Timing curve question?
Le Mans Master
Joined: Apr 2004
Posts: 7,106
Likes: 38
From: Newark DE
Originally Posted by dwncchs
Remember if your using ported vacumn-it is venturi vacumn and it will not diminish with WOT
on my '65 of course it's full vacuum and most people on here because of recommendations from people such as Lars and JohnZ have switched their cars to full manifold vacuum if they had ported vacuum so that's where my brain is usually thinking from.
Racer
Joined: Apr 2006
Posts: 477
Likes: 43
From: Millbrae California
Originally Posted by dwncchs
Remember if your using ported vacumn-it is venturi vacumn and it will not diminish with WOT
Last edited by olescarb; Jun 10, 2006 at 10:46 AM.
Pro
Joined: Jan 2005
Posts: 535
Likes: 0
From: Southaven MS
The vacuum can increases timing when the engine is under low load i.e. it has high vacuum...like when cruising. Once you accelerate and the vacuum decreases then the vacuum can decreases timing, but the mechanical timing is a function of the rpm(indirectly)...as rpm increases then the mechanical increases timing to a certain point...depending on your springs and weights. My understanding is that the vacuum can is for only street and cruising, I do not believe they are used on race cars b/c they stay at high rpms.
As for ported versus manifold ports..I agree with olescarb. The manifold gives full vacuum to the vacuum can while idling. The ported only gives vacuum when the throttle blades are opened up(not sure how much but don't think it needs much). As for vacuum can increasing timing at WOT, I don't believe ported versus manifold matters. The only thing it changes is when the vacuum can gets engine vacuum at low rpm. Anyway hope that makes sense. If I am wrong, I'm sure someone here will correct me.
As for ported versus manifold ports..I agree with olescarb. The manifold gives full vacuum to the vacuum can while idling. The ported only gives vacuum when the throttle blades are opened up(not sure how much but don't think it needs much). As for vacuum can increasing timing at WOT, I don't believe ported versus manifold matters. The only thing it changes is when the vacuum can gets engine vacuum at low rpm. Anyway hope that makes sense. If I am wrong, I'm sure someone here will correct me.
Instructor
Joined: May 2006
Posts: 122
Likes: 1
From: hazlet township nj
Originally Posted by 1972warship
The vacuum can increases timing when the engine is under low load i.e. it has high vacuum...like when cruising. Once you accelerate and the vacuum decreases then the vacuum can decreases timing, but the mechanical timing is a function of the rpm(indirectly)...as rpm increases then the mechanical increases timing to a certain point...depending on your springs and weights. My understanding is that the vacuum can is for only street and cruising, I do not believe they are used on race cars b/c they stay at high rpms.
As for ported versus manifold ports..I agree with olescarb. The manifold gives full vacuum to the vacuum can while idling. The ported only gives vacuum when the throttle blades are opened up(not sure how much but don't think it needs much). As for vacuum can increasing timing at WOT, I don't believe ported versus manifold matters. The only thing it changes is when the vacuum can gets engine vacuum at low rpm. Anyway hope that makes sense. If I am wrong, I'm sure someone here will correct me.
As for ported versus manifold ports..I agree with olescarb. The manifold gives full vacuum to the vacuum can while idling. The ported only gives vacuum when the throttle blades are opened up(not sure how much but don't think it needs much). As for vacuum can increasing timing at WOT, I don't believe ported versus manifold matters. The only thing it changes is when the vacuum can gets engine vacuum at low rpm. Anyway hope that makes sense. If I am wrong, I'm sure someone here will correct me.
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
Originally Posted by 77 zz4
Is using the vacuum can and using vacuum advance the same thing?
Melting Slicks
Joined: Dec 2004
Posts: 2,161
Likes: 0
From: pr, frederick md.
Originally Posted by dwncchs
46 degrees wow that sounds like an awful lot of timing
Thread Starter
Melting Slicks
Joined: Mar 2006
Posts: 3,342
Likes: 7
From: Connecticut
Originally Posted by redc3
this forum is the first and only place ive ever heard od someone running that much timing. it has been news to me and my car/cars wont run like that. ie...knocking and pinging. you cant even measure mech. and or vac advance without a progressive light. i wonder if their using it correctly.
Yes, mine has a back dial and tach
I run my rpm at 2,500 and read the mechanical advance (vacuum unhooked and plugged) and I see 35 degrees....
This is all very confusing to me at this point because the engine runs perfect... no knock/ping (even when intentionally ******* it up a hill in 4th gear)
I dont know anymore..... this gets more confusing with every post I read...
Last edited by Bob Onit; Jun 11, 2006 at 01:49 AM.
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Burning Brakes
Joined: Aug 2005
Posts: 1,126
Likes: 0
From: Hudson Falls, N.Y. 76 Vette Modified L-48
Originally Posted by BarryK
that's correct, at idle the vacuum advance is adding to the timing because of high motor vacuum.
under load or at WOT the motor is producing little or no vacuum so there is no additional vacuum advance and the timing is only from the mechanical advance
under load or at WOT the motor is producing little or no vacuum so there is no additional vacuum advance and the timing is only from the mechanical advance
I run mine off full vac not ported...
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
Originally Posted by Bob Onit
As far as a progressive light goes, I believe you mean an adjstable timing light?
Yes, mine has a back dial and tach
I run my rpm at 2,500 and read the mechanical advance (vacuum unhooked and plugged) and I see 35 degrees....
This is all very confusing to me at this point because the engine runs perfect... no knock/ping (even when intentionally ******* it up a hill in 4th gear)
I dont know anymore..... this gets more confusing with every post I read...
Yes, mine has a back dial and tach
I run my rpm at 2,500 and read the mechanical advance (vacuum unhooked and plugged) and I see 35 degrees....
This is all very confusing to me at this point because the engine runs perfect... no knock/ping (even when intentionally ******* it up a hill in 4th gear)
I dont know anymore..... this gets more confusing with every post I read...
Last edited by bobs77vet; Jun 11, 2006 at 08:07 AM.
Burning Brakes
Joined: Aug 2005
Posts: 1,126
Likes: 0
From: Hudson Falls, N.Y. 76 Vette Modified L-48
Originally Posted by bobs77vet
whats there to know...you have it set perfectly and the proof is in how it runs....read Lars articles hes right on the money....if its not pinging you have it set correctly....and you said yourself its running great...sit back and relax and enjoy it.....remember we are talking about PERFORMANCE tuning here not EMISSIONS tuning or even WARRANTY tuning....
Trying to get the screws out of my dist holding in the vac advance to swap it, and the damn things won't budge....any ideas? I've tried penetrating oil and every straight slot screwdriver I own...
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
GM dist. specs
Originally Posted by Bob Onit
Would 35 degrees with no vacuum advance indicate a curve kit bieng installed ?
Or is this a stock reading
Or is this a stock reading
Last edited by ...Roger...; Jun 11, 2006 at 10:07 AM.
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
Originally Posted by bobs77vet
whats there to know...you have it set perfectly and the proof is in how it runs....read Lars articles hes right on the money....if its not pinging you have it set correctly....and you said yourself its running great...sit back and relax and enjoy it.....remember we are talking about PERFORMANCE tuning here not EMISSIONS tuning or even WARRANTY tuning....
Thread Starter
Melting Slicks
Joined: Mar 2006
Posts: 3,342
Likes: 7
From: Connecticut
Originally Posted by dwncchs
If this is at 2500 then the answer would be yes to a curve kit.initial at 12 book says you should only have 6 degees @ 2410 for a total of 18 at 2410rpm----do what Lars said and remove cent.adv.springs set total ---put springs back in.
Well give that a try
ps: dwncchs I did get your PM sorry I diddnt have time to reply right away
Thank you very much
Thread Starter
Melting Slicks
Joined: Mar 2006
Posts: 3,342
Likes: 7
From: Connecticut
Originally Posted by bobs77vet
whats there to know...you have it set perfectly and the proof is in how it runs....read Lars articles hes right on the money....if its not pinging you have it set correctly....and you said yourself its running great...sit back and relax and enjoy it.....remember we are talking about PERFORMANCE tuning here not EMISSIONS tuning or even WARRANTY tuning....
Thread Starter
Melting Slicks
Joined: Mar 2006
Posts: 3,342
Likes: 7
From: Connecticut
Originally Posted by kb2fzq
Hey Bob,
Trying to get the screws out of my dist holding in the vac advance to swap it, and the damn things won't budge....any ideas? I've tried penetrating oil and every straight slot screwdriver I own...
Trying to get the screws out of my dist holding in the vac advance to swap it, and the damn things won't budge....any ideas? I've tried penetrating oil and every straight slot screwdriver I own...
Why are you swapping it?
Burning Brakes
Joined: Sep 2004
Posts: 1,167
Likes: 2
From: Ft.Worth Texas
Originally Posted by Bob Onit
Let the oil work some more
Why are you swapping it?
Why are you swapping it?
Burning Brakes
Joined: Sep 2004
Posts: 1,167
Likes: 2
From: Ft.Worth Texas
Originally Posted by bobs77vet
well they are related but different....the vacuum cannister is the cone shaped thing on your distributor it is a mechanical part......it has a diaphram in it and as the vacuum goes up or down it moves a long shaft back and forth and that will alter the timing.....this vacuum cannister controls the vacuum advance timing. there are also different types of cannisters that "pull" full vacuum advance at different vaccum levels.....ususally the bigger the cam the less vacuum so you can match the vacuum cannister to your application.....this is a typical high performance set up ...16* initial timing....20* mechanical advance.....16* vacuum advance for a total of 52*.....so when we talk about 36* at 2500 rpm we are adding the 16* initial and the 20* mechanical to get the 36*.....you can alter the springs to make the weights get thrown open faster to reach this adjustment. and you can alter bushings etc ...etc..etc.. just keep reading Lars articles and they will start to make sense
1) 16* initial with vacuum hose to dist. disconnected and hose plugged and engine at idle of aprx. 650 rpm
2) with hose still disconnected and plugged and engine at 2500 rpm you are looking for 36*.....change to lighter or heaver springs to get this and use different stop bushings so the advance maxes out at the 2500 rpm
3) now, when you plug back in the vacuum hose which comes from a ported source, you can possibly see a total advance of 52* at the higher rpm's.
You can see this much advance with either a dial back light or there is a marked tape to place on the balancer that works too. Just get the right tape for the diameter of your harmonic balancer.
Ported vacuum....Definition: Engine vacuum that is available above the throttle plates of a throttle body or carburetor. Used to advance ignition timing on older carbureted engines when the throttle is opened above its idle position.
Manifold vacuum....Definition: Low pressure in an engine intake manifold formed just below the throttle plates. Manifold vacuum is highest at idle and drops under acceleration.
also a good read...http://www.therangerstation.com/Maga...acuumLeaks.htm
Last edited by David Ey; Jun 11, 2006 at 05:03 PM.