My comment was directed at Blulakr's post immediately above mine:How is that possible at idle? The reason it's idling is because the VE is so bad (as regulated by the throttle) that it can't run any faster.
Makes no mention of rpm, just a blanket statement.Vacuum is a good indicator of engine efficiency, meaning the engines ability to draw air into the cylinders on the intake stroke.
Low vacuum\poor efficiency at idle due to camshaft design is a tradeoff for better efficiency at higher rpm.Hmmm, I know of engines that have very low manifold vacuum but VE is over 100%...
Manifold vacuum is just an indication of how tightly the throttle blades are closed, atmospheric pressure vs. manifold depresssion.
True, but his statement is still correct. At any given RPM and throttle opening, an increase in vacuum (lower manifold pressure) indicates increased efficiency. So, if you are tuning idle (say 10° at 800 RPM), and you increase timing, the RPMs will increase. If you then close the throttle blades to return to 800 RPM, the manifold pressure will be lower (more vacuum) showing an increase in efficiency. The same scenario would be true at WOT or part-throttle cruise, etc.
"varied answers" you got one correct answer for (timing affect on vacuum) for question .Hmm, does Joe Sherman linger in general tech? I'm currious on his take considering the varried answers. I know the cam plays a big roll, bit it makes sense that excess residual pressure would cause low vac numbers too...except that ignition timing and cam timing are somewhat seperate entities and the cam profile is what really determines when and how the cylinder fills and evacuates.
Yours? The "yes"? Heres my problem with a simple "yes""varied answers" you got one correct answer for (timing affect on vacuum) for question .
different subject on What Makes Vacuum . lot of things can vary the vacuum reading .
If your engine is mechanically sound with good compression, your cam timing is correct, you have no large vacuum leaks and you're base ignition timing is around 10 btdc then that's the vacuum you're gonna get. I wouldn't change base timing just to improve idle vacuum, you may end up with hard starting and pinging.im trying to find out if i can improve my numbers beyone the 6 inhg im seeing now, and, if improving that number will have a positive affect on the engines performance.
Exactly. If you have .5" at peak hp with your stock engine, and you change to better heads, intake, headers, etc., your efficiency improves (assuming you tuned it) and your vacuum will increase at WOT. At some point your efficiencies become excessively limited by the throttle and a larger carb is required to support the increased efficiency. Again, a sign of increased efficiency is less manifold pressure (greater vacuum). While a bit of a view from the other direction (more power rather than greater vacuum), this still supports the statements made to the OP's original vacuum question.How much manifold vacuum are you expecting at WOT? If more than 1" you have a restriction.
Well DanH...It may not be the "yes" You provided, which was helpful, but i appriciate the elaboration, as I'm trying to sponge as much from you all as i can.I dont know I'd your typing skills limit your responses or if you just know so much you've forgotten that learners are searching for the long answer sometimes. You all helped me, I'm thankful for that. ;-)question on base timing at idle , then replys to WOT . OK
what if the engine is check on Monday vs Friday
typing skills , I suck at it , takes to long.Well DanH...It may not be the "yes" You provided, which was helpful, but i appriciate the elaboration, as I'm trying to sponge as much from you all as i can.I dont know I'd your typing skills limit your responses or if you just know so much you've forgotten that learners are searching for the long answer sometimes. You all helped me, I'm thankful for that. ;-)