Vaccum Pressure

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Shane

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Hey All,

I am glad to report that my car has been running strong lately and I enjoy it every chance I can....weather permitting these days...

I am now focusing on learning how to fine tune things on the car..
Orignially I set the idle mixture screws to three turns out and was running like that.

I now bought a vaccum gage and seen I had around 8 HG( it would move a little towards 7 or 9)

I tinkered with the two adjuster screws and have got it to a little over 10, and it will move over a 11 or just above 9.

It looks like this is as high as it will go.

I just really dont understand what the vaccum does. (i know that sounds stupid, but I was just wondering if someone could give me a basic idea to help me clear up the idea in my head.

Also is just over 10 HG alright?

my car has a 69' 340 motor, ld340, purple cam, 600 holley dp, headers, everything else is stock.

Shane
 
Shane 10-11" of vacuum sounds about right for your engine. When the pistons go down and the intake valve opens a negative pressure forms otherwise called a vacuum. A big cam that has more duration and overlap will have less vacuum. Less vacuum usually means poorer idle, less fuel mileage and less power in the lower rpm range but more top end power. A stock low performance or RV cam will generally have very low overlap thus creating high idle vacuum, better fuel mileage, smooth idle, and more low end power but less top end power.
 
Thanks Fishy68, makes sense when you spell it out like that...

I have my timing at 10 btdc, would it be possible to get a little more vaccum by fine tuning the timing abit?

The car seems to run strong as it is, maybe i should just leave it as it is for now...

thanks

Shane
 
Fishy was right on with his description of what vacuum indicates and how it is produced.

Increasing your timing can improve vacuum as the engine will pickup RPM at a relative throttle plate opening increasing vacuum. Be careful to not go to far. Just go up 2 degrees at a time, re adjust your idle mixture and then go for a test drive and listen for any pinging. If you hear any pingning, drop the timing down a degree at a time until it goes away.

Do have any idea what your total timing is?
 
alright, I will try going up 2 degrees and see what the vaccum gage says...

I am not sure of my total timing, but the timing light I bought does have a dial on it.

Do I get the motor at 3000 rpm, and then turn that dial and see what it says when the timing mark is at 0? or the 10 btdc mark?

Also do I leave the advance to the distributor on or off while checking the total timing?
 
Yes unplug the vacuum from the vac advance. You don't want that in the equation. When you have the dial at zero on your light and for example it shows 12 before tdc and you rev the engine to 3000 then the mark goes out of sight you need to dial the light to get the mark back on 12 before tdc. Then whatever number is showing on the dial, lets say 20 for example, is how much centrifigual advance you have.
 
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