Boost Pressure vs Conditions in Turbo Engines

I was wondering why boost pressure can vary in differing conditions. In my vehicle, I may see as low as 18 psi of boost pressure at 32 degF and as much as 25 psi at 75 degF. Curious as to what can cause this. Thanks!

Hi Clint,

Boost pressure will vary (in an open loop state) based on ambient air temperature (along with other factors like altitude). This is due to the effect air density has on its ability to compress. A cooler, more dense sample of air will compress to a value with less effort than hotter, thinner air.

They odd thing in your questions is you're saying that the hotter air is generating more boost. Are you talking ambient temps or air inlet temps?

Is there a possible boost target air temp compensation in your map?

Reason I ask is, seeing your profile picture is a Subaru, Subaru stock ecus have plenty of boost pressure target compensation maps based on temp, altitude etc.

You will also find with a Subaru they run lower boost in gears like 1st and 2nd (inlet air temps might be lower) and higher levels in 4th+(when inlet air temp is increasing). Simply, this is mainly due to load applied to the turbo. In lower gears there is less load and exhaust heat generate then as you go up in gears and generate more load and heat the boost comes up. This happens in turbo vehicles but is mostly noticeable in Subaru's due to their massive exhaust manifold volumes.

Older (and presumably newer) Audi vehicles used to raise boost and retard timing in hot weather in order to provide the same level of power under any conditions. This was certainly the case with the 100 series S4 (2.2L I5 turbo) and performance was said to be very consistent regardless of the weather.

I imagine any vehicle with closed loop boost control and torque modeling would be at least capable of this behavior. I didn't own my Gen IV Legacy (which had both of those things) long enough to notice it though

Chris,

Thank you for the reply and I apologize for not seeing your response sooner. The theory, as you described, makes perfect sense. I am strictly talking about ambient temperatures (which I suppose can effect intake temperatures as well) and I’m only referencing boost data from 3rd gear street ramp runs.

I do see some compensation tables for boost targets (especially for intake temperature). However, the tables are set up to reduce boost pressure as temperatures increase (probably to reduce the chance of knock). That’s why it seems weird that my car would not build boost as effectively in lower ambient temperatures.

The car was street tuned in cold weather and I’m wondering if that plays a role as well. Perhaps the tune would be more nominal in different weather conditions if the base tune was configured in a more median temperature environment, but who knows...