Pro Comp Electronics Stack Injection

Pro Comp Electronics Stack Injection

I purchased a Pro Comp stack injection system for a Small Block Chevy engine this past November from a distributor in California called Straight Line Performance in Norco California, it was advertised as being usable with a speed density ECU of TPI variant FAST, Holley, or equivalent. It was also advertised as having a vacuum plenum (which would be necessary to run a map sensor.)

I received my system (intake, throttle bodies, fuel rails, and some assorted plumbing) as advertised. I was curious to see if the throttle bodies could be swapped side for side to put the injectors to the inside as Straight Line Performance said they probably could. That was to no avail as the fuel rails would have hit each other and the electrical connections would have also. While the throttle bodies were off I took pictures of the remaining manifold and adapters.

The entire system is made in Australia and shipped here through distributors. The intake was originally designed for IDA style Weber carburation and then adapters were machined to accept this Australian throttle body. As I inspected the system further I observed there was NO vacuum plenum or for that matter no fittings or open bungs for vacuum whatsoever. I contacted Straight Line and asked why the Small Block Chevy system had no vacuum system. He assured me it was there and had 3/8” hole in the runners. With the throttle bodies off and a runner length of only 2 inches any holes would have been readily visible but weren’t there.

I was bound and determined to use this system based on the latest TPI Chevy computer # 1227730 (speed density) with 24lb injectors, with a dedicated custom stand alone harness and a custom burned prom. To accomplish this goal I needed a vacuum source, so I modified the runners to come together underneath the intake and exit on top of the manifold in the rear. After close to 4 hours work this was accomplished.

I installed the entire system and proceeded to fire the unit. After 4 different proms we finally came to a combination that didn’t run so rich, but the main problem was that the system made very little vacuum (10 inches.), right on the borderline for the ability to run a speed density computer. To get it to idle and run at all I had to apply an external vacuum (mighty vac) @ 18inches.

While the system was acquiring operating temperature everything seemed ok, but it was when it went into closed loop things fell apart. The system started to pop back through the throttle bodies in random patterns, it was also popping through the exhaust. Any additional blipping of the throttle would cause additional backfiring. The man I had do the electronic setup has done 100’s of these things commercially and he was as baffled as I was. First we tried another Map sensor, then kicked down the fuel pressure to lean it out (we finally ended up @ 35psi). We replace the computer and 4 custom burned chips. Each chip got successively better but still not actually drivable.

My electronics guy thought it might be that the FI trigger wire (to negative coil tower) might be getting interference from surrounding electronics and suggested I install a MSD 6 AL to acquire a cleaner square wave signal for the FI trigger. After trying that it was to no avail, the system just flat out won’t work with a speed density system. I even looked up the Pro Comp Electronics catalog and they showed a small block Ford intake with throttle bodies and said that a Small Block Chevy system was available. They also claimed that the Chevy system has a vacuum plenum….it doesn’t! The Pro Comp catalog shows a complete turn-key system but doesn’t say if it is a speed density system or if it’s an Alpha-N system (which would make far more sense.)

Alpha-N takes the vacuum out of the equation entirely because it makes decisions based on TPS, ignition pulse, and oxygen sensor input. You might think, “what’s the big deal just get the Alpha-N computer.” The difference is the price…..a factory style TPI speed density computer can be had with prom for about $300……..The Alpha-N systems start at about $1700!

Initially I thought, “How can anyone build an electronic fuel injection manifold with no vacuum ports?” The answer is right there if you pay attention, the manifold was originally designed for carburation (Weber IDA’s.) With a stick shift transmission there would be no need for vacuum. Obviously the transformation to electronic fuel injection was not very well thought out as ANY speed density system requires a map sensor be hooked up to the engine vacuum, and this one has none!

Fortunately my electronic guy has been here and done that before. He was doubtful from the start that a stack injection system could run off a speed density system. They are notoriously low vacuum producers. Thank goodness he has done low buck Alpha-N stack injection systems before. He has worked with the now obsolete Accel Gen 6 ECU before on a retrofitted Hilborn stack setup. The Gen 6 ECU has an Alpha–N capability at a low price, so we found the computer, altered the harness, programmed the new ECU and reinstalled the system.

Today was the turning point, the car acts 100% different with the Alpha-N tune in the Accel Gen 6 box. Idles great and has the crispness that one expects with electronic fuel injection. Jim's Performance had the knowledge to fix a problem caused by a manufacturer that didn’t do the required research and development to properly sort out their finished product. I blame both Pro-Comp (who to this day never returned my phone calls), and Straight Line Performance in Norco California that denied my claim that the intake manifold didn’t have and vacuum provisions (they told me I didn’t know where to look!)

Conclusion:

Would I ever buy another product from Pro-Comp Electronics? The obvious answer is NO! Would I buy anything from the distributor known as Straight Line Performance? This ranks and even bigger NO! The owner was very condescending after he had all my money and ignored any and all my concerns. I now know how to “fix” this poorly executed system, but there are other systems that are designed better and perform better. Maybe the Small Block Ford stack system they make is better and actually HAS a vacuum plenum, but I know the Small Block Chevy system does NOT! Be forewarned the manufacturer and the distributor BOTH confirmed to me that this system would run with a speed density computer…….IT WILL NOT! VERY DISSATISFIED!