[schweigert]

As a heads up, vacuum advance isn't required, even if your distributor has it. I've been doing a lot of research on this myself recently and there's a big debate on the importance of vacuum advance and running ported vs manifold. I've run my car in all three configurations and I'm going to play with the carb a bit more, but I'm going to try full manifold now.

what do you mean ported vs manifold? for the vacuum source of the dizzy?

 

[xj220]

I saw you have EFI now so I don't know about your setup, but at least on carbs (again I'm still learning about all this) there are two ports on the front: one for ported vacuum and the other for manifold vacuum. The difference between the two is that ported doesn't provide any vacuum advance at idle but once the throttle opens then it provides vacuum to advance the timing. Manifold pressure provides a vacuum at idle and throughout the rpm range (until full open when there's no vacuum). There's more info online if you want more details.

 

[GypsyR]

As a heads up, vacuum advance isn't required, even if your distributor has it.

No, you don't HAVE to run it. Either because your electronic fuel injection manages your timing by other means or you don't care about losing an average of about 2MPG.

 

[Pony Gaz]

No, you don't HAVE to run it. Either because your electronic fuel injection manages your timing by other means or you don't care about losing an average of about 2MPG.

Or you dont want a cooler idling engine.

 

[Huskinhano]

I think it's very presumptuous of your mechanic to be making changes without your permission personally. That bothers me.

 

[jcoby]

As a heads up, vacuum advance isn't required, even if your distributor has it. I've been doing a lot of research on this myself recently and there's a big debate on the importance of vacuum advance and running ported vs manifold. I've run my car in all three configurations and I'm going to play with the carb a bit more, but I'm going to try full manifold now.

You need to add timing because it takes time to burn the fuel. You want peak cylinder pressure to happen a bit after TDC. There are two things that come into play: mixture and rpm. A lean mixture is slower burning than a rich one. Higher RPMs need to ignite sooner.

RPM advance is handled by the weights in the distributor. Easy enough.

A typical setup where the carb goes lean during part throttle operations will benefit from having manifold vacuum. Cruising at a ~15:1 or so AFR and light load or at idle lean as possible you can run 40+ total degrees of timing. As you apply throttle vacuum will drop and vacuum advance will decrease until you're running only on mechanical advance. It's (crudely) tying your additional advance to load.

Ported vacuum.. works pretty much the same except you get no vacuum advance at idle and a poorer part throttle vacuum response. Net effect is that you get a worse idle and worse part throttle response, higher EGTs, etc.

I don't see any reason to run ported unless you need to run a rich idle AFR due to a cam or whatever and the extra timing hurts idle quality.

 

[schweigert]

UPDATE: talked to the mechanic, asked him why he didn't reconnect the vacuum hose. it wasn't because he thought the sniper was controlling the timing (which I could understand why he would assume that, the display shows timing on it)

he was saying the vacuum was .... im getting them confused --- manifold? port? his point was the vacuum coming off the sniper was too strong to make a difference with the vacuum advance. does that make sense?

 

[GypsyR]

Not to me, maybe the other guys who have chipped on this. Maybe. I'm sensing a "baffle 'em with bull$4it" defense myself.

 

[Pony Gaz]

Makes no sense to me either. Not the kind of answer I would expect from a reliable mechanic. The Sniper cant produce a stronger vacuum than manifold vacuum. So probably like Gypsy says, he's trying to baffle you with BS.

 

[SVTguy]

I would imagine you should still use that, that line is how your distributor adjusts your advance. With no line connected your timing is "locked in" at base timing through the entire RPM range..

The distributor still has the mechanical advance, running with or with out vacuum advance makes very little noticeable difference on my car.

 

[jcoby]

UPDATE: talked to the mechanic, asked him why he didn't reconnect the vacuum hose. it wasn't because he thought the sniper was controlling the timing (which I could understand why he would assume that, the display shows timing on it)

he was saying the vacuum was .... im getting them confused --- manifold? port? his point was the vacuum coming off the sniper was too strong to make a difference with the vacuum advance. does that make sense?

Not even a little bit. Treat your setup as if you have a carb that happens to be electronically controlled. The Sniper cannot make any more or less vacuum than a carb.

Depending on how your Sniper is setup and what drivetrain you have, it may make little/no difference to not have vacuum advance or it may make a big difference. If the Sniper is set to lean out to 14.7-15:1 or leaner in cruise, you want the vacuum advance. The vacuum advance will also help at idle and off-idle transition if you're running stoich or leaner.

Unless you have a huge-overlap cam and need to run like 12:1 at idle, i'd hook up the vacuum advance to manifold and move on.

 

[cougar70]

schweigert had his Sniper EFI correctly set up to begin with.

 

[schweigert]

schweigert had his Sniper EFI correctly set up to begin with.

I did learn something over at the holley sniper forum -

I might have it plugged into the wrong vacuum port

ignore the red circle - but see the three vacuum ports at the bottom of the bottom image

I have my dizzy vacuum plugged into the smaller one bottom left (dead center), but just learned the port vacuum the one furthest right ... so maybe he was onto something there?

 

[cougar70]

I think the center one is correct. It's easy enough to experiment though. Unused ports on the Sniper should be plugged.

 

[schweigert]

I think the center one is correct. It's easy enough to experiment though. Unused ports on the Sniper should be plugged.

yup- just got confirmation, I had it correct.

the far right one " the (ported) timed spark port is only for emissions/EGR engines"

http://www.chevytalk.org/fusionbb/showpost.php?post/1905429/


side note - that mod danny on the holley sniper forum is a machine... has every answer and replies to everyone within 12 hours