yvonnejim

Just noticed the 1.0 reading on the 160mph "blow-out" run. I seem to always get readings of 0.0 , except when accelerating. I've seen max of 0.8 on heavy acceleration.

Woodster

My car read .7 - .8 bar BEFORE I installed my ECU that was reprogrammed to PSI 480 specs.
I also believe large swings in temperature will effect your max boost levels.(colder = higher)
Having said that, I do not believe you will ever see more than .8 bar in your stock Twin Turbo
(assuming that it is stock)
Cheers,
MK

yvonnejim

But should I usually get readings of 0.0 at steady/any speed?

bigape

Yes, anytime your turbos are not spooling up, you'll be at 0.0 If you're concerned about it, find an open stretch of highway, stick it in 3rd and play with the throttle while monitoring your boost. That should tell you all you need to know. The max that I've gotten out of my stock TT is 0.7 do far. We'll see what the future upgrades bring

Edgy01

Of course, you will get nothing simply when running up the RPMs in the garage or standing still. Need a load on to generate boost.

Dan

yvonnejim

Cruising at a steady 100mph certainly applies a load to the engine, but bar reads 0.0 - or should it still read 0.0

strato58

Steady crusing at 100 without acceleration will show 0bar. It does not take alot of hp to steady cruse at 100....accelerate from that point and look at the guage...my max is .8 on a stock 1996 tt.

omega

boost comes with acc. when your motor feels no load the turbos are resting you will notice boost any accuration as in entering a freeway any time you mash the acc.get up to speep slowly the turbos don't need to work so they don't get up to speed quckly and they will work 07 is the norm for a stock tt

TB993tt

With Motronic control: colder = lower boost since less mass is needed to hit the target parameters. But more power since those parameters (if done properly ) are tuned to DIN power which is calculated at 68DegF.

Dock

In terms of the actual boost the turbos are generating yes, but in terms of what the boost gauge reads I don't think so. Here's how I think the system works...

As far as I know, the turbos generate pressure to get a maximum of ~0.7 bar over atmosphere to the engine. Boost is read just before the throttle body, and this is displayed on the boost gauge on the tach. With no boost generated by the turbos the pressure in the intake is ambient pressure. The higher the altitude and/or the higher the temperature the lower the ambient pressure is in the intake (still reads 0.0 on the tach when not under boost). In either of these cases the turbos have to make more total boost (out of the turbos themselves) to get the pressure in the intake up to the 0.7 bar required by the engine, but the total pressure in the intake is none the less only 0.7 bar, and that's what's read by the driver on the tach. At altitudes below sea level, and/or at cold temperatures, the ambient pressure in the intake is greater than standard (higher than sea level pressure but still 0.0 on the tach when not under boost) and in this case the turbos don't have to generate much boost to attain the engine's required 0.7 bar intake pressure. But the maximum boost in the intake is still ~0.7 bar

If the car is operating normally, the maximum boost read on the tach should be ~0.7 bar regardless of altitude or temperature...at least until you go higher than the turbo's critical altitude (which has to be very high).

TB993tt

I get what you mean and can't disagree. However "you" are talking about quite small variations.
My experience to back up the "mass vs temperature" is based on what I observe on my Motronically controlled motor. OK the motor is not standard but I presume the factory system does this but mine probably exagerates this feature of the Motronic since it is mapped this way, but.....
On a 80DegF ambient day I will observe (on a proper boost guage) up to 1.2bar @ say 5700rpm. In temperatures like we are having in the UK at the moment ~40degc and less, the boost at the same revs will not get over 1bar.
When I first noticed this feature (in the summer) I was excited, thinking some magic bling was giving me 1.2bar so I MUST be making 650hp - until my bubble was burst when it was explained to me - same DIN mapped hp, but more air needed to hit the target power (parameters).
BTW is the stock digi boost guage on the same POS as on the 993tt ?

Dock

The "more air needed" comment means that at higher altitudes and/or higher temperatures the turbos have a higher total output to make up for the loss of atmospheric pressure, because they have to make up the difference between ambient and seal level pressure *and* generate another 0.7 bar on top of that.

At seal level on a standard day the ambient pressure in the intake is 14.7 psi. If we had a direct reading pressure gauge in the cockpit it would read 14.7. Under full boost the gauge would read ~25 since the turbos produce 0.7 bar (10.3 psi) *above* atmosphere. Say the ambient pressure in the intake, due to increased altitude and/or increased temperature, is 12.0 psi. Since the stock 996 Turbo's engine is designed to make maximum power at 0.7 bar above seal level pressure, the turbos *first* have to produce enough pressure to get the pressure in the intake to seal level (+2.7 psi), then produce an additional 0.7 bar (+10.3 psi) above that. So the direct reading pressure gauge (if we had one) would still read ~25 psi (just like at seal level), but the turbos are actually working harder than at seal level, producing a total output of 13 psi versus the 10.3 psi they produce at sea level. So...the "more air" is correct with regard to the total output, but not correct with regard to what the total pressure is in the intake.

TB993tt

So you are saying that it is the temperature's affect on pressure which causes the "more air" and my understanding of the temperature's affect on mass of air is incorrect ?
The guage reads "total pressure in the intake" surely, so how does your explanation account for the different readings when all my readings are taken at the same altitude with only small variations in pressure

Dock

I don't know where the pressure that is displayed in the cockpit of your car is picked up back at the engine. On the 996 Turbo it's right before the throttle body.

I also don't know why your car has different boost readings.

The way I understand the system, the 996 Turbo wants ~0.7 bar pressure above atmosphere delivered to the cylinders. The turbos themselves work as required, based on ambient pressure, to produce this pressure to the cylinders.

TB993tt

I understand and agree with what you are saying, but where does the temperature and the affect it has on mass fit into it - because the ECU reads mass and not boost it allows the boost to increase when temp is hot and air is less dense and reduce when it is colder and air is denser - that is what I see in practice