Vaccum advance fast idle problem
Le Mans Master
Joined: Apr 2000
Posts: 5,202
Likes: 10
From: cincinnati ohio
St. Jude Donor '09
Tim H, You don't have to take our word for it, do a little reading...there are countless articles, dyno tests and the like on getting max performance of a street motor using the entire timing system.......here's just one example...
http://www.chevyhiperformance.com/howto/97438/
http://www.chevyhiperformance.com/howto/97438/
Le Mans Master
Joined: Jul 2005
Posts: 5,752
Likes: 4
From: Austin Texas
Ah the ported vs. manifold debate.
1st of all to the original poster, sounds like you got the vacuum advance hooked up right and your car is acting as expected with the vacuum advance disconnected. If you want to lower idle RPM (sounds like that is your concern) simply lower the idle speed at the carb.
I have mine hooked up to a ported source myself - the reason being my timing is set at the proper and desired starting advance already (16) and, with my idle vacuum with the cam I have, my timing and RPM would bounce around like a ping pong ball as it would never be all "in". So ported works for me.
I would have to get a new vacuum can that brings in all the advance at a lower vacuum for manifold to work, not sure if there would be any benifits. But I agree vacuum advance is benificial for a street driven car. Also, and I am not sure if it's a material issue at all, I never liked the tought of advance diving down (as a result of vacuum decreasing) for that moment as you enter WOT from idle.
1st of all to the original poster, sounds like you got the vacuum advance hooked up right and your car is acting as expected with the vacuum advance disconnected. If you want to lower idle RPM (sounds like that is your concern) simply lower the idle speed at the carb.
I have mine hooked up to a ported source myself - the reason being my timing is set at the proper and desired starting advance already (16) and, with my idle vacuum with the cam I have, my timing and RPM would bounce around like a ping pong ball as it would never be all "in". So ported works for me.
I would have to get a new vacuum can that brings in all the advance at a lower vacuum for manifold to work, not sure if there would be any benifits. But I agree vacuum advance is benificial for a street driven car. Also, and I am not sure if it's a material issue at all, I never liked the tought of advance diving down (as a result of vacuum decreasing) for that moment as you enter WOT from idle.
Race Director
Joined: Nov 2000
Posts: 11,872
Likes: 12
From: Waterloo ontario Canada
The fact that the vacuum advance goes away quickly at full throttle is a good thing, as the cruise total advance (above 36 and below 52) would be too much (damaging) for the engine to handle. That is another reason to use a quick mechanical advance curve and as much initial as your engine can stand, to fill the timing void (caused by the loss of vacuum advance at full throttle)....
This 52 degrees total is a recommended amount. Check out Lars paper.
Pro
Joined: Sep 2002
Posts: 690
Likes: 1
From: near Memphis TN
The fact that the vacuum advance goes away quickly at full throttle is a good thing, as the cruise total advance (above 36 and below 52) would be too much (damaging) for the engine to handle. That is another reason to use a quick mechanical advance curve and as much initial as your engine can stand, to fill the timing void (caused by the loss of vacuum advance at full throttle)....
timing at idle with vc connected to manifold - 20 (as an example)
then from idle you go to WOT - total timing suddenly decreases to pre-vacuum advance level (lets say 4), if but for a moment, then mechanical advance (RPM driven) timing raises it to 36 total.
timing at idle with vc connected to ported - 16
then from idle you go to WOT - total timing does not decrease at all, but increases as mechanical timing raises it to 36 total.
As you point out a quick mechanical advance curve will decrease any effect, if any, of this.
Norval - I think BigG is saying cruise timing 52 is to high for WOT, which I think we all obviously agree with.
Last edited by SpyderD; Aug 30, 2007 at 02:19 PM.
Le Mans Master
Joined: Jul 2005
Posts: 5,752
Likes: 4
From: Austin Texas
Only gave a range 'cuz some engines can't take much cruise advance, though most can. I get a little bucking just at 52.
Intermediate
Joined: Jul 2007
Posts: 39
Likes: 0
From: Quebec Quebec
Also please speak in normal english if you would?[/QUOTE]
I'm a french canadian and I appreciate strongly advices on this forum. Sometimes I may have answers. But according to Tim H, if I don't write perfectly english, it's not a good idea to answer. Or to ask. It's sad for all Corvette lovers Who are speaking another native language. At least I tried to be well understood.
Just too bad !
I'm a french canadian and I appreciate strongly advices on this forum. Sometimes I may have answers. But according to Tim H, if I don't write perfectly english, it's not a good idea to answer. Or to ask. It's sad for all Corvette lovers Who are speaking another native language. At least I tried to be well understood.
Just too bad !
Tech Contributor
Joined: Jun 2004
Posts: 20,931
Likes: 962
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
I'm a french canadian and I appreciate strongly advices on this forum. Sometimes I may have answers. But according to Tim H, if I don't write perfectly english, it's not a good idea to answer. Or to ask. It's sad for all Corvette lovers Who are speaking another native language. At least I tried to be well understood.
Just too bad ![/QUOTE]
Cloue,
I agree. It is unfortunate at times. But, I think you'll find most of us are very patient regarding language issues. It's my guess that most of us in the forum are not bilingually fluent, so we are usually respectful of those who can communicate in multiple languages. The more prevalent problem seems to be those who are technically illiterate, and have a chip on their shoulder regarding others who may have more knowledge on a particular subject. There's a wide range of talent on this forum, and we can all learn new stuff if we keep our ears open and our egos in check. Please don't let that guy disuade you from participating in these discussions.
Mike
Safety Car
Joined: Oct 2000
Posts: 3,593
Likes: 103
From: Southern Indiana
the thermal efficiency of the combustion dropped like a rock
I can read almost anything on here, its the i dotters and t crossers that have to show their smarts when they can't find any other way to contribute!
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Le Mans Master
Joined: Apr 2004
Posts: 7,106
Likes: 38
From: Newark DE
This IS a TECHICAL discussion forum. If the purpose isn't for techincal information than what's the point?
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
Keep up the great posts Mike.
Oh and about the dotting every i and crossing every t...thats just being accurate..try not adjusting every valve or only torqueing half your rod bolts...
Cruising
Joined: Jul 2005
Posts: 10
Likes: 0
From: Saltillo MS
I am just trying to share my experiance with the original poster. I am eternaly greatfull to the members that share their knowledge and time with me and the world and for that
Street Driven Corvettes must operate in varied conditions as oposed to drag queens. By the way, Tim H my wife has a matching set of burgandy shoes and purse to match those valve covers. Get your mechcanic on here so we will all understand what you mean.
Street Driven Corvettes must operate in varied conditions as oposed to drag queens. By the way, Tim H my wife has a matching set of burgandy shoes and purse to match those valve covers. Get your mechcanic on here so we will all understand what you mean.
Instructor
Joined: Apr 2006
Posts: 212
Likes: 0
From: Woodland Park Colorado
My setup liked the vac advance plugged into ported like spyder.Not to disaggree with anyone but in my case the vacuum at idle in gear was 6 due to my cam and living at 8500 ft above sea level and my idle would not stop bouncing. I could have bought a different vac can but now I have no issues with idle or any other speed so why change what works. My timing is set to 12 initial 36 at 3000 rpm and 48 total for the fast burn heads. I also had to change my power valve in my carb to 3.5 and that cured my off idle stumble. These rules on tuning can very depending on sea level, setup,and other conditions and dont work for everyone. The nice thing about my setup is I havent had to touch it for over a month now and can just drive.
Le Mans Master
Joined: Apr 2004
Posts: 7,106
Likes: 38
From: Newark DE
My setup liked the vac advance plugged into ported like spyder.Not to disaggree with anyone but in my case the vacuum at idle in gear was 6 due to my cam and living at 8500 ft above sea level and my idle would not stop bouncing. I could have bought a different vac can but now I have no issues with idle or any other speed so why change what works. My timing is set to 12 initial 36 at 3000 rpm and 48 total for the fast burn heads. I also had to change my power valve in my carb to 3.5 and that cured my off idle stumble. These rules on tuning can very depending on sea level, setup,and other conditions and dont work for everyone. The nice thing about my setup is I havent had to touch it for over a month now and can just drive.
Having said that, even Lars says that while the vast majority of cars will see an improvement by switching your vacuum source to full manifold vacuum each car is different and there are some that seem to respond better by running off the ported vacuum but these are the exception rather than the rule.
Tech Contributor
Joined: Jun 2004
Posts: 20,931
Likes: 962
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Intermediate
Joined: Jul 2007
Posts: 39
Likes: 0
From: Quebec Quebec
because someone gives actual accurate technical information rather than guesses and wrong misinformed information they are than referred to as "i dotters and t crossers" and are made fun of because they actually know what they are talking about??
This IS a TECHICAL discussion forum. If the purpose isn't for techincal information than what's the point?
This IS a TECHICAL discussion forum. If the purpose isn't for techincal information than what's the point?
I understand what Tim K Meant now. Let me say tank you once more to these pros who help rookies like me.
Tech Contributor
Joined: Aug 1999
Posts: 14,382
Likes: 6,401
From: At my Bar drinking and wrenching in Lafayette Colorado
http://lbfun.com/Corvette/Tech/vette...101Article.pdf
I highly recommend everyone read this to gain a basic understanding of timing, vacuum advance, and the concepts of ported versus manifold vacuum.
For additional info, I have tech papers about how to set timing, vacuum advance control unit spec numbers, and engine vacuum concepts. Anyone interested in these papers can e-mail me for copies:
V8FastCars@msn.com
Lars
