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Discussion Starter · #1 ·
I've been told to change my springs in the Summit HEI I have because I'm running Vortec heads and they need a different curve... First, is this true? And second, should I curve it to bring the mech. advance in at a lower RPM or higher RPM? Summit gave me the springs to do just that but it also warns: "Replacement of factory installed springs may cause pre-ignition when pump gas is used. High perf. racing fuel is recommended if curve is modified." Whats that all about? For tech specs, the current curve in the dist. is as follows:

-Mech. Advance 14' @ 2,200 RPM

-Vacuum Advance Vac. 10' @ 11 inches

-Total Advance 24' @ 2,200 RPM

Thanks for any information provided, this one has alot of people around my town stumped.


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CoolAid
'70 Malibu L-31
 

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66 El Camino 57 Chevy pickup 2004 Tahoe
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Coolaid,

What is your static timing? That's the timing you see with the light before revving the engine up.

You'll need to add this figure to the numbers you posted for total.

You did check this stuff with a light, didn't you? I found over a period of too many years that pieces of paper supplied with a distributor rarely bear much resemblence to reality. I recommend you look and see for yourself if you hacen't.

Isn't your location at a high altitude? You can run substantially more advance at high altitude, maybe 4 degrees or more.

Tom
 

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Discussion Starter · #3 ·
I have it timed at 11' right now. And yes, I'm up high at 6,200ft altitude.... I was told today to bring the advance in at a higher RPM but I'm not sure about that yet.... Hope some of this helps.. Thanks

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CoolAid
'70 Malibu L-31
 

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Since Vortec heads deliver the charge to your cylinders more efficiently, the spark doesn't need as much lead time (advance) so you should bring the mech advance in a little slower. Unfortunatly, without a dyno and distributor machine the only way I know to "curve" a distributor is trial and error. As a matter of fact I just installed new springs in my HEI this last weekend.

What I did is: install the medium springs, disconnect and plug the vacuum advance, then set your initial timing 2 degrees above stock. Make sure you set the timing at an RPM LOWER than when the mech advance springs start kicking in, so your not looking at the mech advance starting to come in. With the vacuum still disconnected and plugged, make a WOT accel.. Listen for any spark knock. If you get knock or ping, go to the next heavier springs. If no knock, go to the next lightest springs. Keep lightening the springs until you just start to get ping, then go to next heaviest spring-to eliminate the knock. You have just set your initial and mech advance to the ideal curve for your setup.

Reconnect your vacuum advance and do another WOT accel., again, listening for knock. If no knock- your in business! If you get knock with the vacuum hooked up, adjust it to bring in more vacuum. Hopefully you have an adjustable vacuum advance. If not, GET ONE. If your vac advance is not adjustable you'll have to experiment with different advance units til you hit the one that eliminates the spark knock or ping.

I got incredible results with this procedure. Performance and milage seem to have gone through the roof! Hope it helps.
 

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11 degrees initial, plus 14 centrifugal would be 25 degrees at 2200 rpm. (Unless the 14 degrees is rated at the distributor, which equals 28 crankshaft degrees. Then you've got 39 degrees advance at 2200 rpm, and, yeah, that's too much.) What is the total at 3000 rpm as shown by a timing light with the vacuum advance disconnected? If there's no further advance, I'd say you don't have enough! I'd look for 30-34 degrees at 3000 as a starting point. Tune from there depending on audible knock and/or timeslips and/or dyno results.

Add in whatever amount of vacuum advance the car will tolerate without ping. Some stock units have 55+ degrees advance if you add the vacuum advance to the total centrifugal and initial. No problem. You won't see that in the real world, unless you're engine braking down a steep grade over 3000 rpm. Not likely.
 

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There is more than one way to do this. You can also change the flyweights to achieve a faster or slower timing curve. Lighter weights would slow the timing curve down a bit as OK71Chevl suggests. But as he also stated, it's hard to do without a distributor machine.
 
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