sbc help... how much timing advance???

hey...

I have a freshly rebuilt 305 in my 63 belair taxi... i have just run the cam in and am trying to set my ignition timing...

i'm running a msd street-fire HEI, comp cam (218 duration), edelbrock 600 cfm carb so a pretty much stock 305 giving me 18 inches of manifold vacuum...

I have set the initial timing at 12 degrees BTDC, with another 14 degrees of mechanical advance bringing it up to 26 degrees at about 3000 rpm...

now this is where i could use some advice. when i connect the vacuum advance to the to the ported vacuum port on the carb, i'm getting almost 48 degrees advance at about 3500 rpm which means that my vacuum advance is adding 22 degrees.

i believe i should be running 38 to 40 degrees with vacuum, can anyone confirm or correct me on this ???

i will add that i have not noticed any detonation problems so am i ok to run with 12 initial, 14 degrees of centrifugal and 22 degrees of vacuum (48 all up)???

any info will be much appreciated

 

Seem to me you aren't running very much total advance. Alot of SB Chevies work pretty good at about 30 degrees total mechanical advance. But if it is working good the way it is, maybe you could run it that way without any troubles.

 

I had the best luck running a SBC at about 35 degrees total...all included. Always remember to include your initial into that calculation. I usually don't run that much lead (initial) but that kinda depends on you heads/cam etc. If it runs good at 12 degrees then leave it there.

 

yeah i think i will leave timing how it is at the moment then as it seems to be running ok...
have done a little research and i think the best way to know for sure is to have it tuned hooked up to an emissions tester as it will be my daily driver and i want the maximum fuel economy...

thanks for your replies...

 

Retrod's figures are good ones for most small block Chevy's.

If you're aiming for good gas mileage your highly retarded total timing figure isn't going to deliver it for you and you may find that the engine runs hot.

Get yourself a Motors Manual for the car, look up your engine as it would be in stock form and copy those timing figures.

After you get it running good you can make minor changes one at a time to see if things improve.

Install a vacuum gauge in the car and see what kind of figures you get at idle and on the road.

With your cam and assuming the engine is in good shape and timed near optimum, you should have 10"-12" vacuum at idle - which should be about 600 rpm.

Cruising a level road near sea level altitude at 40 mph with a light throttle setting and maintaining a steady speed, vacuum will be around 17"-18".

It's not enough that the engine runs, it needs to run good....

 

One of the major specs that isn't specified, are the degrees of dist. advance specified in camshaft degrees or crankshaft degrees? Camshaft degrees X 2= crankshaft degrees.
You wanta steer clear of anything above 36-38 degrees crankshaft total mechanical and static, with vacuum plugged off. Many engines can't handle this on pump hi-test of today, and a retard is necessary, possibly only in initial advance with total restored in longer mechanical advance.
Using a set of weights and springs from another Chevy, (396, IIRC), allowed me to use 9.5:1 "power pack" heads (all I had) on a 283 I built for my pickup, on regular gas. But beware hi-rpm detonation, it can destroy a good engine in short order!
Dave

 

Is the mechanical "all in" at 3000? Or have you checked beyond 3K RPM's?

48 degrees or so BTDC with Vac advance might be fine, will help with fuel economy/etc. if you're not getting any part throttle pinging. Just like setting up the dizzy's mechanical advance, gear ratio, car weight, have an effect on how much vacuum advance you can/should be able to run.

I've never tried the "adjustable" Vac advance units, but could be you have one and might just need to adjust it? I'd think that since it's an aftermarket dizzy, it would hav an adjustable can, if not, I don't think they (adjustable vac adv units) are very expensive.

 

Are you dealing with true TDC?

Sometimes the rubber between pieces on Harmonic Dampeners dry rots and allows the outer ring to slip and you'll get strange all-in and initial figures when shooting the timing since you followed an incorrect TDC.