legacycentral bbs

so i went down to the local shop today and was talking to them about injectors. when i told them they would most likely be using 800cc injectors they got a little woried.

so my question is, what is the exact cc per min fuel flow of a 816 cc injector at legacy fuel pressure. that and whats the flow of a standard sti injector 550cc i belive, at legacy fuel presure.

lastly will the 550 cc standard sti yellow be able to support 300hp or 20+ psi in boost on 91 octane

At what pressure was the 816cc/min number measured?

Flow is roughly proportional to the square root of pressure.

So if it flows 816cc/min at 3 bars (which I believe is what most Japanese imports, including new age Subarus, use these days), then at our stock 2.5 bars it'd flow (2.5bar/3bar)^0.5*816cc/min=745cc/min.

And similarly, an injector which flows 550cc/min at 3 bars would flow (2.5bar/3bar)^0.5*550cc/min=502cc/min at 2.5 bars.

A turbo motor usually has a brake specific fuel consumption of around 0.55 lb/hr/hp -- meaning it takes about 0.55 pounds of fuel to produce 1 horsepower for 1 hour. 1 pound per hour of fuel is about 10.5 cc/min. And you don't want to run a duty cycle higher than about 85%. So, let's do some simple math again:

0.55lb/hr/hp * 300hp * 10.5 (cc/min)/(lb/hr) / 85% / 4 injectors = 509 cc/min

So you'd want a minimum of 509cc/min injectors to run 300hp. Probably a bit more, since you're using relatively lower octane fuel and a lot of boost.

Another point -- Subaru put 550cc/min injectors on a car that made 300hp from the factory. Doesn't that mean 550cc/min should handle 300hp?

vrg; that final equation on there is a litte.. borked?
or I'm just doing it wrong I guess.


(.55 * 300 * 10.5) * .85 / 4 = 368.15625

Divide, not multiply, by 0.85.

vrg3 is the master, i got my sti 300hp injectors in my ej22t.

well then. now that thats cleared up, if i have thoes modified so they flow 745cc's at 0 duty cycle will my car be able to idle properly or will it be running rich as a pig

All injectors flow nothing a 0 duty cycle. Do you mean 100% duty cycle?

Are you honestly asking me if the stock ECU can idle properly with injectors that flow twice as much as stock?

No, it can't.

i meant to hit 1 but the basic question is if it can idle at the least possible duty cycle. when i told my friend at the shop i was planing on using 800cc's he said it might be too rich to idle even with the maxium amount of tuning

Jason's talking about the pulse width resolution of the ECU. If the resolution isn't fine enough, then the minimum possible injector pulse width from the ECU will result in the injector spraying way too much fuel at idle. This is assuming the ECU would automatically try and turn down idle pulse width to the minimum.

Anyway, I believe the consensus is, the ECU can not learn what to do with those injectors, especially at idle.

I wonder, what would the ECU end up doing if you replaced the injectors with 2x higher flow injectors, and then replaced the MAF with another sensor that reads twice as much range, then wouldn't you be pretty close with the ECU's base maps....? Maybe just a bit of timing adjustment after that?

I don't know what our stock ECU's pulse width resolution is exactly, but I don't think that even matters unless we figure out how to reprogram the ECU for these injectors.

If the engine runs at all, the ECU can trim back pulse widths by as much as -20% based on feedback from the oxygen sensor. However, with as much fuel as these injectors would flow, a) that would still be way too much, and b) it could very well cause lots of misfires which the oxygen sensor would read as a lean condition, and c) a fuel trim railed at -20% may well throw a trouble code and throw the ECU into open loop.

So, yes, I don't expect the ECU would be able to learn to drive 745cc/min injectors. I haven't tried it myself, though, so if you'd like to try it anyway, please do report back, Jason.

You could try matching a bigger MAF sensor to bigger injectors... In many cases you'd end up with a lot more ignition advance than you ought to have, though... and you'd need to somehow bias things towards running rich because the ECU wouldn't know to do its on-boost-enrichment thing at the right time.

With the right aftermarket ECU, you should be able to achieve a steady idle with 745cc/min injectors. But it might be easier to tune that ECU with smaller injectors and a rising rate fuel pressure regulator.

I'm going to throw my plug in here for engine managment.

DON'T SKIMP ON ENGINE MANAGEMENT!!

You're stock ECU/engine management will not work the way it's supposed to and give you the flexibility it was originally designed to.

Legacy777 wrote:I'm going to throw my plug in here for engine managment.

DON'T SKIMP ON ENGINE MANAGEMENT!!

You're stock ECU/engine management will not work the way it's supposed to and give you the flexibility it was originally designed to.

That seems to be the trend these days, people want to turn up the boost, add bigger MAF sensors and injectors, then expect to get by with the stock engine management; or cheat with a FCD.

I myself won't be putting the turbocharger on my car till I've got the Link ECU in my hands and wired into the car - cheap insurance I say.

I'm not saying I'd actually try that Just curious how well it'd work. It seems as though all you'd need to complete this rough combination is timing control, and only in the retarded direction (Timmy!)

I'd expect such a combination to be very undrivable under partial throttle / normal driving conditions. But it may make for a quick inexpensive race-only vehicle, if nothing else it'd be an interesting experiment. For those of us who like to blow stuff up...

I'm sticking to reasonable boost levels and stock injectors/control myself until I've got tuning capabilities and a wideband O2.

well about putting in bigger injectors in and trying it, heres how it feels with sti injectors.

idle is very rich and every 1000rpm up to 5000rpm is very rich and then when you hit 6000rpm the car boggss out in every gear. when your boost is not on and your takeing it easy it burns more even gas becuse its not getting used up. you need a thick wallet and gas every block.

550 cc injectors should be more than you need unless your planning on running something like a schwitzer turbo. I have 550 cc injectors with my big turbo and they run at -33% duty cycle and the car is still running rich.

sorry about what seems like some misunderstandings. im gonna be tunning with a perfect power for now but im afraid that it wont be able to handle the modded sti injectors that run at 7xx cc. i was also planing on upgrading to a sz49 sometime in the near future but if dan is running 550s on that monster of a thing at 33% duty cycle than i think ill be good for now.

Yes, it's quite possible that even with a piggyback the stock ECU won't be able to maintain driveability with injectors that large. I think you get to be the first to find out.