Looking for help or advice with E38 surging/bucking at low throttle

[smokeshow]

I have a mild LS3 (mild cam, Mamo heads, light flywheel, supporting mods) in my Miata. I had bucking that was most noticeable in 2nd gear, around 1600 rpm, off throttle. It was *horrible* when the car was first running, the car was essentially undriveable in that region. I have it 95% gone by *reducing* timing in that area, which on my car is .08, .12, .16 g/cyl area in the spark table below 2000 rpm. I've seen some guys post that even their stock Corvette or whatever does that a bit, so I'm satisfied.

I *did* try increasing spark there, but the car wouldn't slow off throttle. It's interesting to me that it seems to work for many people ... maybe their vehicles weigh more than 2400#, so that's the difference.

Not slowing down is an airflow problem. You can technically solve it by pulling all the timing out to neuter it completely for torque production - or you can get it up near MBT where it should be. In either case, the torque delta per degree spark advance is less. That's the goal...getting out of that steep torque change region.

[blakesmiley]

I think you should try on the order of 20 degrees additional advance in the problem area. At least.

+20 will put it back around the stock table in that area. I'll go back to stock in the lower airmass rows and add from there to see if it starts clearing up. At the beginning of this I went as high 48* in the problem area with little difference, but that was before a lot of the recent changes. I'll let you all know how it goes as soon it stops raining here haha.

Will try this out.

[grubinski]

Not slowing down is an airflow problem. You can technically solve it by pulling all the timing out to neuter it completely for torque production - or you can get it up near MBT where it should be. In either case, the torque delta per degree spark advance is less. That's the goal...getting out of that steep torque change region.

By "airflow problem" you mean base running airflow is too high? The car idles fine.

Still an interesting suggestion, I may play around with it a bit ... I'm always willing to learn. Thanks!

[smokeshow]

+20 will put it back around the stock table in that area. I'll go back to stock in the lower airmass rows and add from there to see if it starts clearing up. At the beginning of this I went as high 48* in the problem area with little difference, but that was before a lot of the recent changes. I'll let you all know how it goes as soon it stops raining here haha.

Will try this out.

That looks better. I'd even extend it further into the higher airmass area. There's essentially a torque hole between 0.36g and 0.64g.

Every cam like that I've worked with has needed more spark at low speed and low load on pump gas. IVC is super late, so the charge motion is lazy and compression low...almost always benefits from extra spark to get the burn going.

In the end, you may/likely won't be able to get rid of all of the buck. Even stock vehicles will suffer from certain scenarios that excite the drivetrain just right. Its a mass and a spring, so at some point it'll eventually oscillate.

By "airflow problem" you mean base running airflow is too high? The car idles fine.

Still an interesting suggestion, I may play around with it a bit ... I'm always willing to learn. Thanks!

Not base airflow, min airflow. Its essentially an airflow floor that the throttle follows if your idle airflow falls low enough for some reason. Lot of people like to raise it because its a simple hack to get them idling...but you then suffer from the sail-on effect.

[grubinski]

Not base airflow, min airflow. Its essentially an airflow floor that the throttle follows if your idle airflow falls low enough for some reason. Lot of people like to raise it because its a simple hack to get them idling...but you then suffer from the sail-on effect.

Yeah, that's what I meant. I thought that was an idle airflow target, but I have noticed when logging that after a certain point, I could move it lower and the idle airflow would stay where it was.

Now you've got me *very* interested to lower it and see if the car will slow off throttle with more advance.

[blakesmiley]

That looks better. I'd even extend it further into the higher airmass area. There's essentially a torque hole between 0.36g and 0.64g.

Every cam like that I've worked with has needed more spark at low speed and low load on pump gas. IVC is super late, so the charge motion is lazy and compression low...almost always benefits from extra spark to get the burn going.

In the end, you may/likely won't be able to get rid of all of the buck. Even stock vehicles will suffer from certain scenarios that excite the drivetrain just right. Its a mass and a spring, so at some point it'll eventually oscillate.

I think you have me headed in the right direction! Got to try a few variations and its the best its been yet, maybe 50% of what it was.

Seems to me like VE/MAF models and fuel trim swings might be some of what's left.

You're a god amongst men, and we don't deserve you!

Will keep the thread updated with how it ends up.

more advance v3 screenshot.png

[Alvin]

I'm with smokeshow on this one.. More timing in those areas is usually what helps.