What is your idle vacuum-stock engine.

[moparzrule]

Ahh who cares? There's Hot Certified Christians at TD!

[Dusty_Duster]

I get 10-11" of vacuum at idle, but I have an aftermarket cam (don't know which one).

Wouldn't having a larger-diameter throttlebody and/or AIS passage make your idle vacuum lower?

[moparzrule]

I get 10-11" of vacuum at idle, but I have an aftermarket cam (don't know which one).

Slider or roller? No roller cam should make your vacuum that low, the S3 cam usually puts people around 15'' vacuum at idle and thats a pretty wicked cam. And I've seen a super 60 cam go as low as 14''. Is your cam timing right?

Wouldn't having a larger-diameter throttlebody and/or AIS passage make your idle vacuum lower?

No, your idle would be higher if it was getting more air!

[karlak]

Ahh who cares? There's Hot Certified Christians at TD!

Huh??? or is that

[Pat]

bone stock CSX 16-17" on the stock guage...but....I once lined up three autometer guages on a T and idled a car and they all read differently. None of these guages are going to be bang on perfect.

So, my vote is that stock, they should idle anywhere between 15 and 20 inches!

[BadAssPerformance]

Good point on gauge accuracy Pat

I guess we all need to post elevation too?

[supercrackerbox]

Wait, is this poll for what mine is, or what it's supposed to be? My gauge on my Daytona doesn't have any real numbers on the vacuum side, but the OTC scanner said it's reading low (13-15). Drive train is completely stock '87 setup aside from a DSM BOV and a K&N filter.

[Dusty_Duster]

Slider or roller? No roller cam should make your vacuum that low, the S3 cam usually puts people around 15'' vacuum at idle and thats a pretty wicked cam. And I've seen a super 60 cam go as low as 14''. Is your cam timing right?

Cam timing is right on. I'm not sure what kind of cam I have in it. The engine is a '90, I think.

[BadAssPerformance]

As the title says, having a disagreement with JT. I say 18-20, he says 15-18.

50/50? WTF... Looks like we're both right, LOL

[87glhs232]

Then I here by decree that 15-20 is correct!

[BadAssPerformance]

Here, here

[Ghettosmurf]

stock t2 with aftermarket gauge...16-17

[93sundance]

I voted for 18 - 20, because that is what it is always at in my friends SC. (aftermarket gauge)

[88_pacifica]

I voted for 18 - 20, because that is what it is always at in my friends SC. (aftermarket gauge)

Excuse me, but this poll is only open to those with cars and licenses....

I was going to say with jobs too, but, well, you know...

[iTurbo]

My Spirit R/T only pulls 10 in/hg at idle. It's a TIII with LW stage I cams, ported exhaust manifold, and 3" exhaust. Elevation is about 7200 ft.

[Dusty_Duster]

...
No, your idle would be higher if it was getting more air!

Following your line of reasoning, at 30 inHg (0 psi absolute pressure), you would be getting more air then at 18-20 inHg.

I think you just revolutionized turbocharger theory.

[moparzrule]

Following your line of reasoning, at 30 inHg (0 psi absolute pressure), you would be getting more air then at 18-20 inHg.

I think you just revolutionized turbocharger theory.

Umm, I really don't understand how you get that from what I said, in fact it's the opposite. And since when is 30 inHg = 0 PSI absolute? 30 inHg is -14.7 PSI, note the minus in the front.

[Dusty_Duster]

Umm, I really don't understand how you get that from what I said, in fact it's the opposite. And since when is 30 inHg = 0 PSI absolute? 30 inHg is -14.7 PSI, note the minus in the front.

You are correct that 30 inHg of vacuum is -14.7 PSI of relative pressure (relative to atmospheric pressure), which means 0 PSI of absolute pressure, since atmospheric pressure is 14.7 PSI.

In a naturally-aspirated motor, when you go full throttle, the manifold relative pressure approaches 0 inHg of vacuum (which is 14.7 PSI of absolute pressure) because of the larger opening for air to enter the manifold. (The engine doesn't have to "suck" as hard to get air.) According to your reasoning from the post above, a naturally-aspirated motor at full throttle would approach 30 inHg of manifold vacuum, which is 0 psi of absolute pressure.

High-profile cams are designed to have a greater lift and duration in order to get more air into the cylinders at higher RPMs (when the valves don't open and close as long). However, at idle, since the intake valves are open more and for a longer period of time, you have a lower idle vacuum.

[moparzrule]

OMG. Atmospheric pressure is a positive 14.7 PSI, or 0 inHG. 30 inhg is negative 14.7 PSI. Absolute pressure is 0 inHG, or POSITIVE 14.7 PSI. Therefore, when is 30 inHg you are getting 1 atmosphere *less* air because it's in the negative.
Don't know how to explain any simpler than that.

[Dusty_Duster]

OMG. Atmospheric pressure is a positive 14.7 PSI, or 0 inHG. 30 inhg is negative 14.7 PSI. Absolute pressure is 0 inHG, or POSITIVE 14.7 PSI. Therefore, when is 30 inHg you are getting 1 atmosphere *less* air because it's in the negative.
Don't know how to explain any simpler than that.

I think we're arguing here and both saying the exact same thing. I hate it when that happens!