Boost Pressure

[rapserv]

Have fitted a digital boost gauge, coupled directly into the inlet manifold and have done a few test runs with vehicle under various load conditions.
Gauge indicates an ambient pressure just below one atmosphere (around 13.8psi) on idle (we are 200m above sea level here) and shows a maximum pressure achieved during the test drives of 35.5psi.
After reading a number of posts on the forum, I understand that I should expect an ideal boost pressure of around 20psi give or take.
I would assume that this is 20psi above the ambient pressure and I am reading 35.5psi ... therefore, in reality, I actually have about 21.7psi boost.
Is the way I am interpreting this correct .... or not !
If I am correct, is this too much boost? ...I don't have any surge issues at the moment.

[alpick]

see what fitting a boost gauge etc does, makes you doubt the beast and sweat the small stuff.:spudnikworried:

[Blknight.aus]

IMHO your boost gauge is wrong...

at 200m above sea level you should be seeing somthing like 14.4ish psi (as opposed to 14.7@ sea level)

[John W]

Does it read 0 or 14 when the engine is not ticking over?. I do not have one but would have thought that it will measure boost over atmospheric so you are getting one plus atmospheres boost I suspect?

[biggin]

It's hard to believe that your setup measures absolute pressure rather than gauge pressure.

I can see three possiblities:

your sensor and gauge do not match
the gauge is out of calibration
all is correct and your are somehow getting up to 35.5psi:o
Try removing the sensor and check it against a known air pressure, or something.

[Blknight.aus]

that it reads absolute isnt so strange, thats what the ECU is working off of and if its electronic it may have a switch on the back of the gauge that will allow you to flick it from absolute to gauge.

[rapserv]

Does it read 0 or 14 when the engine is not ticking over?. I do not have one but would have thought that it will measure boost over atmospheric so you are getting one plus atmospheres boost I suspect?

Hi John W,
Actually, its reading 13.7 with engine off
13.6 on idle
35.5 max boost achieved

Also, made a mistake with our property altitude ... is 600m ... not 600ft (200m) :-(

[rapserv]

IMHO your boost gauge is wrong...

at 200m above sea level you should be seeing somthing like 14.4ish psi (as opposed to 14.7@ sea level)

sorry Dave,

made a mistake with our altitude ... should be 600m ... not 600ft.

[Blknight.aus]

13.7 is near enough for atmospheric pressure at 600M, sounds about on the money for max boost in a disco td5 to me Deefer TD5's can be made to go consistantly higher as they dont have the wastegate modulator.

I'd have to go back and have a look but I from memory the disco TD5 is allowed some "overboost" in certain circumstances

[mturri]

13.7 is near enough for atmospheric pressure at 600M, sounds about on the money for max boost in a disco td5 to me Deefer TD5's can be made to go consistantly higher as they dont have the wastegate modulator.

I'd have to go back and have a look but I from memory the disco TD5 is allowed some "overboost" in certain circumstances

Almost certain it is actually the other way around: because of the wastegate modulator, the overboost limit on Td5 Discos is set slightly higher (240 kPa) then on Deefers (225 kPa). Stand to be corrected though.

Rgds
Matt

[Blknight.aus]

where that falls over is with tweaking your waste gate actuator..

It may have been just the engine in the disco I was working but the first time I fell foul of the overboosting power surge and cut deal swapping out then running the ECU in a deefer caused no problems but once it was in a disco it would keep hitting off on the overboost. wound the actuator rod out a little and that then seemed to sort the disco.

Both vehicles were manuals.