Jerk On Little Throttle Or Backoff - Do You Jerk Is The Question ;)

[simpletool]

Perhaps you could try moving the vacuum (trigger) for the BOV from the plenum (stock location) to a splitter off the turbo (the one that goes to the wastegater/boost controller).

This MIGHT do 2 things:

1. cause the BOV to open a little earlier ,and

2. perhaps there is a bigger pressure ripple near the turbo outlet pipe that will pop open the BOV with the small pressure differential. (1-2psi)

This shouldn't be too hard to test, let us know how it goes.

To reiterate, you should find the small pressure hose that goes from the BOV to the back of the plenum. Take the plenum end off and plumb it back in as close to the turbo (yet after the compressor) as possible- possibly off a splitter from the boost signal. (Remember to plug the outlet on the plenum).

It might also be that the BOV simply won't budge with such a small pressure change.

OR, it might not be a BOV issue at all.

I'm not an expert, just trying to think it through.

[benl1981]

Had to think about it but you need to take the BOV connection off the plenum..this sets up the pressure difference that causes the BOV to open.

Cheers

[Haysey]

i get jerk, been told its due to the timing getting used to going from 7000rpm to like 4 in a short period of time

[benl1981]

Chris,

Your reversion - is it like a miss?

I can see the AFM voltage jumping all over the place.

Tune shop wants $80 to diagnose it ..

Have many people tuned this out? It happens anywhere from about 0psi to 5 or 6 psi.

[Cubes]

HKS idle stabilizer.

http://www.hksusa.com/products/?id=2175

[benl1981]

Looks like that is just an interceptor like a SAFC. Would rather fix the root cause or tune it out..

Have to get a few more opinions on it..

[driftmeister]

I am using SSQV HKS Blowoff valve and with EVC 5 Boost controller on my 34GTT. The only way to make this jerk dissapear is to reduce the gain on the EVC, however doing this you will not be able to reach you pre-selected boost since the gain is to compensate it. Though this would only happen when my boost is set at .75 to .85 bar. Anything lower then there is no jerk at all. To work around this, i set my boost higher (.8 bar) but still remain the gain setting to reach my .75 bar.

I was told that the boost climb up too fast that we get a slight surge so the ECU will cut the fuel or ignition. Just my 2 cent.

[benl1981]

Is this on the stock turbo and/or stock ECU?

Thanks

[Sydneykid]

Have a look at the values in those load points for both ignition and fuel. I find quite often that dyno tuners don't pay much attention to off throttle load points. So they leave "untuned" points where there are large changes in values between adjoining load points. Look to have some progression, like 15, 18, 22, 25........not 15, 25, 25, 25 etc. The engine doesn't like 10 degrees changes in ignition timing between 2 load points or 40% more/less fuel.

Reversion with a GCG high flow, I don't think so. I have never seen it and I am personally on my 5th GCG high flow on RB20's, RB25's and RB25 Neos. Plus about 20 or so more that I have tuned. They are simply not that big a turbo.

cheers

[benl1981]

Thanks SK. I just had a look at the hand controller. On the fuel maps there is a jump from 100 to 114 going from load point 9 to 10 in the lower rpm range.

That only corresponds to 14% increase does it? Not close the 40% you are referring to

Doesn't seem linear like the other cells.

Anyone able to tell me if this is normal?

The airflow is jumping around with a very steady throttle input...

Short clip of Airflow jumping around

Edited June 13, 2007 by benl1981

[Abo Bob]

I think SK has it. I used to have a pretty much perfect tune til I messed with it. Later I got someone to touch it up. He removed quite a bit of fuel in some low load points. That seemed to introduce this jerk you guys are talking about.

I was going to (one day ) tempoararily richen the whole map to test if it was simply the AFR but I see now it might just be the difference between tuning points.

[benl1981]

Well it looks like I have close to base PFC map around the points where I am having issues..

I tried adding 20% fuel to the whole map - on the HC but didn't really help ..

Thanks

[Cubes]

As I think I said previously.. when i get this little jerk my load points increase, tracked down to the afm being disturbed by the plumb back bov or no bov (when I blocked it off the problem became worse). So watch the map trace when it does it and see if your issue is the same as mine.

A larger and or slightly longer afm to turbo pipe 100% sorts this problem.

BUT SK may be on the ball with his suggestion to check the tuning.

[benl1981]

Cubes did you fix your problem with a longer pipe. THere isn't that much room there though.

Yes I see mine spiking down vey high on the map trace when this happens sometimes. 4800mv by 3000rpm - I don't think so!

What sort of turbo do you have cubes?

Also,

Can other people tell me when they look at there map trace if they give it a bit in 2nd or 3rd for a few seconds (till about 4000rpm) then back off does the map trace jump straight back right up the top. Mine jumps through some other load points on the way, then once it hits the top it sometimes jumps down again for a second even though the throttle is still fully closed. Not sure if it did this with the stock turbo.

At lower load points when I back off it jumps straight back up the top.

Thanks

Edited June 14, 2007 by benl1981

[benl1981]

anyone checked their map tracer lately