Just a follow-up to information/ opinions on this prior EVAC post from August 2023.

In May 2025 I installed a 2 bar map sensor in the oil fill cap on the valve cover. I checked the reading this sensor displayed to the baro reading in the ECU to verify they were in sync.

In my sbc at 1400 rpm idle, the vacuum, though slight, is almost equivalent to zero. At WOT and 7k there is 1 psi of pressure, no vacuum. Closing the throttle at 7k the vacuum goes to 1.5 in Hg during the engine rpm deceleration.

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Originally posted by Eman:
Like Lauren Bacall said, Just put your lips together and blow.

So also good test for gf or wife? Well with wife we know thats gets broken with wedding cake.LOL

Header evac on my engine..

^^^^^ Chad, I’ve seen that number you posted on another website (maybe FB) and is very impressive. It’d be interesting to see the vacuum numbers ( in Hg or kPa) vs rpm from idle to max rpm on a line graph to include decel at the end of the run.

Id be interested in seeing how your evacs are installed, both inside and outside views, if possible.

This message has been edited. Last edited by: FastLane,

Here’s what my evac looks like vs rpm. The Baro reading indicates zero pressure /vacuum.

This message has been edited. Last edited by: markemark,

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Originally posted by SpeierRacingHeads:
Header evac on my engine..

Isn’t vacuum expressed in ‘positive’ inches of mercury? So a negative reading of that unit would be pressure?

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Isn’t vacuum expressed in ‘positive’ inches of mercury? So a negative reading of that unit would be pressure?

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Originally posted by markemark:

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Isn’t vacuum expressed in ‘positive’ inches of mercury? So a negative reading of that unit would be pressure?

I’d never seen one marked that way or seen vacuum numbers discussed in that manner, but I stand corrected.

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I’d never seen one marked that way or seen vacuum numbers discussed in that manner

Agreed, I believe when vacuum is discussed (and sometimes displayed) the number is just assumed negative.

quote:

Originally posted by markemark:
^^^^^ Chad, I’ve seen that number you posted on another website (maybe FB) and is very impressive. It’d be interesting to see the vacuum numbers ( in Hg or kPa) vs rpm from idle to max rpm on a line graph to include decel at the end of the run.

I wouldn't have any idle data..

Also this engine has gas ports and .7/.7/2MM rings.

quote:

Originally posted by FastLane:
Id be interested in seeing how your evacs are installed, both inside and outside views, if possible.

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Originally posted by SpeierRacingHeads:

No flow back valves. Wonder if the tube enters the center of the collector and what it looks like inside the collector.

Chad, are there steel tubes going to both collectors? Do you ever get any oil accumulation in that aluminum junction box?

I have the moroso style check valve on the collector. I use the GM ones. The tube is in the center of the cone in the collector. The other side header is just the normal cover to collector. But yes it will pull oil!

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Originally posted by SpeierRacingHeads:Also this engine has gas ports and .7/.7/2MM rings.

^^^^ Very impressive achieving a solid -5 inHg run averaging with a “single” evac tube. Is this an 1/8 mile run?

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Originally posted by markemark:

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Originally posted by SpeierRacingHeads:Also this engine has gas ports and .7/.7/2MM rings.

^^^^ Very impressive achieving a solid -5 inHg run averaging with a “single” evac tube. Is this an 1/8 mile run?

yes 1/8 mile. But isn't it way more than -5?

In the graph I assume the evac is displayed below the 0 line, but there is no legend value on the left side. Your initial post has a value of -11.15 inHg which I’m assuming is the maximum value.

The 3 lines are scales at -10 to -12.. You can see that at the right of one of the pics above.