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997 Turbo boost figures

Alex L

Alex L

PCGB Member
Member
Posted on '911 Turbo' forum but may be more relevant here:

http://www.porscheclubgbforum.com/tm.asp?m=345357


Hi guys

I believe the 997 Turbo is supposed to get to 1.2 bar boost pressure with Sport Mode and overboost (1.0bar in Normal Mode). The Drivers Guide does show a pic of 1.2 bar with overboost arrow.

Anyway, recently I found a quiet stip of straight dry road, and had a play with the boost figures. I only ever use 99 octane Shell vPower fuel.

Normal Mode
If I put the car in 6th gear, then I can get 0.9 bar for about 1 sec, then it drops to 0.8 bar and stays there.

Sport Mode
Again, in 6th gear, I can get to 1.1 bar for a split-sec with the overboost arrow showing, then it drops to a constant 1.0 bar.

I am wondering if the reasons a lot of the guys with manual cars that are seeing 1.2 bar is because they have reduced the exhaust back-pressure from switching to an aftermarket exhaust?!
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Either that or newer Turbos are being tuned-down slightly compared to early factory productions?!

What do you think? I would interested to know what other 997 Turbo owners have seen.
 
I've only ever seen 1.0 bar, but if it went any higher than that I'd of been watching the road anyhow...[;)]

I never attained the top figure in my 996 so possibly just me, higher gears and flat to the floor do seem to give the higher number readings..

garyw
 
Think I have an answer...

Max boost pressure increases with air temperature (the electronics try to compensate hp loss due to unfavourable environmental conditions...). Thus, in hot weather conditions the boost pressure is higher than in colder climate.
Click to expand...

When I was getting 1.0-1.1 it was only about 5 or 6 degrees C.

Gary: it's not so scary on an empty motorway in 6th gear from about 60 mph. Got to be careful not to hold it for too long though as if caught your license will equally as quickly dissapear [:(] I agree that in lower gears though even 0.8 bar is just a blurrrrrr as the world hurtles backwards around you!

 
Not so sure that is the answer. It depends on the boost control system. The boost is regulated by a wastegate - basically a valve that dumps exhaust gasses away from the turbo. This boost control system will be designed to achieve a set value of boost - i.e. boost will build with engine revs until it reaches the set max boost value when the wastegate will open to regulate the turbo. On a hot day the turbo will have to work harder as the ambient air is less dense so the turbo will have more compressing to do, however it will still achieve the max boost. On a cold day when the air is less dense the turbo will have less work to do to achieve max boost pressure. The chances are the in-dash boost gauge probably isn't that accurate or repeatable, or were you at wide open throttle or just off it? You should be achieving consistent boost pressures in all temps.

The best way to test it is to take it on a rolling road where you will get a trace of boost pressure across the rev range.
 
Well Alex in the pursuit of scientific research, today I tried some hard acceleration in sport mode. The combination of wet roads and self preservation meant I only saw a max of 0.7 bar. The scenery just passes so quickly as the boost hits home! Tried nailing it at 40 mph on the local dual carriageway, resulting in wheel spin and the traction control cutting the power [&:] - was fun though as the car twitches and slithers [8|][8|]
Maybe when the weather calms down and dries up I'll try again.
G
 
ORIGINAL: sawood12

Not so sure that is the answer. It depends on the boost control system. The boost is regulated by a wastegate - basically a valve that dumps exhaust gasses away from the turbo. This boost control system will be designed to achieve a set value of boost - i.e. boost will build with engine revs until it reaches the set max boost value when the wastegate will open to regulate the turbo. On a hot day the turbo will have to work harder as the ambient air is less dense so the turbo will have more compressing to do, however it will still achieve the max boost. On a cold day when the air is less dense the turbo will have less work to do to achieve max boost pressure. The chances are the in-dash boost gauge probably isn't that accurate or repeatable, or were you at wide open throttle or just off it? You should be achieving consistent boost pressures in all temps.

The best way to test it is to take it on a rolling road where you will get a trace of boost pressure across the rev range.
Click to expand...

You might be right. Looking at the beginning of the Rennteam thread on this topic you can see in the pic with 1.2 bar that the outside temperature is only 2 degrees C.

http://www.rennteam.com/showflat.php?Cat=0&Board=997turbo&Number=295901&page=0&fpart=1
 
Did a bit more playing today. Managed to spot the ellusive 1.2 bar at last - albeit only for 1 second or so.
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Strange it doesn't stay for longer even if I kept the gas pedal flat on the floor.
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Alex,
It depends on load. You really need to try it on a incline with the motor on full torque to get the required result. I wouldn't worry too much about the readout - if it feels fast enough it probably is! Enjoy, have fun - but be safe.
Regards,
Clive
PS - I'm very jealous BTW
 
Hi Alex,

I had the same issue with the stock exhaust.

As soon as i had the cargraphic one fitted along with the race air filter the gage would often go past 1.2


Nick
 
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