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.

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.

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
The 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!

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:

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.

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. 

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?

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.

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.

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.

The Cobb AccessPort unleashes a continuous bombardment of information. The screen can be setup to display 6 live engine statistics with each showing min/max figures. This is just the surface; with the touch of a button, it’ll begin recording a variety of parameters of your choosing. That’s a lot of information that was previously inaccessible. What does this mean? What’s will this information tell me? I’m glad you asked!

One of the more fun things to do with all this data is to plot your own dyno curves based on full gear pulls! Do you know when your optimal shift points are? Is it worthwhile revving the engine deep into redline? Where could I gain time? When does my turbo really spool? What’s my baseline power? How much power was gained from the last mod or tune? Once you see the power curve, these questions are easily answered!

So let’s talk about V-Dyno. Just like an in-ground dynamometer it measures the force (torque) applied by the engine to accelerate your car. Let’s say you go out driving, it’s the middle of boost season, and the car is running like the prized thoroughbred it was built to be. Everything feels good, ear to ear smile, but then, what do you tell your buddies? My butt-dyno says I make 347WHP? They’ll laugh you out of the stable. Instead take a data log. Let V-Dyno crunch the numbers, and have a printout that your car is actually making 347WHP! If the datalogs you take are consistent (as described later in the article) and you setup your car’s parameters correctly, Vdyno can be very accurate and repeatable and we have correlated our results to in-ground dyno data.

So how does it all work?

V-dyno analyses engine speed (RPM) and calculates acceleration performance based on the rate of change of engine speed in a single gear. It factors in tire size, weight of the car, gear ratio, ambient air temperature and frontal area of your car and a number of other things such as occupant weight. You’ve probably heard “oh but that was power on such and such dyno so the numbers are skewed this and that way”. Well, that’s also built in. You can select your dyno type and correction factor. There’s a large amount of preset cars in their database but we’ve build a good profile for the VW GTI. You can build the same profile in your own installation of Vdyno using the information below.

How do I collect data?

First, the AP has to be setup to record all the useful parameters. For Vdyno you need to log at least AFR, Accelerator Pedal Position, Boost Pressure, and RPM. You can log others as well, but these are the ones that are used in the calculation. Then, while driving make sure you are in the Gauges screen. The car will need to be cruising in 3rd or 4th gear, around 2500 rpm. Click the center button to start recording. Wait a second until the screen on the AP starts refreshing again, and then stomp on the gas as if it were an on/off switch. Take it to redline, release the gas pedal, shift up, then click the center button again to stop recording. 

Your final results are only as good as the collected data. Here are a few hints to getting repeatable V-dyno results:

• Make sure it’s not windy
• Keep ambient temperatures consistent
• Always collect data on the same stretch of road
• Empty your trunk
• Don’t take passengers along
• Gas tank half full
• Do 2 or 3 passes
• 4th gear logs are better than 3rd
• Stay safe!!!

I have logs, now what?

Transfer them to your computer using the AccessPort manager. Download Virtual Dyno, and set it up with the correct profile for your car. Match the columns up. Then drag and drop the logs in, wait a second and you’ll see your power curve.

But my power curve looks strange.

There are a number of things that will skew the data and so the end result will be way off. If you see strange humps, bumps, and non smooth curves, chances are that the car lost traction, you had clutch slip, there was a big bump in the road or something else went wrong. If you come up with something odd and want to chat about your setup (and how to get more power) drop us a line!

Happy Vdyno-ing!

We have been tuning the MK6 GTI using the COBB AccessPort and working with a bone stock vehicle to start. It is a 2011 3 door DSG model. The car is very pleasant to drive and the first step to improving the experience is to add power safely and improve driveability.

With a bone stock car (what is known as Stage 1) which just includes a reflash of the ECU you can get the majority of the power that the stock turbocharger can offer from just the tune. We started by ramping up performance from stock by adjusting boost, timing, and watching all vital engine parameters such as knock, air fuel ratios, charge air, and engine temperatures. A lot of seat time and stress testing of the calibration backs up its resilience. The car drives as stock until pushed and when it is pushed it delivers a smooth rush of torque that doesn’t fall flat in the upper RPMs.

On 91 octane fuel we were able to add a peak of 32whp and 69 ft-lb of torque over stock and 24whp and 48 ft-lb of torque over the COBB OTS tunes. The overall area under the curve gained however is what makes the car very enjoyable to drive.

The torque ramp from low in the RPM range makes the car quite eager to accelerate and makes passing a breeze. The engine is not very knock prone which is great to see and even on 91 octane fuel we were able to achieve very consistent results.

Before adding any bolt-ons we decided to increase the fuel octane and search for what we can achieve with an E85 blend. Often an E85 blend and tune will net a very nice power increase. With the COBB AP it is also a breeze to switch between different fuel blends and tunes within minutes.

On most direct injection cars, blending some E85 with premium pump gas fuel allows you to tune very close to MBT (mean best torque) and maximize both boost and timing (especially on a stock turbo). For this, we strapped the GTI down on the dyno and mixed 4 gallons of E85 with E10 91 pump gas fuel.

This achieved an E30 fuel blend. On this blend approximately 10% extra fuel volume must be delivered and both the high pressure fuel pump, low pressure fuel pump, and injectors are up to the task of delivering the needed fuel on the OEM turbo.

With the E30 blend we were able to achieve an additional 15whp and 26wtq on the bone stock car.

Boost was maintained at similar levels but we were able to push timing to mean best torque (MBT) meaning that we were able to add timing until we did not gain any further performance while remaining knock free.

At these power levels we are actually leaving some torque on the table between 2700 and 4000 RPM and this is why boost levels remained similar. This is because we have reached the programmed torque limit for the DSG transmission in our test car. This torque limit is set to 350nm and you can see that we are riding this limit in the graph below. The turbo is able to supply more boost and in order to achieve higher torque values in the lower RPMs, a DSG reflash is needed (and coming shortly). The DSG hardware is able to handle the extra torque; we just need to program it as such!

Even without a DSG flash, the car has a lot of potential with just an ECU tune. Adding flow parts will give us some top end performance and we will see an even flatter torque curve. As we continue to develop for the GTI and have applied our years of direct injection tuning expertise, we are now offering pump gas as well as E85 tunes for the MK6 GTI which can either be packaged with a COBB Accessport or bought individually. We are also going to continue updating you on our development and tuning progress. If you have any questions, don’t hesitate to Contact Us!