help for light load bucking with cam

nate · Post by **nate** » Thu Sep 15, 2005 9:41 am

Since this requires a long explanation, I'll give the quick fix first and then explain why I think it works.

Big cam/high compression will require retarding idle and low load timing more than small cam/low compression.
This may work out to something like - F cam 12:1 comp needing -15 degrees timing, E cam 9:1 needing -8 degrees.

My theory - Light load bucking and lopey idles are caused by intake manifold vacuum pulling exhaust (from the
combustion chamber) into the intake port. This causes such a strong dilution of the intake charge that you get
intermittant misfiring.
Engines will stop bucking (and the idle wont sound lopey) once you reach a certian rpm OR load. Why?
I believe that with rpm there becomes a point where the intake charge has enough momentum to resist
backflow. With load, you are reducing intake vacuum which gives it less ability to pull spent gasses out of
the combustion chamber.
Since we can tune the engine, we have direct control over the load it runs at. Retard the timing and you
will increase the "load". This has the direct effect of lowering intake vacuum. Retard the timing enough and
you can get to a point where the reversion gets small enough to not cause misfiring. The retarded timing
also helps smooth out the erratic cylinder filling (which makes things smoother) and tends to "deaden" the
stronger pulses when the cylinder does fill well. I think it also increases the chances of lighting a diluted
mixture due to the effective compression being higher (the ignitable molecules are closer together).
Another consideration is fuel mixture. If the fuel mixture is not quite where you want it, I think it's better
to error on the slightly rich side. This is due to the fact that a lean mixture burns faster (efectively advancing
the timing) and it is more likely to not burn completely (ignitable molecules are farther apart). You can
data log the load-rpm range where the engine bucks, and retard the timing only in those cells. This way when
you roll into the throttle you will still have the advance you need to accelerate. Try to blend the timing on the
border cells so that you dont suddenly jump from 15 degrees of less timing to full timing in one cell.
Thats it. Hopefully you can now idle around a parking lot without feeling like you're about to snap a u-joint.

SHaFT7 · Post by **SHaFT7** » Thu Sep 15, 2005 10:30 am

hrm, i don't understand all of it, but as i'm having very similar issues, its something i'm going to look into :D

CManT1914 · Post by **CManT1914** » Thu Sep 15, 2005 4:31 pm

Good info, most of it makes sense to me. I have bucking below 2000, although it isn't real bad until I hit 1500 and below and I'm off the throttle. Then it can get pretty bad. :(

SHaFT7 · Post by **SHaFT7** » Fri Oct 28, 2005 4:59 pm

you know what's interesting...i have the low load bucking problem, but i also had a huge problem with the car falling on its face on any load over 30% but under WOT. i ended up fixing 95% of it by adding about 8 deg of timing on the 3 timing tables (which i have all set to the sealevel table).

i wonder if reducing the timing back to stock and then lower, in the low load areas of the table will help the bucking.....

i did notice back before i had the tweecer, i ran spout out 20 deg and used a btm to pull timing under boost, that i had almost no low load bucking problems (this was a stock longblock though, and now i have a full 306 combo)

final5-0 · Post by **final5-0** » Sat Oct 29, 2005 8:01 pm

I can say that playing with the spark in the lower load rows helped my low rpm, light load, cruise around town, bucking driving conditions.

It has been so long ago that I played with that stuff I don't really remember for certain but I wanna say I dialed the spark back a bit.

Another thing that helped my combo was I always had a lean spike and a slight dead spot if I matted the skinny pedal from a cruise. I lowered the value in the manifold volume scalar and the wb showed the lean spike was gone and the response picked up a bit as well.

Grady

Methodical · Post by **Methodical** » Sun Oct 30, 2005 2:04 am

Maybe there's something to this. I initially had the global spark adder at 4 with the dist. at 10*. I've been researching this forum and noticed the consensus was to set timing using the tables. Therefore, I eliminated the MBT, BDL tables - I forget the other table right now but you guys know which one I am speaking of by inputting 55s in the entire table. I then reset the global spark adder back to 0 and noticed my idle issues had improved. My idle was pretty good with no bucking but when the idle settled it acted as if it wanted to stall and sometimes it would. Setting timing back to 0 help with that issue. I don't now if it's a coincidence or not but there was an improvement in the idle. To date my car has not stalled (Knock of wood). Now, I am only tweecing the upper 3 rows (part and WOT throttle) of the base spark table. Again, maybe there is something to this. Try it and see what happens. The good thing with self tuning is you can experiment and if it doesn't work just switch back to the tune that works - and what's even better is you can do it on the fly. Just One Man's Opinion. Peace.

CManT1914 · Post by **CManT1914** » Thu Nov 17, 2005 3:52 pm

I went ahead and tried this the other day. I had some minor bucking below 2000 rpms, but more severely below 1500.

I didn't have a datalog to see at what load percentage the bucking was worst at, so I just messed with the tables from 2000rpms and down, and 40% load and down. I basically divided all those figures by half, it might have been too much, but it got rid of 95% of my bucking issues. Now I can go back in and add a little more until I get it to right where I want it.

The only problem I can see this causing is loss of fuel economy, since spark is added to increase effiency right? Anybody know if there's some truth to that?

Either way, this is a good method of helping with the low rpm low load bucking issues us cammed guys have!

Jack Hidley · Post by **Jack Hidley** » Thu Nov 17, 2005 5:14 pm

I'm sure that it will cause some loss in fuel economy, but it's cheaper than replacing u-joints every 100 miles:)

JuiceSC · Post by **JuiceSC** » Thu Nov 17, 2005 5:57 pm

Getting the maf right solved all my low load bucking a while back. With the stock timing tables.

Chucko · Post by **Chucko** » Thu Nov 17, 2005 10:11 pm

Does injector timing have any effect on light load bucking?

CManT1914 · Post by **CManT1914** » Thu Nov 17, 2005 10:36 pm

JuiceSC wrote:Getting the maf right solved all my low load bucking a while back. With the stock timing tables.

Yea, I'm still dialing in my maf transfer. This is just kind of a quick fix until I can get everything else straightened out. Then I plan on putting the tables back to stock (or as close as I can).

Chucko wrote:Does injector timing have any effect on light load bucking?

I'm interested in this as well. I changed my injector timing, but haven't loaded the new tune yet. I will tomorrow hopefully and let you know what happens.

JuiceSC · Post by **JuiceSC** » Fri Nov 18, 2005 7:00 am

Chucko wrote:Does injector timing have any effect on light load bucking?

A while back I would have said "yes". At this point, I'm not so sure. I tried something a few weeks ago and didn't like the way the car ran. Can't quite put my finger on it, the datalog looked fine and the car drove well. But something was slightly different and I didn't like it.

gw342 · Post by **gw342** » Fri Nov 18, 2005 11:20 pm

Well gents, I am not using the Tweecer (but no knocks to you guys-I think its a great tool), but I would like to offer a tid bit here. I had used a PMS for several years and then the AEM. Anyway, I ran the AFM N91 cam (240/248 at 050, 110lsa) which is quite the beast, even in the 11:1 347 I had been running.

The bottom line is, I am agreement with regard to reduced timing and enrichening mixture. Nate is spot-on, I had discussed this with other tuners and concluded the same thing. And it works.

It took the versatility of the AEM for me to test out this theory, and I had great success. The motor bucked below 1800rpm really bad. Eventually, I wound up with approximately 22-24* total in it from off idle (950rpm) ramping up to about 28* at 1800rpm, at low loads. I could literally let the clutch out and the car would motor itself through a flat parking lot with almost zero bucking. Of course, with the AEM I was able to configure my load and rpm breakpoints to really define the target areas and tune the bucking down.

Now, I had pondered over the notion of injector timing and what might happen. With the AEM I was able to try this. My end result was no noticeable difference whatsoever. I did verify the injector timing was at work, by messing with upper rpms and loads, and it did have a negative impact. Down low, however, no change. I am aware of the EECs use of the maf for load and such on the 94-95 cars, and I was using a MAP/speed density arrangement with my AEM. But the basic idea is the same. Perhaps you guys will find different results (and I am very interested in those results), but those were mine.

Good luck.
Gary

CManT1914 · Post by **CManT1914** » Fri Nov 18, 2005 11:32 pm

Gary, thanks for that info on injector timing. I had read somewhere that the Cobra R (ZA0) injector timing (136 in all cells) was supposed to work well for a combo like mine. I was gonna try it, but decided to wait to see if anybody actually noticed a difference by messing with it. Now I think I might run a datalog through EA to see what it comes up with, if I ever feel like it, lol. Sounds like it doesn't really matter.

gw342 · Post by **gw342** » Fri Nov 18, 2005 11:45 pm

Thanks CManT1914.

You know, I forgot to mention in all that stuff, the a/f ratios I found to work well. My motor ran fine at 14.7, however under bucking rpms and loads I found 14.2 to 14.4 was better. Not that much of a change, but it did help. From 14.2 to approximately 13.5 I saw not difference, and below 13.5 the engine would run poorly again. Of course, a/f ratios are just guidlelines, obviously your ride may want something different.

I had read elsewhere about teh injector timing, and discussed with the head AEM tuning engineer about injector timing. He and I drove in my car together, with him at the laptop! We played with injector timing to no avail, which re-affirmed our thinking on what was happening in the engine, as Nate mentioned so well above.

nate · Post by **nate** » Sat Nov 19, 2005 8:33 pm

I just wanted to say thanks for the positive feedback on my origional post. It made my day to hear the positive results some of you have had!
On the subject of injector timing... I seem to be the rare exception here. With the inj timing retarded until sometime after the exhaust valve closes, my low speed driveability (and idle) are terrible. Am I one of the only people experiencing this? Most people seem to have no significant changes in low speed driveability. I havent had time to do enough experimenting to make any educated guesses as to why this is.

CManT1914 · Post by **CManT1914** » Sun Nov 20, 2005 1:07 am

Nate, I'd love to be able to help you out on the subject of injector timing, but I have not messed with mine. (I originally changed it to tha ZA0 settings (heard those were supposed to work good for my setup), but I changed my mind and changed them back)

LEE93COBRA · Post by **LEE93COBRA** » Tue Nov 22, 2005 12:12 pm

my low rpm bucking problemn I had with my car was due to the MAF curve being to rich on the bottem end. Lean'd out the first 7-8 cells and it cleared it up

CManT1914 · Post by **CManT1914** » Tue Nov 22, 2005 11:40 pm

Well I filled up the mustang today. After messing with the low rpm low load timing, I was worried about a loss of fuel economy.

Now this tank of gas also included quite a bit of WOT driving (lots of datalogging), more so than any tank within the last month. I was averaging 14mpg before.

I filled up today and got just under 16mpg on the last tank! I am REALLY pleased, especially considering how hard I was driving the mustang in this time. I think the increase in gas mileage might be contributed to my being able to cruise at a lower rpm. For instance, before I would have to keep the revs above 2000 rpm to stay comfortable. Now I can cruise at any rpm, and I usually am around 1500. So I think that is the major factor in the gas mileage. Even if it's a temporary fix until I get my maf transfer down good, it's definitely worth it!

amvantage · Post by **amvantage** » Thu Jul 06, 2006 6:55 pm

i've got the same problem, i have a tweecer rt it reads the eec and write to the tweecer but i cannot datalog, help?

TripleBlackVert · Post by **TripleBlackVert** » Fri Apr 04, 2008 10:54 pm

i have this as well. very bad bucking/stumbling while cruising at steady throttle on flat ground.

what i noticed about my situation was that at the time of the stumble, my maf reading would drop considerably, sometimes to about 1/2 the value of the surrounding readings. at first, i thought i had a bad maf, but then i remembered that the idle is fine, and so is the acceleration. if the maf was bad, i would also have problems there as well.

my current hypothesis is similar to what nate says, with one additional statement. when the intake vacuum keeps the exhaust from exiting, the overlap in my cam actually allow the exhaust to flow back through the intake valve and into the intake. this would cause the air to stop flowing for a split second, which would explain the observation of the drop in the maf reading at the time of the stumble

i followed the recommendations in other threads and filled all my spark tables except the "borderline" table with 55. then i set my global adder to 0 and lowered the values between 1200 and 2500.

it seems to have helped alot. i have very little stumbling currently, but i have not driven it enough yet to feel confident that it has been taken care of.

thanks EEC Tuning!

A6intruder · Post by **A6intruder** » Sat Apr 12, 2008 12:24 am

I just read through this thread about bucking. I didn't see anyone consider

ITHBMA Air - Throttle Body Air Flow (scalar)
and
Idle - IISC neutral idle air (function)

ASSuming you have your MAF function, injector slopes and timing reasonably close you will see things smooth out if you tweec those two parameters.

My understanding is that these are basically default values used if the EEC doesn't like the quality of data it gets from the MAF at light LOAD.

I know they have always smoothed things out for me when I make hardware changes.

Dart347 · Post by **Dart347** » Tue Apr 22, 2008 11:36 am

I just got my TB Airflow and ISC Neutral Idle Air setup properly, I think, and I still have a slight surging at light loads from off idle to 2k. I datalogged my timing and it was at 40 degrees. I used the real time monitor to watch the Spark-Base Sealevel and Sealevel LOM tables and I see which cells it is using, but none of them go up to 40 degrees. I had about an extra 4 degrees of advance on top of what is in these tables. Where is this extra advance coming from?

I guess that I will try lowering the values in the tables to see if that helps with the surging. It is pretty slight and not very noticeable. It is really hard to make it buck. It only bucks if you are idling in gear and then try to give it a tiny bit of gas and not use the clutch. It has a pretty sensitive gas pedal, so I am not sure if it is actually bucking or if it is just me opening and closing the throttle.

Anyone know if gw342's total timing #'s included the 10degrees of base timing? If so, then I am waaaay higher than his at high 30's to 40.

A6intruder · Post by **A6intruder** » Wed Apr 23, 2008 12:16 am

The EEC ASSumes you have the dizzy set to 10 degrees advance. Look at a stock timing table. Your engine won't neccessarily be that far off of a stack table at the low RPMs you are talking about.

Also, "Bucking" is usually at very light LOAD, barely touching the gas. I had to put an extra spring on my throttle return with my stock throttle body. the stocker must have lost some of its "spring". I found myself with a lot of driver induced "bucking". I can't remember if I went to the hrdware store or if I found some springs at the auto parts store.

Good luck,

Dart347 · Post by **Dart347** » Wed Apr 23, 2008 10:57 am

I see that my extra 4 degrees is coming from the Scaler "Spark-Adder for part throttle." I took 6 degrees out of the light load cells up to 2500 in one of my tunes. One of these days I will see if this helps any. My bucking is Very minimal.

I am using BE and I do understand that all of its numbers include the 10 degrees of base timing that we are supposed to have set. My question was regarding gw342's tune. He said, " I wound up with approximately 22-24* total in it from off idle (950rpm) ramping up to about 28* at 1800rpm, at low loads." He is using an AEM and I was just wondering if these numbers that he stated were just the advance from the EEC or if it included the base. He said total so I am going to assume that these #'s include the base.

patrickmx2 · Post by **patrickmx2** » Tue Apr 29, 2008 11:59 am

I found that with my E303 cam in the past I just reduced my timing in the low loads where I was at 45mph or lower (AOD with 3.31 gears) and where the bucking occurred the worst. I eliminated all bucking and a slightly rich mixture insured fewer misfires than being slightly lean. Most people after that questioned if I had a cam at all.

I've done the same thing with B303's in others cars. Also I noticed bucking is worse on cars with freer flowing exhaust. I don't like drone at all on a 100 mile around trip commute so I installed OEM mufflers on my 90 Fox that came off of a 2003 GT. This helped stabilize the car even more in terms of idle and I could run more timing with less fuel to gain back MPG.

JScritch · Post by **JScritch** » Tue Apr 29, 2008 11:03 pm

Dart347 wrote:I see that my extra 4 degrees is coming from the Scaler "Spark-Adder for part throttle."

Can't you just zero out the spark adder for this or would that cause other problems?

Dart347 · Post by **Dart347** » Wed Apr 30, 2008 10:59 am

Yes I can. That is actually what I did and I also pulled 6 degrees out of the light load low RPM cells. I just haven't driven the car yet to see if it did anything. The races were cancelled last week and I can't go tonight because it is my daughter's 5th birthday today. I won't have time to drive it again until next Wednesday. I will try this tune out on the way to the track and see if it made a difference.

TripleBlackVert · Post by **TripleBlackVert** » Wed Apr 30, 2008 3:20 pm

my bucking actually turned out to be more related to the orientation (aka "clocking") of my mass air meter.

it was at 12:00 and it wasn't seeing the air because of the 90* bend in the tubing in front of it. after i clocked it to 9:00, the bucking went away completely. i even was able to go back to my original spark advance values.

help for light load bucking with cam

stock l/b and intake, same issue with light load bucking

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