follow later;
• Bolt-on wherever possible so others can install it;
• Should not require permanent mods to stock components (so it can be reverted);
• Price-sensitive -- keep as inexpensive as possible.

I quickly learned early on I had one conflict: the D-Jet system uses "low impedance injectors" and the Microsquirt system needs "high impendance injectors". I'll lead you to this link if you want to learn the difference but it basically comes down to electrical resistance. I could use the D-Jet injectors if I added a resistor pack -- which is exactly what VW/Porsche did when they used similar injectors on the L-Jet system for the 1.8L 914 engine. 

FiveO High Impedance Injectors

I was mulling over what to do when I learned of high impedance injectors available from FiveO Motorsport that appeared like they'd bolt right into my D-Jet rails. Physically, they looked the same. Electrically, they'd work with the Microsquirt without a resistor pack.

"Use stock injectors". That was for cost considerations. But also recall one of my other parameters was to be sensible about my choices. FiveO sells their injectors for $77 each and occasionally has a 15% off coupon so let's call it around $300 shipped for a set of four. On the other hand, I had full intention of sending my D-Jet injectors out for cleaning/testing and that would have cost a minium of $125 shipped round-trip. And that assumes that they would not find a bad one (a used replacement would be another $75-150 each, and it would still need to be cleaned/tested.) Plus, I'd need a resistor pack to use the D-Jet low impedance injectors with the Microsquirt (at an unknown cost). 

LS2 Individual Coilpacks

So cost of cleaning plus if just one injector tested bad, plus a resistor pack...and I was approaching the $300 cost for new injectors.  Thus first design parameter overridden: I'm going with the FiveO injectors.

Soon after, yet another design parameter was quickly overridden: using the stock disty. During the initial design phase I thought I'd use individual coil packs (ICPs) for ignition, which would add a few hundred dollars to the design along with its associated wiring and tuning complexity. However, I was looking through DubShops' products and I realized that I didn't need ICPs, that I could use the compact IGN4-VW integrated coil/ignitor as used on the 2002 VW Jetta (among others). Even better, that coil/ignitor was available anywhere and could cost as low as $25 from a Friendly Local Auto Parts Store (FLAPS). I realized that along with a new set of plug wires - something I was going to replace anyway - it would not cost much more to replace the distributor ignition with the IGN4-VW, and I could probably sell that functioning D-Jet distributor for more than those costs. Remove the disty, remove the drive gear and washer, plug the hole (DubShop sells them), make a bracket, mount the coil, and wire it up. Microsquirt drives it just fine. Done.

So I had my basic design parameters of stock induction components with afternarket ECU, sensor, and injectors. A fully-tunable system that uses parts available from any FLAPS.

It was time to start buying parts.

The Sensors

I didn't have the coin to toss everything on the credit card at one time, plus I wasn't that convinced my design had legs. So I decided to start with buying the sensors and seeing how well they fit on the engine.

The Dub Shop's Crank Position Sensor

First item on the agenda, and the kingpin piece around which the whole design revolves (har de har): the crank position sensor (CPS). For the automotive-agnostic, the engine's computer needs a reference point to indicate where the crankshaft is within its rotation. Using that reference, it will know the proper time to fire the injectors and spark plugs. It is the key to the whole package; without it, nothing else will work.

Fortunately, The Dub Shop has developed a solution for the VW/914 Type 4 engine, a CPS that incorporates a Hall Effect magnetic sensor mounted behind the crank pulley. It has 36 teeth (well almost, keep reading) and every time one of the teeth pass by the sensor it registers a small voltage change. The frequency of this change correlates to RPM and can be counted.

The sharp ones among you may ask, "ok, since they're all the same then how does the ECU know where the crankshaft is in its rotation?" Good question! The eagle-eyed among you will notice in that photo that one of the 36 teeth is missing; in the photo above it's down toward the bottom, around the 5 o'clock position. By having one tooth missing from the "36 teeth" the CPS will mark that position as Top Dead Center (TDC) of the engine, the firing position of the #1 spark plug. With that open slot, and regular counting for the rest of the rotation, the ECU knows where the crankshaft is. DubShop's CPS was the first item placed in the shopping cart.

Modified "Coolant" Temperature Sensor

Other items ordered at the same time were sensors that I had to find mounting space for on the engine: manifold pressure sensor (MPS), throttle position sensor (TPS), intake air temp sensor (IAT), modified "coolant" temp sensor (CLT). I also had an expiring coupon from Summit Racing so I used it for an Innovate wideband controller * (also know as "O2 Sensor", measures the air-fuel ratio - AFR - of the exhaust and is used for tuning.) By buying just the required sensors, it minimized the initial outlay of cash while giving me a chance to see if I can mount the components in usable locations.

I was perusing 914 World forums and found that, coincidentally, two other guys were having the same thoughts. EricP started a thread about his Microsquirt build with similar parameters as mine. And one other guy was doing a Megasquirt. I contacted Eric directly and he and I have been having very helpful conversations on our designs; he's certainly been extremely helpful to mine.

I should mention that by this time I'd already removed the drivetrain from the street car. The engine was mounted on the stand, induction and cooling tin fully removed, and I was cleaning things up. Lots of oil leaks (it's an air-cooled VW engine, natch) so I was working on those: gaskets, crank seals both end, pushrod tubes and seals, etc. Plenty of things to keep me occupied. I soon found myself with the "As Long As Syndrome", or "as long as I'm doing this, I may as well do that." "That" became having all the engine sheet metal blasted and powdercoated. I brought everything over to Shoreline Coatings in Branford CT. They did the parts for the race engine a year or so ago, so it was a no-brainer for my street car. Very, very pleased with their work.

But of course, the "As Long As Syndrome" tends to push things back a bit, as it's never resolved immediately, so I focused on cleaning more stuff and working on notes for the general EFI design...and waited for more parts to arrive.

On to Part 4: "The Sensors - cont'd"

*3/28/22 Update: I did not have good service with the Innovate, it seemed to eat sensors. Rumor has it their controller keeps things too hot. Since then I have moved to using a 14poin7 Spartan 2 with excellent results.

Further, I have switched to a 1 Bar GM MAP sensor; since this will never be forced inuction there was no need for a 3Bar since this gives me better graduation over the full MAP and voltage range.

TDC - Top Dead Center, the firing position for #1 cylinder
CPS - Crank Position Sensor, also known as CAS, Crank Angle Sensor
TPS - Throttle Position Sensor
MPS - Manifold Pressure Sensor
IAT - Intake air Temperature sensor
CLT - Coolant temp sensor
AFR - Air-Fuel Ratio
FLAPS - Friendly Local Auto Parts Store