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Old 09-30-2008, 10:20 PM
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Vacuum too low at idle?

Just fired new sbc 406 and it sounds good. However, Iam not sure if the vacuum readings are what they should be for this motor.

406ci / afr 210 heads / super victor manifold/ 830hp series holley/ full roller mechanical - duration 248/254@50 with .577/.583 lift. 1 7/8 in headers. 4:11 rear , 28" tire, t-400 , 4000 stall , 2200 lb with driver.

My vacuum readings are: 7" @ 800rpm
8" @ 1000rpm
16 @ 2000rpm
This is with no load, since the transmission is not yet installed.


Are these readings unusual for this motor????

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Last edited by 406 bug; 10-01-2008 at 09:59 AM.
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Old 09-30-2008, 10:33 PM
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What is your initial timing set at? It will play a big part in the vacuum readings.
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Old 09-30-2008, 10:43 PM
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Quote:
Originally Posted by Jmark
What is your initial timing set at? It will play a big part in the vacuum readings.

I'm just a bit embarrassed here Jmark, since its brand new I eyeballed it at about 10 btdc. I will put a light on it tomorrow. Thanks
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Old 10-01-2008, 08:17 AM
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vacuum to low what is to low? 0 is to low, if you have a good size cam all the timing adjustments in the world will not change it.
My car has similar readings and runs great, it does shake a lot and can be seen rocking back and forth at the lights 900-950 rpm, have to run an electric vacuum pump for the brake booster ............... but that is all in the combination of parts.
Definitely time it with a lite, with a modified motor chances are the vac will still be on the low side compared to a stock motor.
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Old 10-01-2008, 08:41 AM
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406bug, If the manifold vacuum is that low I would replace the power valve to less then 6.5 to 4.5. There is chance the plugs will load up on you too frequent. I have a funny feeling the vaccuum will go below 6.5" when have it in gear.
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Old 10-01-2008, 11:04 AM
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Quote:
Originally Posted by lg1969
406bug, If the manifold vacuum is that low I would replace the power valve to less then 6.5 to 4.5. There is chance the plugs will load up on you too frequent. I have a funny feeling the vaccuum will go below 6.5" when have it in gear.
Thats my fear, when I rebuild the carb- I found 4.5 pvs and figured it would be best to return it to stock specs (6.5s) so I would have a baseline to work from.
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Old 10-01-2008, 11:49 AM
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You did not give the camshaft valve ISA specification. If it is lower then say 110*, and closer to 108*, or 106*, it will have a rough/bumpy idle, with lower vacuum readings.

My 2 cents worth.
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Old 10-01-2008, 11:57 AM
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It will want a lot lot more idle timing than 10deg. Start with the timing locked out
say 34-36deg at idle. Reset the carb and see how much vacuum you have.
Will want a 3.5 or 4.5" power valve for a automatic.
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Old 10-01-2008, 12:04 PM
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your base timing is way to low for that cam, needs to be 18*+ which does help idle Hg....

read the "initial" timing section in this very good article:

http://www.gnetworks.com/v4files/bar...withimages.pdf

minumum operating range of that cam is approx 2500 so read the Hg at 3000+ to indicate cruise Hg....
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Old 10-01-2008, 06:46 PM
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Quote:
Originally Posted by 406 bug

Just fired new sbc 406

My vacuum readings are:
7" @ 800rpm
Wow - I'm surprised you got the beast to idle at 800... Guess those extra inches are at work here.
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Old 10-01-2008, 09:22 PM
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Put the timing light on this pm and the initial timing was higher than I thought.
14deg. bumped it up to 18 and the idle vacuum is 8in@850rpm. Motor seem to be happy at 1000rpm idle where I pull 10in. Based on all the good imput (which I really appreciate) I feel ok with the current idle vacuum. After the trans is installed, I am thinking the idle, in gear, shouldn't cause the vacuum to drop too much with the 4000 stall??? If it does I will go to the 4.5 pv's.
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Old 10-02-2008, 08:21 AM
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Bug,
you do need to re-work the dist due to that cam....

I posted try 18* base because most Chevy dist are 20* or 22* of cent (18*b+22*c=40* TT WOT and that's very likely to much!!!)

your new Hg values are about correct for 18*b and that cam duration....
here's my favorite link for ballparking cam duration versus idle Hg...
click on each graph gives all the motor details for that cam and expected Hg versus idle rpms values when correct for the cam base timing is applied (per the other link article)....

http://www.compcams.com/Technical/DynoSheets/

LOL:
F-bird "likes" mega duration cams on the street (I don't!), I'll leave it to him to write out the dist set up....

ps: if you are going to do much intown/intraffic driving adding a MSD or other multi-spark box is likely a good plan....
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Old 10-02-2008, 08:35 AM
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Sounds like a killer combo. What are you putting this thing in that weighs 2200 pounds?
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Old 10-02-2008, 09:45 AM
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one very important thing that was not mentioned was compression and ring gap. that alone can make a huge difference! I cannot say enough about Gapless rings for idle quality (though most people use them for power), nor the effects of compression on cam timing. What's your cranking compression? cranking compression is an excellent tool for figuring a good place to set initial timing.
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Old 10-02-2008, 10:26 AM
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Quote:
Originally Posted by red65mustang
Bug,
you do need to re-work the dist due to that cam....

I posted try 18* base because most Chevy dist are 20* or 22* of cent (18*b+22*c=40* TT WOT and that's very likely to much!!!)

your new Hg values are about correct for 18*b and that cam duration....
here's my favorite link for ballparking cam duration versus idle Hg...
click on each graph gives all the motor details for that cam and expected Hg versus idle rpms values when correct for the cam base timing is applied (per the other link article)....

http://www.compcams.com/Technical/DynoSheets/

LOL:
F-bird "likes" mega duration cams on the street (I don't!), I'll leave it to him to write out the dist set up....

ps: if you are going to do much intown/intraffic driving adding a MSD or other multi-spark box is likely a good plan....


Thanks for the Info. I am using MSD distributor and box. The plan is to set it up for a total advance of 36 to 38 Deg.
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