Statistics: Posted by DrCharles — Sat Oct 23, 2021 2:16 am
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Ultimately it comes down to cut-and-try. A cam like mine (272@.050) with mid-10's CR is rarely run on the street, and the drag-only cars don't care about anything but starting, idle (maybe) and WOT. No vacuum advance data there...
But I've found by experiment that mine likes a lot of idle advance, will start hot up to 24, and best WOT power for most Mopar big-blocks is between 34 and 38 degrees! The remaining variables are cruise advance (as you posted, something around 50 deg, and mine is 52), and how fast the mechanical advance should come in.
I need some more street driving to see if I picked the right setting for that last one. There's a cruise-in tonight at the Steak & Shake 8 miles away
Statistics: Posted by DrCharles — Fri Oct 22, 2021 4:24 pm
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9.1 compression RV cam
advance comes in slowly
5 degrees initial
32 degrees total
Example 2:
9.5 to 10.5 compression - 230 degree cam at .050" approximately
Mechanical advance comes on faster at first
10-14 degrees initial/slope to 30 degrees at 3000 rpm
34-36 degrees total 4500 to 5000 rpm
Vacuum advance:
Starts 3-5 inches 10-14 inches all in
Cruise 50 degrees approximately
Statistics: Posted by rgalajda — Fri Oct 22, 2021 10:48 am
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I wouldn't expect the head swap to mean the carb tune will need to change. You should be picking up a noticeable amount of power across the effective RPM range, but the driveability won't necessarily change.
Now watch it play out that the ignition requires no changes, but the carb hates the new heads...
Statistics: Posted by BradH — Fri Oct 22, 2021 12:55 am
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But is there likely to be a large change to my timing curve? Seems like 24 to 36, all in by 3000, is a good conservative setup.
Likewise, do you anticipate substantial changes to the carb tune?
Statistics: Posted by DrCharles — Fri Oct 22, 2021 12:28 am
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When combustion is richer than stoichiometric, there is less O2 in the exhaust, since there is not enough O2 to burn all the fuel, so the O2 is all used up, and the meter reads below 14.7. Conversely a lean mixture has a surplus of oxygen in the exhaust since there is not enough fuel to react with it all, so the meter reads above 14.7.
So... wouldn't an improvement in the "burn" reduce the surplus of unreacted oxygen, and cause a richer reading?
Statistics: Posted by DrCharles — Wed Oct 20, 2021 12:37 am
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I am curious too if your plugs looked different. If combustion is more complete with the same AFR will they look as though the cylinder is leaner?
Statistics: Posted by Right hand drive — Wed Oct 20, 2021 12:24 am
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My previous curve was 16 cranking, 27 at idle rpm, but only a slow advance to the low 30's by 4000 rpm. (And manifold vacuum advance up to 16 additional degrees above 12"Hg, which also added about 10 deg at idle). The advance on the heavy spring was so slow that the timing actually reduced at mid- to high rpm due to the delay in the electronics. I just got the darn 950 dialed in, too. Idle around 13.5-14 AFR, cruise 14.5, WOT 12.5.
Then I put the light on it recently and found the above (thought I had 35 by 4000, but even that was only around 32 at a 3000 rpm cruise). So I recurved. Now I have 24 initial (cranking or idle), and 36-37 as high as I want to stand next to it holding the throttle and a timing light
Idles around 35 with the vac can connected, 8.5" Hg.The idle AFR seems to have leaned out about half a point, likewise the cruise. I haven't had it on the highway for some WOT trials yet. But the off-idle and low-speed cruise feel great with much improved throttle response
I assume this change in AFR is normal, but not something I have seen much written about. Thoughts?
Statistics: Posted by DrCharles — Tue Oct 19, 2021 9:44 pm
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