I ditched my turbo.......

[fatmobile]

That's a huge vacuum,.. about what the brake booster pulls.
 Hard to believe it pulls that hard trying to get air,..
 but i've never measured mine.

[rabbitman]

That's a huge vacuum,.. about what the brake booster pulls.
 Hard to believe it pulls that hard trying to get air,..
 but i've never measured mine.

Yeah, I'm perplexed by those numbers.  0 inHg is atmospheric pressure.  29 inHg is HUGE vacuum.

I'm not an expert on this stuff in the slightest but 0 inhg is a complete vacuum and 29 inhg (rounded off) is atmospheric pressure at sea level, it changes slightly with the weather. I'm at 1100ishft so it's a little less.
The manifold pressure gauge says 28.5ish inhg when the engine isn't running.

This has a good picture at the bottom showing how inhg works: http://hyperphysics.phy-astr.gsu.edu/Hbase/pman.html

Us diesel guys don't know much about inhg because of the lack of a throttle plate, I'm only starting to understand it from talking to Dad (he's sometimes a small aircraft pilot) about it. He doesn't understand what a boost gauge is, we see 0psi as being no boost but really it's atmospheric pressure pushing the air into the engine at 14.7psi.

The piston isn't sucking the air in, the atmosphere is pushing air in to fill the void created by the piston going down .

A boosted aircraft (just because they have a intake mani pressure gauge) would read something higher than 29inhg, like 40 or 50inhg......wierd stuff.

Soooo, back to school everyone

[booneylander]

Random. That's the exact gauge I'm using in my golf right now.

[truckinwagen]

most automotive gauges show boost and vacuum as the difference between the pressure in your manifold and the true atmospheric pressure.

our boost gauges show 0 as no boost even though true atmospheric pressure is 14-ish.

we all just assumed you were using a more standard automotive gauge thats all.

[Rabbit on Roids]

it takes no vacuum to pull no mercury (inches of HG) ie, lower number.

if you have a large vacuum, then it pulls lots of mercury, indicating a large vacuum.

so, yea, andrews correct on this. you should have like NO VACUUM in your intake system. the closest you can get to zero vacuum, the better your crap is gonna flow.

[Rabbit on Roids]

yea, if we were talking about vacuum in general, rabbitman would be right. but we are talking about a specific kind of vacuum..

[rabbitman]

yea, if we were talking about vacuum in general, rabbitman would be right. but we are talking about a specific kind of vacuum..

Yeah but with the gauge I'm using I don't really have a choice of which kind of vacuum.

Here's a close pic of my gauge, it's the one on the top and the difference is mine goes to 75inhg rather than 50inhg: http://www.aircraftspruce.com/menus/in/manifoldpressuregauges_uma.html

It's sitting at 29.5inhg because it's almost down at sea level.

So I lost about 2 inhg at 4700rpm and I don't know if that's enough to cause a noticeable power drop.

Andrew, I've seen those gauges like in your picture and they don't make any sense to me , I'll have to show Dad and see what he thinks.

[Patrick]

i think the stock TD manifold is grossly restrictive. is that the one you have on there? anything is better than a stock TD manifold if you dont have boost going through it.

I'm using this gasser mani off a '80 1.6L bunny.

Dude, you need a bigger inlet on that thing! put a pipe on a couple of inches in diameter bigger, should see a lot less vacuum. BTW, what are you using to pipe the air to it? is it collapsing when under load??

[rabbitman]

Patrick, from the airbox I have a 2" aeroduct that goes into a 1.5" rubber hose. It's rather lame I know but I'm sure it's not collapsing.

The difference is that your gauge is an aircraft gauge which measures "absolute" pressure rather than measuring manifold vacuum.

I wish I understood all this better, so it all depends on what ya call 0psi or 0inhg then.

So...........arrrg I just lost my thought.

Andrew on that gauge picture you posted what would it show if it was on a NA gasser that was idling? What about full throttle?
'cause on the aircraft gauge idle would be about 10-12inhg and full throttle would be 28-29ish depending on altitude. Thanks for all yer guyses input

[Rabbit on Roids]

it shows high vacuum at idle, and you want as close to 0 vacuum as possible at full throttle. or boost if you are in a compressed application.

[rabbitman]

Just talked to Dad about it, showed him the pic of the boost/vacuum gauge.

We figured out the numbers labeled inhg are not correct, like when it says 10inhg it's really negative 10psi, and scientifically it would be called 19inhg

[745 turbogreasel]

So If I have this right, you are pulling a 2.25 PSI vac across your intake duct?
If so, I'd class that as 'strangled'

[rabbitman]

Yeah I think I see what you're saying, it not all clear in my mind yet though

That bit about vacuum being compared to a higher pressure is what threw me off with the aircraft gauge then along comes the auto vac gauge and really screwed me up. But I think I kinda got it though.

If I could find a 2 1/16" manifold pressure gauge I'd probably want it, I've not found any and I bet it wouldn't be cheap if it was gettable.

[Rabbit on Roids]

manifold pressure gauge? never heard of one. what do they read? psi like a boost gauge? or what?

[rabbitman]

They read pressure in "inches of mercury" and the zero of the scale starts at zero pressure (or total vacuum).  Because of that, 29.92 inches of mercury of pressure is equal to 1 bar or 14.7 psi or atmospheric pressure at sea level.  A vacuum gauge that has 0 set as atmospheric pressure will read 29.92 inches of mercury at total vacuum and be an inverse of the manifold pressure gauge.  

HAH....THE LIGHT TURNED ON. Yup that makes sense, I was having trouble wrapping my little brain around it.

So If I have this right, you are pulling a 2.25 PSI vac across your intake duct?
If so, I'd class that as 'strangled'

Is that the case? A 2.25 psi vacuum? huh, my brain isn't totally wrapped around this yet I guess Maybe I'll get workin' on a better intake system.