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Old 02-21-2006, 02:54 AM   #1
caterpill
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Unhappy <--Newbie to EM: How does the ECU control boost, and is it possible to: TD04 --> VF34

I've just changed my downpipe today from a HKS to an Invidia bellmouth, in my 04 WRX. While I was in there, I did a little checkup of my stock turbo. I did notice some exhaust smoke if I idle too long and also some unique instances where it would fog up the back of my car, etc. It doesn't seem to smoke all the time, but sometimes it does. It seems to have axial shaft play, (up/down/side/side) but no forward and back.

I just assume my turbo's bearings are on it's way out, at 56,000 miles (about 10,000mi with a Cobb Stage 2), so I decided I oughta learn a bit or two about any upgrades.

What I was curious about, was, since i'm keeping a close eye on any particular noises or excessive smoking to indicate a broken turbo, i'm somewhat ready to swap the turbo out for something else.

My concern is that I am not familiar with the exact science of turbocharging and boost control... Now, I do know a very simple part, and that is, the wastegate bleeds off boost via vacuum control. LOL.

But I do not understand what happens when I swap out my stock TD04 with a VF34 from a STi. I believe the bigger turbo will overboost, the fuel system will not compensate for that, and blow up my engine, correct? Can someone explain why? Why doesn't the wastegate bleed off the extra boost to keep the boost level at 14.7 psi?

Also, I was thinking, from a reliability standpoint, wouldn't it be better to run a slightly bigger turbo with a slightly larger capacity, than a TD04 at the brink of efficiency or capacity? For instance, I currently have a Cobb Accessport and a Stage 2 OTS map. I guess from a reliability standpoint, the turbo was designed to boost 14.7psi for the WRX, but the Stage 2 map increases it to ~16. This, is known to be within the limits of the stock turbo if not at the limits of efficiency. However, in the event that I need a new turbo, because of wear and tear, wouldn't it be better to purchase a VF34 and run it at 16psi? I would think it would last longer if not be much, much more reliable. Then there is the efficiency factor; is the VF34 very inefficient at 16psi that would make me waste gas, etc etc.? Or are there important reasons why no one does it?

I just don't understand the concepts too well and I would greatly appreciate if anyone could clear it up for me a little; I am at the point where I know my turbo is failing, but I would rather not buy another stock turbo if it'll wear out in short order.

Thank You In Advance,
Dan
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Old 02-21-2006, 09:11 AM   #2
Spec C Wannabe
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Quote:
Originally Posted by caterpill
But I do not understand what happens when I swap out my stock TD04 with a VF34 from a STi. I believe the bigger turbo will overboost, the fuel system will not compensate for that, and blow up my engine, correct? Can someone explain why? Why doesn't the wastegate bleed off the extra boost to keep the boost level at 14.7 psi?

Also, I was thinking, from a reliability standpoint, wouldn't it be better to run a slightly bigger turbo with a slightly larger capacity, than a TD04 at the brink of efficiency or capacity? For instance, I currently have a Cobb Accessport and a Stage 2 OTS map. I guess from a reliability standpoint, the turbo was designed to boost 14.7psi for the WRX, but the Stage 2 map increases it to ~16. This, is known to be within the limits of the stock turbo if not at the limits of efficiency. However, in the event that I need a new turbo, because of wear and tear, wouldn't it be better to purchase a VF34 and run it at 16psi? I would think it would last longer if not be much, much more reliable. Then there is the efficiency factor; is the VF34 very inefficient at 16psi that would make me waste gas, etc etc.? Or are there important reasons why no one does it?
You will not overboost by solely changing turbo. Actually you will run less boost since VF34 has bigger, heavier turbine and thus more exhaust flow is need inorder to keep the same boost level as TD04. The real problem is the "overflow" of air intake generated by VF34. Wastegate solenoid is programed from the factory to match the boost characteristics of TD04. Without changing program in the ECU, wastegate will work in the same pattern (interms of wastegate duty cycle) no matter what turbo you throw in.

VF34 blowing 16 psi of boost has much higher flow rate (in Cubic Foot per Minute) than TD04 at the same psi. Since flow rate is higher, injectors need to work harder to keep Air to Fuel Ratio within the set value. Again new ecu programming and new injectors are needed to take care of said overflow issue.
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Old 02-21-2006, 04:12 PM   #3
ride5000
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Quote:
Originally Posted by Spec C Wannabe
Again new ecu programming and new injectors are needed to take care of said overflow issue.
not necessarily.

compare an oem turbo car running at 20* F ambient at sea level to one running at 80* ambient at 5k feet of elevation.

the cold dense air case will be pumping a LOT more air mass than the hot rarified air case. the ecus are the same in both cars with the same maps. how is the car able to run safely in both cases?

because the MAF sensor is doing its job!

now, is it likely that the injectors will run out of on time once you put a vf34 on the car? yes. stock cars run out of injector pulse width on tweaked stage 2 setups.

will that mean leaner afrs? yes, as the ecu cannot command the injectors to flow any additional fuel.

will that mean dangerous afrs? could be, but it could actually go either way. despite the fact that the vf34 is now pushing in more air, that air will also be colder, and a lot less likely to detonate in the first place. in the end you could probably get away with a leaner afr anyway. that shifts the issue to the definition of the phrase "dangerous afrs." the difference between oem pig rich <10:1 afrs and a nice 11.5:1 is a lot of pulse width, and you could run a vf34 at 1 bar on 11.5:1 all day long.

i am in no way condoning the fitting of bigger turbos to otherwise stock cars willy nilly. at a minimum i would want a capable datalogger, some kind of knock monitoring/indication, and a wideband afr meter. there are certainly cases of folks who have run setups that were ridiculed by the general public as completely untenable yet not only run but actually performed well. the knee jerk reaction is to dismiss it as either luck or ticking time bombs, but i've got many tens of thousands of lines of ecu log data that indicate the bugeye ecu is DAMNED resilient to mechanical mods, and actually works shockingly well at running the car.

however, in the original poster's case, he has a post bugeye ecu. in that case all bets are off due to the craptacular open loop delay coded in, perhaps to dissuade owners from modifying the cars, or perhaps to comply with emissions standards... who knows? in any event, it really is best to get some kind of engine management system down before you mechanically modify your post bugeye car, especially if you run <93 octane.

hth
ken
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Old 02-21-2006, 08:42 PM   #4
caterpill
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Hmm.. interesting.

So I was somewhat right about my engine blowing up, but totally wrong that it is a function of airflow difference that kills my engine, not the manifold pressure.

I see.. I think I better read up a bit more.

But any turbo upgrade would require larger injectors; but then that would also require some other type of user tunable engine management rather then a Accessport, huh?

I think cobb has a map for a vf series turbo and bigger injectors; but technically if I were not to spend any other money on engine management, the ONLY way to do it would be to use the VF map from cobb tuning correct?

Otherwise, I would need something like a UTEC or a tuner to get me up and running with a map?

I do realize that venturing into a larger turbo is actually more then I bargained for. I was only wondering of possibly running a larger turbo below it's capacity for more reliability, but it seems that even that method would be hard.

So, the bottom line is, getting a new / different turbo will cause problems if I leave all else equal like my accessport, since the new turbo will have different flow characteristics, correct? so unless I have a way to adjust and monitor air/fuel ratio among other things, it would probably be best to stick with a TD04?

I know this is venturing out of EM a little bit, but would my accessport with a stage 2 map be unaffected if I were to purchase a monster ported TD04? OR, would I have to upgrade other things to support the different flow characteristics of the ported turbo? (or is the supposedly faster spoolup time of a ported turbo not enough to do any hurt?)

Thanks for the help, I really appreciate it. I am still a newbie in many aspects despite the damned "scooby specialist" label.
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Old 02-22-2006, 08:19 AM   #5
ride5000
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if you're trying to decide whether or not you should make the leap, consider getting a pull off vf39 from an sti, get a set of 4 pink injectors, and buy/run the accessport stage 2.5 map. from other testimonials the vf39 will run stably on the 2.5 map, despite it having been "tuned" for a vf30/34. you're looking at a bit less than a grand for brand new pinks and an 04 rex access point, and if you look around you could probably get a vf39 for <300 bucks. $1300 is not too shabby for 270whp.
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Old 02-22-2006, 10:58 PM   #6
jjimpreza
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Good thread . I too am in the same boat as the thread starter stated and wanted to make mention of another route to go with the AP if you wanted to run a map different than the current VF series offered by Cobb.

You can always get a Protune but that is alittle more than I would like to spend and you might not have access to a Protuner.

So The other option that I have heard great reviews from people about would be maps from PDXtuning.com. They will make you maps based on the list of mods that you provide. So you could go with a TD05 or even the monster TD04 and have them make custom maps. They don't need your car just the list of what you have on it to create the map. I hear they are charging $150 for five maps and send them to you via email. Just another thought.

But I sit here in the same boat as you with regards to the turbo issue and what way to go based on the EM. VF series or TD04 monster or TD05 decisions....decisions
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Old 02-22-2006, 11:13 PM   #7
caterpill
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I was merely covering my ass in case my turbo decides to just start leaking oil through the seals; because I knew so little about basic EM and how different turbos bring up different problems, I wanted to clarify the topic a little bit for myself..

Now I do understand that it isn't as simple as I initially thought, but my best cost-effective path to go would probably be a ported TD04 or even, in the worst case scenario, a stock TD04.

I initially had hoped the cobb stage 2 wouldn't be as harsh as it seems to be on the stock turbo, but I think as my car passes the 56,000 mile marker (04') the turbo is showing signs of tiredness.

Although my car doesn't really smoke that much, I did have an intermittent off-throttle smoking problem. (ex: as rpm's decelerate)

Technically the reason for purchasing the Accessport was the stupid OL/CL issue for 04' wrx owners; I wanted to make sure I didn't run lean. But now I think I jumped into a price range out of my league.. with higher-stressing mods such as an aftermarket turbo, the necessity for devices like a wideband o2 sensor, EGT gauge and other tuning tools which I probably won't be able to afford strays me away and tells me I should stick to the stock turbo.
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Old 02-23-2006, 01:43 AM   #8
jjimpreza
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Quote:
Originally Posted by caterpill
TD04 or even, in the worst case scenario, a stock TD04.

I initially had hoped the cobb stage 2 wouldn't be as harsh as it seems to be on the stock turbo, but I think as my car passes the 56,000 mile marker (04') the turbo is showing signs of tiredness.
Well I don't think the Cobb stage 2 is to harsh as I have no EM, just a TBE and I have the same problem. I have 66K on my 03 and have only had the TBE on for the last 10K. If anything the AP EM will probably make the car run safer than the mods without the EM.

Well if cost is and issue you could either buy a TD04 used (super cheap) or get a TD04 from Deadbolt(free rebuilds for life) then keep your exisitng management.

Last edited by jjimpreza; 02-23-2006 at 01:50 AM.
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