Can high humidity effect boost reading?

[RedLined]

Hey guys, since i got my car back from getting the broken half shaft fixed, it hasn't been boosting up to 12.5 psi like it used to. I'm not sure wether its a mechanical problem or if maybe its because its been really humid out here the past few weeks. So my question is this, would my boost gauge be able to pick up the drop in boost due to the humidity or is there something wrong with my car/boost gauge? Thanks.

[11psi5.0]

boost is relative to atmospheric pressure, so depending on what the barometer is reading that day, it should reflect on your boost gauge, somewhat.

[Kevinedwards05]

Hmm...let me think here.....a super charger acts much like a jet engine.....sucks, squeezes, and then blows......A jet engines performance parameters change based on the N1 of the day...in other words.....barometric pressure really spices things up a bit, and you may not be able to run the engine and get your full 100% efficiency based on your N1 of the day. There are many times when you can only get 91% efficiency....so...your supercharger is going to act much the same way.....Any questions?

[Redline686]

I live in St. Louis where its always humid and I've only seen a max of 10, i thought it was just a leak somewhere but maybe its the weather.

[Kevinedwards05]

You can easily run your engine and spray a soapy solution around all of your connections and mount points for the supercharger and answer that question yourself.

[yrethguad]

12 psi is 12 psi. If the ECU is taking it's reading from the same place as your gauge, they should match. Things break though. Gauges, sensors, etc.

[Raven]

on non humid cooler days, i hit 11/11.5 psi. on humid days like today here in MN, i hit 10 psi at most

[yrethguad]

That's weird as hell. The ECU must sense humidity somehow, or perhaps the sensor is affected by it.

[Kevinedwards05]

I could come up with some documentation on that, but it has to do with air density. The ECU is a mystery to me still, but I know that even the best aircraft computers cannot make an engine perform at 100% efficiency unless the conditions are perfect. The Super Charger acts much like the jet engine in that it is sucking air in, compressing it (which in turn heats the crap out of it), and then pushes that air into the intake with force. The boost pressure is affected by the pressure ratio of the super charger. It is the form of the ration of exhaust total pressure (what's being shoved into the intake) to the intake total pressure. (What's being sucked into the super charger. Higher the humidity, lower the inlet pressure.

[SCdyne]

A positive displacement blower doesn't compress the air entering the engine. It just displaces a given volume of air per rotor revolution and over time that volume is more than can be consumed by the motor. This causes a backup in the system and is shown as manifold pressure above atmospheric (boost).
That being said temperature has a major impact on boosted engine performance and that is especially true when you are limited by a mechanical link between supercharger rotor speeds and engine RPM.

Additionally 0 on the boost gauge isn't absolute 0. We're talking about the difference between gauge pressure and absolute pressure. 0 on your boost gauge is whatever your pressure for your altitude, humidity, etc is on a given day and the average gauge (even digital) is only accurate to 1.5 psi if you are lucky.

Beyond everything else when it's pushing 100 F ambient temperature outside you are starting off the whole power process with less oxygen. Add the fact that you may be seeing 20%-40% humidity (something that has never happened here in the Mojave Desert since 1992, but we are dealing with now) and another item is taking up space where oxygen would normally be.
[This is one reason why I'm not a fan of water injection and proof as to why it's a waste of time]

So when you look at all the things that go into the air entering your engine it's no wonder it doesn't seem as powerful on hot humid days. The only solution is get more oxygen into the motor, which is where a smaller pulley and/or Turbo comes into the scene..