MK7 GTI Step Colder Spark Plugs and Compression Test

When turning up the power on a vehicle such as we are doing via the COBB Accessport, inevitably more heat is generated. With more heat in the combustion chamber, you may start to move outside the optimal heat range of the OEM plugs and thus have an increased risk of pre-ignition. For more in-depth spark plug tech check out our spark plug article here. Because of this our red development MK7 GTI is getting a fresh set of plugs.

Knock, Pre-Ignition and Heat Range

Do not confuse pre-ignition and detonation (or knock). Knock is what the knock sensor picks up and you can datalog this as ignition corrections during a pull. Knock is caused by multiple flame fronts starting AFTER the initial flame front was started by the spark plug. Replacing the spark plugs with a step colder will not prevent knock – knock is a result of fuel used, tune, and temperatures – and I mention this because we get asked all the time if replacing the plugs will reduce knock. It will not. However it will reduce the CHANCE for pre-ignition and pre-ignition is much more serious than knock and more damaging should it occur. The knock sensors can’t pick up pre-ignition because pre-ignition happens when the air-fuel mixtures ignites BEFORE the spark event and will usually result in a hole in a piston or broken rod. A colder spark plug helps this because it pulls more heat into the head of the engine from the combustion chamber and is less likely to act as a point of ignition before it is triggered by the ECU.

Generally speaking when starting to tune a car, we go for a step colder plug than factory to avoid pre-ignition. Going too cold can prevent the plug getting hot enough to burn off deposits in daily driving and can result in fouling and misfire – so a compromise must be struck for a street car. For the MK7 GTI we chose the popular Denso IKH24KT.mk7-gti-stratified-spark-plugs-compression-1The Install

Installing the spark plugs is fairly straightforward. You will need to remove the engine cover, the coil pack harness, and then pull each individual coil pack. 


Once the spark plugs have been pulled it’s a good time to give them a look-over. Our plugs were very clean with no speckles or excessive deposits. They were burning cleanly without obvious evidence or oil consumption or knock. The orange deposit is due to running some race fuel additive (and normal if doing so). You can see that the new step colder plugs have a slightly recessed tip which is a feature of colder plugs.


Compression Test (while the plugs are out)

If you have a compression tester handy, this is a great time to perform a quick compression test of your GTI. Very little is required to do so. You simply plug the compression tester in each spark plug hole and crank the engine while holding the accelerator pedal to the floor. I crank until the compression tester needle stops rising and this is around 12-15 cranks per cylinder.


What you are looking for in terms of the compression test results is consistency across all the cylinders. The absolute number will depend on the tester used and altitude above sea level. Our development vehicle was very healthy with a 190psi reading across all four cylinders.


Before putting the new spark plugs in and torquing them down to 18-20ft/lb it is worth checking their gap using a feeler gauge. We recommend 0.024-0.026″ (0.60-0.66mm) to prevent misfiring under heavy load. The gap will naturally open up over time as they wear out and heat cycle. The new Densos did not require any adjustment of the gap.


Once the plugs are back in along with the coil packs and harness, you’re done! You’ve now checked the health of your motor with a quick compression test, looked at how clean it is running via inspecting the old plugs, and freshened up the ignition system with new plugs that are more tolerant to tuned, higher performance driving.

The Stratified Team

Does a leaner Air-Fuel Ratio result in more power on the MK7 GTI?

Calibrating an engine revolves around solid data analysis and testing of any hypotheses. In the case of the MK7 GTI, the OEM ECU calibration commands a relatively lean air-fuel ratio (AFR) target during wide open throttle operation. For the majority of a pull, unless one of the temperature failsafe thresholds are triggered, the ECU will command a 14.7 AFR tapering to mid 13s by redline.
It has to be understood that these targets are within the context of the OEM tune. The boost pressure targets, ignition timing targets, and variable timing targets are all taken into consideration when selecting the appropriate air fuel ratio. On top of this, economical operation is a strong requirement for the OEM.


When pushing the car beyond the factory setup in order extract maximum effort from the engine, all variables including the air-fuel ratio must be reconsidered and retested.
When commanding a higher output the engine requires more in-cylinder cooling in order to avoid detonation (knock) while maintaining maximum performance. This is the case with other internal combustion engines we have worked with but we had to put the hypothesis to the test for the EA888 Gen3 as well.


The COBB Accessport gives us a very powerful analysis and datalogging tool at a high sample rate of 12hz (that means we get 12 datapoints per second for each sensor or actuator signal we log)
We tested this hypothesis in a controlled environment where we only varied air-fuel ratios. We made sure that this was done on a hot day with 91 octane fuel that is knock prone. This was done such that we can clearly see the effect of the air-fuel ratio without any other variables interfering.
Here you can see that the richer run did produce more power. The question now is why? We looked at the knock traces from both runs for to get an answer to this as boost and temperatures were identical. The graph below answers this question.
As you can see, the leaner AFRs do not offer the necessary in-cylinder cooling to prevent the engine from knocking. Yes, the engine knocks in both runs because we have setup the test as such to ensure we see a contrast between the two sets of data. However you can see that the richer air-fuel ratios result in a diminishing knock trend which in turn results in the engine making more power and staying safer!


Finally, we have to ensure the data is repeatable. For this we have two back to back runs of the richer tune. You can see that the data is repeatable within 2-3whp which gives us further confidence in the test results
vw-gti-mk7-stratified-tune-dyno-repeatableThe conclusion to all this – when calibrating a vehicle outside of the OEM parameters, all variables and hypotheses need testing in order to optimize the new set up.
Happy Tuning,
The Stratified Team

Upgrading the Volkswagen GTI FMIC

I will preface this with the fact that the OEM front mount intercooler (FMIC) on the VW GTI is very good for an OEM unit. Here’s a little comparison between the MK6 GTI and the OEM unit on a MK3 Focus ST. vw-gti-vs-focus-st-intercooler-efficiencyWhen you are looking at the performance of an intercooler you want to look at the difference in temperature between the beginning and end of a wide open throttle run. This change in temperature will tell you whether the FMIC is maintaining a cold air charge or whether it is struggling. The OEM Focus ST FMIC in this case is clearly struggling in comparison to the GTI and this comes down to the size. The GTI/Golf R FMIC is very large by comparison – having the same frontal area as the radiator!

Our COBB Tuned development GTI received its final piece to being fully bolted with the F23T turbo. The Golf R FMIC. The install is quite straightforward. There are a lot of screws to take out of the front end to get to the FMIC (bumper and crash bar must come out) as it is between the AC condenser and radiator so if you DIY this install make sure you keep track of these. Having a second pair of hands and a couple of floor jacks to support the cooling stack as you take out and put in the FMIC helps.


I really do like the design of the cooling stack. Lots of airflow in the front end of this car. The R FMIC has a core that is 5mm thicker and it has 31 rows versus the GTI 27 rows.


Some data comparisons indicate that on a 4th gear pull the change in temperature from the beginning to the end has dropped from 17.5* F to 9.5* F with the R FMIC. A second advantage to the better flowing and larger R FMIC is a smaller pressure drop across its core. Due to the lower pressure drop and lower temperatures, at the same boost pressure the turbo is more efficient and we also gained around 4-5 g/s of airflow.


I do recommend this upgrade for stock and upgraded turbos alike. The GTI FMIC is quite good but the R FMIC is a direct fit, relatively inexpensive, and will be worth around 5-10whp and be more resilient during multiple pulls.

VW GTI MK6 Cobb Turboback Installed

Last weekend we had the opportunity to install the COBB turboback exhaust on our Stratified Tuned MK6 GTI. This is a very nice piece of kit and it consists of a full 3″ exhaust from the turbo back replacing the OEM 2.4″ exhaust.

VW GTI exhaust tips COBB Stratified

This short video overviews the constructions of the exhaust and its features:

Now … what about performance? Before getting to the nitty-gritty; where is the performance gained with the turboback? There are two avenues actually.

The first is the drop in post turbine pressure. By replacing the two OEM catalytic converters with a single high flow unit and enlarging the piping diameter from 2.4″ to 3.0″ you are dropping the pressure right after the turbine of the turbo. This in turn increases the pressure DIFFERENTIAL between the exhaust gases before and after the turbo. The end result is that the turbine has more energy input and that in turn means faster spool and more air delivered to the engine via the compressor which is now spun faster throughout the entire rev range.

VW GTI downpipw COBB Stratified

The second area where performance is gained is a drop in pumping losses. The engine must pump exhaust out of its cylinders and that is energy that is wasted. By making the process easier, the engine doesn’t waste as much energy doing this and that translates into more power making its way to the wheels.

After verifying repeatability and ensuring that all other variables are kept consistent (temperature, 91 octane fuel) the downpipe and catback gained +13whp and +18wtq (red line). This is very much in line with our expectations. A catless downpipe will add a little more but at this point it’s the turbocharger that’s the biggest restriction in the exhaust stream. The car previously had a Stratified Tune via the COBB AP, COBB intake and diverter valve replaced with no other modifications
VW MK6 GTI Stg2 91 Dyno StratifiedThe exhaust sound is very much a personal preference … so I will another video do the talking.

What’s next? Time to get more consistent torque delivery with a look at DSG tuning. After this, revisiting E85, intercooling, AND and then moving onto a larger turbo! Stay tuned!


Volkswagen / Audi Diverter Valves: How they work, the limitations, and what you need for your TFSI

VW has been using an electronic diverter valve (DV) on their turbo TSI/FSI engines for a long time. This is a hotly discussed topic in the community as the valves sometimes fail and VW has several revisions of the valve.

As many of you know the purpose of the diverter valve is to vent excess boost when the throttle is closed (you lift off the accelerator or change gear). Without venting this excess pressure, the turbo experiences surge which is actually damaging to the turbo. You can hear this as a fluttering sound.

Due to this it is very important for the DV to function properly. By the same token, when the throttle is open and you are building boost, this valve must stay shut and not vent boost from the high pressure side to the low pressure side. Venting this boost under wide open throttle causes a boost leak and a drop in power.

Let’s start with the basics and outline the components of the valve:

VW Diverter Valve Operation Diagram

The valve in the image above is known as Rev G of the valve and it is found in the earlier Audi and VW cars. Before going any further we have to discuss just how the valve works. All the electronic valves work on the same principle so this applies to all of them.

The light blue piston seal at the bottom of the valve presses against the high pressure side opening inside the compressor housing. This seals the high pressure chamber from the low pressure chamber that connects to the intake right in front of the turbo. This connection between the high and low pressure is all inside the turbo compressor housing and the DV either allows air to pass or not between the two pressure areas.

VW Diverter Valve Operation Explained

The top of the valve houses a solenoid that pulls the piston up when energized opening the valve. The spring helps return the valve to the sealed position and also keeps the valve closed as boost builds up.

Here’s where things get interesting. The spring is much too soft to prevent boost from pushing the valve open. Because of this VW has designed a set of holes at the bottom of the piston shown below. VW Diverter Equalization Holes

These opening allow boost pressure to build insider the internal valve chamber. This chamber is sealed from the low pressure side by the orange diaphragm. This pressure that builds up inside the valve equalizes the forces on the valve and keeps it shut under boost. Once you lift off the throttle, the solenoid pulls the valve open (with a good amount of force and speed) releasing pressure to the intake from both the charge piping as well as the internal valve chamber. The spring then returns the valve to the sealed position.

So how do these valves fail?

VW Diverter Valve Tear

One obvious failure mode is a tear in the orange diaphragm. Once the diaphragm is torn boost will leak from the internal chamber to the low pressure side and the piston itself won’t be held shut as tightly.

VW/Audi have built a more robust valve with the Rev D pictured on the left below. This valve does away with the diaphragm that is prone to failure. There is more to it than just this.

VW Diverter Rev D vs Rev G

Boost leaks and undesirable valve “lifts” happen during transients. That is when you are just building boost, or when you are quickly back on the throttle after a gear shift. During times like these one of two things can happen:

One is that the valve does not return quickly enough to the seated/sealed position. The stiffness of the spring along with the mass of the piston determine this speed. The Rev D valve has a stiffer spring and a faster returning piston.

Secondly, the piston can lift if the internal valve chamber does not fill quickly enough with pressurized air due to the pressure imbalance. You will notice that the Rev D valve has more openings (6 versus 4) than the older valve allowing for faster pressure equalization.

Keep in mind that if the valve does not seal or lifts when it’s not supposed to, you are likely to not just experience low boost, but also a low boost code (P0299). You are also likely to hear a high pitched noise under high boost if you have an aftermarket air intake installed.

VW Diverter Location

If you experience the error code, intermittent boost, or loud whistling, it’s time to check your DV. Upgrading to the Rev D valve can address some of the reliability issues of the older diaphragm based valve. However the Rev D also has potential issues. One is that it doesn’t have a good seal against the compressor housing potentially allowing some boost to leak. Another is that the spring may still be too soft when you are building boost in excess of 25psi quickly.

There are several aftermarket options out there. The key is to have the fastest acting valve with the least amount of surge that still seals. The ECU monitors pre-throttle boost and can also trigger fault codes if the valve is not fast enough or too stiff and there are pressure spikes in the charge piping so keep this in mind. As always, it’s all about building the right solution for your specific needs.

If you have any questions or would like to discuss options and tuning, don’t hesitate to Contact Us.