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Discussion Starter · #1 ·
Will initial timing have an affect on idle vacuum?
 

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Vacuum is a good indicator of engine efficiency, meaning the engines ability to draw air into the cylinders on the intake stroke.

If the ignition timing is off then the combustion process is either too late or too early. Both will leave pressure in the cylinder that is not evacuated fully on the exhaust stroke. That residual pressure means less air will be drawn in on the next intake stroke.
 

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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.
 

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Discussion Starter · #6 ·
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.
 

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Hmmm, I know of engines that have very low manifold vacuum but VE is over 100%...
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.

David
 

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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.

David
My comment was directed at Blulakr's post immediately above mine:
Vacuum is a good indicator of engine efficiency, meaning the engines ability to draw air into the cylinders on the intake stroke.
Makes no mention of rpm, just a blanket statement.
 

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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.
Low vacuum\poor efficiency at idle due to camshaft design is a tradeoff for better efficiency at higher rpm.
 

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My comment was directed at Blulakr's post immediately above mine:
Vacuum is a good indicator of engine efficiency, meaning the engines ability to draw air into the cylinders on the intake stroke.
Makes no mention of rpm, just a blanket statement.
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.

David
 

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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.
"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 .
 

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Discussion Starter · #12 ·
"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 .
Yours? The "yes"? Heres my problem with a simple "yes"
Im not 100% new to engines.But, this is the highest performance level ive worked with so far. I have a lot of "knowledge" on the performance, but not nearly, not even close!, to what A LOT of other guys have, nor have i put said knowledge into practice in such a way that i could consider it "Performance wisdom." Therefore, I am defering to others experience in hopes to learn something i can apply in the future, so i dont have to bug ya'll with seemingly basic questions to seasoned builders and dyno tuners. Now then,

I know that several variables can change the vacuum reading at a given rpm, specifically idle. I know that MY particular cam is going to cause a low vacuum reading. I know that elevation,Idle mixture adjustment, cubic inches, valve adjustment, idle speed and now timing adjustment(add whatever i missed) affect the vacuum reading at idle.

Because I dont have a chassis dyno, or stretch of unpatrolled road at my disposal, im trying to cover EVERY possible angle that i can address in a carport. I know I'm going on a search for any vacuum leaks. If i find none, 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.
 

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from a stock engine to the highest n/a race engine ... it is a simple ..Yes.

sorry thats all there is to it .
 

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Advancing timing CAN increase vacuum at idle and part throttle low engine speed.

Does it always increase vacuum @ idle? No.

The question was generic. The answers I've seen...some of them are too specific in relation to the original question.

The VE does have an effect on vacuum however, you can't go by that totally. Some engines are over 100% VE at wide open throttle but not very many of those have much, if any, vacuum at idle/low speeds. Mine is over 100% VE, as many of the little SBF's I've built over the years have been, but idle vacuum is pretty sorry. Current 438" only carries 4-6"/Hg at idle (1400 RPM).
 

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The same scenario would be true at WOT or part-throttle cruise, etc.
How much manifold vacuum are you expecting at WOT? If more than 1" you have a restriction.
 

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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.
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.

I believe cam manufacturers provide idle vacuum specs??
 

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How much manifold vacuum are you expecting at WOT? If more than 1" you have a restriction.
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.

General
: Not to muddy the picture for others, but various efficiencies give different effects and not all are beneficial to a particular purpose if there is compromise (and there always is). An example would be a hot cam in a bone-stock street car, that while it might see an improvement (with better vacuum and increased torque) at some point in the RPM range, the overall effect would probably be negative. This of course leads to matching the beneficial efficiencies into one assembly (intake, cam, heads, displacement, etc.), then the assemblies into supporting groups (engine, drivetrain, chassis, cooling, etc) making a complimentary package that works together - the complete car package focused on the design goals.

David
 

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question on base timing at idle , then replys to WOT . OK

what if the engine is check on Monday vs Friday
 

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Discussion Starter · #19 ·
question on base timing at idle , then replys to WOT . OK

what if the engine is check on Monday vs Friday
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. ;-)
 

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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. ;-)
typing skills , I suck at it , takes to long.

one of your responces , you hit on what effects different vacuum levels .learning to apply the knowledge is the key . take reading a book on doing the wild thing or swimming , lets just say hands on is the final teacher and the best

BTW , Monday/Friday , it can change readings . you did give the answer to why in a post.

edit , add , the long answer , can be more confusing . thats way algebra isn,t part of 1st grade teaching
 
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