GN7 and others please help me out.
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GN7 and others please help me out.

  1. #1
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    Default GN7 and others please help me out.

    I am trying to tune my hot rod. I am running a GM 350ho crate engine (vortec heads). First question, should I have the vacuum advance connected to manifold vacuum or ported/timed vacuum?

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    Already miss the 310/562 2manymustangs's Avatar
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    my 2 cents, ported vaccum source...
    Pat Eason, the voice of reason...

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    ok. can you explain why. This seems to be a big debate. people say that other than idle that manifold and ported is the same...others say that manifold is on and off based on load while ported is always climbing to a point. This engine idles better on manifold since it is advancing...but when I mash the throttle there is a momentary delay. Maybe I can tune the delay out??? Here is a link supporting MANIFOLD VACUUM http://rockridgefarm.com/vettdoc/Timing_101.pdf Thoughts?

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    Quote Originally Posted by dnewps View Post
    ok. can you explain why. This seems to be a big debate. people say that other than idle that manifold and ported is the same...others say that manifold is on and off based on load while ported is always climbing to a point. This engine idles better on manifold since it is advancing...but when I mash the throttle there is a momentary delay. Maybe I can tune the delay out??? Here is a link supporting MANIFOLD VACUUM http://rockridgefarm.com/vettdoc/Timing_101.pdf Thoughts?
    For the most part, ported is an emissions arrangement. It was never used in the 60s.
    Port does not "keep" climbing, but is somewhat progressive, but it is a short progression. It doesn't take much throttle to have it equal manifold vac. I have never seen an engine with even a semi mild cam that didn't run better with manifold vac.



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    Quote Originally Posted by gn7 View Post
    For the most part, ported is an emissions arrangement. It was never used in the 60s.
    Port does not "keep" climbing, but is somewhat progressive, but it is a short progression. It doesn't take much throttle to have it equal manifold vac. I have never seen an engine with even a semi mild cam that didn't run better with manifold vac.
    ok so I am going to try to tune this with manifold vacuum. Is my first step to plug the advance and set total timing? GM calls for 32 on this engine...Is that what I should use? Roughly at what rpm should the timing be all in? its a light vette with a 4 spd but tall gears (I think 3.88 from a 69).

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    Quote Originally Posted by dnewps View Post
    ok so I am going to try to tune this with manifold vacuum. Is my first step to plug the advance and set total timing? GM calls for 32 on this engine...Is that what I should use? Roughly at what rpm should the timing be all in? its a light vette with a 4 spd but tall gears (I think 3.88 from a 69).
    Disconnected. If you leave it connected, the slight throttle opening it takes to hit the desired RPM is so slight that it will make enough vacuum to add to the timing and throw off timing and it will hit max before it would if you were driving the thing.
    You are trying to set the max mechanical until it hits the stop.
    Don't be surprised if the timing is still climbing until 3000 or even 3500.



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    Last edited by gn7; 08-13-2013 at 02:12 PM.

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    Nevermind. I'd listen to what GN said.
    Last edited by 67weiman; 08-13-2013 at 02:11 PM.

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    Whew.... reread your first post. I was trying to figure out why you wanted vacuum advance on a BOAT. Then I saw HOTROD.
    Carry on.
    Wags

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    Quote Originally Posted by wagspe208 View Post
    Whew.... reread your first post. I was trying to figure out why you wanted vacuum advance on a BOAT. Then I saw HOTROD.
    Carry on.
    Wags
    I tripped over that myself Wags. Something didn't add up about a crate 350 HO in a boat except maybe a Bayliner, and then what difference would it matter how the advance was hooked up. Who would care.
    Had to be a car.



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    Quote Originally Posted by gn7 View Post
    I tripped over that myself Wags. Something didn't add up about a crate 350 HO in a boat except maybe a Bayliner, and then what difference would it matter how the advance was hooked up. Who would care.
    Had to be a car.
    Yes its a car....ok disconnected. should total be 32? the distributor I have is from sumitt...specs say 14 deg mechanical @2200 rpm with 10-11 deg vacuum advance (I don't think it is adjustable). This is not the same distributor that GM calls for, that has 22 deg mech @4600rpm with 20deg vacuum advance @ 7.5hg. If I set it at 32 and it has 14 mech then my initial will be 18...too much? Time for a new distributor?

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    Quote Originally Posted by dnewps View Post
    Yes its a car....ok disconnected. should total be 32? the distributor I have is from sumitt...specs say 14 deg mechanical @2200 rpm with 10-11 deg vacuum advance (I don't think it is adjustable). This is not the same distributor that GM calls for, that has 22 deg mech @4600rpm with 20deg vacuum advance @ 7.5hg. If I set it at 32 and it has 14 mech then my initial will be 18...too much? Time for a new distributor?
    No wonder people recurved their GM distributors. Imagine near zero vac @full throttle and full advance not happening until 4600!!!!

    NO 18 initial is not too much. It will help it idle better. I would get a new distributor. I would try that one first.



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    Last edited by gn7; 08-13-2013 at 02:52 PM.

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    Quote Originally Posted by gn7 View Post
    No wonder people recurved their GM distributors. Imagine near zero vac @full throttle and full advance not happening until 4600!!!!

    NO 18 initial is not too much. It will help it idle better. I would get a new distributor. I would try that one first.
    I have it pretty dialed in at 34 total and 20 initial (vacuum advanced disconnected) 30 at idle with the vacuum advance connected to manifold. This distributor does not have the mechanical tach provision I need for the oem gauge and the little electric autometer I had just took a shit. So time for a new distributor. how much mechanical advance should I get and where should it max out? how much vacuum advance? Since the vacuum advance is now working at idle should I set the idle screws (max vacuum method) with the vacuum advance connected? I was thinking of using a "T" connection.
    Last edited by dnewps; 08-13-2013 at 05:34 PM.

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    Quote Originally Posted by dnewps View Post
    Yes its a car....ok disconnected. should total be 32? the distributor I have is from sumitt...specs say 14 deg mechanical @2200 rpm with 10-11 deg vacuum advance (I don't think it is adjustable). This is not the same distributor that GM calls for, that has 22 deg mech @4600rpm with 20deg vacuum advance @ 7.5hg. If I set it at 32 and it has 14 mech then my initial will be 18...too much? Time for a new distributor?
    Take the distributor to a shop that has a distributor machine (sunnen) and get the curve checked with and without vac! Then if the person or persons know what the hell the are doing they can tell you if it needs ported or non ported (vacuum pulling timing back for idle and start) OR get a degreed timing light and figure it out on your'e own. Good luck!

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    Quote Originally Posted by ol guy View Post
    Take the distributor to a shop that has a distributor machine (sunnen) and get the curve checked with and without vac! Then if the person or persons know what the hell the are doing they can tell you if it needs ported or non ported (vacuum pulling timing back for idle and start) OR get a degreed timing light and figure it out on your'e own. Good luck!
    I had it running pretty good on ported but it idled a little rough at 15 deg initial...the manifold vac advances the idle to 30 and it idles much smoother. hit the throttle and the timing backs off then climbs with rpm. what I am trying to figure out at this point is what the curve should be without using a dyno. how much total mechanical advance and how much vacuum advance is ideal. I have given up on the shops since I cannot find one that even understands manifold vac vs ported....its worse than the chevy ford debate.

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