If you are here, you probably have a car with a fast spooling turbo and installed an aftermarket boost controller. This issue is common with cars that boosts at very low RPMs after installing a manual boost controller.
How low? A stock Daihatsu YRV K3-VET is no longer in vacuum ~900 RPM and makes 1 bar of boost ~2800 RPM. Here’s a dyno courtesy of my friend, on a modified K3-VET.
To understand PTFB, you must first understand how the stock ECU control boost. The ECU will read throttle position, current gear, RPM, intake air temperature, etc. and boost accordingly. Stock ECU boost control is very sophisticated. Because the stock ECU controls fuel and ignition, everything works great – when boost increases, the ECU adds additional fuel.
So what is PTFB?
It means having full boost at only partial throttle. Using the K3-VET with a manual boost controller as an example, cruising at 100km/h @ 4th gear is 2800 RPM. You step on the throttle slightly and the car goes into full boost.
That is awesome! Right?
No, not if you just want to maintain your speed at 100km/h, stay in vacuum and cruise.
Why does PTFB happen?
Because the boost controller does not take into consideration throttle position and current gear, among other things. PTFB most likely occurs with manual boost controllers.
So is PTFB a problem?
If you did not tune for PTFB, yes it is a major problem.
When cruising, your boost controller will put your car in boost, but the stock ECU is in closed loop and will try to keep air fuel ratios stoichiometric. You will run lean, exhaust gas temperature will increase and you WILL kill your engine eventually.
If you tuned for PTFB, your fuel consumption will be higher, and your throttle/acceleration will be less linear.
So PTFB is great if I’m tuned right?
The simple answer is no.
For a car that’s a daily commuter, it is undesirable as fuel economy is poorer. Your throttle control will also be less linear.
Imagine your grandmother is driving your car. She slows down to enter a sweeping corner. She exits & steps on the throttle to get back to cruising speed. The RPM was ~2300, PTFB kicks in for 0.5 bar of boost, and suddenly she’s flying out the corner.
Is it a bad thing? Depends, if grandma was having fun it would be great. But the sudden power surge might have caught grandma off guard and catch the guard rail instead.
On the track, PTFB in FWD cars might cause wheel spin, but not a big deal. In a RWD, the driver looses the ability to control tyre slip angle with throttle and the sudden surge of power may just sent the car spinning out.
So how do I increase boost?
You can still use a manual boost controller or simple electronic boost controller provided you tune to compensate for PTFB. If you do not want PTFB, sophisticated boost controllers such as the HKS EVC 6 or the Apex’i AVC-R coupled with a piggyback to tune fuel would work. The best way would be to flash the stock ECU if you can, otherwise get a piggyback or standalone that controls both boost and fuel with throttle input as load.
How about increasing boost on the Daihatsu YRV K3-VET engine in the Myvi Turbo?
One of the first mods I did for Myvi Turbo was to install a Turbosmart Boost Tee manual boost controller. Big mistake. PTFB and the car was hesitating and struggling. I thought if I increased boost by just a little the stock ECU can compensate for it. I can tell you from experience it won’t work.