...decrease manifold vacuum. I don't have hard numbers, but I noticed a decrease in pressure (closer to atmospher) after porting my plenum and going to siamesed runners. Should this happen? How do I increase vacuum? Different cam?

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Bob '89 Formula CC350
Last(lost)Member of the Elusive Great Lakes Crew
My car's homepage

 

I can't imagine you'd loose any vacuum with the relatively small amount of material you'd remove. I'd check for leaks around the gaskets after the plenum work.

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Rob P
89RSconvtZZ4TPI
92Z28convt5spd
71Impala convt 402BB
BETTER DRIVING THRU SUPERIOR HORSEPOWER!

 

I wouldn't consider it small. Besides cleaning all the flash out and smoothing things, I virtually eliminated all the bosses between the runner ports. I also siamesed them. My runners are also siamesed down to the intake. I removed at least a large coffee can of material. No joke.
So, given everything else the same, would a larger plenum volume reduce vacuum?

 

Keep looking you have a leak if its droped with no other mods

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92 Z28 5.7 TPI AUTO 105k factory motor and trans.

SLP headers and runners, B&M 2400 stall, shift kit,LCA brackets, EBACH spring kit, NITTO'S, NOS

best 60ft 1.87 1/4 12.48 @ 110.81

 

rcr:
1. the single passage runners will 'decrease'
intake signal due to the lost of 'tuned' intake
pulses. thus slowing the low-end but increasing the mid/upper rpm. like going from
a dual plane manifold to a single plane.
2. tuning vacuum signal is the combination of
cam/intake and runner volume.
the larger = less vacuum signal
the smaller = more vacuum signal
hope this helps...
airdeano
ps. did you score the hot cam and springs?
if you did your a dog!! j/k LOLOL
i was going to get them too...