JustinL

That's the point of this discussion. What you're saying is the wrong answer to why the boost gauge reads higher.

I like Laust's answer the best now... as I've been thinking my wastegate spring theory is bunk because the exhaust would heat up that thing way more than the atmosphere anyway.

Regardless, EBC is where it's at to maintain proper boost in all conditions.

Justin

Jake951

I think Tazman may be onto something here. The boost controller senses a different pressure than what we actually measure as boost in the intake manifold. The difference is the pressure drop across the intercooler. My take is that the ideal gas law says there has to be a pressure drop because the air is cooler downstream from the intercooler in the intake manifold. The pressure drop is compensated exactly by an increase in the density of the cooler air. Therefore the boost controller, which senses pressure before the intercooler, is actually sensing the wrong pressure if the goal is to maintain constant boost in the intake manifold in all ambient temperatures! I would guess the problem gets worse at higher ambient temperatures because there would be a bigger temperature drop across the intercooler and therefore a bigger pressure drop.

In short, my guess is that it's a boost regulation issue because of the imperfect way MBC is implemented in our cars.

With a bigger turbo that makes power at lower boost, the problem should be less severe because the boost pressures are lower, which results in less heating of the air.

Does this make sense or am I full of hot air?

TurboTommy

Not so Jake 951,

The pressure drop through an IC is always the same regardless of ambient temps.

This all definitely has to do with the work needed from the turbo to compress air of different temperatures. There's even a few references in Hugh M.'s book about correction factors used for turbocharger speeds in different ambient temps.

toddk911

"Don't make it so complicated. The denser the air, the more O2 in the combustion chamber. That's about it."

EXACTLY

Only 16 oz of water will fit in a 16oz glass. But if you heat up that same 16oz of water to the point where the molecular structure expands, that same 16oz of water will no longer fit in that 16oz glass. Only say 15 or 14 oz will fit.

And the reverse would be if you cool down that 16oz of water enough, you could get a shade over 16oz in the same glass.

Not convert the glass to the motor and the water to the boost.

To quote myself from earlier it is all about density of air. The greater the density, the higher the volume per X amount of space. And the density of air is directly perportionate to tempurature.

Jake951

I don't have Hugh M.'s book. Is there a Web site which has the same information?

Can I assume that an EBC plus a good dual-port WG should not have this problem when properly set up?

Chas

so it depends on the horse then?

special tool

A horse is a horse

toddk911

Unless it's the famous Mr. Ed

hosrom_951

The cooler the air gets into an engine, the more power. On turbocharged cars, we have the intercooler to cool the hot compressed air once it leaves the turbocharger. It then gets cooled and enters in engine. In cooler climates, the air gets cooled easily and quickly, and reaches a lower temperature than in warmer/hotter climates. It would be better to run a bigger intercooler in warmer climates, to cool off as much air as possible, though cooler cliamtes would cool that same amount of air, much MUCH cooler than warmer climates.

JustinL

Todd, You have to specific with your units. What weighs more a pound of bricks or a pound of feathers? You are using oz. interchangeably with fl.oz., which you can't do. 1L of water will fill a 1L container regardless of temperature. 1Kg of water may or may not fill a 1L container depending on temperature.

I'll go with your analogy though to illustrate the other side of the argument: You have a 1L container that has a drain at the bottom that opens at a certain pressure (lets say 15Psi). If you fill the container with hot or cold water, the valve still opens at 15psi and closes at 14.9psi. Thus the pressure measurement at the location of the valve should stay close to 15 psi.
What some of us are considering is that the signal to bleed off pressure is somehow comprimised so the "valve" is open wider than it should be.

I hope that makes sense to people other than myself.

Justin

GaryK

I live at 6200ft and have a simple Reliaboost manual controller on my car with a T04E and a few other mods. Though I typically run 15psig boost, on cold mornings, I can see 17psig in the higher gears. So, if I'm planning on driving in the winter (when the roads are clear), I adjust the Reliaboost to bring the boost down to the level I want.

The lower barometric pressure at this altitude means the pressure ratio for the turbo to produce an absolute say 15psi boost is higher than at sea level. So, to produce an absolute 15psi boost at 6000ft is equivalent to roughly 18psi at sea level. It's easy to see this since I have both the in-dash absolute boost gauge and an aftermarket gauge plumbed into the line from the intake manifold to the ECU. When the in-dash gauge reads 1 bar, my other gauge reads 3psi.


It's wise to be moderate with boost levels here if you are using a small turbo, like the K03s fitted to many new VW/Audi's. But, then again, spool-up suffers noticeably here compared to sea level. It's all about air density.

There is nothing that will kill boost and power worse than a hot dry day at high altitude.

turbite

Jake also makes a good point.
The intercooler may be more efficient at higher temps, and if you're measuring before the intercooler then this might be one variable in the boost control. Is the intercooler more effecient at higher ambient temps? Its effeciency
I also think Laust has a good point that the exhaust temp will be higher due to their being less air per fuel, resulting in the wg opening sooner.

Does the stock setup control boost relative to the ambient pressuer?

I see there's some confusion about the stupid horse.
The horse was just an analogy to say that the issue is _not_ in the turbo or density or whatever, but a variable in the boost control somewhere.

toddk,horsom: Remember we're talking about psi here, not how much air gets through the engine.
The question is "why do you make less _PSI_ at higher temperatures."
note the difference between that and "why do I make less power at higher temps."

Ben Z.

To me, Laust's is the only logical explanation thus far.

Tomas L

The intercooler should have nothing to do with this. The volume air flow is more or less constant regardless of ambient temperature. This means that the pressure drop is more or less the same. The cooling effect is also the same since the temp difference betwwen the compressed air and the cooling air is the same.

Peckster and Toddk911: sorry but you are way off.

Laust's theory is so far the only reasonable. I do have one addition to it. If the exhaust pressure increases more gas will be forced out through the wastegate even if it's not forced to open more by the pressure. The pressure differential over the wg is higher and more gas will then flow throug it even when the opening is unchanged.

I do have another theory.
The wastegate is really not a pressure regulator. It's a pressure controlled valve. When the control pressure reaches a certain pressure the valve lifts from the seat. It does not however open fully immediately. It opens a certain amount for each additional psi of control pressure until it's fully opened.
On a cold day when the air is denser the exhaust flow should be greater than on a hot day. Therefore it's required more control pressure (boost) to the wg to open it enough to bleed off the required amount of exhaust gas.

Tomas

JustinL

I'll see if I can clarify Jake's theory:

1) There is more of a pressure drop across the intercooler on cold days.

2)if pre-intercooled air temp and pressure is essentially the same on a hot day and a cold day, the signal to the wastegate is the same and the density and mass of the air entreing the intercooler is unchanged.

3) On a cold day, the intecooler is able to drop the temp of the charge air more dramatically. PV=nRT V,n,R are constant in this case so if you lower T more on a cold day than a hot day you get a bigger pressure drop across the IC.

4) Since you measure boost after the intercooler and control it before, there is bound to be a pressure difference on hot and cold days... but the ammount may be negligable as I made a lot of assumptions, but it may contribute to the phenomenon.

Justin