boost presure [Archive] - SaabCentral Forums

: boost presure


mark1405
16th June 2006, 07:12 AM
ive been watching my boost aguge for a few days now and im noticing that after the initial boost of between 14-15 psi the boost then drops down to 10psi and stays there right up till the red line, does anyone have any ideas why i cant hold at 15psi as im running a mbc and if i turn it up anymore i will hit the fuel cut off as the initial boost will go above 15.5psi? i have also replaced the vacuume lines tyhe other day but the problem was there b4 i changes them.

zer0daze
16th June 2006, 08:12 AM
ive been watching my boost aguge for a few days now and im noticing that after the initial boost of between 14-15 psi the boost then drops down to 10psi and stays there right up till the red line, does anyone have any ideas why i cant hold at 15psi as im running a mbc and if i turn it up anymore i will hit the fuel cut off as the initial boost will go above 15.5psi? i have also replaced the vacuume lines tyhe other day but the problem was there b4 i changes them.

couple of reasons.

i assume you are using the port ono the turbo for your MBC. this is the first problem. pressure at the turbo is not the same as pressure in the manifold. the air there is turbulant, unorganized (or is that disorganized? or are either one of them really words?), and hot. in the manifold it has already seen all the pressure drops and fluctuations from the intercooler, charge pipe bends, and all the other crap thats in the way.

ok, so thats fixable. heres the other issues that arent as easy to fix.

the turbo is too damn small! the engine is out flowing the turbo, and above 4500-5k that t25 is a hair dryer.

T-5 is another issue. i had an obd2 scanner/datalogger plugged in the other day. it does some retarded stuff with the timing maps. i have a feeling its a similar to a common issue in the DSM community. phantom knock
i dont think this is the cause of not holding boost to redline, but part of what causes the fluctuations you see (where its bouncing from 10-12ish)

theres other things too.

a soft/worn wastegate spring maybe


before moving my MBC i would get boost like this
gears 1 and 2 would get 12
gear 3 would come up to 15, and then fall and dance around 12
gear 4 would bump 15, and hold then14

i didnt not spell check this... its too damn early. so grammer nazis DIAF
:)

mark1405
16th June 2006, 09:30 AM
yes it a hose from the turbo to the mbc then the other side of the mbc to the waste gate, other mods i currently have are an jr cone filter 3in cat back system and hopefully of next week it will be a viggen intercooler. i have tapped the line to the map sensor for my boost gauge

zer0daze
16th June 2006, 10:07 AM
something like this i think is a good idea for these cars

http://www.accountwizard.com/webimages/goldeneaglemfg/gvm100-b.jpg

http://www.accountwizard.com/clients/shop.asp?web=goldeneaglemfg&outsidelink=class&class=Vacuum+Manifold&subclass=Vacuum+Manifold