In low load I am experiencing bucking.

My log this morning of event shows 16 mph in 2nd gear of 4(manual), 1200-1400 RPM, 40-50 MAP, closed loop, I have a timing of 34-36 degrees.

Trans and drivetrain is original with 125K on it.


Should I pull some timing?

 

Wow ... that does seem like a lot of timing at that RPM .... I thought "all in" should occur at 28 to 30k ??? I'm still very close to idle SA (20) there ....???

 

Could try that. Does seem like alot of timing for 1400rpms at any KPA, What sort of AFR or BLM numbers are you seeing.

What mask are you running, i'd like to look and see what the stock timing is under a load/rpm like that. You said MAP, so 7730 or 7727 ecm?

You could toss up a Datalog for us to look at in excel format.

 

A tuning general rule of thumb says at that map and rpm 10-25* is a safe place to start.

 

Thank you.

I am using a timing map provided to me by Fast 355. Aluminum heads 9.5/1 static CR. I am using late edition Edel RPM heads with 10.0/1 SCR. Actually that SA Map of his is more agressive than what I am using today. I did not copy it exactly. I am almost as aggressive. Maybe 95% of values.

I have zero KC's.

I see the iron headed LT1 SA MAP has 27.5 deg SA at 1400/50.

ARAP alum head L98 is approx 40-41 at same rpm/map.

I will pull timing in 5deg increments and see how it responds.

My datalog of bucking event shows nothing. I am in CL there. BLM 124-132 day to day. I idle at 21 d OL. I transition to CL at about 12 MPH.

 

I've had that much timing in my bins with no issue. If it takes it, its ok but alittle less wouldnt hurt. I idle at 28-30 deg

Stock ARAP bins are like that too but MAF. I've had 39-41 deg in a 360" motor just off idle of 800 rpm and the car loved it. Big cam it needed timing to move around

I'd say first try less timing and if that dont work, try a tad more if it takes it. Then try richening it up. Richer seems to work better for me

 

My cam is 224/230 @ .05 114 LSA

 

Obviously the lowest map for that rpm is what you want, torque is key.

 

Would not lower timing values produce lower MAP/higher VAC reads? If so then I should pull timing as I stated. I will try incrementally.

 

I know for idle tuning, more timing generally gets me more vacuum. Timing is torque so i would think more increases vacuum at rpms or reduces map. Problem is fueling could be off at those points so more timing may not help solve problem alone. It may want more fuel. My car fells better being richer

 

No not necessarily, it all depends on the total setup, heads, cam, intake, exhaust and even gears. Example, aluminum heads on an otherwise identical setup will require more timing to get the same kpa as the iron heads when swapped. In this example its the thermal efficency of the heads metal that changes, dissipating the heat and lowering pressure in the chamber quicker. But even exhaust with more back pressure will raise temps in the chamber, gears too, they change the load, the intake will change torque range. As you can see every setup is different, thats why I never just swap over timing tables. The numbers I posted are just a starting point, I have used a landscape that I copied from a EFI live tuning books, its just a general starting point. If it helps I'll post the pic of it, I use it all the time.


I have a question for you, did this just start happening out of the blue or have you changed things and now its bucking?

 

This season of driving/tuning started with going to a more aggressive timing table. I started tuning with a stock LO3 table years back. The bucking was less with stock L03 table. I added timing to it mostly at higher map >80 and WOT. Eventually I determined the car was sluggish in every day driving as well as spirited entering xway getting up to speed. I found a LT1 iron head SA table and found it to be way more aggressive for SA especially in low load areas. Used it and no KC and engine picked up responsiveness. It became very clear more timing works. Went to ARAP but only about 1/2 way from LT1 to it. Engine picked up more crispness and apparent TQ. Now I am almost at ARAP level of SA.

Adding more fuel may help in OL (<12 mph). It bucks there too. I recall taking fuel out of VE tables as was rich at idle(OL) yet commanded to 14.0/1 in OL A/F tables. I may need to add more fuel at RPM/MAP off idle in transition. Manual trans.

 

What's the IAC doing when it bucks? Are you seeing lots of transient movement there as well? Might need to tighten it's range if it is still stock.

 

Good point. I will look at log and see where it is going. I dont expect a lot of movement but I too could slow down its happyness and see how it reacts. One change at a time. 1. I did pull timing 5d but not yet loaded .bin
2. I will fatten up VE some off idle
3. I may run OL >> 12 mph maybe to 20 mph so as to not see the transition OL to CL
4. IAC happyness reduced?

all done individually.

 

I had a somewhat similar issue issue awhile back in that there was too gross of a change between data points for a given RPM/Kpa area in terms of tabled fuel and spark. Look for abrupt changes in your spark and fuel tables as well. Should be seeing easy slopes not walls or cliffs.

An example would be a spark table that goes from 22* to 32* degrees in one or two data points in any direction. It becomes close to being an on/off switch, as there are virtually no intermediary steps available between high and low data.

Idle areas can be like that as well. I had to create kind of a flat spot or bowl that the idle could sit in, around my ideal rpm/kpa point. Otherwise it would be hunting constantly as it went from place to place on fuel and spark charts. I had in my mind that an idle point is exactly that, a POINT on the chart, and it in reality, it turns out that it's an AREA, not a point.

 

So you always had this problem with this setup? what trans and what gears are you running?

 

I checked the slope of my prior SA table B4 I pulled some timing in the area of bucking. It was very smooth transition xcept idle is 21 and then jumps to 26 or so in OL off idle in trasition. But that is not where the bucking occurs. Bucking is specific to 1st gear of 4 at speeds mentioned. Like in a parking lot of roads in my condo complex crawling along trying not to upset neighbors.

gears are 3.07. 4 speed manual. Not sure what first gear is in gearbox. Pretty much typical.

I think it got worse this season after I upped SA table globally cept WOT I am 35.5 d sa and always was. Gas mileage improved considerably due to higher SA.

 

[QUOTE]What's the IAC doing when it bucks? Are you seeing lots of transient movement there as well? Might need to tighten it's range if it is still stock. /QUOTE]

IAC not moving unless I increase TPS% per my logs. IAC is at 3 steps and then TPS moves 4% and the AE is on and IAC moves to 13. This is a 7.4L TB on a 350 cid so maybe the passafeway/pindle is larger. I will reduce it 20%.

I see IAC TF TPS% was set to 2. I moved to 1.

TF min steps was 2 I moved to 1.

I see IAC TF decay filter which may determine the lenght of IAC TF event. Not sure which way to move it. Set to stock LO3 at high 9.75% and low 12.48. Which way to move that to allow a faster decay of event?