Thread: Turbo whistle and blow off valve

Nice.....
Ive been wondering why all my mates 6cyl turbo diesels 1hz(t) , td42(t) 2h(t) have straight through 3" exhausts and if they flog them along under load then back off they make a loud blow off whistle noise that sounds weird but it sounds like it comes from the exhaust side of things so I'm wondering if it's something to do with manifold pressure and a millisecond of gasses passing through the turbine from full load to nothing...I'm not sure as mine does not do that as mine makes other noises

Also with that factory relief valve I have not checked but can it be adjusted ? I have 2 laying around one off a sd33t and one of the factory 4bd1t not that I think it is needed but would be funny

Although the previous comments may be crap and it may be to do with the wastegate size,type,position in housing etc etc as I think most of the turbo kits are Schweitzer ....

Also forgot to ask did you have a big smile when "hooning" around the streets ?

Hi Murray

for us uninform is there a reason you need a blow off valve as i thought the waste gate would do this job?

when i put my 300tdi on a dyno the shop said i needed a blow off valve fitted if i want to run at 15psi.....i couldn't convince the owner that 15 was a factory setting, good thing the young fellow on the floor was happy to look and learn

Blow off valves are needed with petrol engines because they have a throttle in the intake system that shuts when you lift your right foot.

They serve little purpose on a diesel that has the intake sytem wide open at all times, other than limiting pressure in the inlet manifold (most of us don't want that).

Originally Posted by weeds

Hi Murray

for us uninform is there a reason you need a blow off valve as i thought the waste gate would do this job?

when i put my 300tdi on a dyno the shop said i needed a blow off valve fitted if i want to run at 15psi.....i couldn't convince the owner that 15 was a factory setting, good thing the young fellow on the floor was happy to look and learn

Not need on a diesel IMO but I'm sure there may be some benefit somehow ....
Nothing a good size wastegate won't do and personally would prefer a external wastegate so that the drive pressure bypasses the turbo all together and doesn't keep pushing the turbo...I'm asuming the relief valve will be tuned in conjunction with the standard wastegate..

Originally Posted by weeds

Hi Murray

for us uninform is there a reason you need a blow off valve as i thought the waste gate would do this job?

when i put my 300tdi on a dyno the shop said i needed a blow off valve fitted if i want to run at 15psi.....i couldn't convince the owner that 15 was a factory setting, good thing the young fellow on the floor was happy to look and learn

Not need on a diesel IMO but I'm sure there may be some benefit somehow ....
Nothing a good size wastegate won't do and personally would prefer a external wastegate so that the drive pressure bypasses the turbo all together and doesn't keep pushing the turbo...I'm asuming the relief valve will be tuned in conjunction with the standard wastegate to eliminate some sort of problem...

from what I know (and that's not all that much really)

petrol turbo cars have more of a need for a blow off valve because when the accelerator is lifted, it of course closes the accelerator butterfly and causes pressure to build up quite quickly.

where as a diesel simply cuts/reduces fuel input and the air from the turbo is still free to enter the engine (of course less air as the rpm has dropped)

from what I have heard the blow off valves also reduces turbo lag as the waste gate doesn't open as much and the turbo doesn't spool down quite as much.
also less resistance is put on the turbo as the pressure doesn't all of a sudden skyrocket as the accelerator butterfly slams shut (which would also cause the turbo to spool down quicker)

very happily to be corrected if I have some/all aspects wrong.

starting to really like the 4bd1t's
would love to drive/ride in one sometime to see what all the fuss is about

I know now that a blow off valve is not needed on a diesel, but when I did the original conversion, there was one on the truck crossover pipe, so I thought I better leave it in the system just in case it was important......I've learned a lot about my engine since then.
I never hear it blow with the full exhaust system on.

Cheers, Murray