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Old 06-02-2008, 10:53 AM
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How to boost vacuum

I have just put together and fired my new racing motor! After firing it I have found that the vacuum is just a shade on the low side. I am required to pull 15" at 800 RPM. When the engine is cold or just up to operating temp I am getting 13.5. After hot lapping I am getting very close. It is between 14.5 and 15. I need to get a little boost that will keep me over the top of that magic number. Technically the measurement should always be taken after the end of my race so I should always be hot when the check happens.

Here's what I know. I have a set of rhoads original lifters in right now. I have 50 weight racing oil and I am going to changed to 20w 50 hoping that they will bleed a little more due to the weight of the oil. (Please no comments about how the 50 in both oils should make them equal. I have seen performance gains on a dyno by switching ) I was in a real rush while putting the motor into my car and so right now I only have 2 foot down pipes on my exhaust and am wondering if when I get my full pipes on there if it will help the vacuum by balancing the back pressure/scavenging effect.

What I don't know. Right now I don't have my vacuum advance hooked up and am running 32* of timing at idle. Could I see a pickup of vacuum by hooking that up and running the initial timing at a lower number? Are there any other tricks that I can use to pick up vacuum? I run a team of 4 cars and we have a reputation of being above board and very insistent about the class rules. We can't afford to be questioned!
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Old 06-02-2008, 11:21 AM
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Set your total at 36 and see what you have this should do it. If not leave your total at 36 and hook vac advance up to intake vacuum, this will raise the vacuum at check time and will drop out when you are on the hammer as there will be no vac at full throttle when racing.
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Old 06-03-2008, 08:36 AM
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Ok so just to be clear when you say total timing are you refering to the timing after the centrafugal has kicked in? Or just the timing with with the vac advance unhooked?

But you are saying that I will get more vac if the vac advance is connected and retarding the advance at idle?
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Old 06-03-2008, 10:55 AM
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Ok for total I mean with vac unplugged and port capped, then run her up to about 3000 or where ever your centrifigal is all in set it to 36 degrees, then plug the vac advance back into intake vacuum, this will advance timing even further at idle but will drop out when on the hammer, thus leaving you at 36 when racing only advancing at idle and upping your reading.Again just set it at 36 like I said and check you may not need the vac adv. hooked up ,your timing at 32 is slow.
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Old 06-03-2008, 10:57 AM
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Having vac advance will slightly increase RPM for a given throttle setting, which will increase vacuum in the manifold. Question is: how much lead will be best & at how many inches...

Pick a canister that pegs near your present idle vac reading. A good one might be 1975 Cad 500 49 state. Having a strong return spring, It will not affect your timing under full throttle.


There are adjustable units from Crane, Accel, etc., they are 25-30 bucks.

Here is some great canister info..

http://www.corvetteclub.org.uk/kb.php?mode=article&k=30
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Old 06-03-2008, 01:34 PM
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more idle timing will increase vacuum.

have you tried adjusting the idle mixture screws for highest vacuum?

some lower lift rockers on the intake will help as a last resort.

A cam with wider lobe separation will do it.

is you gage correct?
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Old 06-03-2008, 02:52 PM
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It is likely your timing at idle is overadvanced at idle. Only looking at a vacuum gauge and playing with a timing light will answer your question.
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Old 06-03-2008, 10:33 PM
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I would not think you have excessive advance..

BTW, Bull, those Rhoads lifters are the Bomb, huh!
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Old 06-03-2008, 11:32 PM
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It would be unusual to have timing at 32 degrees being optimum at 800 RPM. That is likely overadvanced at that point.
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Old 06-04-2008, 12:55 AM
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I ran 36* locked out with ported vacuum adding an additional 12* at cruise. Initial timing is SO inconsequential to engine operation provided you get the proper part-throttle lead and WOT lead.

More idle advance will help vacuum. Wider LSA will help idle vacuum. Another thing you can do is advance the cam a bit. It might tend to remove some of the high-rpm advantage you have, but it will help vacuum.

But... if you can engineer a setup where you get moderate advance during cranking (28-32*) and use manifold vacuum to add more at idle, you can all but close the throttles which dramatically improves idle vacuum.
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Old 06-04-2008, 01:32 AM
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Just an example.. a 69 Camaro 350 vac canister provides 12 degrees of advance at about 15-16 inches. However, you'd double that measurement at the crank. So, if you had an initial timing of 14 BTC, you'd add 24 more with the vac advance, giving you 38. In my experience, a 350 will take 50 degrees easily.

I just installed a Crane adjustable canister, and I liked it. I think it would be a good choice for you, because you can adjust it to your vacuum, and limit the travel as well. Only snag is: When you are adjusting the travel limit, you change the base timing, so put a light on it before & after.
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Old 06-04-2008, 09:22 AM
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It is stupid to even mention a vac can install on a race motor. All yiou need to do is understand what the vacuum rule is versus how a race engine operates to know that it's a silly option.
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Old 06-04-2008, 09:44 AM
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its an unnecessary option on a race engine, but not silly. It certainly doesn't hurt, and in his situation (trying to get more vacuum) it might be something that makes his car pass tech without any bad side effects.
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Old 06-04-2008, 10:11 AM
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Quote:
Originally Posted by curtis73
its an unnecessary option on a race engine, but not silly. It certainly doesn't hurt, and in his situation (trying to get more vacuum) it might be something that makes his car pass tech without any bad side effects.

Why is it not silly. Based on his post he is already at 32 degrees advanced at idle, what good is a vac can going to do?

He needs to set his base timing to get vacuum as high as possible, as well as tune in the idle circuit as best he can, then dial in the mechanical advace via weights and bushings to get total timing at the setting which makes best power.
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Old 06-04-2008, 10:23 AM
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Quote:
Originally Posted by Rick WI
He needs to set his base timing to get vacuum as high as possible, as well as tune in the idle circuit as best he can, then dial in the mechanical advace via weights and bushings to get total timing at the setting which makes best power.

Umm... Sir...

He actually needs to set his base timing and mechanical advance to get the POWER as high as possible, and then alter his vacuum advance to obtain the 15"+ of vacuum that his organization requires.

Although some may insist that this is stupid, it will allow him to run legal, which is what he was looking for. I personally prefer vac advance, even on race motors and boats, as I feel it lowers the thermal load and provides better transitional crispness.

just my 2c...
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