Heff Posted May 5, 2011 Share Posted May 5, 2011 ok I'm working on a 327 sbc that came in a project car I'm working on for my son. I don't know alot about the internals other than it has domed pistons from looking into #1 plug hole, double hump heads, a fairly large solid cam of some sort, roller rockers, edelbrock torquer intake, headers, HEI, and a 600 cfm summit carb. My problem is now that I'm able to get it to idle down with the new carb, I can't get the vacuum advance to work because of the low vacuum (about 7" at idle). I put a curve kit in the distributor, and set my initial at 16 with all in around 35. My question is can i just run it this way with no vacuum advance hooked up? Quote Link to comment Share on other sites More sharing options...
Andreas Posted May 6, 2011 Share Posted May 6, 2011 Yeah you can drive it like that but: Quote: Vacuum timing advance The second method used to advance the ignition timing is called vacuum timing advance. This method is almost always used in addition to mechanical timing advance. It generally increases fuel economy and driveability, particularly at lean mixtures. Vacuum advance works by using a manifold vacuum source to advance the timing at low to mid engine load conditions by rotating the position sensor (contact points, hall effect or optical sensor, reluctor stator, etc) mounting plate in the distributor with respect to the distributor shaft. Vacuum advance is diminished at wide open throttle (WOT), causing the timing advance to return to the base advance in addition to the mechanical advance Quote Link to comment Share on other sites More sharing options...
wallaby Posted May 6, 2011 Share Posted May 6, 2011 They do make adjustable advance cans... LINKY But I've found that sometimes infinite adjustment just means greater odds of never getting it right. LOL There are probably load/speed ranges in your engine where the vacuum is high enough to have the vacuum advance working...just not at idle. I'd be worried that if you tailor the advance can to function at idle, it might over-function at other ranges. For street driving, you can get a much smoother engine that gives good mileage at cruise rpms if the vacuum advance is hooked up & working. In your situation you may have to deal with it not working at idle, but I'd leave it hooked up for the advantages at cruise. The adjustable cans have a screw that adjusts the tension on the internal spring, so you can dial in how much vacuum is needed to bring in advance. It's possible that the kits' included instructions give enough info on how to get it right, or the Summit parts guy might even be able to help you out. Really I'd say blocking it off is a last resort. It used to be common practice, and they still sell distributors without vacuum advance provisions, but those are meant for usage primarily at WOT only...when a vacuum advance wouldn't function anyway. Quote Link to comment Share on other sites More sharing options...
Mike 57 Posted May 6, 2011 Share Posted May 6, 2011 My concern is the amount of compression of the engine. If it has domed pistons and double hump heads the compression could be 11 to 1 or maybe more. It sounds like you have a healthy cam and it is bleeding off cylinder pressure at low RPMs. As RPMs increase and the cam stars to work you could have some issues with preingnition. I run an adjustable advance unit and I only pull in about 8 degrees, I run a pretty stout cam and at cruise I am right on the edge of ping and I am running aluminum heads. Don't get me wrong if you can run a vacuum I would but it may take some tuning. Quote Link to comment Share on other sites More sharing options...
Heff Posted May 6, 2011 Author Share Posted May 6, 2011 Thanks Mark I may call summit and see what they say, the adjustable one's I've looked at read like they only adjust for how much they advance, buy i should talk to some one who knows for sure. Everything I've been reading on this tells me I want to run the vacuum advance for a street driven car per Andreas reply, but for now I think I will leave it the way it is until I get the front clip finished up and can actually drive it down the road and see what it does. My guess is that I'll end up putting a smaller cam in it, I was just hoping I could make it work the way it is. This was one of those project cars that I bought from a guy who bought it from a guy, and has been sitting for 15 years, and I was told "I don't think the motor will run" but I thought to myself it's a sbc these things will run upside down under water, so once I got the correct style spark plugs in it, got rid of the resistor wire they had running to the HEI, and took off the massive 750 double pumper that was almost seized up she fired right up with 60psi oil pressure, now I'm just trying to get it fine tuned. Quote Link to comment Share on other sites More sharing options...
Andreas Posted May 6, 2011 Share Posted May 6, 2011 Or do what I am just installing in my Monte: http://www.firstgenmc.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=311549#Post311549 No more mechanical or vacuum advance, just a laptop! Quote Link to comment Share on other sites More sharing options...
Heff Posted May 7, 2011 Author Share Posted May 7, 2011 interesting I may look into it down the line. Quote Link to comment Share on other sites More sharing options...
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