TooSlo86
Club Member
Ok guys and gals, I’m going to try to make some meaningful sense to extract ALL of your knowledge :naughty: This should really stir up some differing opinions (and kill alot of time at work reading all of it), but judging from the threads all over the forums, ALOT of people suffer from the same condition, nearly as many claim they can “tune for it” even with more radical cams, but there is no SOLID information on how. This will be sort of a two-fold subject. Part A will let you project your theory on the physical/mechanical happenings taking place to cause the instability, and Part B will be what you have found to be a successful “work around” for solving the condition. The subject at matter being; tight LSA, larger duration cams on street cars, specifically under light throttle/load in low rpm conditions. Better known as BUCKING :icon_evil.
So to give a little background on why this is bothering me, I went from a 220/[email protected] 112LSA cam to a 238/[email protected] 109LSA installed on a 105 ICL, custom with low lift, and now have SERIOUS low load/speed/light throttle drive-ability issues. (Minor even at 2100-2200 cruising at 60mph) Before you say it’s the nature of the beast, I ask you to answer how it is others can run larger cams successfully under these conditions. Aside from the bucking this cam has VERY good street manners considering it’s size. I have tried very low timing in the area with horrendous results, I have tried high timing, I have ran it as rich as 12.5:1 under the condition, tried various TPS to throttle blade conditions, decreased TP threshold required for recognition from CT to PT, messed with my ISC flow scalar until I went nuts, all with less than ideal results, verified/re-verified my dizzy was stabbed correctly, tried firing the injectors on varying delays, and I’m sure more I can’t think of. I will say from datalogging and watching the screen and w/b, during the bucking my spark is stable, my AFR is stable, and my TPS voltage is nice and smooth during a bucking event. My tune just prior to the cam swap was one that I had been running for quite some time, it ran great on the street, got good fuel mileage, ran hard at the track and made good #’s on the dyno, I used this tune with only minor modifications after the swap.
So what is actually mechanically and or physically taking place in the engine that causes this? On carbed cars it’s my understanding that it is the loss (or disruption) of the vacuum signal through the carb upsetting it’s ability to effectively draw “metered” fuel. On an EFI application where the fuel is injected under pressure I can’t make the connection. Is it that the reversion back into the intake is making it to the maf, upsetting values? (doesn’t look like it on logs) and I can’t believe that due to the stable AFR under the condition. Is it the reversion back into the intake feeding “dirty air” into the cylinder that is currently in the overlap stage entering the intake stroke, or is it the reversion into the intake diluting the cylinder that is well into it’s intake stroke? Or is it none of the above? I’m trying to grasp the underlying cause to better understand what could or would help to alleviate the condition, which brings me to part B;
What have you learned is the most effective combatant against light throttle bucking at low load/rpm’s? Maybe manipulating the volume, or flow characteristics in the intake system? Thinking outside the box on required timing in those areas? Dumping copious amounts of fuel into it in those areas? Pulling fuel? What are your thoughts? I’m open to anything, and I have been using Binary Editor and a Moates Quarterhorse for 3 years and have a pretty good understanding (not a total tuning newbie) so I can try just about anything in the tune.
Thanks, and WAKE UP! It's over.
So to give a little background on why this is bothering me, I went from a 220/[email protected] 112LSA cam to a 238/[email protected] 109LSA installed on a 105 ICL, custom with low lift, and now have SERIOUS low load/speed/light throttle drive-ability issues. (Minor even at 2100-2200 cruising at 60mph) Before you say it’s the nature of the beast, I ask you to answer how it is others can run larger cams successfully under these conditions. Aside from the bucking this cam has VERY good street manners considering it’s size. I have tried very low timing in the area with horrendous results, I have tried high timing, I have ran it as rich as 12.5:1 under the condition, tried various TPS to throttle blade conditions, decreased TP threshold required for recognition from CT to PT, messed with my ISC flow scalar until I went nuts, all with less than ideal results, verified/re-verified my dizzy was stabbed correctly, tried firing the injectors on varying delays, and I’m sure more I can’t think of. I will say from datalogging and watching the screen and w/b, during the bucking my spark is stable, my AFR is stable, and my TPS voltage is nice and smooth during a bucking event. My tune just prior to the cam swap was one that I had been running for quite some time, it ran great on the street, got good fuel mileage, ran hard at the track and made good #’s on the dyno, I used this tune with only minor modifications after the swap.
So what is actually mechanically and or physically taking place in the engine that causes this? On carbed cars it’s my understanding that it is the loss (or disruption) of the vacuum signal through the carb upsetting it’s ability to effectively draw “metered” fuel. On an EFI application where the fuel is injected under pressure I can’t make the connection. Is it that the reversion back into the intake is making it to the maf, upsetting values? (doesn’t look like it on logs) and I can’t believe that due to the stable AFR under the condition. Is it the reversion back into the intake feeding “dirty air” into the cylinder that is currently in the overlap stage entering the intake stroke, or is it the reversion into the intake diluting the cylinder that is well into it’s intake stroke? Or is it none of the above? I’m trying to grasp the underlying cause to better understand what could or would help to alleviate the condition, which brings me to part B;
What have you learned is the most effective combatant against light throttle bucking at low load/rpm’s? Maybe manipulating the volume, or flow characteristics in the intake system? Thinking outside the box on required timing in those areas? Dumping copious amounts of fuel into it in those areas? Pulling fuel? What are your thoughts? I’m open to anything, and I have been using Binary Editor and a Moates Quarterhorse for 3 years and have a pretty good understanding (not a total tuning newbie) so I can try just about anything in the tune.
Thanks, and WAKE UP! It's over.
Last edited: