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teamjapspeed February 27, 2015 News, Products Comments Off on JAPSPEED GT86: DECAT MANIFOLD & DOWNPIPE TESTING

Since kitting out our GT86 with our Japspeed Sports Cat Manifold and Sports Cat Downpipe we’ve enjoyed increased power output, improved throttle response and of course, a much more enjoyable soundtrack!


The benefits of the Japspeed Sports Cat exhaust components is that they stay on the right side of the law, as by fitting them you will still pass an MOT’s requirement for a catalytic converter to be present (if the car was supplied with one from factory) and they will also keep your emissions levels in check.

However, since fitting the Sports Cat Manifold & Downpipe to our own car, we’ve been frequently asked whether or not we will be producing a more simplistic decat version of both parts. We subsequently released our GT86 Decat Manifold and GT86 Decat Downpipe and, in between jobs on our new 370Z drift car build, got them both fitted to our car.


Of course, there’d be no point in fitting them without performing the appropriate tests afterwards, so the car was taken back to AC Speedtech for another ECUTek remap, to see how the Decat components responded and affected the car’s performance.


The car began its mapping session on the dyno before being taken out on the road for some live road mapping. As Andy from AC Speedtech explained, “the car was first mapped on a dyno, before being taken out on the road so that the map could be refined for real world driving. One area I addressed once the car was on the road was the timing, which I had advanced on the dyno. Out on the road however, it was apparent that we needed to ease off the timing a little to ensure that the car would still perform at its best in all conditions, regardless of the ambient  temperature and other factors that can affect performance. In essence, I made sure that there was a suitable safety margin in place to ensure engine reliability and longevity.”

While mapping the car, adjustments were made to the variable valve timing and ignition timing. The timing was advanced around 3 degrees throughout the mid range and top end of the rev range, but it was also advanced to a lesser degree at the bottom end so as to ensure that drivability wasn’t affected.

The result was a much stronger torque curve mid range, while the lack of low end power that was demonstrated in our previous dyno graph has now also been seen to.

As Andy explained, he believes the infamous factory torque dip is caused as a result of the stock manifold, downpipe and catalytic converters, along with Toyota/Subaru’s requirement to keep the car’s emissions below a certain level at a specific rpm range. While our Sports Cat Manifold and Downpipe helped to eradicate the issue, they were still a tad restrictive and thus left us unable to remove the torque dip without resorting to a more extreme map (with a much lower bottom end torque figure).

By fitting our Decat items and mapping the car accordingly the car’s torque remains strong in the mid-range while holding torque all the way through to the rev limiter. From enjoying a number of spirited drives in the car we can’t help but agree with what the dyno said – the car’s fun factor has improved ten-fold and it’s an awful lot easier to give it some abuse without dropping off the power band. We also tried a few sixth gear pulls from low speed on the motorway, just to see how the car would respond. While before it would have bogged down at low rpm, now it was pulling like a train right through the rev range.

After mapping we took the car back to John Clarkson Autos who, if you remember, is where we originally took the car for dyno testing when we first fitted the Sports Cats andAEM Induction Kit. We returned to John Clarkson earlier today purely to ensure that the car was dyno’d on the same dyno that it always has been, so as to ensure that our figures are relevant to one another and show true power and torque figures.

The end result was a peak power figure or 225.6 bhp, with torque peaking at 176.1 lb/ft at 6504rpm.


The graph below shows the comparison between the most recent dyno chart (with the Decat components, Japspeed Cat Back Exhaust and AEM Induction Kit fitted) and the previous dyno chart, where the car was fitted with the Sports Cats, Japspeed Cat Back Exhaust and AEM Induction Kit and recorded a peak power output of 217.7 bhp and a peak torque figure of 173.7 lb/ft. More importantly though, you can see the massive difference between the torque curves (the red line shows the previous graph and the blue represents the latest results with the Decats fitted).


All of our Japspeed Toyota GT86/Subaru BRZ exhaust components are available from our online store.

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