940 2.3 boost issues

[Mattty]

Wondering if anyone could help me diagnose my boost issues in my 1997 940 2.3. Stock everything apart from a 16t/15g hybrid turbo and a ball/spring type MBC set at 14psi. Planning on putting a 3" downpipe on very soon and I also have an ecu chip to go on but want to wait until my boost issue was sorted.

Basically when I accelerate in 1st/2nd gear, my boost spikes to 12psi then settles and holds 9/10psi to redline. However when accelerating in 3rd/4th gear everything seems to boost as normal and boost builds to 14psi and stays there to redline.

I thought it might be the fact that boost is building too quickly in 1st/2nd and leaking(?) so I just tried to very slowly build boost in 2nd, but as soon as it hits 9/10psi it just stays there and wont go past.

Scratching my head a bit at this as I'm new to boosted cars so any suggestions as to what it could be or what to look for are helpful.

Cheers

[martin_r_smith]

Sounds perfectly normal to me. In 1st and 2nd there is insufficient load to sustain high boost. If its holding 14psi in 3rd and 4th then you have no issues at all.

[Mattty]

Originally Posted by martin_r_smith

Sounds perfectly normal to me. In 1st and 2nd there is insufficient load to sustain high boost. If its holding 14psi in 3rd and 4th then you have no issues at all.

Thanks for the reply. Still trying to wrap my head around controlling boost; does that mean that if I set, in theory, my MBC to 20psi and it held this in 3rd and 4th, would it be able to hold in more than the 10psi in 1st and 2nd?

Edit - Just took it out for another spin and realised it doesn't actually hold 14psi to redline. When I tested it in 4th it held 14psi to around 4-5k then dropped back off to 10psi. My bad. Still only climbs to 9/10psi in 1st and 2nd gear, however.

[Laird Scooby]

Originally Posted by Mattty

Thanks for the reply. Still trying to wrap my head around controlling boost; does that mean that if I set, in theory, my MBC to 20psi and it held this in 3rd and 4th, would it be able to hold in more than the 10psi in 1st and 2nd?

Edit - Just took it out for another spin and realised it doesn't actually hold 14psi to redline. When I tested it in 4th it held 14psi to around 4-5k then dropped back off to 10psi. My bad. Still only climbs to 9/10psi in 1st and 2nd gear, however.

The thing that spins the turbo up in the first place is the expansion of hot exhaust gases. If you're only generating enough load on the engine that it pushes out just enough hot exhaust gases in 1st and 2nd to build 10psi, setting your BMC further above what you've already got won't create any more boost.

To see what i mean on a more practical level, find a steep hill near you, drive along at 30mph in 4th then when you start going uphill and losing speed, floor the loud pedal (still in 4th) and watch the boost gauge.
You should see it climb fairly steadily to 14psi or whatever your MBC is set at.

Now do the same but in 3rd gear.

Then do the same, in 3rd gear again but only squeeze the loud pedal enough to just accelerate up the hill - not losing or maintaining speed but only just accelerating.

This will give you three different load levels, the lightest load will be the last one and the engine won't have to work so hard so there will be less hot exhaust gases to flow through the turbo impeller, expanding as they go. As such, you'll get less boost.

[TonyS9]

What is your standard boost?

The B230FK actuator is not very stable boosting to 10+psi, B230FT is more stable.

Also 14psi is a bit high for standard fueling IMO, but not sure.

[martin_r_smith]

Originally Posted by Mattty

Thanks for the reply. Still trying to wrap my head around controlling boost; does that mean that if I set, in theory, my MBC to 20psi and it held this in 3rd and 4th, would it be able to hold in more than the 10psi in 1st and 2nd?

Edit - Just took it out for another spin and realised it doesn't actually hold 14psi to redline. When I tested it in 4th it held 14psi to around 4-5k then dropped back off to 10psi. My bad. Still only climbs to 9/10psi in 1st and 2nd gear, however.

Sounds completely normal, again down to load. In the top end of 4th past 5k your not passing the air through the engine (mainly due to cam restrictions but also restriction in the exhaust manifold, turbine housing and downpipe) and therefore not producing exhaust gasses to spin the turbine and so less boost.

If for some reason you want more boost up top then you have options:

Fit a cam that works past 5k - v cam is cheap and good

Fit an adjustable cam gear to shift the power band where you want it

Fit a turbo with a larger turbine housing (15g/16T/18T/19T are easy fit)

Port the exhaust manifold (big restriction here)

Fit a larger diameter downpipe

Or do all the above with some bigger injectors a decent tune, basic chips like TLAO will work but a custom tune is far superior, and then be happy with 240bhp.

Just increasing the boost to 20psi as you suggested to get your target of 14psi is a bad idea.

[Mattty]

Originally Posted by martin_r_smith

Sounds completely normal, again down to load. In the top end of 4th past 5k your not passing the air through the engine (mainly due to cam restrictions but also restriction in the exhaust manifold, turbine housing and downpipe) and therefore not producing exhaust gasses to spin the turbine and so less boost.

If for some reason you want more boost up top then you have options:

Fit a cam that works past 5k - v cam is cheap and good

Fit an adjustable cam gear to shift the power band where you want it

Fit a turbo with a larger turbine housing (15g/16T/18T/19T are easy fit)

Port the exhaust manifold (big restriction here)

Fit a larger diameter downpipe

Or do all the above with some bigger injectors a decent tune, basic chips like TLAO will work but a custom tune is far superior, and then be happy with 240bhp.

Just increasing the boost to 20psi as you suggested to get your target of 14psi is a bad idea.

Thanks, that was super helpful!