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Username Post: Fine Tuning BBC - Timing & Vac Adv        (Topic#199586)
Diehard50248 
Forum Newbie
Posts: 72

Loc: Twin Cities
Reg: 01-13-08
09-24-08 07:42 PM - Post#1528944    

This is a compilation of previous posts from questions I've asked in the 1965 & 1966 Fullsize Chevy Forum. I think my questions are more "performance" related, so I'm bringing it here...

If possible I want my 496 to idle smoothly, use full vacuum advance, have zero pinging, and run like a raped ape. I'm beginning to think getting all three may be unrealistic, but I want some confirmation before I start to settle.

Here are the facts & the numbers:
1966 Impala Convertible
496 BBC
Comp Cams 11-246-3 ([url]http://www.compcams.com/Cam_Specs/CamDetails.aspx?csid=396&sb=0[/u rl])
8.5:1 or 9.0:1 compression (I don't remember which but I know its low)
Iron Heads
Headers
3.31 Gear - Posi
"stock" stall converter
780 Holley w/ Vac Secondaries (3310-1)
Edelbrock Air Gap Intake
Summit Style HEI Distributor (Large Cap)
92 Octane Fuel

Timing Curve:
RPM / Mech Adv
500 / +0
1000 / +0
1500 / +13
2000 / +17
2500 / +19
3000 / +21
3500 / +21

Vacuum Readings: Adjustable VA, limited to 8* at crank
Idle in park: 16" HG
Idle in drive: 10" HG
Partial Throttle: 8" HG
Steady Cruise: 18" HG
Coasting: 23" HG
WOT: 2" HG

I think I figured out why I can't seem to get a good idle and avoid partial throttle pinging. My in-gear idle vacuum and my partial throttle vacuum levels are so close I can't get my vacuum advance "all in" at idle, AND "all out" at partial throttle. What can I do to fix this situation?

To explain it using the numbers - My in-gear idle vacuum is 10" HG. To get the vac advance "all in" at idle means it should be all in by 8" HG. My vacuum under load is also 8" HG. To get vacuum advance "out" under partial throttle (to avoid pinging) the vacuum advance needs to drop out. Do you see the dilemma?

I'm starting to think I may be forced to use "ported" vacuum for the vacuum advance. I'll lose the benefits of more advance at idle, but its the only way I can see to avoid over advancing my engine at partial throttle. Do you guys agree with this conclusion? Is there anything I can do to keep full manifold vacuum for the advance but still have it drop out at partial throttle? Any advice would be greatly appreciated.

Jeff


1966 Impala Convertible - 496 BBC Stroker
A picture of "Paula"


 
steeler_fan 
Contributor
Posts: 132
steeler_fan
Loc: Folsom Ca
Reg: 02-16-07
09-24-08 08:37 PM - Post#1528964    
    In response to Diehard50248

Jeff

here is a corvette site with a great set of articles on timing and vacuum advance. But there is one particular vacuum advance unit that is referenced for high performance big blocks.
VC1810 B28. This Vacuum can could be the ticket for you. i attached both the site with the various articles, and the 2nd site is the specific one for vacuum advance and different parts with the specs.


http://www.corvette-restoration.com/resources/how_...

http://corvette-restorati on.com/resources/technica...

steeler_fan
and yes the offense sucked on sunday night

 
jimmmy57 
Contributor
Posts: 362
jimmmy57
Loc: Australia
Reg: 03-08-08
09-26-08 01:30 AM - Post#1529700    
    In response to steeler_fan

Hi Jeff,
Great combo you have there , except that it's a big camshaft for a "stock" convertor. How about leaving the vacuum advance alone and install a 2500 rpm stall convertor to let the engine idle in gear and have good vacuum, should bring the car to life also
Jimmy

"I've told you a million times, , Don't exagerate"
http://www.picturetrail.com/gallery/view?uid=1087 1...


 
Diehard50248 
Forum Newbie
Posts: 72

Loc: Twin Cities
Reg: 01-13-08
09-26-08 04:03 AM - Post#1529736    
    In response to jimmmy57

  • Diehard50248;2009483 Said:
I think I figured out why I can't seem to get a good idle and avoid partial throttle pinging. My in-gear idle vacuum and my partial throttle vacuum levels are so close I can't get my vacuum advance "all in" at idle, AND "all out" at partial throttle. What can I do to fix this situation? Does this mean I need to give up on using full manifold vacuum for my vacuum advance?

To explain it using the numbers - My in-gear idle vacuum is 10" HG. To get the idle vac advance "all in" at idle means it should be all in by 8" HG (2" below idle). My vacuum under partial throttle load is also 8" HG. To get vacuum advance "out" under partial throttle (to avoid pinging) the vacuum advance needs to drop out. Do you see the dilemma?

I'm starting to think I may be forced to use "ported" vacuum for the vacuum advance. I'll lose the benefits of more advance at idle, but its the only way I can see to avoid over advancing my engine at partial throttle. Do you guys agree with this conclusion? Is there anything I can do to keep full manifold vacuum for the advance but still have it drop out at partial throttle? Any advice would be appreciated.

Jeff



Good News / Bad News:

I set intial timing at 20* and plugged the VA. It didn't ping, so I hooked the VA back on and hooked it up to ported vacuum. I adjusted the VA to just start advancing at 8" HG. It worked! No partial throttle pinging (because the VA drops out), and no WOT pinging (I guess my engine doesn't mine 40/41* total timing). The only thing I'm still not happy with is idle. Even with 20* intial timing it still isn't as smooth as I'd like it. I'm confident it would smooth out if I could just use full manifold vacuum, but it doesn't look like that's going to work in my case. If there's a way to have full manifold vacuum "all in" at 8" (for idle) and "all out" at 8" HG (for partial throttle), please let me know.

1966 Impala Convertible - 496 BBC Stroker
A picture of "Paula"


 
Jim.Biron 
"7th Year" Platinum Supporting Member
Posts: 2524
Jim.Biron
Age: 64
Loc: Saugerties, NY
Reg: 04-10-08
09-26-08 04:18 AM - Post#1529746    
    In response to Diehard50248

  • Diehard50248 Said:
  • Diehard50248;2009483 Said:
I think I figured out why I can't seem to get a good idle and avoid partial throttle pinging. My in-gear idle vacuum and my partial throttle vacuum levels are so close I can't get my vacuum advance "all in" at idle, AND "all out" at partial throttle. What can I do to fix this situation? Does this mean I need to give up on using full manifold vacuum for my vacuum advance?

To explain it using the numbers - My in-gear idle vacuum is 10" HG. To get the idle vac advance "all in" at idle means it should be all in by 8" HG (2" below idle). My vacuum under partial throttle load is also 8" HG. To get vacuum advance "out" under partial throttle (to avoid pinging) the vacuum advance needs to drop out. Do you see the dilemma?

I'm starting to think I may be forced to use "ported" vacuum for the vacuum advance. I'll lose the benefits of more advance at idle, but its the only way I can see to avoid over advancing my engine at partial throttle. Do you guys agree with this conclusion? Is there anything I can do to keep full manifold vacuum for the advance but still have it drop out at partial throttle? Any advice would be appreciated.

Jeff



Good News / Bad News:

I set intial timing at 20* and plugged the VA. It didn't ping, so I hooked the VA back on and hooked it up to ported vacuum. I adjusted the VA to just start advancing at 8" HG. It worked! No partial throttle pinging (because the VA drops out), and no WOT pinging (I guess my engine doesn't mine 40/41* total timing). The only thing I'm still not happy with is idle. Even with 20* intial timing it still isn't as smooth as I'd like it. I'm confident it would smooth out if I could just use full manifold vacuum, but it doesn't look like that's going to work in my case. If there's a way to have full manifold vacuum "all in" at 8" (for idle) and "all out" at 8" HG (for partial throttle), please let me know.





Take a look at the HEI thread and it's posts. Sounds like you and I have about the same problem. What happens if you run all mechanical giving you the total you want and no vacuum?
Jim
www.HudsonValleyCruise.com


 
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