Sources for mechanical Manifold pressure gauge around 1-1/4" diameter

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lakeracer69

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I've looked around and found some in the 2-1/8" range, but would really like one that is smaller ( panel space is very tight). I want one that doesn't require electricity to operate.

Something that is reasonable quality and cost effective too. I found a 2-1/4" inch one at Summit racing for around $50.00.
I realistically need something smaller. Any suggestions?

Thanks
 

Marc W

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Beware the one at Summit Racing. I have one, it is a boost gauge for supercharged engines.
 

Marc W

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Yes, the #82321 is for boosted engines. The clue is the pressure range: 2-30 psi. I have one that I mistakenly bought. Since I have it, I am going to put it on my diesel PU.

Matthew, a vacuum gauge is different. It measures the difference between manifold pressure and ambient pressure so it will vary with altitude. You can use it but you have to correct for altitude. A manifold pressure gauge measures absolute pressure.
 

cluttonfred

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Sorry, but how can manifold pressure not be impacted by ambient pressure? Does a proper manifold pressure gauge have some sort of sealed chamber for the baseline?

UMA has an *electronic* 1-1/4” manifold pressure gauge series but their mechanical series only goes down to 2-1/4”.
 

Marc W

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You are right. A manifold pressure gauge does have a sealed chamber. "But wait, there's more!" The better manifold pressure gauges will also be compensated for temperature and altitude.
 

cluttonfred

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OK, how far off is a simple vacuum gauge from a proper manifold pressure gauge? Just curious if it makes a big difference.
 

TiPi

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A vacuum gauge is a differential pressure gauge, displaying the pressure difference between the vacuum port and the local ambient pressure. At sea level, no big deal. At a density altitude of about 7,500’, the ambient pressure is roughly 75% of sea level. The vacuum gauge will now show a pressure difference relative to that. The MAP will show relative to sea level.
The MAP can be used as a rough gauge for power, the vacuum gauge gives you basically a throttle position. The throttle position is not related to power at altitude.
 

cluttonfred

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Thanks, that’s helpful. Presumably the actual pressure in the intake manifold is also impacted by the barometric pressure whether due to weather or altitude?
 

TiPi

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That’s correct. On my 912, I have around 28” at WOT and sea level. Climbing to 9,999’, the MAP will gradually decrease to about 20” (still at WOT). A vacuum gauge would still show the same as at sea level, very little vacuum.
 
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