I have had my car pro tuned for about 6 months now and I have a weird stumble after I have been cruising. It is always around 2500 rpm and I am in vacuum not boost. When I roll on or off the gas at these rpm's it feel like a miss fire or fuel cut or maybe even staved for air as the car stumbles.

Now I know this is probably a stupid question but would a drop in or air intake smooth this out?

Or

Do I need to adjust my cruising trims or....?

I have checked all my vacuum lines and tighter the hell out of all my couplers. I pull like -25 vacuum on decel.

 

sounds like a Mivec issue

 

you need to take a log..

 

My car still does this on the stock tune, but it's around 2800 RPM. Specific requested torque range, vacuum. I figure a tune will help. Is this not a common problem? My car has had it as long as I can remember.

 

I have the same problem! every time I try and log it, I cannot catch the stumble... Any ideas? or is it hard to tell without a log? Thanks in advance for any help.

 

we really need a log

 

You have -25 on decel?!?! I've never seen it go below -12, please post a log this.

 

I have the same issue after getting the synapse DV, I can easily replicated the issue too:

1) Cruising along, 55-70 miles down the highway
2) Steps on gas slightly, 0-7 psi boost
3) Let's go of the gas to slow down
4) huge hit in the face, (stumble)
5) I can avoid the problem by not boosting

Looking at my AFR, it looks like when I'm stepping on the gas to maintain speed on some uphills, the AFR is stupid lean 15.5-16.5, causing the "stumble".

Any ideas?

 

After educating myself a bit I can probably add a little here. What I experience is surge. After installing a Synapse DV and getting tuned I have a LOT more of it, almost always experienced in 5th gear, RPM varies, but throttle pressure is consistent.

Without a log to show the actual metrics, I'm guessing I'm just a little bit into boost (~2-5 psi?) where I experience the surge. Pull mode, port B only, no preload. Car bucks and everything. Not a huge deal in the beginning but now it's getting really annoying. Putting the car into 4th gear at ~65 mph is not an ideal workaround.

 

you can avoid the surge by instead of letting your foot partially off the throttle, take it all the way off to let all the pressurized air vent, then put your foot back where it was.

Its a pain in the ***, but the additional low end torque is worth it

 

It's so consistent though that after letting off the gas, if I resume throttle pressure it will immediately surge again. I agree, I definitely don't want to lose that low end. I read up on the Synapse DV config. thread and it seems like folks didn't really get a better result in push mode. I'll still give it a shot, but it does sound like you lose some response in push mode.

 

I thought it's cause you were feeling the transitions between vacuum and boost right? Maybe thats what surge means?

 

Imagine the way our turbocharger's compressor is driven, right back to basics: The turbine wheel is spun by exhaust gas, utilising also the energy stored in the exhaust gas as latent heat.

This force on the turbine shaft generates torque at the compressor end as they are physically connected. Remember that part as its the key element, Torque

Ok, Now lets imagine our turbocharger compressing air, forcing it into the engine, the engine is utilising it.. what happens then? Ok, The engine expells it, and our turbine housing pressure goes up accordingly, the torque applied at the turbine wheel increases, increasing the spin speed and compressor efficiency at the other end. Great chain of events!

End Result? boost is climbing nicely.

Ok, Now imagine this scenario: The turbo begins to spin, as above, but the compressor wheel is generating more air than we can move and the torque required to spin that huge compressor is not being met? Remember that unless the air is processed by the engine, and expelled as hot, rapidly moving air into the turbine housing, our turbine shaft torque will not increase to spin the compressor faster, and it may well diminish.. our compressor is slowing, braked massively by the compressed air its trying to overcome and increase.....

Result? Compressor starts to slow a little.. (beginings of a compressor stall)

Ok, So now what? The engine is still rotating and consuming air, but the turbo has stopped producing an excess, so our engine has now had chance to consume what excess was available and the inlet pressure is now diminishing, the turbine torque is now increasing again...

VERY IMPORTANT: Remember also that at same time, compressor resistance to spin has also dropped due to housing no longer being as pressurised and as a result, the torque required to spin it has dropped massively..

Boom.. Suddenly the Turbine wheels torque massively exceeds the compressors resistance and the turbo spins to speed in an instant!! Sounds great, rapid boost climb!! The compressors acceerating at over 2G

BUT: We just hit the same problem again, our huge compressor has made so much boost, so fast, and is trying so hard to push the massive volume or air, that our YB cant use it.... so we start to slow again........

So we went: Fast, slow, fast, slow, or in other words: The turbo is Surging. As you can imagine, it continues this cycle until it hits the revs your engine consumes all the air.. then your away!!

 

Try using both A and B ports with the splitter. I kind of got this using only the B port. There's also a different spring that you can buy that's supposed to help too.