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Old 05-04-2004, 12:08 AM
Sudden Psycho 31's Avatar
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vacuum vs. timing

i have found very little on this subject on the internet.
a few vague ones at best

does anyone have any experience with changing timing at idle and effects on vacuum.
i must have 15 in hg at 1000 rpm. i cannot set my total advance where i want it with the cam i have.
they make a dash mounted 15 deg advance /retard.


i know that i can set it where i want and check the vacuum manually... i guess the question is has any one used one of these timing devices on their vehicle?
thanks ahead

31

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Old 05-04-2004, 01:45 AM
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what`s your engine combo?
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Old 05-04-2004, 09:51 AM
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Quote:
i cannot set my total advance where i want it with the cam i have.

Why?

What are we workin on here? Can you get more specific with the intent as well as what you have already tried.

We love long post's.
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Old 05-04-2004, 10:12 AM
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i switched to this new user name last night because i did not like it, my previous one was bskaggs, and i only had a handful of posts.
anyway

i am running a dirt track circle car. we only a few rules.vacuum can not exceed 15 in at 1000 rpm.
no aluminum
no headers

what i have
align bored 400, decked, .030 over
xtreme comp cam 268h grind, 224/230
modified vortecs
6 in crower sportsmans
kb hypers
GMPP cast vortec
600 DP holley

i was just curious if anyone has run an in-dash timing adjuster?
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Old 05-04-2004, 04:47 PM
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Since this is a hydraulic cam, set the lash as loose as possible (I would try zero lash + 1/16 to 1/8 turn of the nut).

Make sure your engine is running at peak efficiency - fresh plugs, etc. The more power the engine puts out the less throttle opening is needed to sustain idle RPM, and the more vacuum you generate.

You can also use a distributor with vacuum advance. Adjustable vacuum advance allows you to tune it in better. Connect the vacuum advance to direct manifold vacuum so that you get full advance at idle. Back off your mechanical timing to prevent max timing from exceeding about 36 degrees, and get the mechanical advance curve tuned to come in as quickly as possible without pinging in the 2000-3000 RPM range.

Other than that, Rhoades or Crane fast bleed lifters will make the cam seem milder and build a little idle vacuum (usually around 1" more) but many reports indicate these lifters will cost you a little top end power.


A friend did some extensive R+D on cams for vacuum rules.

I will see if he will let me post any of it.
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Old 05-05-2004, 07:43 AM
Technical Support Barry Grant
 
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Vacuum vs. Timing

Most tracks that have a vacuum rule have it as a minimum rule, not a maximum, you may want to double check that first.

What we've done in the past in these situations is to lock the timing in, and use a distributor with two pick-ups in the housing. You can change the timing from one pick-up to the other. This should get you where you need to be, and will allow you to switch between the pick-ups depending on track conditions.
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Old 05-05-2004, 10:39 AM
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Correction

tech, you are right! it is minimum vacuum. i even reread before i post and missed it.
sloppy typing, sloppy brain, tired, whisky... are just a few possible excuses.

i don't understand the two pickup thing...
see above excuses, LOL

i understand locking down the advance to where it is straight up through the rpm band.

thanks
31
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Old 05-05-2004, 11:06 AM
Technical Support Barry Grant
 
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Vacuum.

Changing the timing will change the amount of vacuum the engine has.

If they are only checking your vacuum specifically at 1000 RPM you could possibly change the timing by switching pickups (via a switch in the drivers compartment) this would increase your engine vacuum.

Another thing to consider is making sure your carburetor is adjusted properly if your butterflies are out of adjustment this would affect the amount of vacuum the engine is producing. If the carburetor is too rich or too lean at idle it will also affect the amount of vacuum the engine is producing at idle.
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