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Discussion Starter #1
So my td04 blew up on my 04 FXT. I've decided while I have it out I might as well put a vf48 in. I will eventually be doing STI up and down pipes, tmic, scoop, and splitter, as well as a pro tune. However, I need to get this thing back running asap and some of these parts are kinda hard to source, not to mention hard on the wallet. In the meantime, would I be ok running everything stock with the vf48? I would imagine the ecu wouldn't have a problem with it as it will just be looking to maintain the stock boost levels, AFRs, temps, etc, but I figured I'd double check with you guys
 

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I think you'd still need a tune, since the ECU has limits on wastegate duty cycle and the turbo dynamics would be all wrong. However, I'm sure you could find a tuner to give you an inexpensive e-tune for just a VF48, just to make sure the boost stays at stock levels.

The cheapest approach would be to just source a good used TD04, they're usually under $100 since no one really wants them.
 

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Discussion Starter #3
I already have the vf48, so might as well use it since that's the end goal anyway. I have a Tactrix cable so I could probably get it tuned well enough to run til I get the rest of the parts and do a pro tune, but I'll probably still check with the shop to see how much they'd charge to do this tune.
 

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In that case you're most of the way there already! If you're interested in learning, you could always copy the tables from a factory VF48 rom over to yours and "tune" it yourself, though that's a little dicey without a wideband to watch. An e-tune is just as good as a pro tune, or nearly, it just takes longer generally.
 

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Discussion Starter #5
Ah, I should mention I have a wideband as well. Haha
 

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I wouldn’t run it without tune. Tunes aren’t that expensive or you could experiment with your aforementioned cable and software.
 

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Buy a TD04 for now and wait till you have EVERYTHING to go with the VF48. Do it once and do it right. Add a 3 port boost control solenoid and a bigger fuel pump to your list and you're good to go.
 

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Discussion Starter #8
Buy a TD04 for now and wait till you have EVERYTHING to go with the VF48. Do it once and do it right. Add a 3 port boost control solenoid and a bigger fuel pump to your list and you're good to go.
The grimmspeed ebcs and walbro 255 are on my list as well. I plan on getting all the rest of the parts together within the next month, I just need my car running preferably this weekend, which is why I'd like to get a conservative tune together for the vf48.
 

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Short answer: Don't.

If you still decide to do it, then at least zero out your target boost and wastegate duty cycle maps first.

For your future plans, to make proper use of the vf48, you will need to at least change to 565cc top feed injectors and rails. This is more for safety than power. That vf48 will happily push more air than the stock 550cc injectors can safely provide fuel for. Then engine go Boom.

Curb your thirst for power, do more homework. Your wallet will thank you in the end.

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Discussion Starter #10
Short answer: Don't.

If you still decide to do it, then at least zero out your target boost and wastegate duty cycle maps first.

For your future plans, to make proper use of the vf48, you will need to at least change to 565cc top feed injectors and rails. This is more for safety than power. That vf48 will happily push more air than the stock 550cc injectors can safely provide fuel for. Then engine go Boom.

Curb your thirst for power, do more homework. Your wallet will thank you in the end.

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Why do you say don't? I'm not really trying to make crazy power here, it's an off-road rig, just want a little more to help push these big wheels. I went through all the vf series turbo specs and the 48 was the one that fit the application best. I understand that best practice is having all the supporting mods and I plan on doing that asap. I have an STI uppipe and tmic on their way right now and I'm actively sourcing the rest of the parts. However, as I stated, I need it running asap, so in the meantime I don't see how running it with a conservative tune could cause problems? I get what you're saying about the turbo being capable of more than I need, but if I'm not pushing it to those figures what damage can it cause?
 

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Discussion Starter #11
By the way, I'm not trying to be a dick, I'm honestly asking. I've done tons of research but I can always learn more from people that know more than I do.
 

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Short answer: Don't.

If you still decide to do it, then at least zero out your target boost and wastegate duty cycle maps first.

For your future plans, to make proper use of the vf48, you will need to at least change to 565cc top feed injectors and rails. This is more for safety than power. That vf48 will happily push more air than the stock 550cc injectors can safely provide fuel for. Then engine go Boom.

Curb your thirst for power, do more homework. Your wallet will thank you in the end.

Sent from my SM-G975U using Tapatalk
Do 565cc side feeds somehow flow less than 565cc top feeds? The SG5 FXT has the same side feed injectors the STI's came with, and the VF48 is just a very slightly improved version of the VF39. A VF48 will push stock injectors to about 90% duty cycle, maybe 95% if you're pushing to the very limit of the compressor map. The fuel pump does need to be upgraded, but the injectors are just fine.
 

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Discussion Starter #13
Do 565cc side feeds somehow flow less than 565cc top feeds? The SG5 FXT has the same side feed injectors the STI's came with, and the VF48 is just a very slightly improved version of the VF39. A VF48 will push stock injectors to about 90% duty cycle, maybe 95% if you're pushing to the very limit of the compressor map. The fuel pump does need to be upgraded, but the injectors are just fine.

That was the impression I was under, but it never hurts to ask.
 

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I started with "dont" because your question indicated that you don't understand everything you need to accomplish your goal.

If you go back to my post, I stated that if you decide to go against the general advice and do it anyway; then at least zero out the boost target and wgdc maps. This should be more than enough to prevent major trouble, until you can get a pro to tune it.

If you are able to build a safe tune yourself, then why post your original question? You would already know what is safe, and how you would get there.

The stock xt sides feeds are not 565cc. They were actually 550cc and had issues with leaking when the pump was at 100%; and had issues leaning out past about 80٪ duty cycle. Why do you think Subaru stopped using them?

Besides, we are talking over the internet here folks. I have no way of knowing the condition of the 15 year old injectors and internals of some rando engine. Do you really think I want to be responsible for someone blowing up their engine?


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I say go for it. Stock boost is stock boost whether it's a TD03 or a GT 35R. I haven't heard a single naysayer mention airflow because if they did their answer would be yes. Boost is just the resistance to flow.

You are correct in that the fuel trims as well as the boost control tables have enough adjustment in them to deal with small VE and airflow changes. Usually %10.

The comment about 550 vs 565 injectors is hilarious. What's 15cc of fuel worth? About 10hp?
 

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You are right, the ECU can account for some volumetric changes. Most of the stock ECU maps I have seen from Subaru leave about 15~25% headroom on the injector duty cycle. Combined with ignition timing adjustments and turbo dynamics there is actually a lot of resilience built in.

What you missed on my previous post was that I was referring to the fact that Subaru found the 550cc injectors so problematic after just 2 years in production cars that they stopped using them.

By putting in a turbo with a different repsonse profile to the one the ECU is configured for, the ecu is going to be trying to pull fuel where it would normally be adding it and add fuel where it would be pulling it. And trying to command 15+ year old injectors with known issues to keep up with it.

All of this on 15+ year old stock internals in unknown condition.

There are tragic stories on just about every other Subaru forum of people blowing up their engines by following the advice of other people on the internet saying "you should be fine, go for it". What is wrong with trying to prevent someones engine from becoming another statistic?

Again, referring to my original post, if you put in the vf48, then at least zero out the target boost and wgdc maps.


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Discussion Starter #17
You are right, the ECU can account for some volumetric changes. Most of the stock ECU maps I have seen from Subaru leave about 15~25% headroom on the injector duty cycle. Combined with ignition timing adjustments and turbo dynamics there is actually a lot of resilience built in.

What you missed on my previous post was that I was referring to the fact that Subaru found the 550cc injectors so problematic after just 2 years in production cars that they stopped using them.

By putting in a turbo with a different repsonse profile to the one the ECU is configured for, the ecu is going to be trying to pull fuel where it would normally be adding it and add fuel where it would be pulling it. And trying to command 15+ year old injectors with known issues to keep up with it.

All of this on 15+ year old stock internals in unknown condition.

There are tragic stories on just about every other Subaru forum of people blowing up their engines by following the advice of other people on the internet saying "you should be fine, go for it". What is wrong with trying to prevent someones engine from becoming another statistic?

Again, referring to my original post, if you put in the vf48, then at least zero out the target boost and wgdc maps.


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I definitely appreciate you erring on the side of caution. Everything you've said makes complete sense.
 

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@kharmicresonance Do you have a source for the information on defective side feed injectors? You make it sound like they either won't close, and won't open, or will not flow what they are rated for. Do you have any flow bench testing to support this? What about the multitude of forester and STI owners who are running VF48's or similar on stock side feeds with an upgraded fuel pump?

What makes side feeds so bad, and top feeds so good? I'm always interesting in knowing more about subaru's, but what you're saying is highly unsubstantiated and not something I've seen corroborated anywhere else.


Also, unless you have other numbers, I found flow rates for the yellow side feeds anywhere from 530 to 565cc's, and generally your average set of injectors will likely be +/- 10 if not +/- 20 within the set. In light of this, I think it should be fairly obvious that a nominal 15cc difference is meaningless, regardless of its existence.
 

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Uh? You are correct, in that I specifically do not have physical evidence of the issues with the yellow side feed injectors.

On the other hand, You can go through this forums history, as well as nasioc and rs25, plus a dozen other toyota/nissan forums from around 2003-2008 and find plenty of anecdotal evidence of leaking side-feeds. This was a common problem in high performance applications. The side-feeds and their rails just leaked under high pressure.

As for the injectors themselves, you wont find as many forum posts, but those same forums still document how the side feeds would lean out especially as they got older. Many shops that serviced injectors found that the coils in the side feeds would break down and their resistance would shoot up - making them lean out. In a lot of cases the injectors would just outright fail - right from the factory. It was a common practice to order a whole new set of injectors for a weekend of racing then throw them away. This was one of the main reasons why so many aftermarket kits popped up to convert to top feed. And just to note, a lot of injector shops still, to this very day, refuse to service side-feeds. They will only sell you a new matched set.

I am not here to spread BS. I will not knowingly talk out my @$$. I genuinely just want to help people who love their subies as I have since 1999 with my first Forester.

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I did some looking around (20 or so minutes of non-exhaustive googling) and I cannot find anything about people having issues with yellow, OEM sidefeeds. A few with leaking between the rail and injector, but that's got nothing to do with injector performance. I could not find anything about them leaning as they get older, in fact Witchhunter has a page (WitchHunter Performance - Subaru Fuel Injector Modifications) which does list flow issues with other injectors, but discusses the yellow sidefeeds with no mention of any issue. I found one mention of them getting clogged, but again, that's got nothing to do with performance.

I was under the impression that the top feed conversions were due to the vastly larger aftermarket support for topfeeds, since there are far more applications (both subaru and other) that use topfeeds vs sidefeeds. I was unable to find anyone who doesn't service sidefeeds, in fact most places appear to advertise that both top and side feeds are the same price for service, or that side feeds are slightly higher due to including a larger number of new parts.

Maybe I'm missing something here, but given that an injector is simply a coil attached to a valve, wouldn't coil resistance increasing lead to higher impedance and over-driving, thereby ramping open sooner and causing a rich condition, rather than lean? If the resistance decreased, then it would take more current to drive, and potentially open slower, incompletely, or not at all? Though of course injector impedance should be matched to the driver for best results, obviously.


The reason for my interest in this specific topic is twofold, first it's interesting and more info is always good, and second I and others that I know are currently running yellow sidefeeds with VF39/48's with no issues (85-90% max duty cycle on a fairly rich tune) and I'd be very interested in being prepared for future problems!
 
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