Another fun update, this time eBay 720cc RX8 mod injectors!
That is to say, a set of Denso injectors for an RX8 modified to flow 720cc @ 43.5psi static fuel pressure purchased on eBay. For $200.
What could go wrong, right?!
Starting with the the basic question, why? Because I wanted to. Had a couple of hundred extra bucks worth of fun money, decided that doing the Pepsi Challenge with a cheap set of injectors from eBay and sharing my experience would be fun. I went into this knowing full well that I might be investing in complete garbage.
Next question, why these 720cc injectors? My eventual horsepower target on pump-gas puts me at around 700-750cc with a ~60% duty cycle. These particular injectors happen fall into that sweet spot. They're also plug-and-play in a BP, no fitting adapters or harness pig-tails required.
Last question, what did I expect before starting? I expected they'd be junk, would idle rough, wouldn't give stable AFRs, and be a nightmare to tune. I started with the expectation that they wouldn't work as required and that I'd have to bench the car while ordering up a set of ID725s or Deatchwerks 700s.
So what did I actually get for my $200? I got four Denso injectors of some part number I don't recall off the top of my head that were quite clean and inscribed a number from one to four with a fine point Sharpie. That number corresponded to a column on a flow sheet included with the . They also came with fresh seals. For the sake of safety I ordered a second set o seals--spares are good.
The test sheet claimed two flowed bang-on 720cc, one flowed 723, and the last flowed 726. Yes, I was skeptical about these flow claims but went with it because that's part of the Pepsi Challenge.
Installation was simple because injectors. (I mean, it's three bolts and they're all out in a '97 BP.) I won't dig too much into that other than to say make sure that you check the Denso connectors for damage (my number two connector is hosed, being held on the injector by friction alone) and fish any stray injector seat seals out of the holes with a pick.
As a small side note, I installed the two 720s at cylinders one and two, the 723 at cylinder three, and the 726 at cylinder four. I figured it wouldn't hurt having the slightly fatter injectors squirting into the hotter cylinders.
Pull fuel pump relay, relieve fuel system pressure...
Remove fuel rail and injectors, fish injector seat seals out of the holes with a pick...
Lightly lube the new seals and o-rings on the replacement injectors...
Grumble about brittle, cracked, and broken Denso connectors, buy a whole new set of four on eBay to replace them all in a future project.
Once it was all buttoned up I adjusted the required fuel setting on my MegaSquirt for the bigger injectors and cranked it over. Took a little bit for it to get going but once it did things mellowed out into a nice even if kind of rich idle. After warmup I cruised it around for a bit letting TunerStudio work on adjusting the map as it though necessary.
For the most part it worked well, the live analyzer leaned out the fatness in the map pretty well but there was a persistent problem where the injectors would go pig-rich for one event a couple of milliseconds after a quick throttle input. Smooth throttle, no issue. Stabbing the throttle, no bueno. The car would stumble, almost stall, then rocket away as if it'd finally figured out how to make the AFRs work out. I spent a good chunk of time trying to sort out accel enrichment or other potential problem sources with no luck. Maybe this was just the crappy behavior I was expecting to see in these things?
As a last ditch effort, I decided to scrap my tune and start over from a fresh base map. (Long story but I accidentally chose the 94/95 turbo base map but it worked so I went with it.) With the base map setup for the correct displacement and required fuel the car started and idled just fine. I took it around the block again to tune the low end and found none of the tip-in stumbling/super rich condition that I had before. I ran it around for 15 minutes on the live analyzer and it got reasonably good.
That's right. The $200 720cc modified RX8 injector set actually worked and behaved reasonably. I didn't spend a lot of time behind the seat logging and tuning, I was just pleasantly surprised and wanted to end my day on that good note!
This morning (Sunday) I was up early and in the car with an Android tablet plugged into the MS to pull logs. I had zero intention of doing any tuning on my drive, I didn't want the distraction of it. Instead, just drove and logged everything for about a half an hour. Managed to cover almost every target cell in the map. Over breakfast at a diner I poured over the log results which really weren't too bad at all.
Nothing overly lean, only a couple of rich spots down low. No stumbles or hesitations that I could see in any of the logs from this run. (Or feel while thrashing it around.) Fed through MegaLogViewer's analyzer the recommended changes leaned the map out slightly pretty much everywhere. I'm going to go with this recommendation.
I won't spend a lot more time obsessing about this particular tune, it's good enough for now. There's not a lot of point in trying to extract every ounce of NA power I can when I'm going to be going turbo soon. Post turbo installation I'll have it run in on a dunno with a professional tuner manning the helm.
Anyway, the conclusion is...I dunno. Sometimes eBay injectors aren't complete garbage? I don't completely trust them [yet] but proven to work so far. I don't know if I'd go full boost with them but I wouldn't hesitate to run wastegate pressure (7psi) with them on an initial turbo installation.
Even at that, I'll still probably end up with a set of Injector Dynamics ID1050x injectors because sexy modern spray patterns are sexy.
