Ambient Air Regulating Boost?

So I drove up to Maryland over break to visit family and right when I got up there I started noticing what I thought was a boost leak. I had just installed my SVC IC a few weeks prior so figured a hose got loose. While I was there temps were down to 20 at night and only around 30-40 during the day. In those temps I could not get about 12 PSI and instead of that sexy turbo squeal it sounded like a balloon deflating when the boost peaked around 12.

Coming back down to Florida, I immediately checked all the piping. Adjusted one that maybe had a tiny tiny leak from the clamp slipping a little, smoke tested and had no leaks. I had my 20k service with Ford scheduled so had them look at it just to be safe. They didn’t run any type of tests other than visual and check for codes, but said that ambient air controls max boost pressure which is what I was experiencing.

A few questions about that:
1. I do have a Cobb stage 2 93 octane tune on, does that make any adjustments to that ambient air driven boost limit?
2. For those guys living up North, how low do you typically see this capping your boost?
3. Does that weird balloon deflating noise happen to others?

Everything appears to be fine now. Got the tune back loaded (removed for service) and in Florida temps I peak at 22-23 psi when really hammering down on the truck. It just seemed like a weird situation though, don’t exactly have the piece of mind with it.

Oh quick summary of engine-related mods:
- Cobb Stage 2 93 Octane Tune
- SVC IC
- AFE Magnum Force Intake
- Turbo silencer delete
- Boomba BOV adapter

Anecdotally, on a stock (stage 0) tune and a stage 1 tune, boost reaches ~20psi in cold weather (15-40 degrees) which is all winter for me. It does sound like a leak to me they way you described it.

Is the tune their basic OTS version, or has it been tweaked at all?

The air temperature really doesn't affect the boost amount, BUT the barometric pressure will certainly change the boost levels seen on a gauge. The Boost value is the sum of a subtraction of the local Barometric Pressure from the Absolute Pressure in the manifold. At higher elevations (lower baro pressure) the turbocharger must spin faster to get meet the boost target. There is a specific table in AT that allows you to set a compensation based on the baro pressure.

So..., you indeed may have a boost leak. The colder temps can definitely affect the sealing of hoses, gaskets, etc., when they are compromised already. A correct seal will not be affected by ambient temps. Your Boomba blow-off valve adapter is the likely leaky culprit.

You can see practically any parameter in real-time on the AP, and record 20+ in a datalog.

Ambient conditions will change target boost pressure in a few different ways - through charge air temperature (post intercooler air temp.), intake air temperature (pre turbo air temp.), barometric pressure, and the torque model.

At high charge air temperatures, target load will be decreased which will also decrease your boost pressure target. Also, when intake air temperatures are very high, an airflow-based limit can decrease target load and boost pressure. At low barometric pressure, a baro.-based compensation will decrease airflow limits and target load to keep the shaft speed of the turbo in-check, since lower baro. pressure will result in lower compressor inlet pressure. Without lowering boost targets with significantly lower compressor inlet pressure, the pressure ratio across the compressor will increase significantly along with turbo shaft speed. All of these limit features are in the factory calibration and are maintained in our OTS calibrations.

In cold conditions, charge air temperature based timing compensations will increase ignition timing. When this happens, the torque model can reduce target boost pressure to make sure that torque and power output remain consistent and safe. I doubt that ignition timing would be advanced enough in 30-40*F ambient such that the torque model was capping boost pressure to 12 PSI, so I would agree that you most likely have/had a boost leak.

Two other things to consider - 1) if you were in 2WD, traction control could have been cutting in and closing the throttle to stop wheel slip. As you might expect, this will significantly cut down on boost pressure. 2) boost pressure targets are not constant through the rev range. Our OTS, like many other calibrations, runs a pretty steep taper of boost pressure towards redline. So if you were only going WOT at higher RPM, you wouldn't see the same peak boost pressure on the Accessport gauge screen min/max values as you would if you did a pull through the entire rev range.

If you want any more help diagnosing the issue, feel free to send in a datalog of a 4th gear pull from 2500RPM to redline to [email protected]. Make sure that you're using the default datalogging list. We can take a peek and see if we can offer any more specific advice.

-Sam@COBB

I just got educated. This just brought together many of the bits and pieces I was and wasn’t aware of into clear view. Awesome.