For a couple of years, I’ve been reading fellow enthusiast cautions about using the Cobb “bundled” tunes that come with the Cobb Accessport. These are tunes that may be preinstalled on an Accessport, or can be freely downloaded from the Cobb website.
People have stated: It’s not “refined”. It doesn’t make good “power”. Cobb tunes are garbage. It causes too much “knock”. “you’ll shoot your eye out” – A Christmas Story reference.
Despite having an Accessport I’d never tried the Cobb bundled tunes, I had gone straight to a Custom tune and never looked back.
Seeing these suggestions being given by fellow enthusiasts to another enthusiast who was asking for advice was the incentive I needed to try out the Cobb OTS file.
“Never run a Cobb tune…” It doesn’t get any more cautionary than that. I wanted to see what the issue with the Cobb tune is.
To collect data on the Cobb IS38 tune for a GTI I conducted five (5) full-throttle pulls in third gear starting around 1700-2500 rpm and letting up around 6400-6600 rpm. The ambient temperature was 77 F.
To make a comparison I found a data log from using the EQT Custom IS38 Tune that I have for this Shuenk IS38+ turbo. The EQT data log contains six pulls and was recorded on a day when the ambient temperature was 82 F.
The GTI is equipped with an HPA High-Pressure Fuel Pump (HPFP) which is not identified as a hardware upgrade the Cobb tune is made for. The EQT Custom Tune was developed using the aftermarket HPFP
The following charts compare the Cobb OTS IS38 tune for the GTI and a Custom EQT tune for a GTI with IS38+.
Solid lines are Cobb, dashed lines are EQT.
The Cobb tune builds boost to 24 psi and holds that to around 4500 RPM when it starts tapering, ending around 20 psi at 6500 rpm.
In comparison, the EQT tune builds boost to 28 psi and begins to taper around 5200 rpm, ending around 23.5 psi at 6500 rpm.
The next chart compares the Ignition Timing Angle:
The EQT tune consistently operates with an additional 4 degrees of timing advance, with the difference closing to around 2 degrees by 5900 RPM, and then opening again nearing the 6500 RPM conclusions.
The next chart shows Timing Retard with the Cobb IS38 tune. During the six pulls, there is one instance of one cylinder having a timing retard just over 3 degrees.
Next is the EQT tune Timing Retard. The maximum value recorded during six pulls is 2.5 degrees.
Notably, the EQT retards timing more near the redline whereas the Cobb tune retards timing right as boost peaks.
The last chart compares Air to Fuel Ratios:
Note: The line types are reversed in this chart, Cobb is the dashed lines and EQT is the solid lines.
During the initial boost build-up, the Cobb has a slightly richer AFR. The EQT tune then becomes slightly richer, with both tunes having a similar AFR by the 6500 RPM conclusion.
The Cobb off-the-shelf IS38 tune for the Mk7 GTI was flashed to my IS38+ equipped GTI to record engine operating data during a series of full-throttle third gear pulls.
The goal was to collect data to help me to understand the cautionary comments that I have read from other enthusiasts about using the Cobb tune.
I was not surprised to find that the Cobb OTS tune operates with less Boost Pressure and a lower Ignition Timing Angle than the EQT custom tune. Differences in AFR do not seem dramatic to me.
The Cobb OTS IS38 tune doesn’t perform in a way that causes me concern. I expect an OTS tune, that would potentially be used on any GTI with an IS38, would be more conservative than a custom tune.
Cobb specifically mentions custom mapping as a benefit of their product, though that is not a service they provide, they do provide a directory of Cobb dealers, such as EQT Tuning and Stratified Automotive Controls, two tuners I have custom tunes from.
Based on the data I recorded, the free Cobb OTS tune does make less power than the $500 EQT custom tune (surprise!).
Virtual Dyno was used to process the data logs and return an estimate for wheel horsepower. The average of the peak wheel horsepower for the respective tunes is shown in the box plots:
On average the EQT tune produced 50 whp more than the Cobb OTS tune.
Note: As a reminder, the GTI used in this test is equipped with the HPA HPFP, which is not an upgrade the Cobb IS38 tune is designed for. It may be possible that the pump is adversely affecting the outcome, though my guess is this is unlikely, and even if it is having an affect, likely would not explain the entire power difference.
Although the Cobb OTS tune produces less wheel horsepower than the EQT custom tune, nothing I observed would lead me to suggest another enthusiast never use a Cobb tune, that recommendation seems unwarranted.
This file may no longer be hosted at the Cobb website. This is the Cobb USDM IS38 tune for my Mk7 GTI.