Originally posted by Will: the BARO reading taken from the MAP sensor before the engine turns compensates barometric pressure.
I'll occasionally turn the key to run while the car is in motion, in gear, the wheels driving the engine.
Historically, I don't think there's ever been a restriction on switching on the ignition to a car, besides having an appropriate amount of timing advance. If the OEMs added a startup requirement while retaining the same user interface (steering wheel, pedals, shifter), that's bad user interface design.
My expectation as a user is that if a device looks the same from the outside, I should be able to use it in the same way as usual!
Anyway, if a fuel-injected car drives wonky after I drive it, I'll now have a possible explanation for things... I am kind of shocked at a product initialization that depends on non-obvious user actions.
My truck doesn't mind being turned on/off while rolling, but it has a MAF.
I worked on the injection timing in the last few days.
In the previous datalog I posted, you'll notice short-duration lean AFR spikes when I got back on the throttle after a shift.
No matter what I did with the acceleration enrichment settings, I could not tune out the lean spikes. I surmised that with the uniform 360° BTDC compression injection timing, the squirt (with the acceleration enrichment shot) wasn't making it into the cylinder on time, leaving the first cycle after a shift lean.
I played with the injection timing in 45° increments. Advancing the 3000+ RPM injection timing to 405° worked best:
Advance beyond 405° was not beneficial. The more the fuel is advanced relative to the intake event, the more time there is for an airflow change to occur between the squirt (and thus fuel mass calculation) and the intake event. Transient response was less good with more advance.
At low speeds / idle, I retained the default 360° timing:
Initially, I thought that I should finish the squirt just before the intake valve closing. This would minimize the time during which a sudden airflow change could cause the injected fuel mass to be incorrect for the air mass that's about to be sucked into the cylinder.
Squirting on a wide-open intake valve gave lean spikes. I think that the fuel was entering the combustion chamber (on time) as large droplets, not burning completely, and then exhausted as unburned HC.
I think that my philosophy for injection timing is to do it as late as possible, while still finishing the squirt on a closed (or barely opened) intake valve. This would apply to the White Bug; other engines may react differently?
You know all that is going to change once you install the bigger injectors, you'll have to re tune again.
Advance beyond 405° was not beneficial. The more the fuel is advanced relative to the intake event, the more time there is for an airflow change to occur between the squirt (and thus fuel mass calculation) and the intake event. Transient response was less good with more advance.
At low speeds / idle, I retained the default 360° timing:
Initially, I thought that I should finish the squirt just before the intake valve closing. This would minimize the time during which a sudden airflow change could cause the injected fuel mass to be incorrect for the air mass that's about to be sucked into the cylinder.
Squirting on a wide-open intake valve gave lean spikes. I think that the fuel was entering the combustion chamber (on time) as large droplets, not burning completely, and then exhausted as unburned HC.
I think that my philosophy for injection timing is to do it as late as possible, while still finishing the squirt on a closed (or barely opened) intake valve. This would apply to the White Bug; other engines may react differently?
The 405 number is where to stop the injector on time before TDC compression/power? And you're saying that you want to complete the injection before the intake valve opens?
Originally posted by La fiera: You know all that is going to change once you install the bigger injectors, you'll have to re tune again.
PS They are on your way.
I think that a tuner becomes a good tuner by tuning many different kinds of engines/setups.
Since I am an inexperienced tuner, getting my current setup working is giving me useful experience. So even if I have to retune later with a different setup, I do not feel that my present activities are a waste of time.
I can't complain about getting parts sent to me!
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Originally posted by Will: The 405 number is where to stop the injector on time before TDC compression/power?
Yes, that's right. Given the desired injector on-time, engine speed, and engine acceleration, the ECU schedules the injector turn-on such that the turn-off occurs at the angle specified in the look-up-table.
The injector turn-off angle is not exact; priority is given to the correct injector on-time, to the detriment of end-of-injection timing accuracy.
Ignition dwell works a bit differently; priority is given to firing the ignition coil at the correct angle, to the detriment of dwell time accuracy.
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Originally posted by Will: And you're saying that you want to complete the injection before the intake valve opens?
Yes, that's what's working "best" on my Fiero, for now.
I define "best" in terms of minimizing the lean spikes when I stomp on the throttle following gearchanges on a 1-2-3-4 WOT run from idle. I kept the same acceleration enrichment settings as I varied the injection timing. I was working in increments of 45°.
I am not studying the influence of injection timing on steady-state power.
Thursday night I noticed a ticking sound occur. It was mostly prominent at idle, going away with RPMs. Maybe it existed before, but not at a level to notice.
The local tuning shop heard the car, and the owner said that the tick was the sound of a bad lifter.
After taking off the valve covers, I found a VERY worn rocker pivot ball:
I also found coked oil on the rocker itself:
Maybe it's time to switch to synthetic oil.
To be continued...
[This message has been edited by pmbrunelle (edited 07-04-2020).]
Gross! were all of them like that or just the one?
------------------ "I am not what you so glibly call to be a civilized man. I have broken with society for reasons which I alone am able to appreciate. I am therefore not subject to it's stupid laws, and I ask you to never allude to them in my presence again."
Gross! were all of them like that or just the one?
All of the rockers had some degree of coked oil, but this one was the worst. The coking seems to be concentrated near the heat generated by friction of the ball pivot point.
As for the other balls, I'm not sure, I haven't taken apart the others yet. I was busy today working on my friend's 3.5 Fiero swap. This rocker is the only one that was loose though; when I took off the valve covers, I was able to wiggle this one by hand. The others felt normal.
The 3.5 already has roller trunnion rockers from the factory, so this is a problem you won't have to deal with.
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Originally posted by La fiera: Maybe the pusrod was plugged up with debris?! What kind of oil did you use?
I took out the pushrod, and I was able to see through it.
I noticed the tick at about 1800 km. I limped home and the engine now has 2042 km.
I cut down on the ZDDP additive (Redline Break-In Additive) because I read that too much ZDDP isn't good long-term. Maybe that was a mistake. I don't know how much phosphorus/zinc is already in the oil as-purchased.
Before taking things apart further to study the balls, I will put back the failed rocker+ball, re-adjust the lifters, and then spin the oil pump (with a drill + modified distributor) to see if oil comes out of all 12 pushrods.
ZDDP is a very good extreme pressure protector but is very inneffient. How so you may ask? The more pressure is exerted on ZDDP the more drag it develops between the sliding surfaces and that drag can induce overheating of parts, and that's not counting the energy wasted trying to overcome that friction. I've seen many new engines ruined because of overdose of ZDDP But not all is lost. There are two elements that do just the oposite, the more pressure exerted in between the sliding surfaces, the slickery and slipery those elements get thus reducing friction to a very minimum amout. Enter Molibdenum disulfide and Tungsten disulfide. The first one is availabe in powder form or part of the chemestry of the oil, in other words the oil has additives to keep the Molibdemum suspended in the oil. The problem with using it in powder form is that Molibdenum is a heavy solid element and if pouedr in oil it will just sit at the bottom. I personally use M0S2 solid powder to mix it with assembly lube and bearing lube. I also have a propietary blend of Tungsten disulfide that stays suspended in the oil and does not sink like if used in powder form and that I add to all the oils that go on my engines.
So, if you want to have a long lasting engine regardless is a street or performance engine, specially if you use the old flat tapped cam design the best oil you can buy is:
I personally use this oil for break-in of my engines, that's how good it is!
Spinning the oil pump with my priming tool, I see oil coming out of all 12 pushrods.
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Originally posted by La fiera: The more pressure is exerted on ZDDP the more drag it develops between the sliding surfaces and that drag can induce overheating of parts
From the oil coking near the pivot (and blueing of the rocker), we see that there was overheating due to excess friction. Everything goes downhill once there's overheating, so anything to reduce friction (and therefore heat) will help.
The other key element that affects the overheating is RPM; I am not sure if my tests in the 5000+ RPM range are too much for ball-pivot rockers.
I guess you don't add any extra additives to the Liqui-Moly oil in the picture? You use it straight from the bottle, or you still add your WS2 to it? This oil seems to be available in Canada.
I'm not sure if I should retry with brand-new rockers (the same as what I have now) and better oil, or if I should look into roller rockers right away.
[This message has been edited by pmbrunelle (edited 07-05-2020).]
Spinning the oil pump with my priming tool, I see oil coming out of all 12 pushrods.
From the oil coking near the pivot (and blueing of the rocker), we see that there was overheating due to excess friction. Everything goes downhill once there's overheating, so anything to reduce friction (and therefore heat) will help.
The other key element that affects the overheating is RPM; I am not sure if my tests in the 5000+ RPM range are too much for ball-pivot rockers.
I guess you don't add any extra additives to the Liqui-Moly oil in the picture? You use it straight from the bottle, or you still add your WS2 to it? This oil seems to be available in Canada.
I'm not sure if I should retry with brand-new rockers (the same as what I have now) and better oil, or if I should look into roller rockers right away.
If I were you I replace all the new rockers and balls. With this Liqui Moly you don't have to add anyting else. It has 1100ppm Phosphorous and 1200ppm zinc and about 900ppm Mos2.
[This message has been edited by La fiera (edited 07-05-2020).]
old school hotrodders used to add grooves to the rocker balls to improve oiling, the downside though, is that the load is not higher on the contacting areas. I wasn't really concerned for my engine, more just trying to get more insight as to what the cause is. whats your valve adjustment procedure?
------------------ "I am not what you so glibly call to be a civilized man. I have broken with society for reasons which I alone am able to appreciate. I am therefore not subject to it's stupid laws, and I ask you to never allude to them in my presence again."
Originally posted by pmbrunelle: The other key element that affects the overheating is RPM; I am not sure if my tests in the 5000+ RPM range are too much for ball-pivot rockers.
The 2.8 in my Formula has 240,000 miles. It sees 6000 RPM pretty much every time I drive it. I didn't see anything obvious when I resealed the valve covers a couple of years ago.
Originally posted by ericjon262: old school hotrodders used to add grooves to the rocker balls to improve oiling, the downside though, is that the load is not higher on the contacting areas.
You can buy balls that are already grooved out of the package. Actually, the ball in the picture above used to have four equally spaced grooves. On one side, the ball wore down so much that the groove vanished. I'll probably use the same grooved balls again; they come in the package with the rockers.
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Originally posted by ericjon262: whats your valve adjustment procedure?
First, I make sure that I'm on the base circle for the valve I want to adjust.
I find zero lash by wiggling the pushrod end-to-end as I tighten the nut; when I stop feeling movement, that's zero lash.
The spec of the Crower lifter is to push in the plunger 0.040" +/- 0.020" beyond zero lash. With my 24 TPI rocker stud and 1.52 rocker ratio, I have to turn the nut 208° to hit 0.040". I rounded that to 7/12ths of a turn, and then eyeballed it. I might aim towards the upper end of the allowable preload range this time; not sure yet.
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Originally posted by Will: The 2.8 in my Formula has 240,000 miles. It sees 6000 RPM pretty much every time I drive it. I didn't see anything obvious when I resealed the valve covers a couple of years ago.
But that's for short bursts, right? I don't know if the thermal mass of the rocker + ball would allow it to absorb the friction work without much temperature rise.
To collect data to tune the VE(RPM, MAP) lookup table, I maintain a particular RPM (corresponding to a column in the table) with the brake pedal, and I slowly press the throttle until WOT, then I slowly release the throttle. With the slow throttle movement I traverse the MAP rows for a given RPM. The apply/release cycle takes maybe 10 seconds.
So I don't know if this use case of mine is tougher than "normal use", whatever that may be. Still easier than a top speed run I guess? Fieros can survive top speed runs without ruining their rockers, right? Well now my VE table is fairly well sorted out, I shouldn't have to do much of that anymore.
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Originally posted by La fiera: So, if you want to have a long lasting engine regardless is a street or performance engine, specially if you use the old flat tapped cam design the best oil you can buy is:
I personally use this oil for break-in of my engines, that's how good it is!
Nearby garage Momentum Motorsport will have a bottle of 10W-40 with MoS2 Anti-Friction ready for me tomorrow morning: https://mmsportmtl.com/
Grease loaded with MoS2 saved my butt with a wear problem at work (versus grease without it), so I have faith that simply changing the oil could make a big difference.
[This message has been edited by pmbrunelle (edited 07-06-2020).]
I guess you don't add any extra additives to the Liqui-Moly oil in the picture? You use it straight from the bottle, or you still add your WS2 to it? This oil seems to be available in Canada.
After a long time in the mail, today I received higher flow fuel injectors from La Fiera. Thank you. In the spirit of sharing parts, tonight I will be carefully packaging a Fiero part for ericjon262.
I got the Liqui Moly oil today. It is expensive compared to Canadian Tire stuff. Cheaper than new rocker arms though... those haven't yet arrived.
In the interest of changing only one thing at a time, I won't install the fuel injectors at the same time as my replacement rockers. When we try to make too many changes at once, we become lost, and no longer understand what we are doing.
Also, my dad wants to build a fuel injector test jig, so that will be a project for us to do together before installing the injectors.
Awesome!!! I'm the same way, one thing at a time so I can keep track of my progress. That's nice you are working with your dad on this project. Sadly I lost my dad 2 years ago and I miss him dearly. He was a car guy, an engineer. The first time he heard the 3.4 start he was all giggles! Enjoy you dad while you can and I'm glad I can help on reaching your goals!
I installed replacement rocker arms and the car is running again, for now!
I did not use the Comp Cams break-in lube that cam with the rocker arm set. From its MSDS, the Comp Cams lube contains (non-exhaustive list): ZINC DITHIOPHOSPHATE PARAFFINIC PETROLEUM OIL ANTHRAQUINONE DYE
If it is true that my rocker arms failed due to excess friction from a ZDDP overdose, then I should avoid using the Comp Cams lube, like I used during the initial engine assembly.
Anyway, repeating the same procedures and expecting different results doesn't make sense, so something had to change. I primed the oil pump with the drill until I saw the gray Liqui Moly oil flooding the rocker arms. I did not mix any additives into the oil. Three hours after priming (I didn't want the Liqui Moly to drain away for the first start), I started the engine and went on a half-hour ride, keeping the RPM between 1500 and 3500 RPM.
I will keep the RPMs low for the next 500 km or so, to give the chance to the parts to wear into each other. After the wearing-in, friction should be reduced, then I should be able to spin the engine to 6000 RPM without overheating the rockers (hopefully).
If these ball-and-socket rockers fail again I will probably look into installing roller rockers.
[This message has been edited by pmbrunelle (edited 07-11-2020).]
After a long time in the mail, today I received higher flow fuel injectors from La Fiera. Thank you. In the spirit of sharing parts, tonight I will be carefully packaging a Fiero part for ericjon262.
I got the Liqui Moly oil today. It is expensive compared to Canadian Tire stuff. Cheaper than new rocker arms though... those haven't yet arrived.
In the interest of changing only one thing at a time, I won't install the fuel injectors at the same time as my replacement rockers. When we try to make too many changes at once, we become lost, and no longer understand what we are doing.
Also, my dad wants to build a fuel injector test jig, so that will be a project for us to do together before installing the injectors.
I'm looking forward to said part! I'll remember it and continue the trend! One of the things I'm looking forward to about moving back to my hometown next year, is that I'llbe able to tackle projects with my dad again, and maybe conquer another crazy roadtrip with him like the one we took to Alaska!
I hope the new rockers work out! that's a frustrating sequence of events for sure. It actually is something I'm a bit worried about with my car, but for different reasons, the bolt threads have a tendancy to pull out of the heads on the later engines like mine. if it happens I'll have to pull the engine back out and install timeserts.
------------------ "I am not what you so glibly call to be a civilized man. I have broken with society for reasons which I alone am able to appreciate. I am therefore not subject to it's stupid laws, and I ask you to never allude to them in my presence again."
This time, with about 200 km (driving calmly) on the all-new rockers+balls and Liqui Moly oil, the #4 exhaust rocker ball died... Last time it was the #2 exhaust rocker, so at least the problem doesn't seem to be associated with any particular cylinder. This time, only the failed rocker had coked oil on it; the remaining rockers were clean.
Comp Cams suggested that my high-volume oil pump may be forcing the oil to shoot out of the pushrods too strong, overshooting the rockers and simply hitting the valve covers. In this case, drippers would be needed to redirect the oil to the rocker balls.
I guess the hypothesis is plausible, but at the same time, I thought that HV oil pumps were a common mod on the 2.8 Fiero. Why would my Fiero be the only one that needs drippers on its valve covers? To see how strong the oil sprays I might try to spin the oil pump at 1500 RPM; representative of 3000 RPM highway cruise.
Since the problem isn't tied to any particular cylinder, it may be a random infant mortality problem of the ball+rocker; maybe if I use one of my four spare rockers (I had a V8 kit), the ball will successfully break in. The chance of failure when breaking in 1 vs 12 new rockers is much reduced.
Have you tried turning the engine over by hand and checking for binding or anything unusual? Running the pump to verify is a good idea. Another possibility is that you got part of a batch that wasn’t hardened properly? It’s also interesting that both failures happened on an exhaust valve.
[This message has been edited by Honest Don (edited 07-18-2020).]
I grabbed a stock 2.8 Fiero head from my spare parts collection for examination/brainstorming.
I noticed an important difference between the Comp Cams Magnum rocker arm and the stock GM rocker arm.
On the Magnum rocker arm, the oil hole is approximately aligned with the pushrod axis. Oil would squirt straight up towards the valve cover.
On the GM rocker arm, the oil hole is oriented to squirt oil towards the rocker arm stud.
So it seems likely I'm not adequately oiling the rocker ball area...
Options: 1. Re-install a new Magnum rocker arm/ball replace the failed one. Might get lucky this time, but overall this setup seems marginal. 2. Weld a steel cap over the Magnum rockers to redirect oil towards the ball. Not sure about the metallurgy of this idea. The rocker arm is a hard metal; I can't cut it with a file. 3. Make aluminium oil drippers, then weld the drippers to the valve covers. Not sure about welding to porous aluminium that may be oil-contaminated. Probably have to refinish the valve covers after. Drippers bring vibration concerns, and packaging difficulty. 4. Install Crane gold roller rockers. May fit beneath the stock valve covers. Must re-verify the valvetrain geometry, and possibly buy pushrods of a different length. 5. Install stock replacement Melling rockers. Pretty certain (almost 100% sure) they will fit beneath stock valve covers. Must re-verify the valvetrain geometry, and possibly buy pushrods of a different length.
I prefer options 4 and 5 for the moment.
[This message has been edited by pmbrunelle (edited 07-18-2020).]
Have you tried turning the engine over by hand and checking for binding or anything unusual? Running the pump to verify is a good idea. Another possibility is that you got part of a batch that wasn’t hardened properly? It’s also interesting that both failures happened on an exhaust valve.
Yeah, I've been turning the engine with a rachet wrench, but noticed nothing.
The most obvious symptom with a fubared rocker arm ball is that I can wiggle the rocker arm with my fingers; it's that loose!
Exhaust rockers apparently run hotter than intake rockers. If you have a failed exhaust rocker ball, it is suggested to transfer a successfully broken-in intake rocker to the exhaust position, and then place the brand-new rocker+ball at an intake position; the less demanding position.
Comp Cams suggested that my high-volume oil pump may be forcing the oil to shoot out of the pushrods too strong, overshooting the rockers and simply hitting the valve covers. In this case, drippers would be needed to redirect the oil to the rocker balls.
I think the same as comp cams. Their rocker arms have the lubrication hole in a straight line with the push rod while the original ones have the lateral hole and not centered with the push rod and the jet is directed towards the rocker ball which prevents an oil jet it only flows towards the balls.
These are 2 different things between these 2 rockers.
In a normal v6 engine when you remove the valve cover and look at the lubrication of the rockers, you only see a flow of oil going towards the rocker ball.
In a normal v6 engine when you remove the valve cover and look at the lubrication of the rockers, you only see a flow of oil going towards the rocker ball.
My thoughts as well. I've pre-oiled push-rod engines, to include V6/60's, on several occasions, all with stock rocker components. I've never seen oil "squirt" out the top of a pushrod, but I have seen it dribble in a steady flow out the lubrication hole in the rocker arm and pool in the rocker arm around the rocker ball, immersing that interface in oil.
Pre-oiling accomplished with a 1/2" trigger variable drill motor that I started slowly until the pump had oil, then just held at max RPM until I had oil out the top of every pushrod.
I've also installed high volume oil pumps with stock pressure springs in two different high mileage V6/60's and not had any problems.
Sure looks like it's that lubrication hole, though. It's worth noting that GM's oil hole location makes it IMPOSSIBLE for the oil to squirt out the end of the pushrod and miss the rocker ball. How did Comp screw up something that simple?
[This message has been edited by Will (edited 07-18-2020).]
Originally posted by fieroguru: Just another data point where aftermarket R&D doesn't match GMs...
Maybe the rockers were developed for SBCs with drippers, and then assumed to work on the 60° V6
Still, I have only read positive reviews of the Magnum rockers on PFF, so it is strange that my experience differs from the norm.
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Originally posted by Honest Don: It’s a chore, but verification of exactly what the oil flow is doing would be worth the mess.
I ran the idea of running the engine without valve covers through my dad; he thought it would be a good way to set my car on fire when the oil hits the 350 °C exhaust manifolds
I decided to use a can of WD-40 (with the red straw) to spray WD-40 up a pushrod. There is quite a difference between the Comp Cams rocker arm and the GM rocker arm:
If I remain with ball pivot rockers, I must decide whether to keep using grooved balls, or switch to smooth balls (like stock Fiero). Earlier in this thread, ericjon262 suggested grooved balls might be a good thing to allow oil to flow into the contact region.
On the other hand, grooved balls may allow the oil to drain away from the rocker arm too quickly, preventing the formation of an oil puddle (does this happen?) above the ball.
What's better? Grooved or smooth?
Edit to add: I could easily test the drain-away time between a grooved and a smooth ball with my cylinder head on the bench. However, the rockers would not be moving.
[This message has been edited by pmbrunelle (edited 07-18-2020).]
If you start the car when cool and let it run w/o the valve cover off, it will confirm what the issue is before the exhaust manifolds start getting too hot. You could install a shield over the exhaust if you wanted an extra safety. It is pretty cool to see the rockers in motion at idle and some issues are very obvious, like this one...
Another approach you can try is to slot or elongate the hole torwards the ball. By doing this the velocity of the oil from the pushrods will decrease at the rocker oil hole because it's bigger. Oil at lower velocity will not fly off missing the ball but spew like a volcano filling the rocker bowl with oil.
I put the engine back together with stock replacement Melling MRK-407 rocker arms and smooth (non-grooved) pivot balls. The Melling rocker has its oil hole oriented towards the rocker stud.
So the engine is running again, and it sounds normal.
Two questions remain:
1. Will the pivot balls live this time? I will find out by driving the car.
2. Will this engine breathe less well, because the stamped rocker may have less consistent (i.e. lower) ratios? My WOT tune was pretty good (hitting the AFR target) with the Comp Cams rockers. With the stamped rockers, if the engine runs richer than before at WOT, that means that the engine isn't breathing as much air as it did before. I will check this shortly.
I think that I want solid lifters (either roller or flat tappet) for my next project. Hydraulic lifters are a huge PITA in the sense that it's hard to verify valvetrain operation with the engine off. Solid lifters work the same with or without oil pressure.
[This message has been edited by pmbrunelle (edited 07-26-2020).]
In order to meet the AFR target, I cut the fueling: 1% @ 5500 RPM 4% @ 6000 RPM
I am slightly disappointed, but a reliable (still to be proven) engine beats a better-breathing engine with a lifespan of a several hundred kilometres!
The advantage of these is that they have a tiny hole (.024) that delivers pressurized oil between the lobe and the lifter to keept the cam lobe and lifter always lubricated. I've always used these in my Supernatural engines and cams look like new. Every time I check the lash is on point. They are not dependant on splash oil, the are pressured fed.
I started to experiment with the boost control solenoid yesterday.
With the wastegate spring alone, MAP is around 150 kPa.
I finally enabled the boost control feature of the MS3, and I increased the MAP to about 175 kPa. Now the injectors are really maxed out (94% duty cycle observed).
I then went to the dragstrip to see what it would do: https://www.youtube.com/watch?v=sucaTBVG6K4 The timing equipment seems to be in disrepair (timeslips missing information), so I didn't get too much value from that dragstrip evening...
On the butt dyno, the boost increase didn't seem to increase performance very much. I think that's because I was retarding the ignition timing too much with boost. I uploaded a new ignition timing table with less boost retard to the car.
I also advanced the ignition timing in the 5000 to 6000 RPM range, since the engine is breathing less well in this range. A less dense air-fuel mixture (and therefore a slower-moving flame) needs to be ignited earlier to perform the most work on the piston. I looked at the VE table for indications on how to do this.
Here is the revised ignition table I just loaded into the car: There have been many revisions. This is is certainly not the last!
You will notice that at low loads, the timing is quite retarded relative to MBT.
The car used to have a bucking sensation at low loads below 2500 RPM... driving with more throttle would eliminate that, but I did want to be able to drive calmly and smoothly.
Retarding the timing at low loads cured that problem. I am not certain of the root cause, but I do have my suspicions.
Apparently the bucking is something that happens when people install aggressive cams; too much exhaust gas remains from the previous cycle and dilutes the air-fuel charge. Maybe by retarding the timing, the cylinder pressure is higher at the moment the exhaust valve opens, and the cylinder is scavenged better.
Probably my low 7.4 compression was a factor, leaving room for more exhaust gas to stay inside the cylinder (instead of being pushed out), but the car drives fine now.
Oh, I forgot to add that the stock-style stamped rockers are doing just fine!
[This message has been edited by pmbrunelle (edited 08-09-2020).]
