Hey Everyone! Dave here with another quick Tech Tip!
As the title says, we’re going to be talking about boost taps and how they can produce false data which can cause your car to run poorly!
Now there are many good reasons to have a Boost tap. Maybe you’re using it for a standalone gauge setup, or need a boost reference for Water Methanol Injection.
Regardless of your reason for needing one, there is a right and a wrong way to access manifold vacuum.
Unfortunately VW didn’t bless us with an easy access vacuum port unlike the older vehicles which were full of overly complicated vacuum plumbing. While we’re thankful for the cleaner engine bays and less lines to worry about cracking or needing replacing, there really aren’t many good ways to get an accurate source of Manifold Pressure other than our electronic TMAP Sensor located on top of the intake manifold.
1.8T 20V Vacuum Diagram
One of the market offerings does utilize the original sensor hole, but it comes with a dire side effect which is what this article is primarily talking about.
While these spacer style of boost taps may seem like a great idea since you dont need to puncture a hole in the side of your manifold, they have a dire side effect that you may not know about.
By moving the TMAP from your intake manifolds airflow stream, your sensor picks up a lot of extra heat that isn’t really in the manifold. (False Data)
But what makes a TMAP different from a MAP Sensor? Are they the same thing?
A TMAP is a Temperature / Manifold Absolute Pressure Sensor, meaning it not only registers manifold pressure in absolute (PsiA), it is also your Inlet Air Temperature Sensor.
Pro Tip: “Boost” is not Manifold Absolute Pressure. Boost is Manifold Relative Pressure because it is “Relative” to atmospheric pressure
So what happens if I use this style of boost tap that relocates the TMAP?
The TMAP being spaced out of the air flow stream actually causes the sensor to read temperature incorrectly as seen in this data log plot below.
The Ambient temperature is the air temperature outside as picked up by the sensor located by your intercooler behind the front grille. This sensor said it was an 82° day outside and while maybe 20-30° degrees above ambient is fairly normal, 158° Inlet Air Temperatures are not!
The sensor being removed from the airflow caused it to read abnormally high which will trigger ignition timing retard causing the car to make less power.
What style of Boost Tap should I buy?
If you absolutely HAVE to have a boost tap, we suggest boost taps that do NOT relocate the TMAP at all. A great example would be the APR part as shown below available at EQTuning.com. Click Here
How exactly does my car make less power due to this false temperature reading?
Ever since the dawn of Electronic Fuel Injection (EFI) the engine computers have always had to know what the air temperature was for various different compensations to help the car run at its best regardless of the weather conditions.
One of these tables is an Air Temperature Vs. Ignition Timing Compensation table.
The example below is from a 100% stock 2005 Subaru STi viewed in EcuFlash Software.
You can see that this factory Subaru tune pulls up to 8.09° of ignition timing as a protection mechanism
Below is another example from a Mk5 Volkswagen GTI views in Syvecs SCal Software.
You can see based on the example’s above that everything from a completely stock tune to a full blown standalone engine management system retards ignition timing as the Inlet Temperature increases.
Timing is simply one means to protect itself from damage, it may also add additional fueling to try and cool down the charge to combat the high temperature.
As a rule of thumb for all car enthusiasts, we know Cold Air = Good. Hot Air = Bad.
The ECU will do its best to protect your engine from damage during hot conditions by reducing its overall engine power with the ignition strategy shown in the tables above.
Without accurate information from all of its sensors, the ECU cannot perform its most crucial task of keeping your engine within safe operating parameters.
10° of ignition retard as seen in the Mk5 GTI example is definitely something you’ll feel in the drivers seat going down the road.
Thanks for joining our Tech Tip and we hope this helps you!
– Dave
