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Car Stutters Big, Changing At High Rpm


123456
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Hmm, so mods are as listed in my signature. Basically changing from 1st to 2nd or 2nd to 3rd at about 6.5-7k rpm the car feels like for a fraction of a second it's starved of oxygen, and kind of jolts, then it jumps forward. It does it randomly, at different temperatures, not on every shift. Mainly when im shifting fast to.

I have a few ideas on what it could be, but i want other peoples opinions. thx

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Mine does this aswell, but only when changing gears fast (it hesitates for a split second before power comes on strong).

I always put it down to the BOV not letting enough air out on gearchanges when the throttle is closed, stopping the flow of air to the engine for a split second.

That air then has to be brought back to speed once the throttle is opened after the gearchange, hence the slight hesistation before power comes on (lack of air speed initially).

I could be totally wrong though.

Fixxxer :laugh:

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I was actually thinking on the contrary, that it was the BOV emptying too much air out (i.e. a leak?), that resulted in starvation after the change as the piping was refilled.

What your saying makes more sense.. maybe my BOV is on the way out, people say they are good to about 230rwkw normally

Anyone else?

Edited by 123456
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A BOV doesn't effect the speed of the air on the way to the engine.

All it does is release pressure so that the blades of the turbo can keep spinning.

If you had a hose spraying water into your garden and you quickly stuck a hole in the side of it, you aren't increasing the speed of the water out the nossle.

How long since your re-map. Might wanna check out some tuning issues first.

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Um about 2-3 months, and only thing i've changed since remap is the dump and a new cat, which didn't make the surging any worse, any other suggestions? Just feels like it's starved of air.. or fuel after change, could it be timing related or not?

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To continue on from what I was saying before, and what I think happens is that the BOV will release pressure or 'boost' in the pipes after a gearchange.

After getting back on the gas again, the throttle is reopened, turbo shaft speed has also decreased by being off the gas (by how much I don't know), but the pressurised air that was in there just before the gearchange is gone/vented. The turbo has to then build pressure in the system, which shows up as a hesitation on quick gearchanges.

Therefore I think that a large FMIC setup with large pipes will exacerbate this problem.

Fixxxer

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You are talking about turbo lag yes I noticed. Its not "stopping the flow of air" is what I was saying. If you think about it, with a less effective bov or no bov, the engine see's boost for longer, the only thing that is aided, is the speed of the impellor.

I'd be checking the AFR. You say yourself it feels like a afr issue. Thats the only way to be sure I reckon.

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when you change gears quickly the bov doesn't have much time to dump air, so there isn't much of a pressure loss in the piping.

i'd possible point the finger at the boost controller. the air between the controller and the wastegate isn't able to escape fast enough, so the wastegate is still partly open when you get on the gas again, so you have less boost until the wastegate is fully shut.

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Ah yeh, thats true, if changing quick it wouldnt have much time to vent.. Do you think it's worth getting a new controller then? Or are you saying boost controllers in general can cause this problem? The joint that tuned it seemed to think that they (Can't remember brand, they were $29, had a long topic.. turbosmart?) were pretty decent

Heres a dyno graph if that helps (before new CAT and Split dump)

mydynokt1.png

thx

Edited by 123456
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You need to be able to change the gears with out shutting the throttle... eg Flatchange!

And inless you want things to break I surgest not doing that inless you have a ecu that will allow you to do it also...

Or buy an auto!

Other than that you proberly have a fuel issue; tuning/supply

Edited by cameron
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Is that a dyno sheet for ants?

Everyone is thinking turbo lag... I don't know if he is describing that. Jolts and feels like it starved... then jumps forward, sounds like something being too rich...

I still reckon check out the AFR's and see if its gone a bit silly due to a small problem somewhere along the line.

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A atmospheric BOV vents measured air to the atmosphere. Your cars air flow meter has measured X amount of air in the system which requires Y amount of fuel. When people screw around by dumping the measured air out of the system - the car does not like it..... It still dumps Y amount of fuel into the cylinders.

Cheers

Sumo

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the gtr was doing that for a while, just lacking power and stuttering under load. wasnt very violent, inet pressure stayed up, although the turbo's sounded like they were choking. it did it mostly when they would start to wind up. traced it down to a faulty O2 sensor...

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