Originally Posted by Warwick Morris

Apologies for the long post. I thought now was the time given the 425 hp (420 bhp) 718 Clubsport is out now.

I took delivery of my Guards Red GT4 in September 2016; and yes it did loose 3rd gear and the gearbox was replaced. The car went well enough but like many here, I found it lacking; I am a track-focussed driver. So suspension setup came in for attention first. You all know the story; rear toe links, front end kit, spherical bushings, CMS roll bars and DSC suspension module. F -3.5 and R -3.0 and the car finally handles well but there is still a bit more understeer than I’d like; but with limited tyre choice (for 19” and 20” wheels), it’s probably as good as it gets.

An IPD plenum and 82mm TB came early in the piece as well as a single mass LWFW and new clutch. The power has never troubled the LSD in my experience so left that alone. To be frank, I can’t say the IPD/TB did much for the power on their own so I then looked at tunes. My first go was with a well know CA outfit selling me one with the updating device. I started dyno testing the car at this point to track my upgrades. A Porsche specialist outfit I’ve dealt with for 13 years did all the dyno runs. Their dyno is in an enclosed room and runs near constant year-round temperature (though humidity will vary of course). The company is a respected tuner who have developed custom tunes for many Porsches including the winning car in the 2018 World Time Attack event. Their dyno produces consistent results over time and has a well-established multiplier of 1.31*RW power to get crank power.

In short I undertook the following successive engine mods.

1. IPD/TB noted above

2. IPD/TB with off the shelf West Coast USA tune.

3. Custom tune by my local specialist (they did it to generate a tune they could sell to other customers)

4. Added Fabspeed catted headers but with the same custom tune

5. Installed an X51 kit supplied by Porsche plus South-East Coast US company intellectual property for the install and their latest custom Cobb tune (so I now have an EVOMS and Cobb downloader!). This work was undertaken by another local Porsche specialist who have a particular interest in all things Cayman. They used another dyno which confirmed all the correct operation of the “flaps” and we smoked tested the plenum for seal – all good. So new heads (with new cams) and correctly operating X51 intake/plenum. The results were disappointing so we re-checked with my normal dyno which confirmed the first results

6. After some alarm at the results both shops (they sort of cooperate even though they are competitors) agreed that the X51 plenum could be slowing down the intake airflow compared with the IPD/TB. We removed the X51 plenum and left the X51 heads. Then re-instated the custom tune. While results were much improved, torque and power was down a bit below 5800 rpm and quite a bit around 3500 rpm. Maximum power was quite good but the car would throw the occasional CEL with the fault being “running too rich”.

7. Not satisfied. I asked my custom tech tuner to delve deep into the ECU and try to resolve the issues. He located an X51 GTS ECU dump and found over 20 different maps were changed between the GT4 and GTS including even things like the cruise control and oil thermostats. As I understand it, they found that the GT4 Siemens (as apposed to the familiar Bosch unit in the GTS) computer has a number of range limits that do not permit the engine to operate at the expanded volumetric efficiency with the new cams. Having copied over the required code from the GTS file, it was then possible to tune the ignition, fuel and cam maps for my GT4. The actual tune then only took 5 hours on the dyno but the ECU code changes took 45 hours.

So what does all this mean.


Attached are 4 plots with multiple data and some of the runs are repeated. As stated, this particular dyno runs a 1.31 RW to FW factor for most Porsche cars. Plots 2 (blue trace Feb 18) and 4 (green trace March 18) show 2 different stock cars with zero mods; both cars were well run-in. Peak power is 301 and 299 bhp; call it 300 Wbhp=393 bhp. Porsche quote a stock car at 380 bhp (385 hp or PS). Allowing for Porsche traditional conservativeness and the excellent quality 98RON fuel we get in Australia, around 390 at the crank seems reasonable for a stock car. Not all the descriptions on the plots are accurate but the dates (in day/mth/yr format) are.



So here is a summary of what I have found. I stress that this is what I have found and others may have different findings. BUT I do stress that all the testing is by disinterested parties paid by me.



Peak power levels are (converted crank bhp in brackets):

1. Standard GT4 300 bhp (393) – plots 2 and 4

2. IPD/TB no tune 315 bhp (413) – plot 1

3. IPD/TB with W Coast USA off shelf tune 317 bhp (415) - plot 1 – Note the HUGE dip from 3500 to 4000 rpm

4. IPD/TB with local custom tune 319 bhp(418) – plot 1 – we increased the rev limiter to 8000 which is actually still quite conservative for this motor.

5. IPD/TB with Fabspeed plus local tune 320 bhp(419) – plot 1 and 2

6. Please note that at this point, quite a lot of driving was done and one of the Fabspeed cats partially collapsed so we removed all the ceramic innards and they were now empty like race headers with a small expansion chamber. We had welded on small extension tubes for the post Cat O2 sensors so no CEL’s.

7. X51 kit with heads and intake+ Cobb 334 bhp(438) – plot 2 – Note the huge loss of power and torque below 7000rpm even compared with a stock car. A terrible result. When driving on the road it feels fast becuase of the zing at the top end, but so much power is lost everywhere else it is quite a bit slower.

8. X51 heads but IPD/TB tune as per 4/5. 334 bhp(438) – plot 2 – note recovery of nearly all the power and torque from 4, then the increase from 6200rpm and same power as with the X51 intake/plenum

9. X51 Heads + IPD/TB plus GTS ECU hack 342 bhp(448) – plot 3 – note on plot 3 recovery of all the power and torque from 5

10. Plot 4 is a comparison with 9 above and a stock car. Very impressive gains from 3000 to limit and overall crank gains of 55 bhp (14%)

I have not track-tested the final results, but in Nov 2016 my top speed on Conrad straight at Bathurst was 252kph and in Nov 2018 (with X51 heads and IPD/TB but before we cracked the ECU) it was 263kph (at rev limit in 4th just turning into Castrol Chase). The car was competitive with 3.8l 997 GT3s. 991 GT3s will still walk away from it.


This has cost a ridiculous amount of money but I have enjoyed the process. Would I have been better off buying a 991 GT3….yes but the GT4 will stay as my track rat. I am fortunately taking delivery of a new 991.2 GT3 RS hopefully in February but will be leaving it as a road warrior and sometimes gentleman’s track day car.

Originally Posted by IPSA

First thanks for sharing this valuable info with the community. This is the type of info that sets RL apart from the other cites that focus on other things.

I'm intriqued by the x 51 manifold issues you identify. Lately there seems to be some info from some reasonably reliable sources that the x 51 cam and heads may be overkill and the magic is in the x 51 plenum. If the factory heads are good to beyond reasonable HP targets for the platform and the cams only open up the extreme top end, it would be fair to believe that the x 51 intake is adding a decent amount of the additional hp and tq. However based on your post it seems the x 51 plenum may be persnickety and also not as helpful as some suggest.

Do you have any more info or thoughts about the x 51 plenum versus the ipd?


Thanks again.

Originally Posted by Warwick Morris

What you state is opposite of what I found. I obviously did a lot of research before I went ahead and others have stated the X51 kit looses some mid range but gives more top end; I found the plenum actually robs a hell of a lot of mid range and the top end boost is all down the heads and cams.

What I did not expect was how much mid range was lost compared with the IPD/TB.

When we removed the X51 plenum and intake assembly and went back to the IPD/TB plus just X51 heads/cams, with the ECU tune sorted, we ended up with the best of both worlds. i.e. we kept (and increased) all the mid range and added all the top end from the full X51 install (compare Plots 2 and 4 for proof).

From all of this I can only conclude that the X51 intake only detracts from the performance of the GT4 and that the IPD plenum is a very effective intake system (you can see this in Plot 1 comparing a standard car to my car with just the IPD/TB mod and no tune). So my conclusion is that all the top end power increase with the X51 kit comes form just the higher lift and extended duration cams (and of course the polished inlet tracts in the new heads).

Also note that the whole purpose of variable inlet tracts is usually to increase mid range by lengthening the tracts (just look at the operation of variable length inlet tracts on any superbike) which is what the X51 plenum is supposed to do but actually fails to do so in the GT4 application.

When you buy the X51 kit you get an amazing mass of parts (it is a very big crate!). My exploration has shown you only need the new heads and can make a coffee table out of the beautifully cast plenum and intake tracts. I priced just the heads and cams on their own from Porsche and they actually cost more than the full kit so you might as well get the big box anyway.

Originally Posted by GoKart Mozart

Awesome info. Thanks for sharing. I am considering the IPD Plenum/TB/Tune and from what you discovered it does help to smooth things out in the middle of the torque curve.

Just to clarify my objective. I am fully aware that headers are the best way to add power on this car, but because of CA smog and Laguna Seca sound restrictions, I'm not sure I want or can go there without a lot of hassle. While we all would like to add as much power with mods as possible, my main issue is the dip in torque in the mid range. If I could minimize that without touching the headers, I think I would be pretty content. Coming from a S54 M3, where the power/torque builds linear, the dip between 3-5k in the GT4 seems very odd. Yes, it has much more low end torque than the S54, but then disappoints briefly by falling off.

Originally Posted by Diablo Dude

Just an FYI as far as going the header route in CALIFORNIA with regards to noise.

We have a new law in CALIFORNIA for 2019 that winds up being a $1,000 mandatory fine for every time a police officer pulls you over and finds that you have modified your exhaust to emit more noise, or excessive noise. It is no longer a correctible offense, It is no longer a "fix-it" ticket. It's a mandatory fine now. Moreover, it appears as though law enforcement has been given a "grant" to focus on enforcing this new law... committing man-power to their cause.

Originally Posted by Reborn996

I am confused on the IPD plenum with GT3 TB results. There have been multiple posts on the forums that said there was only negligible or no gains with those. Even Dundon Motorsports said the plenum and throttle body were not a restriction until high 400HP output for the GT4. So what is the true story on these items ???♂

Originally Posted by groundhog