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2007 Forester XT
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311 Posts
Discussion Starter #1
So as many people do, we do our best thinking while driving.

While analyzing my ridiculously unacceptable pulsing throttle response, I wondered if there is such a phenomenon of pump cavitation on the turbo. Physically and logically I would say "ABSOLUTELY YES!"
I am running a Cobb stage 2 which REQUIRES a stock airbox and stock style air filter, which may be restrictive enough to create an excessive vacuum and cause the turbo to cavitate.

I could definitely understand the cyclical pulsing of high vacuum back and forth as the turbo loses its "prime".

This pulsing throttle response always occurs at moderate and steady acceleration "under boost".

Under aggressive acceleration pulsing does not occur, and I could see the turbo losing "prime" continuously which would make sense why it wouldn't cycle/pulse under extreme acceleration. I could imagine that I am still obtaining boost due to the wastegate compensating. Logically this all makes sense.

I'd love to hear logical, physics based, intelligent opinions, and real world experience on the topic. Ideas?
 

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2007 Forester XT
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311 Posts
Discussion Starter #3
Well, that is logical, physics based, intelligent, and real world, haha... Fair enough!
 

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2008 FSXT M/T
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4,167 Posts
sorry for the generic answer but you are just overthinking things.

the OTS maps are generic maps created to work w/ all cars w/ somewhat safety in mind. however not every car responds the same way hence needing a vehicle specific protune map. basically all cobb does is add some boost and fuel here and there, neglecting to alter certain tables that are necessary for a nice smooth ride - in some cases it can be downright dangerous. one car might run lean, while the next is rich...

im guess the fluctuations you are experiencing are boost map or turbo dynamic related. also most tuners just tune for WOT pulls and dont spend much time on partial throttle areas. IMHO you should consult a tuner a discuss this w/ him, tell him your goals and go from there.
 

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2007 Forester XT
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311 Posts
Discussion Starter #5
I have been told before that I am overthinking things, possible, but my first goal here was to honestly think things through. I'm not one to just toss out the money to have someone else tell me there's nothing they can do for it, $500 please...that's just in my nature.
There is a wealth of expertise here, and I'm hoping to milk out something that I could possibly fix myself, someone who has had a similar issue or thought. I'm all for getting tuned, don't get me wrong, but I'd sure be a fool if it was something simple and I never asked. It's 2010 and the economy sucks, trying to save some dough and be smart.
 

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2008 FSXT M/T
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4,167 Posts
contact other tuners, not every1 costs $500. you can get a mail tune for $150 bucks and be satisfied. what are your exact mods? unless you have a boost leak or installed parts wrong i gurantee its tune related.

a modified car is a major investment, dont cheap out in anyway or you bought the wrong car... if the economy is bad and you cant afford to do it correctly - dont even bother, or you are setting yourself up for disaster.

and always keep in the back of your mind, when you mod anything it has the potential to break no matter how much research, overthinking or expertise. FYI if you want it to last forever leave it alone.

/story.
 

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99 UK S-turbo
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8,566 Posts
An air pump cannot cavitate.......only something pumping liquid can cavitate (think about the word for a second!)

A compressor can go into surge, the classic chuff-chuff-chuff-chuff-chuff-chuff-chuff-chuff-chuff noise you get from an old GroupB rally car, if it ain't making the noise, it aint surging, besides the characteristic frequancy of surge would be too high to feel at the wheels!

Simon
 

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2008 FXT
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128 Posts
OK, i'm having the same problem, under moderate acceleration between 3000 and 3500 rpm i can actually watch the tach move up and down 2-300 rpm. This is on both the cobb ots stage 1 AND an email tune. If i reflash to stock the problem is gone. any more ideas??
 

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Administrator
2007 Forester Sports XT 4EAT
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39,502 Posts
get a protune, the OTS maps are junk...
I totally agree! I tried running the COBB AccessPORT OTS Stage 1 map & it was "crap"! My Forester ran way better running the married "stock" map, which I did until I got a COBB Surgeline dyno Protune. You can view my write up here. What a difference the dyno Protune made! I'm running nearly stock with a custom dyno Protune Stage 1 map & I'm in no real hurry to move the the next power MOD. :biggrin:

Bobby...

['07 FSXT MODding Journal] ['03 X MODding Journal]
 

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OK, i'm having the same problem, under moderate acceleration between 3000 and 3500 rpm i can actually watch the tach move up and down 2-300 rpm. This is on both the cobb ots stage 1 AND an email tune. If i reflash to stock the problem is gone. any more ideas??
Do you hear wastegate flutter? Might want to tighten the WGA.


Also the boost control solenoid could be gunked up causing it to operate strangely at part throttle. Or the front O2 sensor, maf sensor, DBW throttle mapping etc. Could even be a leak in a breather or pcv hose, I had a tear in one that I fixed recently and it fixed a lot of the hesitation I was getting at initial throttle tip in.
 

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04 forester xt
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969 Posts
i agree on it probably being wastegate flutter. im getting it pretty bad on my protuned stage 2. doug at topspeed said he tightened the wastegate all the way and im only getting 12psi max. is this because the forester's have a weak and different actuator than the wrx's? is that why they can up the boost more than we can on the td04? it sounds like a flapping noise, like a card placed on a bicycle wheel that flaps againt the spokes. it does it under light to moderate load.

upgrading the turbo will get rid of this issue. i dont think vf have this issue
 

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2008 FXT
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128 Posts
I don't hear any noise or flutter. The tuner said he couldn't do anything about it and it must be a mechanical issue. But how do i take it to the dealer and tell them there is a problem if it isn't present with a stock tune, only a stage 1 tune??
 

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Premium Member
2008 FSXT 4EAT
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4,143 Posts
I don't hear any noise or flutter. The tuner said he couldn't do anything about it and it must be a mechanical issue. But how do i take it to the dealer and tell them there is a problem if it isn't present with a stock tune, only a stage 1 tune??
Technically, yes, it is a mechanical issue, because you have to physically twist the arm on the wastegate actuator to tighten it. No email/dyno/road tune will fix the resistance on the arm. :)

The actuator was designed for stock boost levels, so when those are exceeded, the wastegate will not stay closed, as it otherwise would. This is why external wastegates are becoming popular. It's a standalone valve that will only open when the boost pressure exceeds the resistance of the spring inside the EWG housing.
 

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2008 FXT
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128 Posts
The boost level while the pulsing is happening is only between 6 and 9 psi. Throttle position is approx 50% so it is not occurring under full throttle/boost. Under the same conditions using the stock map there is no problem. Full throttle pulls are smooth and i hit 14 psi no problem.
 

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Premium Member
2017 VW Golf SportWagen 5MT
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10,784 Posts
Does wastegate flutter cause hesitation? I thought it was just flapping sounds the stock turbo makes.

My car has had hesitation issues all along, with different tunes and turbos. It got more pronounced with more power. I'm fairly certain it's not flutter of any sort, feels more like timing-related or something electronically-controlled, not mechanical. Stage 2 '05 FXT I drove a while back didn't do this. My friend's '07 STi doesn't do this either. I drove Matt's '08 and it does it too (not much, but it's there -- I can tell) -- his car is stock. Legacy GT people have been complaining about this problem since '06 ('07 models).

Stan
 

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2007 Forester XT
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311 Posts
Discussion Starter #19
That's exactly why carburetors should make a comeback! Electronics are for the birds!
 

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Premium Member
2017 VW Golf SportWagen 5MT
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10,784 Posts
I like carbs, they are fun to tinker with and can be very nice when dialed in. Problem with them is you are always rich somewhere and there is no compensation for altitude changes. I like earlier fuel-injected cars (mid 80s - early 90s), before they got too fancy.

Stan
 
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