How much is enough total timing? Is there a generic "best practice" for small block engines for advance curves?


[Modified by BB2B, 11:33 AM 8/22/2001]

 

A rule of thumb is 36 degrees with your vac hose pluged.

 

thanks. At what RPM?

 

As stated above, 36 degrees total mechanical advance, when the centrifugal is all the way in and with the vacuum disconnected, is a very good general spec for any GM V8, providing you're not running the fast-burn style heads. Fast-burns will take less total timing (about 34 max). If you're at higher elevations, you can run a little more (up to 38 or 40). If you have access to a track, you can start with 36 total, then advance and retard 2 degrees at a time to maximize trap speed. You'll find that max trap speed will put your total timing somewhere between 34 and 38 degrees in virtually all cases.

For peak performance, you want this total timing to be all in at or around your torque peak - about 2500 rpm in most cases. Lighter cars can take a quicker advance, whereas heavier cars and trucks may need a slower curve to prevent detonation. The Vettes perform very well with the centrifugal fully in anywhere between 2000 - 2800 rpm.

With a quick, aggressive curve like this, it is important to match a complimenting vacuum advance curve to the mechanical curve. A GM V8 will take up to 52 degrees of total mechanical and vacuum advance at light throttle cruise speed. If you're turning 2500 - 2800 rpm at cruise, that means that you have 36 degrees of mechanical advance slammed in at cruise speed. You cannot run a vacuum advance producing more than 16 degrees of advance to avoid exceeding the 52-degree limit. If you run more than 52 degrees total advance at cruise, the car will start to "buck" and "jerk" while running down the highway - very common problem when using a stock vacuum advance control unit with a re-curved mechanical advance curve (most stock vac advance units produce 18 - 24 degrees of vac advance). Many people will disconnect the vac advance to "improve" performance and eliminate the "jerking." This, however, has an adverse effect on mileage and cruise rpm throttle response. You're much better off installing a vac advance that produces 16 degrees of advance than to eliminate the vac advance. I have parts numbers for advance units that will produce this spec if you need it.

 

Thanks Lars, that is exactly the info I was looking for. The part numbers for the advance units would be helpful.

 

Are you running HEI or a standard point-style distributor? The vacuum advance units are different for the two distributor styles...

 

stock points type.

 

Sorry to jump in on this post but I just bought an MSD tach drive distributor for my 69 BB and was trying to figure out how to tailor the advance with the included bushings and springs. Lars do you happen to know what the vacuum canister supplied with this unit is giving. There are no specs for how much advance it is giving and it is not adjustable. The spec sheet for my 502 crate motor says 36 degrees at 3600 rpms, should I still bring it on at 2500 rpms or so.. Thanks.

 

Also, removal of vacuum to the advance canister places the rotor (out of Phase). Re-phasing rotor is fairly easy, though requires moving the breaker plate to proper positioning and fastening, as in welding etc. Every GM distributor we build exibits this modification. Cutting 2, 1-inch holes between 2 adjacent terminals (on the side) of a distributor cap, and watching the amount of out of phase action taking place via a timing light will shock you.(No pun intended). We also weld OEM canister vacuum canister rod in perfect phase, then retain original look, but no bogus phasing.


[Modified by Magic Fingers, 9:31 PM 8/25/2001]

 

You can limit the degrees of advance of your factory vacuum advance. Shorten the slot where the rod pulls. Or attach some kind of stop to prevent the rod from pulling all the way. I cut my factory advance in half by reducing the rod slot by 50%. Its not so important exactly how much vacuum advance you have, just that you have some, and not too much.

 

Lars,
Sorry to cut in, but I have been trying to find a good setting for my timing, seeing past post, I have gained respect from your opinions and knowledge, I have a '69 with 350ci. automatic, it has a "Mallory" magnetic breakerless distributor and has no vacuum advance, is this distributor a good application for my small block, It seems to do well with the time set at 14 deg. at idle (700rpm+/-), can I get max performance from my engine with this distributor? if so how?
Thanks in advance, Jeff :chevy

 

For those of you who requested some info on timing, timing settings, and vacuum advance specs & settings, I have e-mailed out a few articles on these subjects, including part numbers and specs on the vac advance units. If anyone else needs this info, please drop me an e-mail, and I will be glad to send it out to you.

 

Thanks,Lars
I'll see if I can put this to use :yesnod:
Jeff