Timing tips
What numbers are most useful to you on the cam specs? I've got the cam card here. The short version is 267/275 intake/exhaust duration and 0.549/0.536 lift intake/exhaust. LSA is 110 as it turns out. It says 4 degrees of advance ground in. 106 intake centerline and 114 exhaust.
From my understanding it was about the biggest cam you can use with "stock" springs and pistons. It doesn't have much for vacuum and idles around 50-60% load (which is basically 50-60 kPa absolute pressure, where 103 kPa is WOT).
Refresh your screen.
I know nothing about those hemis.
Does that engine have VVT ?
My head is not getting sense of;
267/275/110, together with .549/.536 lift and
"It doesn't have much for vacuum"
Unless those are durations at some other than "normal" advertised lifts.
>At the usual .006/.008 tappet rise, that is a very conservative cam, and should be pulling a very large vacuum like 17 inches (guessing)
>But if that is at .050, then it is a very big cam, which would have a very low vacuum.
>So as it is, the numbers are meaningless to me. They need to have the accompanying qualifier.
In my street experience with iron wedge-heads on small-blocks, they all like about the same WOT power-timing, after 3600, in the range of 34 to at most 38 degrees. The only experience I have with alloy heads is my personal ride a 360LA, which is happy at ~32/34.
that said;
" Bucking", which is what I call what you are experiencing during very-low rpm cruising, is actually misfiring that is usually caused by running lean with insufficient timing. It seems to me that the air-molecules are too far apart, for the very few and far-apart, fuel-molecules to find them. So the fire starts but sputters out, therefore little to no effective pressure is made, and that piston now has to be carried by the others, costing the engine power, and you feel it as "bucking", especially if multiple cylinders are doing it.
So then, by moving down a gear, several things happen; you move up the fuel delivery curve therefore running less lean, and usually, the engine will have more timing up there, AND you get a lil more flywheel out of it. But more importantly, the higher rpm kills the reversion that happens with a very late intake valve closure, as the piston comes up and trys to ram it back up into the intake.
Or, I suppose, one or more of your injectors is not creating a fine-enough spray.
To run very lean, the molecules have to very tiny and well dispersed.
It may even be that they are being injected too late.
Or the overlap cycle is yanking a portion of them out into the headers, before the beginning of the intake cycle.
With a big cam, and low rpm, your O2 sensor may be as good as useless.
With headers, if there is any air infiltration whatsoever upstream of the O2 sensor, it will lie to you, and more importantly, it will lie to the computer. If the misfired fuel explodes in the header, two things will happen; that pipe will not scavenge on the next cycle because it destroys the vacuum cycle; and the exhaust in there now, is NOT representative of what should be in there.
This can also happen if the timing is too late, and the mixture has Not finished burning before the exhaust valve opens, and if it finds oxygen in the header.
If you are running exhaust logs, then this late-fire can ignite a misfired slug that is already in the log from another cylinder. On the overlap cycle, with a big cam, and at low rpm, this fire can zip over the top of the piston and try to get up into the intake.
These things are why I asked, "How does it cruise one gear down?"
This is how I do it, you can try it;
in Neutral/Park, slowly rev the engine up while watching the manifold vacuum. As the rpm climbs, the vacuum should steadily increase, as you are shutting the door to reversion, which is the fuel charge being bounced back up into the intake by the rising piston, before the late-closing intake valve actually closes.
Eventually the vacuum will hit a plateau, and then begin to decrease again; don't try to find that point if you haven't reached it be 3000rpm,lol. At the bottom of the plateau, or close to it, is where you want to cruise at. This is the lowest rpm that your engine has stopped sending just-inducted A/F charge back up into the intake, and all the fuel charge is going in the same direction and hopefully all of it is being trapped in the cylinder.
With a big street cam this may be as high as 2400rpm. With a small cam, like what a 318 has, this may be very close to idle!. With a typical street cam of 220 to 230 intake duration at .050, it will likely be around 1900, give or take a couple of hundred.
After this Fix the engine at whatever rpm created the highest vacuum. Now begin increasing the timing. As long as the rpm keeps on increasing, keep giving it more. This may take as much as 50/55 degrees. To speed his process up, jump the timing in steps of 3 or 4 degrees, until the gains are inconclusive. When that starts, back up 3 degrees. The number you get is, IS ideal, for that rpm and load.
After this, you gotta use logic to fill in the blanks.
Press refresh,
as I edited after first post, cuz I effed a few things up, lol.
