Pulse feel under moderate to heavy throttle at low-ish RPM

[TDWPgtp]

Hey guys I recently purchased an '11 Legacy GT. This is the first turbocharged car that I have driven for more than a few minutes at a time, so bear with me. I noticed a few days ago that when I give it a fair amount of throttle (or more) around 2000 rpm, I feel a slight pulsing feeling of power until it builds boost a few thousand rpm later. It seems to happen when I probably could/should have downshifted another gear, but it isn't yet bogged down either. Is this a characteristic of these drivetrains, or does it sound like something is wrong?

 

If you have any questions or need any clarification, let me know. All help is appreciated!

[Sarang]

Yes that is a thing. Check the "stumble" thread a little further down here.

Mine got way better with a pro tune.

Don you have a Cobb Accessport yet?

Also, fyi, you'll get better responses in the fifth gen section.

[TDWPgtp]

Awesome, thanks. I didn't know exactly how to describe it, and when I searched for "turbo surge" and some other things, I found unrelated issues. I will have to read that thread at lunch or after work. Before I do, I figured I'd ask, is this bad for anything? Or is it just kind of an annoyance?

 

And as far as I know my car is completely stock. I do not have plans to tune at this point, but that can always change.

[Max Capacity]

FWIW, I highly recommend you get the car off the factory MAP. It's said it has the same timing flaw as the 4th Gen's

 

http://legacygt.com/forums/showthread.php/dangerous-cylinder-trims-5th-gen-lgts-193842.html

 

That's also the guy you want to get your tune from.

[Sarang]

No pressure, but even just a simple tune makes a huge difference on our cars. Spool up is better and the torque rush is delightful.

[LatentWagen]

As said, yes, you should get off the stock tune ASAP, as a custom tune will improve your driving experience a lot. It'll also save the motor, in the long run.

 

But you might have the most luck if you change your driving habits a little.

 

Like, don't give it heavy throttle at 2000 RPM, unless you're in low gear. Driving like this is what eventually kills these motors, IMO. Downshift at least once if you want to mash the gas, I use a 3000 RPM minimum for the sake of mechanical sympathy.

[TDWPgtp]

I try to make sure I downshift far enough, but occasionally I'll find myself in a gear higher than I should be. I don't think the problem is limited to being in too low of a gear, however. I see it when I am accelerating very gently in 3rd around 3000rpm (maybe a bit under), but not every time. To be honest, I haven't figured out EXACTLY what conditions cause it with my car. It is fairly rare for me so far.

 

And after reading through the stumble thread, I have a couple questions. 1) it seems that almost, if not all, of the cars in that thread are modded to some degree. Several people mention that it typically pertains to stage 2+. Does that suggest that there are some possible mods done to my car, or is it still possible to that it is happening to a stock car?

 

2) There is a lot of information in that thread that I am unfamiliar with (pretty much anything tuning related). I intend to at least learn bare minimum information, but for now I feel stupid. Is it decided that in most cases, relocating the vacuum to the plenum and/or increasing fuel line length fixes the issue? And is this still applicable to a 5th gen?

 

3) Would I still be best posting my questions in the 5th gen forums?

 

4) Is there any specific threads someone can point me towards to help educate me on available tunes and their benefits? At this point, I am not super concerned with making more power (although it'd be nice), but rather improving the reliability and lifetime of my motor.

[LatentWagen]

Stock cars as well as modded have shown the surge, so you wouldn't be alone there.

 

Subaru changed around the fuel systems from year to year (and model to model), quite a bit. The STi fuel pressure regulator is a common mod, as are longer or "tuned" length fuel hoses. The problem boils down to harmonics, from what I understand. As you've read, relocating the FPR vacuum signal line gives a "cleaner" signal, so that's a good first step, but it doesn't fix the underlying issue. Nothing will, unfortunately, because the harmonic will always be there, and the ECU will always compensate. I think the worst cases of surge come when the compensation loop frequency matches up with the wavelength of FP variation, and amplifies.

 

The 5th gen guys would definitely be able to help you more specifically, without a doubt. Big power builds tend to be more likely to address this problem so you may want to hunt them down.

 

In terms of tuning, there are a few very well reviewed vendor/tuners here. Head over to the tuning section and read up. Names like Tuning Alliance and Cryo will keep coming up for a reason. Give those guys a shout and discuss your goals, as a first step.

[Sarang]

I think it's worth asking in the fifth gen section, you'll get more fifth genners looking at it for sure. If you don't want to start a new thread, the random thoughts thread is a good place.

My understanding is that our ej255 engine is pretty much the same as the fourth gens, but our turbo and header situation is very different. That means that sometimes what goes for fourth gens goes for us, but sometimes ours is completely different.

[Tuning Alliance]

FWIW, I highly recommend you get the car off the factory MAP. It's said it has the same timing flaw as the 4th Gen's

 

http://legacygt.com/forums/showthread.php/dangerous-cylinder-trims-5th-gen-lgts-193842.html

 

That's also the guy you want to get your tune from.

 

^^

 

Time for a custom etune, we can get you setup properly, shoot us a PM and can discuss

 

-Brian

[SeeeeeYa]

As said, yes, you should get off the stock tune ASAP, as a custom tune will improve your driving experience a lot. It'll also save the motor, in the long run.

 

But you might have the most luck if you change your driving habits a little.

 

Like, don't give it heavy throttle at 2000 RPM, unless you're in low gear. Driving like this is what eventually kills these motors, IMO. Downshift at least once if you want to mash the gas, I use a 3000 RPM minimum for the sake of mechanical sympathy.

 

ANY tune aside, the above applies.