Re: Vacuum Advance and why to use it on the street
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: pxtx</div><div class="ubbcode-body">...
Eric, the way you wrote your, it almost sounded like you were talking about hooking the intake to ported vacuum, not manifold vacuum. I hope you don't take this the wrong way- more saying this for the sake of those reading. Ported Vac= emissions cars (with EGR) and Manifold Vac= muscle cars. There are some people who do not follow this but they appear to be trying to use a "custom tune" for help with stock type tire drag racing and probably shouldn't be mixed in here as that could be a great seperate thread....</div></div>
Tried it both ways, ported and manifold, no difference either way...still un-driveable.
I have my own theories why this (and several similar builds I've been involved on) exhibited similar "symptoms". Maybe I'm right, maybe I'm out to lunch, all I know is I've found methods that consistently work for me. Your mileage may vary (pun somewhat intended [img]<<GRAEMLIN_URL>>/wink.gif[/img] ).
Engines with limited low rpm vacuum (read: big cams) seem to be extremely sensitive to the (often drastic) timing changes that can result from the vacuum advance system. Just changing the base timing from 12° to 14° on my current 496 results in ~100 rpm gain in idle speed and requires re-setting the idle trim screws on the carb. If 2° has that pronounced of an effect, imagine what a wildly-swinging timing curve does as the vacuum advance adds and removes timing as the engine begins surging. Mechanical advance isn't affected by vacuum, only rpm, and as such eliminates that variable.
Besides, the car can blow the tires off pretty much at will in the first 3 gears as it sits now...how much more low end does it "need"? lol. [img]<<GRAEMLIN_URL>>/grin.gif[/img]
Going slightly off-topic but along the subject of variables on differing combinations, here's a little something to ponder over...
The 3418 Holley on my 496 was pig fat rich across the board in stock configuration on this engine and required significant recalibration. I pulled 3 jet sizes out of it on the dyno and picked up significant power (and it was still rich, I just ran out of dyno time to do further tuning). Once the engine was in the car I found out that it also needed a ton of fuel pulled out of the idle/transition circuits to be happy.
That said, I built another 3418 for a car that belongs to a member here with a stock L78, and aside from dropping the jetting down several sizes, it runs tip top for them.
Again, to reiterate... every combination is different, there is no "right" or "wrong" way to approach the tune under <span style="font-weight: bold"><span style="text-decoration: underline">all</span></span> circumstances. You have to experiment to see what does (and doesn't!) work. [img]<<GRAEMLIN_URL>>/smile.gif[/img]
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