Not Making Target Boost

[SecondhandSnake]

Issue: Car should be making 15psi but I'm not seeing it.

Configuration: Two stage compound. First stage is twin 66mm turbos wastegated at 5psi, referenced to stage 1 boost output. Second stage is a ported Eaton M112, stock pulley size. Boost gauge is a stock gauge with the Hilton high boost overlay.

Opening it up, 75%+ throttle, 4th gear up to 4000 RPM, only seeing around 10psi peak on the gauge, boost going up with RPM. Datalog revealed peak load of 1.45, ~425hp worth of air, well below target.
Doing a quick 1st-2nd rip it didn't even show any boost at all, but was pulling hard enough to lock up the seatbelts and pop the gauges out of the center stack. It doesn't spike the boost gauge up to 8psi from the blower alone with a blip of the throttle like it did when it was blower only.

Boost gauge has been calibrated using shop air. It is within 1-2psi of what the instrumented gauge read.

Turbos are verified to be putting out their target 5psi by 50% throttle and 2500RPM with a second boost gauge.

Possibilities:
1.) 2nd stage not performing as expected (blower). Boost bypass is disabled, but is there any way it could be physically not making 8psi on its own? Even without the turbos it should be making close to that almost immediately, but the gauge seems to respond more to the turbos. At the same time they're only putting out 5psi, so it is compounding them somewhat, but slower than expected. Not to mention they light much earlier than they should on their own, again confirming the blower is doing something, just not as fast as expected. Stepping to a smaller pulley is an option, but would introduce more inefficiency and heat to an already very hot configuration.

2.) 1st stage reference needs to be reconfigured. Right now the turbos are wastegated to 5psi and referenced to their output (1st stage), hoping the pressure ratio from the blower will get them to 15psi at the manifold. This could be swapped to be wastegated at 15psi, referencing the manifold, so any slack the blower may cause, they will pick up and make sure it gets to 15psi, even if there's no compounding.

3.) Boost gauge issue, not reflecting actual value. I hooked the reference end from the lower manifold to shop air and calibrated it, so it should be accurate. The airflow data seems to confirm that it is reasonably accurate as well, comparing load values to boost values.

 

[hotcobra03]

Not sure if this helps.

What changes if any to exhaust.

This is for the response of boost

Mine stock I’ve noticed boost not going as high

New I was seeing 10 with long tunes which reading says I should lose 2lbs from flow.

How long does a blower last?

I’ve been seeing only 8. And that’s getting on it

I went to a 4lb lower. Being cage was shot out.

Now I see 12.

But I noticed I see 8 or more with just 1/4 pedal

 

[SecondhandSnake]

The big change to the exhaust was going from longtubes to forward facing tubular headers with some 66mm snails, and 3" downpipes. So there is some more restriction to it than there used to be.

But I've had a nagging thought about the bypass valve. Looking at the vacuum diagram, I'm wondering if even though it's plumbed like stock it isn't acting as it should. Most notably the lower port on the canister is normally routed to the intake tube, which under load is going to see slight vacuum at most (in stock trim). Well, as mentioned above it's going to be getting hit with 5psi now from the 1st stage. I'm wondering if it isn't getting popped open from boost through the lower port. Doing my research it looks like that port is more than often vented to atmosphere. I might plug that line, vent the port, and see if it hits harder. Beyond that I'll check the bypass valve in general, making sure it's closing and opening when it should.

 

[4sdvenom]

I would have to look at the vacuum and boost lines for the bypass again but it sounds like that may be the issue. Bypass is open while trying to make boost.

 

[SecondhandSnake]

Forgot to update this one, so just for posterity for anyone looking up compound configurations-

1.) Bypass was functioning properly. To rule that out I did configure it to reference atmosphere, as a lot of other installations do. A little response was gained, nothing major.

2.) The big difference came in re configuring the wastegates. Maybe it's not as simple to treat the blower as a defined pressure ratio and rely on that. It's much easier to just set the wastegates at 15psi, reference the manifold pressure (post blower), and that way you know what you're getting, regardless of how the work is being split. Either way it hits hard and fast now. It may not hit as fast as a pullied blower, but it feels a lot more powerful and it's still plenty to knock the tires loose on a whim.