Evo 8 HARD bucking on tip in

Hi folks, I have this heavy bucking issue when I’m into the power, say I’m at 50 mph and 1 bar of boost, engine pulls hard no complaints there…now if I let off the throttle and apply a small amount of throttle the bucks violently until I either let off totally or give it more throttle and then it’s back to normal.

the faster I’m going and the higher the revs and boost pressure are the more violent the issue is.

It seems like if I haven’t let off enough or let of quick enough and we’ll just imagine for arguments sake , that there’s still pressurised air in the pipework/inlet manifold. I can here the turbo almost getting rid of its air then when I apply more throttle it quickly starts to go into boost then it bucks like crazy!

some Evo people have suggested it’s a BOV issue, (I’m running a vent to atmosphere valve and stand alone management, no AFM, manifold absolute pressure is used as load reference) I temporarily pulled the vacuum line off the BOV and the bucking wasn’t as bad, so it makes you think it’s something air related.

I do run a fairly small turbo, GTX2867 .72 rear, so it does respond VERY quickly! I can lift off and instantly reapply the throttle and it’s back up to target pressure INSTANTLY! I’m running a max of 1.6 bar although I can be as low as 0.8 bar and the issue is still there. I can’t see it being a surge issue as the problem can be there at low boost pressure plus the compressor housing has an integral Anti surge design.

stock inlet manifold with a 78mm Bosch E throttle, I’ve played about with my throttle targets to see if I could see if anything changed, it didn’t, I tried cutting the target in half (100% is now actually 50% throttle blade opening) again, no change.

My BOV is attached to the stock location so approximately 150mm from the throttle body, Vacuum line attached to one of the nipples on the plenum, it’s a Forge twin piston vent to atmo valve just for reference, it won’t ‘hiss’ unless I’m running at least 1 bar of boost, although I’m tried pulling the vac line - could it be something as simple as a BOV issue?? I know on stock Ecu the bucking thing is quite common and it seems to be related the BOV issue running in conjunction with a Air flow meter.

Air intake is a simple 4inch bend to match the turbo inlet, and a large K&N filter

If it matters, It’s on Maxxecu Race, E throttle.

Any help would be much appreciated as this is driving me mad!! I’ve provided as much info as possible, the car runs and drives superb under any other condition.

Thanks

Can you capture a log of the problem happening? What is happening with the boost and/or manifold pressure? What does the lambda reading show? What is the ignition timing when the bucking is occurring? I'm guessing pulses in the intake manifold are causing big fueling and/or ignition changes which leads to a torque variation (i.e. bucking).

It's possible you are just into an area of the fuel and ignition tables that aren't tuned well. Do you normally have overrun fuel cut-off? Perhaps by not closing the throttle completely, running on the lowest load area of the fuel / ignition table, and this needs tuning.

In general avoiding large changes in either ignition or fuel over the area it is operating will reduce the torque variations.

Hi David and thanks for the Response, I understand everything you mentioned, the ign timing around the problem area are the sites that are tuned, any sites that are not accessible during the tuning all have sensible numbers, there are no big jumps and generally the ignition table has a nice shape to it, again i understand what you mean.

I do have a log of the problem and i don’t know why I didn’t just attach it to my description!

decel fuel cut off is disabled as i like to have a bit of fuel running through the engine, it may help with cooling after it’s been driven hard plus i like the burbles (no silly fake pops & bangs though! It does that naturally now and again when the exhaust is hot) I could understand it if I had a couple of rows of negative timing along the bottom but I don’t.

As for manifold pressure - if it’s happening along the bottom of the fuel & ignition tables then I’d assume it would be in vacuum?? Unless I’ve read your reply wrong.

I think the best thing to do is to go out a get a fresh log and make it happen a good bit so that it’s easy to spot in the log file, at least it will take all the guesswork out of it!

There’s nothing radically different in my tables to what you’d ’normally’ expect to see. I keep thinking back to when the car was 100% stock and it would do the bucking but not when I’m tipping into the high boost/rpm/load areas, stock it was horrible to drive at very low throttle and rpm like for instance if you were crawling through traffic kind of thing, although this now is very violent it does feel like the same thing, there’s got to be something related there!

Bear with me and I’ll upload a sizeable log, I assume it’s fairly simple to add files to this forum??

Thansk

I wonder if this is a form of compressor surge, does your ECU allow reducing the boost target with throttle position? You could let the wastegate dump some of the pressure so the blow off valve didn't have to do all the work.

Get a good log, if you can't up load it directly (see the "Choose File" button at the bottom of the page), then try putting it in a zipped folder, and uploading the zip file.

Another choice is to upload it to somewhere like Google Drive, and make it sharable to "anyone with a link", then paste the link into your forum post.

David - Yes the does allow that, I’m going out tonight to get a log, this will be that one time it doesn’t do it lol

Yeah, as said in my opening post, I’m with you and thinking it may be surge but even at only 0.9 bar?? Guess we’ll find out! I am due to fit my Artec Manifold and G25-660 turbo so, though it would be nice to know why it’s doing it, even just for curiosity sake, knowledge is power they say!

thanks, speak soon this time armed with some proper data