Darwantae951

951Roadrunner

This post is interesting, but i think a few issues are suspect...Tell me if I am wrong....
When you turn the key to turn on gauges, but engine not started, the boost gauge reads one bar. this reflects being at sea level with no flow and ambient pressure of 1 atm. As soon as you start the motor, the boost gauge reads 0.4 bar. At that time, the engine is sucking air more than the airbox will allow. Air is flowing but the motor is creating a vacuum...kinda like when your cheeks collapse when you suck hard on a cocktail straw...there is air flow AND vacuum in your mouth, and in your intake manifold.
As you accelerate the motor, the vacuum actually increases...this is why people put on cone filters...more flow and less vacuum...like sucking on a bigger straw. Without the turbo, the manifold pressure will always be less than one bar. We may choose to say "Boost" is defined as manifold pressure greater than 1 Bar, but the only reason that manifold pressure EVER gets to or above 1 bar is due to the forced induction pressure being created by the turbo. As soon as the manifold pressure rises above 0.4 Bar, the turbo is no longer "freewheeling", but rather is active in increasing manifold pressure, and overcoming the vacuum caused by the motor's demand for air via the restrictive airbox.
How this relates to engine warmup safety i don't know, but tell me if the above is wrong...

951Roadrunner

This post is interesting, but i think a few issues are suspect...Tell me if I am wrong....
When you turn the key to turn on gauges, but engine not started, the boost gauge reads one bar. this reflects being at sea level with no flow and ambient pressure of 1 atm. As soon as you start the motor, the boost gauge reads 0.4 bar. At that time, the engine is sucking air more than the airbox will allow. Air is flowing but the motor is creating a vacuum...kinda like when your cheeks collapse when you suck hard on a cocktail straw...there is air flow AND vacuum in your mouth, and in your intake manifold.
As you accelerate the motor, the vacuum actually increases...this is why people put on cone filters...more flow and less vacuum...like sucking on a bigger straw. Without the turbo, the manifold pressure will always be less than one bar. We may choose to say "Boost" is defined as manifold pressure greater than 1 Bar, but the only reason that manifold pressure EVER gets to or above 1 bar is due to the forced induction pressure being created by the turbo. As soon as the manifold pressure rises above 0.4 Bar, the turbo is no longer "freewheeling", but rather is active in increasing manifold pressure, and overcoming the vacuum caused by the motor's demand for air via the restrictive airbox.
How this relates to engine warmup safety i don't know, but tell me if the above is wrong...

Reimu

At idle the engine shouldn't be sucking much air at all since the throttle plate is closed..

fortysixandtwo

Well..sort of. Vacuum will fluctuate on a normally aspirated engine as well. At full throttle, the manifold of a higher performance engine will be almost at atmospheric pressure (reading ~1 bar on our dash). This is because the throttle plate is fully open, reducing the restriction and therefore reducing vacuum. So, on our cars, even when the boost gauge is reading above 0.4 bar and below 1 bar, the turbo may not necessarily be doing any appreciable work.

As for the engine warmup safety. Engines are designed for working at operating temperature, as well as the oil. Mechanical clearances in the engine and material properties are different when cold. For example, since the block is made of aluminum and most of the fasteners made of steel, things like the head gasket will have more clamping force acting on it when at operating temperatures (aluminum expands / contracts more than steel, with temperature change). Aluminum, steel, and cast iron become more brittle at very low temperatures. High physical load on these components when very cold could cause a failure that would not happen otherwise at operating temperature. High engine load also equates to high thermal load. Rapid heating of these, very cold, parts can also cause high internal stresses and part failure. The spark plugs are not a fan of rapid thermal load either.

Now that I have you a little freaked out, "cold" isn't really defined. A "cold" temperature of 50F is a lot different than a "cold" temperature of -50F. These cars were distributed to different climates all over the world. Cold for someone living in Hawaii is different than cold in the Nordic countries. As a manufacturer, its easier for Porsche to just say warm your car up, than to say: If the temperature is X, idle your car for X seconds before driving and don't exceed X throttle position or load until X seconds of driving. A potential owner would balk at the sight of such a table.

When "cold", drive it like there's a police cruiser behind you, and you'll be fine.

TurboTommy

The amount of vacuum (or the absolute pressure) in the manifold has nothing to do with the air box, or cone filter, or if the turbo is working or not.
It has to do with the pressure differential across the throttle plate.

951Roadrunner

Thank you all for pointing out the incomplete understanding of motor physics...true understanding is always a good thing, and also helps in rationally picking mods and understanding the risks/benefits..
sorry to hijack the discussion on warmup, but a few more things...
the 951 boost gauge has a range of 0.4 to 2 bar...when this is at 1 bar, does that mean manifold pressure is at atmospheric?
my LITTLE brain totally missed the role of the open throttle on increasing airflow and eliminating vacuum. My bad! I am still confused a bit...if an open throttle totally eliminated the vacuum caused by the pistons moving, why does a less restrictive airbox/etc. result in increased power?

fortysixandtwo

Yes. The 951 boost gauge reads vacuum and boost, so 1bar is almost atmospheric (1bar= 0.987atm). You can view the boost gauge like its a barometer for the intake manifold.

A closed or nearly closed throttle plate is the largest restriction in the intake system, but its not the only one. Anything from the point air starts to enter the car could be considered part of the intake system (body panel seams, fender liner gaps, bottom of engine bay, etc....) to the intake valve, can be considered part of the intake. Any restriction along this path adds to the overall pressure drop of the system and shows up as manifold vacuum. The more air you can get into an engine, the more power it can potentially make.

Earlier, a drink straw analogy was used, which works well here. Lets say the engine has a fixed air capacity, and the intake system is changed to be less restrictive (bigger straw), the engine now has the ability to ingest more air at any given point. Now, lets say you have a fixed beverage capacity. The straw is the intake system before the throttle plate, your mouth is the throttle plate, and whatever muscle makes you suck in, is the piston. With the same effort, if you drink though the tiny straw that comes in a mixed drink at the bar, you will not get the same amount of fluid as if you were drinking through one the giant, I must get high fructose corn syrup in me NOW, straws that you find at fast food restaurants.

A beer funnel, would be an example of forced induction drinking.