boooost!!

what is the safest max boost pressure on a standard turbo?

also what is the factory pressure?

is it best to get a reading when accelerating or holding your foot on the brake and holding the throttle full open? i get slightly higher pressure doing to later, (10psi) as opposed to accelerating. (8-9psi)

The method using the brake is the correct way of testing, but it must be done quickly!

I've heard it stated alot that 10psi is the standard setting for a GTA, but the workshop manual mentions a setting value of 950 to 980 mbar which works out at about 14psi! Around 13-14psi, coicindentally, is the figure I've heard mentioned alot as the maximum setting that can be applied to a GTA before the safety pressure switch cuts fuel.

No doubt D.G. can explain further.....

The pressure actually at the turbo is 13.4psi (980milibars), but with the pressure drop of the intake pipes, intercooler and throttle body, the actual boost pressure at the manifold is 850mb peak holding at around 650mb (10psi). The ECU has a fuel cut out at 14.7 psi, but you cant run this boost with out a piggy back device, because the boost spiking above if trying to hold at 14psi will peak to around 16psi initially and trip the cutout.
Also on colder days with denser ambient air, it is easier to trip the cutout.

Best reading for boost, is full throttle in forth.

As for the safest boost pressure on the standard turbo, well its near its limit at standard boost... It will cope with a big increase in midrange torque but still struggle up top...

Years ago I experimented with different boost pressures and on a turbo with the standard size .36 housing I found the most pressure you could run without causing the car to develop unpleasant on boost characteristics was about 1.1 bar. Above this the small turbo created something called boost surge. The characteristics of this are loss of power under full load, lots of heat and some unpleasant noises! Weirdest thing was holding it at full throttle and it going ok (but no better than standard really) then backing off the throttle to say 2/3 open and feeling accelerate like someone just dropped in a 50 shot of nitrous! I was told that it was caused simply by too much air trying to be forced through the turbo causing the airflow to do strange things, as you come off the throttle a bit the turbo drops back into a zone where it can run efficiently and away you go. Would be interesting to get some cars with different size turbos down the rolling road and see how the boost and power graphs compare.

Thats actually the compressor side rather than the exhaust. The small turbine housing will just restrict power in a linear fashion up the rev range, depending on size. Whats happening, when you back off the throttle slightly, like you said, is the compressor coming back into a better efficiency zone, as the engine is consuming slightly less air due to the throttle restriction, because as mentioned, the compressor is out of range even above standard boost.

Or it could have been, that if the car was running increased fueling, with say higher fuel pressure, coming off to 2/3rds throttle will disenage the wide open throttle switch, and run slightly leaner. If the car was overfueling in the first place, then the lower level of fuel may have been the correct ratio for more power.