Re: got an 02 wrx last week

**zax** · 08-29-2012 12:20 PM

Originally Posted by MainFrame

If you take a stock car and change nothing but the injectors and properly scale them in the map, then how is it going to use any more fuel? I guarantee you I can switch a car from 440cc injectors to 1200cc injectors and it will have (basically) identical AFR. So if you are not using any more fuel, then how is there greater demand on flow from the pump?

Now if you install a larger turbo, you're going to need more fuel.

You're idealizing the fuel pump! The pump has an efficiency range. It will only pressurize the fuel to a certain degree. If it cannot provide the continual flux that is demanded by the injectors, you will not achieve proper injector flow. Back to my river and dam reference... the fuel pump capability also represents the HEIGHT of the dam and the pressure potential. The pressure potential is the difference in the fuel pressure of the pressurized fuel and the unpressurized fuel when the system has reached equilibrium. This is what is controlled by the FPR. The larger capacity fuel pump has a greater capability for producing a static pressure potential with a given input current. Like a turbo, the fuel pump has an efficiency range. If you exceed the pump efficiency, you will sheer the fluid [being pumped] and create a cavitation event. At this point, the pump is being overdriven and will impede the flow of fuel into the injectors. I am NOT arguing that this will necessarily happen with the stock Fuel pump and 1200cc injectors, but you can see my point if you were to go to the extremes of this example.... take a 5000cc/min injector and a 120LPH fuel pump as an extreme example. My point, Robin, is that you must match the fuel pump to the injectors regardless of the IDC to a certain degree.

**MainFrame** · 08-29-2012 12:21 PM

Originally Posted by Heide264

Overall fuel usage will be the same assuming you can get it idling correct and you have enough resolution to do so. Normally you aren't so lucky with this.

I've ran stock injectors, 565cc, 800cc, 1300cc, and now 1250cc. There was never any trouble getting it idling with any of them. As far as dispersion, most tuners I've talked to would rather have ID injectors than stock.

I never said you should run injectors 10 times larger than you need, just that it's better to go on the larger side. For one, if you're constantly running over 90% IDC you're going to damage your injectors. Secondly, if you have a boost control malfunction and accidentally overboost suddenly, you're going to want to have spare injector to be able to keep fueling the engine, otherwise it'll go extremely lean and damage the engine.

**MainFrame** · 08-29-2012 12:23 PM

Originally Posted by zax

My point, Robin, is that you must match the fuel pump to the injectors regardless of the IDC to a certain degree.

Well duh!

I wouldn't run injectors that are going to be under 60% max IDC.

**Heide264** · 08-29-2012 12:27 PM

Originally Posted by MainFrame

If there were a river and a dam.. you haven't changed turbos, so there should be no increase in demand for fuel, hence the stream down river stays the same. I think we can agree that if the pump is large enough for the turbo, this means the river up stream from the dam is larger than the stream down river.

Now if you're letting through the exact same amount of water, why does it matter what size the hole is? If the flow down stream from the dam has not increased, how could the river up stream go dry? It still has the same amount of water in it as before.. If it's going dry, but there's no more water going down stream, where does the rest of that water go?

It's similar to comparing displacement to distance traveled. Ones a path integral, ones an overall delta.

Why does a 45 CFM capable turbo at 15psi provides so much more power than a 15 CFM capable turbo at 15psi? Outside of any efficiency changes and temperature effects, it still remains that 15psi is not just 15psi. When a valve opens quickly, and a cylinder sucks air in, a small 'bubble' of low pressure (e.g. vacuum) is formed. Right there, your cylinder is no longer benefiting much from the pressure upstream even 6" or 3". The air that is right in the combustion chamber isn't under the positive manifold pressure until the dynamics shake out... which is substantial at 5,000RPM per say. Providing the additional CFM allows the turbo to fill the low pressure 'bubbles' much better at the moment a valve opens or a cylinder pulls in a glob of air.

Say you wave your hand through a tank of water quickly - you'll notice a low pressure 'bubble' behind it pulling your hand back. Its a similar issue. Fluids aren't ideal - there are standing waves all through there, and the pressure isn't even throughout a given body of fluid. Fuel or air - take your pick.

**Heide264** · 08-29-2012 12:30 PM

Originally Posted by MainFrame

I've ran stock injectors, 565cc, 800cc, 1300cc, and now 1250cc. There was never any trouble getting it idling with any of them. As far as dispersion, most tuners I've talked to would rather have ID injectors than stock.

I never said you should run injectors 10 times larger than you need, just that it's better to go on the larger side. For one, if you're constantly running over 90% IDC you're going to damage your injectors. Secondly, if you have a boost control malfunction and accidentally overboost suddenly, you're going to want to have spare injector to be able to keep fueling the engine, otherwise it'll go extremely lean and damage the engine.

I think in Subarus we are fairly lucky with this. When I said normally, I wasn't being careful with my words. Generally every piece of literature I've seen this is stressed heavily. And I agree, you do want to aim for 80% max, 90 is pushing it.

**MainFrame** · 08-29-2012 12:44 PM

Even from the factory they go oversized on the injectors, otherwise you wouldn't be able to get any gains from going stage two without swapping injectors.

It's also my opinion you should always go oversized on the fuel pump as well.. but to quell the arguments, I'm not saying everyone should run a fuel pump off of a f-16 fighter jet. (I still don't know who makes 5000cc Subaru injectors)

**zax** · 08-29-2012 12:56 PM

Originally Posted by Heide264

Say you wave your hand through a tank of water quickly - you'll notice a low pressure 'bubble' behind it pulling your hand back. Its a similar issue. Fluids aren't ideal - there are standing waves all through there, and the pressure isn't even throughout a given body of fluid. Fuel or air - take your pick.

Originally Posted by zax

you will sheer the fluid [being pumped] and create a cavitation event..

Rev. Mod. Phys. 74, 425 (2002): Single-bubble sonoluminescence

**Heide264** · 08-29-2012 01:14 PM

Originally Posted by MainFrame

Even from the factory they go oversized on the injectors, otherwise you wouldn't be able to get any gains from going stage two without swapping injectors.

It's also my opinion you should always go oversized on the fuel pump as well.. but to quell the arguments, I'm not saying everyone should run a fuel pump off of a f-16 fighter jet. (I still don't know who makes 5000cc Subaru injectors)

As long as your regulator can bleed off enough volume of fuel, there is nothing wrong with going with an overly large pump in my opinion. I think it's a good idea on cars like ours that have smaller rails and a fully 'sequential' fuel system.

I think we are on the same page, I was just playing devil's advocate so people can visualize the issue a bit better. I never used to think of the fuel system much until I read some non-Subaru oriented stuff... Then realized many of the issues that they have on a large scale... we still have (albeit minor) and are commonly overlooked. There isn't a line that says "yup, it's oversized"... its just a game to get the most appropriately (whatever the definition) sized ones you can for the given job.

Side note - 5000ccs would be crazy haha. I did see some absurdly large ones such as 2000cc or maybe 3000cc on sale though. Older literature I've found suggests adding extra injectors beyond much lower flows than I expected - sometimes as low as 1000cc or 1400ccs. I think injector tolerances are much tighter now, which helps a lot. Interesting concept to get the best of both worlds if you have the ECU to control it.

**MainFrame** · 08-29-2012 02:31 PM

Originally Posted by zax

Rev. Mod. Phys. 74, 425 (2002): Single-bubble sonoluminescence

Would it really create cavitation though? I can understand if the fuel in the rail was under zero pressure and was being sucked into the cylinder, but in this case it's the other way around. Although I suppose it's the same either way, it's just going from higher pressure zone to lower pressure zone. But wouldn't the difference in pressure be greater at the pump itself, creating a cavitation event there?

I get the argument about decreased resolution, but is this even a real problem? I mean, if it was such an issue, then why did my car run fine on 1250cc injectors? How are there so many 500+whp cars out there that don't have any problems idling or under low load? The fact of the matter is in the real world larger injectors work just fine.. if everyone with ID1000s were having these theoretical problems you are presenting why does no one ever complain about it?

And I stand by my opinion that you're always better off going larger with injectors. Sure, you can get by for some period of time running injectors at 100% IDC, but you would still be better off with larger than simply "sufficient" injectors that would only max out at 60-80% IDC.

**Heide264** · 08-29-2012 02:53 PM

Originally Posted by MainFrame

I get the argument about decreased resolution, but is this even a real problem? I mean, if it was such an issue, then why did my car run fine on 1250cc injectors? How are there so many 500+whp cars out there that don't have any problems idling or under low load? The fact of the matter is in the real world larger injectors work just fine.. if everyone with ID1000s were having these theoretical problems you are presenting why does no one ever complain about it?

And I stand by my opinion that you're always better off going larger with injectors. Sure, you can get by for some period of time running injectors at 100% IDC, but you would still be better off with larger than simply "sufficient" injectors that would only max out at 60-80% IDC.

I think you'd be 'okay' (being relative) with up to 1200ccs anymore personally. I plan on going ID1000s for room for expansion. I just know it won't be as nice of a fit as if I had ID750s or so. The trade off is worth it in my mind for the ability run e85 or upgrade the turbo down the road. Whether that trade off is worth it for others... or when scaled up to the next level (1000s Vs 1400s)... that isn't my call. I really don't have the personal experience to speak on that end of things at all.

**MainFrame** · 08-29-2012 03:19 PM

Obviously I wouldn't recommend getting injectors that are known to have issues. Such as ID2000s. These have idle issues when coupled with 93 octane and 16-bit ECU because you aren't able to set the pulse width low enough. But I wouldn't recommend them because they don't run right, not because of their size. If there were 565cc injectors that had issues with working properly I wouldn't recommend running those either.

I believe you could put my fuel system, which is good for up to 550whp on a low reading dyno running e85, on an otherwise stock bugeye, and after tuning it would run just fine. It would be WAY overkill, but it would work without issues, so what's the problem with going just a little bigger than what is absolutely necessary?

**Heide264** · 08-29-2012 05:31 PM

It'd work. That's what I am saying. Maybe not as smoothly as a stock one. But yes, it would be functional no doubt.

If you smacked on 1400cc injectors without a pump.. Or huge pump without an fpr... You may hit issues. They may only be apparent at 30% duty cycles and appear to just be knock. Who knows.

If you have a full supporting system... Go for it. I just meant to put out some of the arguments for not over sizing your injectors.

For what it's worth, I plan on upgrading the full system, but many people run 1000cc injectors with a pump no problem at all. Then again, some tuners take otherwise fine injectors and have to fight with them constantly due to a huge number of factors. The first situation being clearly the more common :-).

**djuhnk** · 09-07-2012 09:41 AM

some good info on injectors. i will probably get over 750ccs

on a side note, i found a vf30 for a pretty good price, what do you guys think about these turbos? thinkin about nabbing it. and also, when i put on my catless uppipe how would i remove the check engine light besides waiting till it is tuned out?