vacuum advance question

speedracer · March 20, 2006, 10:26:34 AM

OK guys the speed shop guys that built my 350 engine has all the vacuum ports plugged except the large one on the rear of my 750 Holley (vac sec). So the distributor vac advance is plugged too! They told me because of the high proformance cam (eldebrock) and also they said they set the timing faster than normal and all this was done in the name of proformance. Now the large port on the rear of the carb is going to the valve cover and they have a vacuum line running from the intake to the brake booster and that's it. Question is this right? To me its not. They said to keep the idle set at around a 1000 rpm.
Oh my trannie is a 700r4. All advice and info would be greatly appreciated! Thanks, johnny :(U)

C9 · March 20, 2006, 10:52:56 AM

Did they re-curve the distributor?

How much initial advance?

How much total (all-in) advance?

Faster is an interesting term for advance in a distributor - which is the way I'm taking it.

How big is the cam?

A 1000 rpm idle for a street engine strikes me as a bit extreme.

If you don't run vacuum advance on the street, more than likely the engine will overheat in traffic or when idling.

Here's some info on timing a street engine you may enjoy.

Go here: http://www.jalopyjournal.com/forum/showthread.php?t=16056&highlight=Got+Time%3F

Just cuz it says Speed Shop on the door doesn't mean the info coming out is correct....

speedracer · March 20, 2006, 12:36:12 PM

Quote from: "C9"Did they re-curve the distributor?

How much initial advance?

How much total (all-in) advance?

Faster is an interesting term for advance in a distributor - which is the way I'm taking it.

How big is the cam?

A 1000 rpm idle for a street engine strikes me as a bit extreme. I don't know about recurving the distributor, what or how do you do that? I think all they did is just turn it to advance it. I'll put a light on it and tell ya how much advance.
The cam and lifters was bought as a proformance package with dual plane intake, 750 carb and aluminum heads, they guareente 410 hp plus but it only dyno at 350 hp at the wheels. I think they said that 1000 rpm was needed to make up the vacuum loss. Now your right it does run around 200 degrees and higher in traffic.
Thanks, johnny

If you don't run vacuum advance on the street, more than likely the engine will overheat in traffic or when idling.

Here's some info on timing a street engine you may enjoy.

Go here: http://www.jalopyjournal.com/forum/showthread.php?t=16056&highlight=Got+Time%3F

Just cuz it says Speed Shop on the door doesn't mean the info coming out is correct....

C9 · March 20, 2006, 12:50:25 PM

Distributor curves are simply centrifugal advance amount plotted against rpm.

A typical stock curve may be all-in (IE: max advance) at 2800-3200 rpm.

A drag racing curve may be all-in at 1800-2200 rpm.

Performance engine street advance curves usually fall somewhere in between.

Just depends on what you're trying to accomplish.

Depending on cam specs you may have enough vacuum at idle to run power brakes with no problems.

A stock or very mild cam will pull 18"-19" vacuum at idle.
A bigger cam, 280-290 degrees (advertised duration) will pull 10"-12" of vacuum at adle.
Assuming an idle of 600 rpm or so.

If you run the big cam engine up a few hundred rpm - 800-1000 or so - you'll pull about 18"-19" of vacuum.

All vacuum levels noted assume a well sealed engine.
IE: good rings and valves.

EMSjunkie · March 20, 2006, 11:47:37 PM

I though I was having Deja Vu here for a minute..........
then I realized I was.

http://www.roddingroundtable.com/forums/viewtopic.php?t=4182&highlight=vacuum+advance

Vance

speedracer · March 21, 2006, 01:01:10 AM

Quote from: "C9"Distributor curves are simply centrifugal advance amount plotted against rpm.

A typical stock curve may be all-in (IE: max advance) at 2800-3200 rpm.

A drag racing curve may be all-in at 1800-2200 rpm.

Performance engine street advance curves usually fall somewhere in between.

Just depends on what you're trying to accomplish.

Depending on cam specs you may have enough vacuum at idle to run power brakes with no problems.

A stock or very mild cam will pull 18"-19" vacuum at idle.
A bigger cam, 280-290 degrees (advertised duration) will pull 10"-12" of vacuum at adle.
Assuming an idle of 600 rpm or so.

If you run the big cam engine up a few hundred rpm - 800-1000 or so - you'll pull about 18"-19" of vacuum.

All vacuum levels noted assume a well sealed engine.
IE: good rings and valves.

OK Think I got all that, wish I could find the * specs on the cam, its here some place. Oh yea what do ya think about my speed shop plugging all the vacuum ports and pluging the distributor too? I figure its all because of the type cam I have. Are you famulor with that Eldelbrock Proformance Package I was talking about? The Holley 750 is off the engine right now, I took it down to another shop to have it tricked out (jetted) this guy is famous (old dragster gearhead) worked for and with Big Daddy Garlitts! So atleast I know the carburator will be right.

C9 · March 21, 2006, 09:23:44 AM

QuoteOK Think I got all that, wish I could find the * specs on the cam, its here some place. Oh yea what do ya think about my speed shop plugging all the vacuum ports and pluging the distributor too? I figure its all because of the type cam I have. Are you famulor with that Eldelbrock Proformance Package I was talking about? The Holley 750 is off the engine right now, I took it down to another shop to have it tricked out (jetted) this guy is famous (old dragster gearhead) worked for and with Big Daddy Garlitts! So atleast I know the carburator will be right.

Depends on how the Speed Shop plugged the vacuum ports.
If it's just screw in plugs no big deal.
Same with the rubber or plastic covers that go over the bibs.
Either, easily removed.

Plugging the dist vacuum can makes me wonder.
If they did some trick with epoxy you can drill it out.
Don't go too deep with the drill.

If the vacuum can is completely boogered up, install a new one.
Not too expensive and easily done.

speedracer · March 21, 2006, 03:52:05 PM

Quote from: "C9"
QuoteOK Think I got all that, wish I could find the * specs on the cam, its here some place. Oh yea what do ya think about my speed shop plugging all the vacuum ports and pluging the distributor too? I figure its all because of the type cam I have. Are you famulor with that Eldelbrock Proformance Package I was talking about? The Holley 750 is off the engine right now, I took it down to another shop to have it tricked out (jetted) this guy is famous (old dragster gearhead) worked for and with Big Daddy Garlitts! So atleast I know the carburator will be right.

Depends on how the Speed Shop plugged the vacuum ports.
If it's just screw in plugs no big deal.
Same with the rubber or plastic covers that go over the bibs.
Either, easily removed.

Plugging the dist vacuum can makes me wonder.
If they did some trick with epoxy you can drill it out.
Don't go too deep with the drill.

If the vacuum can is completely boogered up, install a new one.
Not too expensive and easily done.

Yea the plugs are the rubber ones and a large vacuum can was installed, it definitly helped my brakes. If I just go ahead a time the engine myself with a light install a vacuum line from the distributor to a vacuum bib that has constant vacuum (not port) of course not installed during timing the engine. And set the timing will I screw anything up? Right now the engine runs hot in traffic and has a serious after run when I shut the engine off, oh yea maybe set the idol at around 700 rpm vs 1100. What do ya think?

enjenjo · March 21, 2006, 08:09:06 PM

The key here is the engine was built by a speed shop. They built you a racing engine. So 1100rpm idle and overheating at idle are acceptable. If you are going to drive it on the street, you want the vacuum advance. There are low vacuum, vacuum advances available that will work with most cams. Since you will have the vacuum advance at idle, you can use less initial advance, and set the ignition for the total advance reccommended.

speedracer · March 21, 2006, 11:53:10 PM

Quote from: "enjenjo"The key here is the engine was built by a speed shop. They built you a racing engine. So 1100rpm idle and overheating at idle are acceptable. If you are going to drive it on the street, you want the vacuum advance. There are low vacuum, vacuum advances available that will work with most cams. Since you will have the vacuum advance at idle, you can use less initial advance, and set the ignition for the total advance reccommended.

Thats what i'm thinking and maybe to the engine will stop when I turn the key off instead of all that after run that it has now. I just never heard of plugging the vacuum advance but like ya said it was a speed shop that built the motor!
Oh and thanks a million for the link of timing, even I could understand it!!