GT4 or modded Cayman R - Page 9 - Boxster/Cayman

2,410 posts

172 months

Tuesday 27th January 2015

So back to that engine....

It was an interesting thing running that engine after it was built. The experience gives me some pause over the upcoming GT4 and also undeniably informs at least some of my view on modifying modern normally aspirated Porsche cars.

The relationship between a car and its engine is ultimately much more complex than I think many consider and extends far beyond cubic capacity and available power. On paper numbers dont really tell one much unfortunately.
That engine had everything done correctly and without question was significantly more powerful, more athletic, sharper and more robust than it was before and yet…although the engine made the car significantly faster, it felt like driving a turbo diesel car in terms of power delivery for want of a better description. Gone was the nimbleness of drive completely.

It is a strange one to try and explain clearly and concisely so instead I will try to make my point with two graphs.
These graphs are what seem to be called “thrust curve” overlays in the car world. Its essentially a graph showing maximum force available at any given speed. Force = mass x acceleration, therefore Acceleration = Force/mass. As a result this graph can be used to show how much acceleration a car will have and to compare acceleration potential between cars (assuming no significant mass difference and assuming both have reasonably similar drag coefficients).
It is a useful tool to look at as a means to explain what happens when driving an engine that cannot be had from looking at headline figures or a dyno sheet. A dyno after all tells you nothing about the time it took to go from 2000 rpm to 6000 rpm. Only that it did so and what power it made along the way. I am going to use them here to try to illustrate what would take a long time to type.

That M9X engine on paper, with nearly 400HP and a touch over 320 lb ft of torque at the wheels was only a smidgen below the numbers at the wheel that I have for a 3.8 GT3 engine. I have a dyno for a 3.8 with an exhaust and filter that has 16HP more, but around 20 lb ft torque less so pretty close (again don’t get hung up on the numbers, dynos are what they are and its not the point of this post). Below is the overlay of the “thrust curves” for the two cars. The M9X car is in blue. One wants the highest line.

Now here is another overlay thats particularly interesting, this time using a 997.1 GT3 with a 3.6 engine. It was giving wheel numbers that were almost 20HP and 40 lb ft down on the M9X engine dyno posted earlier. Again the M9X car is in blue, again the line is on the bottom. If we were to plot these, taking speeds over a lap where things were changing back and forth the gap between the two would be considerable.

So what does it mean for the GT4. Well the short is that while a bigger engine will undoubtedly bring more power and more speed, be careful to not get sucked in by the numbers and properly evaluate what it is bringing to the car and how they work together. This will be even more true if it ends up being capped in terms of output in some way. It may all combine in one of those serendipitous moments where something incredible appears, but it may also be fundamentally the opposite of what is expected.

And for this thread, well I long ago took the view that the best approach to modifying modern naturally aspirated Porsche cars is to do what you can to enhance the subtleties that make these cars what they are. Enhance the balance and the essence of whats there. Lighten, sharpen and improve, add to the drama and the excitement. These are the things that make the car feel increasingly special. Ultimately, the quest for more power can sometimes be a counterproductive one in the end.

Edited by fioran0 on Tuesday 27th January 01:05

Steve Rance

5,446 posts

231 months