new2me-tt

Now I'm a little concerned that I see a noticeable decrease in boost levels (and seat-of-the-pants performance) when it's hot outside. Will adding a "high flow exhaust" to my ECU flash make it so that there will be no loss of performance when it's hot outside, similar to Dock?

TB993tt

Boost should not go down when its hot, it should usually actually go UP

This is because the ECU generally has to provide a greater air mass of hotter air (relative to colder air) to hit the boost targets in the program - If you observe this, it doesn't mean you are getting more power when its hot (you will always get less power relatively) .

Dock

Here's the way I think it works...The boost we read in the cockpit is measured in the intake just before the throttle body. At sea level on a standard day (59 degrees F) the pressure there (at idle) is 14.7 psi. If the temperature is warmer than 59 degrees then the pressure in the intake is less than 14.7 psi. Even though the ambient pressure is different (lower) on the warmer than standard day, the boost reading in the cockpit in both of these cases (assuming no boost from the turbos is being commanded), is 0.0 bar. But is the boost we read in the cockpit different in either of these cases when the ECU is commanding maximum boost? Take for example an ECU that is programmed to deliver 1.0 bar above seal level pressure. On the standard temperature day the turbos will generate boost to achieve a 1.0 bar reading just before the throttle body. On the warmer than standard day, the turbos will do exactly the same thing...generate boost to 1.0 bar as measured prior to the throttle body. The difference is that on the warmer than standard day the turbos will be working harder to generate the 1.0 bar of boost because they first have to generate enough boost to raise the ambient pressure to 14.7 psi, then generate the additional 1.0 bar of boost to achieve the ECU's demand for 1.0 bar at the throttle body. The boost the driver reads in the cockpit is still 1.0 bar, just like it is on the standard temperature day. The different is, on the warmer day the turbos are working harder to achieve the 1.0 bar.

TB993tt

Hey Dock, have we been here before ?
I am out of my depth here, but does the ambient pressure vary enough due to temperature changes to make a meaningful difference to how hard the turbo works ?
My "theory" is based on watching the boost gauge (proper analogue not factory), and when the ambient is high I see noticably more boost at all revs than when cooler - I summised this was the Motronic doing its trick with measuring the air mass and adjusting to hit the programmed target numbers ?

On the other end of this "theory", when the air is cold and dense I observe less boost - on some tuned engines, (and in particular those with fixed boost controllers which have been remapped on chassis dynos under warm conditions,) the mass of the air in cold conditions can be too much (even with the same boost) and cause the engine to protect itself with a spark interupter - quite a few of the fixed boost 993tt boys have experienced this (including me when I was running that config in '97)

Dock

Yes, I think so.


This is exactly what I was saying...higher temperatures mean more boost from the turbos. But this higher boost from the turbos does not mean there is a correspondingly higher total boost level in the intake.

The key here is the difference between the boost output of the turbos themselves, and the total boost reading in the intake.

At sea level on a 59 degree F day ("standard" atmospheric temperature), the ambient pressure in the intake is 14.7 psi. On a warmer than standard day the ambient pressure may be 14.5 psi. In the example I mentioned in my post above, the ECU wants 1.0 bar above standard (29 psi total) delivered to the engine in both temperature (pressure) examples, so on the 59 degree day the turbos have to produce 14.7 psi, while on the warmer than standard day, the turbos have to produce an output pressure of 14.9 psi to achieve 14.7 at the throttle body...0.2 psi to reach standard pressure, plus an additional 14.7 psi. In this case, if you had a direct boost reading at the turbo, it would read 14.9 psi; however, the boost reading at the throttle body would be 14.7 psi, or 1.0 bar.

new2me-tt

So I think I understand ... When it's hot, the turbos are working harder to produce the required boost but still read only the true maximun boost (1.0 on a standard 1.0 bar program), but are spinning faster to do so. When it's colder, the turbos work less (don't have to spin quite as fast) to make to the same required boost (due to denser air), and the reading should still be 1.0 bar maximun (for a 1.0 bar ECU).

So that fact that I am seeing 0.1 to 0.2 bar less boost at peak when it's 15-20 degrees hotter may indicate that my turbos can't keep up?!?!?

TB993tt

This illustrates what I meant (I think ) 14.9PSI compared to 14.7PSI is would probably not even register a difference on the factory boost gauge, it is 1.01bar vs 1.0bar - not a meaningful difference.
The kind of variations I have seen are something like 1bar (14.7PSI) becoming 1.1bar (16.2PSI) when the ambient goes from say 55DegF to 80DegF

TB993tt

Just to confirm - the readings I am observing are seen when using quite big turbos, certainly big enough to "keep up" and provide a lot of boost regardless of temp/pressure - what Dock decribes for stock smaller turbos trying to maintain a higher than stock boost level with lower mass (than DIN) air may well be happening.
new2me-tt
You have to forget about boost and think about air mass, this is what the ECU bases its calculations on rather than actual boost - think of it that way and it becomes clearer

Dock

Where in the system are you picking up the boost reading?

Dock

Based on my understanding of the system, what you describe is true.

TB993tt

intake just after the intercooler.

Basal Skull

I'm a little confused about boost readings also. At idle/cruising I see about 20 mmHg (vaccum) and seem to get about 15 psi at full throttle. But, my digital reading reads 0 bar when I see 20 mmHg on my mechanical guage. I assume the mechanical guage is sensing vaccum that the digital guage cannot register, I have to check to see if the digital guage starts to go up over 0 when the mechanical guage passes 0, not sure it does.... pressure at idle is deffinitely not atmosphere, there is a vaccum. I don't think the factory boost guage can tell/register so small differences in pressures either. Here are some pics, cruising and at wot third gear, here's where I attached the boost guage tubing per what I read on other's posts who have done this before.

Dock

What does your mechanical guage read with the ignition switch "on" but with the engine not running?

Basal Skull

Getting a heache thinking about this - but 1 bar (or 14.7 psi) isn't the same in terms of air 'mass' for different temperatures and in warmer weather, the ECU is actually sensing air 'mass' and not boost so it requires/allows more boost to make up for the less dense air?

Basal Skull

Dock, pressures read 0 (0 mmhg/0 psi) with ignition on but not running. Haven't noticed a difference in warm or cold weather, assuming it just compares it to outside pressure as a reference. So when on full boost on a warm day and my mechanical boost guage is comparing boost pressures to 'outside' pressure (less dense air both in and out side of the engine), what should it read? higher then when cold - can't quite think this all the way through. Or, is all this related more to inlet temps/intercooler efficiency etc.