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Discussion Starter · #1 ·
Half the people I talk too say you cut your idle manifold vacuum in half, and round down to the nearest .5, so with 10hg you want a 4.5pv, etc. The other half of the people I talk to say you want 2 hg below your idle manifold vacuum, so with 10hg you would want a 7.5pv.

Who is correct, and why?
 

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I read an article where a vacuum gauge is installed in the car and you use a narrow band O2 moniter. You cruise around in your basic operating rpm checking your gauges. Keep a record of your range of vacuum levels from regular driving. You want your PV number below these levels. The A/F ratio will be at 14.7:1 at cruise to 12.8:1 under power which is when the PV opens.
 

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Half the people I talk too say you cut your idle manifold vacuum in half, and round down to the nearest .5, so with 10hg you want a 4.5pv, etc. The other half of the people I talk to say you want 2 hg below your idle manifold vacuum, so with 10hg you would want a 7.5pv.

Who is correct, and why?
If you use example #2 what if your engine vacuum is 15 hg at idle and you -2 hg the largest PV number you can get is 10.5PV Everytime your engine vacuum drops below 10.5 the power valve opens feeding more fuel. If driving in a hilly countryside and the vacuum stays at 8.5 cruising this would be rough on gas mileage. If using example #1 with the same engine vacuum and 15 hg*2 = 7.5PV. If driving in the same conditions the PV never opens until 7.5 hg so you have a lean condition which could cause severe complications.
 

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also remember the larger the pv the eailier the pv will open,so earlier your in rich ment will be example, a 10.5 will open before a 6.5, because as the throttle opens and vacume drop's, and your running 15g's as you can see as the vacume goes down, it will reach the 10.5 before the 6.5...You want to split it in half or sligtly above.. if your running 15 at idle, half is 7.5, or a 8.5 depending on the rest of the setup
 

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Half the people I talk too say you cut your idle manifold vacuum in half, and round down to the nearest .5, so with 10hg you want a 4.5pv, etc. The other half of the people I talk to say you want 2 hg below your idle manifold vacuum, so with 10hg you would want a 7.5pv.

Who is correct, and why?
Both, I hate this; "but it all depends". Sounds like Einstein talking about relativity doesn't it?

It gets down to more than one way to set up the carb. To simplify; if the main metering is rich then you can delay the power circuit to a lower vacuum before it's necessary to turn it on. If you're running on the lean side say for good cruise gas mileage, then you may want the power system to come on earlier to cover power and anti-detonation requirements say when pulling a trailer or heavy load in this case having it come on earlier but not under cruise vacuum would be desirable.

If your idle vacuum is only 10 inches then this speaks to a large cam. Large cams speak to high performance so you want the power enrichment to come on earlier. But not at cruise if this is a street strip machine; so you need to get your cruise vacuum reading as well, which might be higher than idle vacuum. This is an effect of gearing which is a big player in this, therefore, a major reason why hard, fast rules are dangerous to apply.

A full up race engine that never sees the street tends to block off the power system and use really rich main metering instead, but these engine are either idling or at WOT so in-between metering like cruise isn't a consideration while eliminating the power circuit is one less potential failure point.

Certainly if you can do all of this tuning with an exhaust gas analyzer all the better?

Bogie
 

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Discussion Starter · #6 ·
Both, I hate this; "but it all depends". Sounds like Einstein talking about relativity doesn't it?

It gets down to more than one way to set up the carb. To simplify; if the main metering is rich then you can delay the power circuit to a lower vacuum before it's necessary to turn it on. If you're running on the lean side say for good cruise gas mileage, then you may want the power system to come on earlier to cover power and anti-detonation requirements say when pulling a trailer or heavy load in this case having it come on earlier but not under cruise vacuum would be desirable.

If your idle vacuum is only 10 inches then this speaks to a large cam. Large cams speak to high performance so you want the power enrichment to come on earlier. But not at cruise if this is a street strip machine; so you need to get your cruise vacuum reading as well, which might be higher than idle vacuum. This is an effect of gearing which is a big player in this, therefore, a major reason why hard, fast rules are dangerous to apply.

A full up race engine that never sees the street tends to block off the power system and use really rich main metering instead, but these engine are either idling or at WOT so in-between metering like cruise isn't a consideration while eliminating the power circuit is one less potential failure point.

Certainly if you can do all of this tuning with an exhaust gas analyzer all the better?

Bogie
Think I'm gonna get my buddy to hold the vacuum gauge while we take a ride today. Get some numbers to work with.
 

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idle vacuum doesn't have a damn thing to do with PV selection. Drive your car on the highway at your normal cruising speed on a level road, what is your vacuum at that moment? Now subtract 3 or 4 and you'll have a good number to start with. Going smaller will slightly help mpg's going higher will slightly help power.
 

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idle vacuum doesn't have a damn thing to do with PV selection. Drive your car on the highway at your normal cruising speed on a level road, what is your vacuum at that moment? Now subtract 3 or 4 and you'll have a good number to start with. Going smaller will slightly help mpg's going higher will slightly help power.

lol'n guess you've never had a nasty engine that idles real ruff
 
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