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Discussion Starter · #1 ·
I’m new to the Suby world. My question is:

I’m planning on getting a Cobb turbo back exhaust system, air oil separator, cylinder number 4 mod. I’m also going to be getting it pro tune. I was thinking on getting a Cobb air intake. The shop that is doing the work said that I need to change the fuel pump and injectors if I want to put on a Cobb air intake. Is this true? Or is this just an upsell?


Thanks
 

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i'm pretty sure for the 15+ va chassis it needs pump/injectors to fully get everything out of even a turboback exhaust....

yamahaSHO on here would be able to answer this better than I.
 

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You dont need to but it will be nice to have to maximize your gains.
 

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You don't need to change the fuel system. You will need a Pro-tune though. COBB doesn't offer an OTS map for this configuration because the fuel system is close to maxed out. If you need an e-tune we're happy to help, we've tuned tons of these on our dyno and by e-tune.
 

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+1 to the maxed fuel system.
You can ‘do it’ but cobb themselves didnt want to put their name on a tune that allows it for liability reasons.
While they are covering their asses cause they need a much larger margin of error to cover many cars all over the place with different fuel quality with one tune, I personally wouldn’t do it without upgrading.
Expect to spend another 1kish if u wana do a fuel pump and 1050’s, at which point u run into the ‘might as well’ mods like an ebcs, inlet, and fpr.
I was in a similar but different ship on my GR a few months ago where I had installed ELH so I ‘could leave it as it is’ but i just went all out for piece of mind and optimization.
 

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I'd like to point out we're tuning to IDC & A/F and not pushing past the fuel systems limits. Our tunes are not on the edge. What this means is you won't get any big power gains with the intake, but it will be safe and reliable.
 

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Discussion Starter · #7 ·
+1 to the maxed fuel system.
You can ‘do it’ but cobb themselves didnt want to put their name on a tune that allows it for liability reasons.
While they are covering their asses cause they need a much larger margin of error to cover many cars all over the place with different fuel quality with one tune, I personally wouldn’t do it without upgrading.
Expect to spend another 1kish if u wana do a fuel pump and 1050’s, at which point u run into the ‘might as well’ mods like an ebcs, inlet, and fpr.
I was in a similar but different ship on my GR a few months ago where I had installed ELH so I ‘could leave it as it is’ but i just went all out for piece of mind and optimization.
My pro tuner said that I can get by with just changing my fuel pump I don’t have to change my injectors. How true is that?
 

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You need more than a pump to upgrade the fuel system. A pump alone won't help at all.

You can tune around this and just limit power to safe levels for the OEM fuel system.

My pro tuner said that I can get by with just changing my fuel pump I don’t have to change my injectors. How true is that?
 

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My pro tuner said that I can get by with just changing my fuel pump I don’t have to change my injectors. How true is that?
As John mentioned, pump isn't what controls fuel, the injectors are.
Pump just supplies more pressure to the fuel rail, it's the injectors that allow for greater injection as required, with less effort.
I dunno off the top of my head, but I think the sti injectors are around 560's.
80% injector duty cycle on 560's is only 42% on the 1050's.
100% on 560's is only 53% on 1050's.
While you can get up to 100-120%+ on the stock turbo with full bolt on, which is like 65% on the 1050's, you'll probably never get past that unless you upgrade the turbo.
The point is margin for error.
The fuel pump upgrade is just to make sure the 1050 fuel injectors get enough fuel to ride that 50-100% line comfortably.
The pump upgrade may add a little bit of bolstering to those stock injectors being maxed out at 100%+ but it's still not going to solve the problem.
One is more water pressure in the house, one is a larger nozzle on the end of the hose.
Can't really dump more water if the nozzle is too small just because the pressure is a bit higher.
 

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Unless your fuel pressure is falling off, which it isn't, the pump has no effect.

Running over 100% duty cycle is injecting fuel on the wrong cycle. Can you do it, yes. Should you, no! I see 100+% IDC talked about on here often and how Subaru isn't calculating the IDC properly.. They are. I've verified it with o-scope on our dyno. Lots of ECUs will allow 100+% IDC, they also allow 40+ degrees of ignition timing at full boost. Can and should are different things.
 

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100% indicated on A Subaru is not 100% IDC. If that were the case, you'd see it leaning out with more RPM or boost, once you've hit 100%. I've taken IDC as high as 137% without leaning out, so 110% would not even concern me.

You could alway put an aftermarket ECU in and see what your actual IDC is, referenced to static (when the injectors never close).

Cliffs: no, you do not need to upgrade those parts. At most, a fuel pump is a cheap upgrade and will lower your IDC (if you can't get past the #), if you size it properly.
 

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Anything over 100% is just spraying on a closed valve and waiting for the next intake cycle. That's why it doesn't appear to lean out, however your fuel delivery quality is rubbish.

My oscilloscope verifies the IDC of the injectors to be correct. I'm not guessing or speculating.

100% indicated on A Subaru is not 100% IDC. If that were the case, you'd see it leaning out with more RPM or boost, once you've hit 100%. I've taken IDC as high as 137% without leaning out, so 110% would not even concern me.

You could alway put an aftermarket ECU in and see what your actual IDC is, referenced to static (when the injectors never close).

Cliffs: no, you do not need to upgrade those parts. At most, a fuel pump is a cheap upgrade and will lower your IDC (if you can't get past the #), if you size it properly.
 

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That's not an uncommon thing. It does not 'appear' to lean out because the fuel still makes it into the cylinder. This also aids in cleaning and cooling of said valve. It likely delivers the fuel a little more like what a carb would do... Something I actually learned in a PRI class a few years ago, that sort of delivery makes more power.

It's a short time and it's still followed with an atomized spray.
 

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Anything over 100% is just spraying on a closed valve and waiting for the next intake cycle. That's why it doesn't appear to lean out, however your fuel delivery quality is rubbish.

My oscilloscope verifies the IDC of the injectors to be correct. I'm not guessing or speculating.
Very curious on this. Im not huge on using scopes since its always a huge pain in the ass to set up and do. Although when done correctly, they really do shed light on to things. In this case pulse width modulation on the injector IDC. So do you see a differance in the signal wave when its at 100% and above? Curious to know if the injector is really working harder at 100% and above when compared to a lower IDC.

When at 100% IDC and above, wouldnt spraying on a closed valve cause for more fuel than needed at the start of the next combustion cycle? At some point wouldnt this lead to a bad combustion due to excessive fuel?
 

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Very curious on this. Im not huge on using scopes since its always a huge pain in the ass to set up and do. Although when done correctly, they really do shed light on to things. In this case pulse width modulation on the injector IDC. So do you see a differance in the signal wave when its at 100% and above? Curious to know if the injector is really working harder at 100% and above when compared to a lower IDC.

When at 100% IDC and above, wouldnt spraying on a closed valve cause for more fuel than needed at the start of the next combustion cycle? At some point wouldnt this lead to a bad combustion due to excessive fuel?
I can't answer this question with certainty but based on my understanding, if you require 120% IDC the first stroke gets 100%, and 20% hangs on the valve, but then the next stroke gets that 20% that was backed up, plus the new 100%, and another 20% gets held back again.
In turn, you do get your 120% delivered, but 20% of it wasn't really delivered well so as mentioned, the fuel delivery is not great and not quite as consistent.
Why this doesn't cause lean conditions on the first stroke is mainly due to the fact that the car doesn't go 0-120 right away. You have to ramp into it, so even if the last stroke was 115%, you still get 15% added to the next 100 and then the 20% kicks in the next cycle.
Basically, your ignition timing would then be off slightly by one stroke, which now that I think about it, may cause a very slight lean condition when you floor it, and a very slight rich condition on decel when you go from 120%, hang 20%, drop down to 60%, etc...
I don't know the science, I'm just thinking out loud 😂
Until today, I thought 120% meant it just pushed 1.2x the recommended max fuel for the injector per cycle, putting more strain on the hardware, rather than injecting the 100% max beyond the intake stroke duration and so on the backs of the closed valves.
 

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Very curious on this. Im not huge on using scopes since its always a huge pain in the ass to set up and do. Although when done correctly, they really do shed light on to things. In this case pulse width modulation on the injector IDC. So do you see a differance in the signal wave when its at 100% and above? Curious to know if the injector is really working harder at 100% and above when compared to a lower IDC.

When at 100% IDC and above, wouldnt spraying on a closed valve cause for more fuel than needed at the start of the next combustion cycle? At some point wouldnt this lead to a bad combustion due to excessive fuel?
0% & 100% are flat lines on the scope. All the way off or all the way on.

Spraying on a closed valve is going to puddle fuel on the intake valve and that will be added to the fuel from the injector on the following intake stroke. The mass of the fuel on the stroke may be adequate, likely over abundant. The quality of the fuel mixture is suspect IMHO though. We endurance race, so fuel efficiency is really important, if we pit early the race is over. In my 27 years (yes I'm old) of empirical dyno tuning and racing experience tells me that running puddled fuel on the intake valve is less efficient. Maybe it works, but that doesn't mean it's the right way.
 

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There really is no issue spraying on a closed intake valves. Many OEM's do this on purpose. The puddling we're thinking of hear is at such as fast rate, its not really 'puddling', which a hot, closed valve is generally starting to help atomize the fuel already. As long as you're not doing it when the exhaust is open, there is really zero issue in doing this.

If we take my personal high of 137% IDC and do the math (assuming 137% is the max it can do), that's actually right at 73% IDC, which is well before it goes static (injector no longer closes). At that point, I would be more focused on calculating where the exhaust valve is open and whether or not you're injecting there.

Unless you're sending fuel out of the exhaust, you're not wasting fuel hitting the back of the intake valve.

So no, if your bolt-on car is hitting 110%, especially on a street car, I would not be the least bit concerned.
 

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Discussion Starter · #18 ·
As John mentioned, pump isn't what controls fuel, the injectors are.
Pump just supplies more pressure to the fuel rail, it's the injectors that allow for greater injection as required, with less effort.
I dunno off the top of my head, but I think the sti injectors are around 560's.
80% injector duty cycle on 560's is only 42% on the 1050's.
100% on 560's is only 53% on 1050's.
While you can get up to 100-120%+ on the stock turbo with full bolt on, which is like 65% on the 1050's, you'll probably never get past that unless you upgrade the turbo.
The point is margin for error.
The fuel pump upgrade is just to make sure the 1050 fuel injectors get enough fuel to ride that 50-100% line comfortably.
The pump upgrade may add a little bit of bolstering to those stock injectors being maxed out at 100%+ but it's still not going to solve the problem.
One is more water pressure in the house, one is a larger nozzle on the end of the hose.
Can't really dump more water if the nozzle is too small just because the pressure is a bit higher.
Thank you very much , I appreciate your explanation , Makes sense
 

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Discussion Starter · #19 ·
You don't need to change the fuel system. You will need a Pro-tune though. COBB doesn't offer an OTS map for this configuration because the fuel system is close to maxed out. If you need an e-tune we're happy to help, we've tuned tons of these on our dyno and by e-tune.
Thanks Joshua for your knowledge. I would love to get my car pro. tuned by you but I live in Canada . LOL close to Toronto . I found a place , my car goes in on Monday. I’m getting - Turbo back exhaust, cyl 4 mod, A/O separator , Air intake , fuel pump (I guess) and a pro tune .
 

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fuel pump is a good call with stock injectors on a dyno tuned stage 2 setup. is the shop you're going to a subaru specialist? if not an e-tune from someone that is would probably be better.
 
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