Is it a single tr or 2 carbs? As far as vac goes you're kickin my azz I've never seen more than 11 ish on my junk. I think it's safe to say that the carb will "see" less vac when going from, say a low rise(vic jr ect) to a tr set up.

Josh can tell ya this......You at least have the dizzy at the right end

The more intake runner volume the lower the vacuum reading will be..Simple physics.

Timing like enginedoctor said will also have a huge effect on vac...As does exhaust system and efficiency..

Idle vacuum is irrelevent (unless you have power brakes LOL). Where a vacuum reading is relevent is WFO.

We normally do a 2500 RPM and a WFO reading to use for carb tuning..If the vacuum levels are too low (read; overcarbed) install a flame arrestor (or two)    It's a little backwoods, but works great.

GT

i agree with tunnel i should lose vacuum...hence i think that i had a vacuum leak with the prevous mid rise intake!!!! this is what brough up the question in general...i do recall vacuum being important at tuning the carb at idle with idle screw at idle...wow that was alot of idle!!!!...in all honesty i was just curious because the only time i ran a vacuum check on the motor was at idle so i did the same with the tunnel ram!!!! ...shit it supposed to be winter and i am still tunin my boat...i think i will half to go out this weekend now dammit (sarcasim)...honestly i think tinkering with my boat right now is the only thing keeping me sane anymore!!!!

from my understanding the idle vacuum is important for selecting your power valves for your carbonator...also did a plug reading today and they look pretty decent...the color change was right on the 90 degree bend and from what i have read that means the timing is very happy, but i like opinions

Nope.....With all do respect, the power valve should be sized for the "transition" between the high vacuum (low load level) and low vacuum level (high engine load...

So that being said, you should look for a vacuum reading at about mid throttle loaded at or near 2500 RPM, you want the PV to typically hold at or near that level, soooo If you see vacuum in the mid 5's, I would say go with a 6.5 (PV's are common in 1/2 " increments)..The vacuum should really fluctuate at that RPM because there is a transition on a 4V from primaries to secondaries (on a dual four deal may happen twice), it would also not scare me to see two different PV's in each carb, but not normally recommended..Maybe for progressive linkage deals....

GT

what about elimintating pvs all together??? or maybe just the secondaries?  ?

i thought it was from my understanding that from idle you divide in half your vacuum and that is a safe pv size to go with, but im willing to listen to more...

Your method would work great in a car.... A jet boat has a load on it from the second the engine starts, so trailer idle will have way more vacuum than lake idle..

You coudl eliminate the PV's all together, but it needs to run fat on the bottom end and mileage will suffer and it will be fat unless it BTTW...( Balls To The Walls) AKA "to the wood"   


Also, sometimes peeps want to lock out the timing (not a fan  ) and that will make a PV that much more necessary...

I am a very picky driver, I need the throttle to "be there" when I mash the go pedal, I don't like having to compensate for delayed throttle response....This is just me and it is because I learned to drive whitewater with a fuel injected big block, I would literally have to relearn how to drive a tunnel w/two fours....However on a lake, just screwing around, I'm all over it..

You will find that I am a gnat's ass kind of tuner, I like things to be just so, so I may not be the one to point you in the right direction for your particular setup.


GT