bucking @ light throttle

[C7vette2014]

Ahh lol..damn. okay ill fix it...you guys are awesome figuring out these little mistakes. Not sure how long it would have stayed like this. Im gonna smooth out my VTT some more and fix that VVE area and report back.

[C7vette2014]

This is a 17 vette I'm tuning right now with the 225 cam, long tubes and a manual tranny.

VE table

Attachment 146417


Airmass Torque Model

Attachment 146418

MAP Torque Model

Attachment 146419

Log with light throttle. I should note where you see it dropping slightly to the left, he's actually closing throttle down to 3% at those points, so timing is stable. Fueling and final torque is also still getting corrected and will continue getting tweaked until fueling is right.

Attachment 146420

His idle timing and throttle. Again, fueling and torque are still getting tweaked.

Attachment 146421

So again, fix all of these and it should get you pretty close. Everything changes torque. Don't care if it's a cai. If it requires remapping the air models, torque is changing. It's just some things are more intrusive to it.

Hey greg, do you got a screen shot of your -10 area on your vtt? Wanna see if its just as smooth...i got all mine pretty much smoothed out except that one.

[GHuggins]

Here you go. They aren't really smooth though. They were shaped off of the VE changes and just worked out that way for this cam. They do curve side to side and vertically. Just keep in mind that anything that affects fueling changes torque, so all the way to injection timing and O2 settings. Hopefully all of this will help you.

Airmass 225 2.jpg Airmass 225 3.jpg Map 225 2.jpg Map 225 3.jpg

[danmw2003]

I feel like injector duty cycle being so low, likely near its lower limit causes some of this. Injectors cant possibly be accurate at or near its lower limit. I could be wrong about this, but I suspect its not helping.

[C7vette2014]

They were shaped off of the VE changes and just worked out that way for this cam.

what do you mean by this? can you go in more depth please.

[M3T1]

Without digging too deep, torque model and DD are the big two players. If you have a high stall converter there are a couple of background tables that can cause this in lower gears. I know your VE is wrong - look at it in 3d format - big spike in the lighter throttle lower load that needs to be fixed. I know the torque model is wrong - nose diving pretty good for idle without any corrections off idle - doesn't coincide with your air models. DD looks stock below high loads. All of those will play into what you're seeing and feeling. Then sometimes it's dang near impossible to get rid of all of it....

I second this. However, it is possible to get rid of it almost entirely. It just takes a long time. Get your fueling dialed in first (VVE and MAF), and then see what torque management is doing. One thing at a time. You have no corrections besides idle, as GHuggins mentioned. So, you're going to have to raise/lower the torque tables in the region the behavior is occurring. If it's pulling timing on you, it thinks you're making more power than you're commanding. If it's adding timing, it's the opposite. Typically with a cam, you have to lower more than just the idle region, as you're pulling more vacuum at lower RPMs. If it's fueling, you'll be fighting the closed loop being wonky and all over the place. If it's Torque Management, it'll pull timing, then add, then pull, etc. and it'll buck like you're describing. If it's a combination of both, it'll drive like absolute shit.

[C7vette2014]

I second this. However, it is possible to get rid of it almost entirely. It just takes a long time. Get your fueling dialed in first (VVE and MAF), and then see what torque management is doing. One thing at a time. You have no corrections besides idle, as GHuggins mentioned. So, you're going to have to raise/lower the torque tables in the region the behavior is occurring. If it's pulling timing on you, it thinks you're making more power than you're commanding. If it's adding timing, it's the opposite. Typically with a cam, you have to lower more than just the idle region, as you're pulling more vacuum at lower RPMs. If it's fueling, you'll be fighting the closed loop being wonky and all over the place. If it's Torque Management, it'll pull timing, then add, then pull, etc. and it'll buck like you're describing. If it's a combination of both, it'll drive like absolute shit.

This is alot of good info. After correcting my VVE table with the correct axis, i will have to start over again and dial in my VVE. Then ill attempt to massage my vtt"s again. Thanks for the info!

[edcmat-l1]

I second this. However, it is possible to get rid of it almost entirely. It just takes a long time. Get your fueling dialed in first (VVE and MAF), and then see what torque management is doing. One thing at a time. You have no corrections besides idle, as GHuggins mentioned. So, you're going to have to raise/lower the torque tables in the region the behavior is occurring. If it's pulling timing on you, it thinks you're making more power than you're commanding. If it's adding timing, it's the opposite. Typically with a cam, you have to lower more than just the idle region, as you're pulling more vacuum at lower RPMs. If it's fueling, you'll be fighting the closed loop being wonky and all over the place. If it's Torque Management, it'll pull timing, then add, then pull, etc. and it'll buck like you're describing. If it's a combination of both, it'll drive like absolute shit.

This is alot of good info. After correcting my VVE table with the correct axis, i will have to start over again and dial in my VVE. Then ill attempt to massage my vtt"s again. Thanks for the info!

Except for the fact it's at least partially incorrect. More camshaft equals less vacuum at lower engine speeds. Always.

[M3T1]

Except for the fact it's at least partially incorrect. More camshaft equals less vacuum at lower engine speeds. Always.

Sorry, I should have phrased it better. I meant 'lower' vacuum. When I said 'pulling more vacuum', I meant that the vacuum is more intense, meaning a lower value. I phrased that poorly. You are correct. That is why you have to lower the torque model in the associated region so that the engine expects to make less power in those regions.

[gtstorey]

Vacuum won't be more "intense". You can actually get into cams that won't let vacuum assist brakes work correctly because the vacuum is less.

[edcmat-l1]

Sorry, I should have phrased it better. I meant 'lower' vacuum. When I said 'pulling more vacuum', I meant that the vacuum is more intense, meaning a lower value. I phrased that poorly. You are correct. That is why you have to lower the torque model in the associated region so that the engine expects to make less power in those regions.

With a larger cam, there is LESS vacuum, not more. The vacuum is LESS intense, not more.

[smokeshow]

Never gonna be a good idea to set the high speed airflow transition right around where you have a problem like this. 1200rpm is where dynamic airflow disables... And is the cam supposed to be locked out? Not sure why you've got a VVT cam but all the actuator diagnostics are disabled. You wouldn't know if the actuator had a problem. And the cam is definitely moving when this is happening....

[M3T1]

With a larger cam, there is LESS vacuum, not more. The vacuum is LESS intense, not more.

Vacuum is defined as lower barometric pressure than the local atmospheric pressure. So, the more of a vacuum you have, the less pressure you have. If the vacuum was less intense, you'd be closer to atmospheric pressure AKA a higher intake manifold pressure reading. I'm not sure where the confusion lies. As stated by myself and others above, you will have less manifold pressure at idle with a larger camshaft. I think we are all describing the same effect differently.

[blindsquirrel]

you will have less manifold pressure at idle with a larger camshaft.

Higher MAP, less vacuum, closer to normal atmo... that's not less pressure. Less negative pressure as referenced to baro, maybe.

[M3T1]

Higher MAP, less vacuum, closer to normal atmo... that's not less pressure. Less negative pressure as referenced to baro, maybe.

Interesting. I thought the higher MAP was due to having to raise the idle RPM with a larger camshaft. I would have thought that the overlap between intake and exhaust valve would cause pressure loss, as you'd be allowing air to escape.

[ns158sl]

Higher map with a Cam is byproduct of natural egr. Need more airflow from the inefficiencies of the camshaft to meet the same engine speed/torque at idle regions. More throttle/airflow at idle will reduce restriction to the intake, which would be less vacuum/lower MAP. Or so i think. Some dude on here might chime in and give the explanation with physics involved lol.

[blindsquirrel]

Interesting. I thought the higher MAP was due to having to raise the idle RPM with a larger camshaft. I would have thought that the overlap between intake and exhaust valve would cause pressure loss, as you'd be allowing air to escape.

Nope, raising idle speed & adding timing increases vacuum - but only up to a point. Beyond some point opening the throttle more will not increase vacuum (numerically lower MAP reading), and adding more spark timing doesn't add any more RPM.

[C7vette2014]

Never gonna be a good idea to set the high speed airflow transition right around where you have a problem like this. 1200rpm is where dynamic airflow disables... And is the cam supposed to be locked out? Not sure why you've got a VVT cam but all the actuator diagnostics are disabled. You wouldn't know if the actuator had a problem. And the cam is definitely moving when this is happening....

Where do you reconmend the transition rpm should be at? Stock i think wants it at like 4k, cant be for sure, not in front of my pc atm. The cam i have isnt a VVT cam, ita suppose to be phased locked.

Anywho, ive been dialing in the VVE closer and closer and its definitely calmed down the bucking alot! I still see TMA being pulled, but its seldom and less timing....more like 3-5 degrees instead of 10-15 like in my original log. Im gonna keep at it.

Once im happy with my VVE table, im gonna try multiplying my DD table in the lower section about 1.02 and see if tgat helps.

[GHuggins]

You should be able to use the DD tool now and once you're nearly done to do the final corrections to the DD table. It actually works really nicely and Michael did some great work making it.

Here's a shortcut for it - https://order66.shinyapps.io/driver_demand_editor/

[C7vette2014]

You should be able to use the DD tool now and once you're nearly done to do the final corrections to the DD table. It actually works really nicely and Michael did some great work making it.

Here's a shortcut for it - https://order66.shinyapps.io/driver_demand_editor/

Awesome, ill take a look at it ! Thanks Ghuggins