[Berliner Bel Air]

I've put my 283 with Vortec 305 heads and 9.1:1 CR, 240H comp cam, headers, HEI, on a dyno and found that 32 degrees initial + centrifugal gives best max power. That's what I expected, about 4 degrees less than the old style heads.

On old style heads the all-in timing including vacuum advance at cruise should be 52 degrees. What is the equivalent number for Vortec heads?

Vacuum advance is all in at 60 mph (I've checked it with a vacuum guage, it is pulling fully at cruise) and about 2200 rpm at this speed, the Q-jet tuned as lean as it will go without negatively affecting drivability.

I edited this because I'm not getting any answers to this simple question.

thanks

[Berliner Bel Air]

Simple question guys, how about it? How much less timing is required at cruise by a Vortec head vs. a standard old-style Chevy head?

[427v8]

Looks like you're gonna have to tell us!

Run some different numbers and see what happens to MPG.

[500BOMBER]

In my own experience, the best timing for fuel mileage occurs about 2* under spark knock.

[curtis73]

agreed. best timing for mileage is the same as proper timing for the combo

[red65mustang]

Best mpg @ 2200rpm = the highest Hg you can get while actually driving......

duct tape a vacuum guage to the windshield hooked up to manifold vacuum and go for test drives on the same road with more and less total timing to find the "real world" best/highest Hg possible for your particular car (gears/tires/brake drag/tranny slip/etc)

[6426yy]

Just curious, what kind of power did your combo make?

[Berliner Bel Air]

I like the idea with the vacuum guage! I'll try that out very soon, sounds like another great use of the old fashioned vacuum guage (makes me wonder why I didn't think of this myself... especially because I do use the guage to set idle mixture and initial timing).

My combination made 220 hp at the motor at about 4600 rpm. The graph was climbling steeply but I had a bad oil leak unto the exhaust so it starting smoking heavily. Then the fuel pressure regulator was set way too low, so it also seemed to start running out of gas (this did happen on the road later). In sept. I'll put it on the dyno again (once I finally get around to fixing the oil leak). I expect it to put out at least 250, probably more, because I can keep up with my friends 57 Chevy with a 350 in it!

[red65mustang]

Berliner, your welcome....

mounting a vacuum guage in the car is the single most effective "gas saver" you can have(my $.02).....it can "retrain" your size 12 lead foot to go easier on the "go" pedal and keep the Hg above 12-14Hg accelerating= much much better mpg

but my cars always have one because it is a great "real world/real time" diagnostic tool for the condition of the whole motor, just a couple of many examples:

Hg=25 or more on a mild downshift=rings and valves are sealing fine.....
vacuum at idle in drive drops off a pound=look for a minor vac leak.....
needle not steady=maybe a misfire or vac leak.....(that you won't feel)