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Folks - I have been going through loads of data on this site and there is plenty of it if you look in the right places.
My aim is to understand a few bits and bobs in regard to the new GT4 particularly the engine performance characteristics and how that may relate to gearing for example.
The first data points are the engine dyno curves as presented by Porsche (the data has been digitised from these curves to better compare them with other data sets).
This graph present the baseline data for the 981 GT4 (Porsche in green) and baseline data from the unmodified BGB Motorsport GT4 (Maha dyno in blue) - x axis RPM, y axis HP and Tqe ft lb
The Maha run delivers peak RWHP of ~ 320 - note the curves are different but broadly similar too - the car is not underated as one would expect from Porsche.
So what will the 718 GT4 engine deliver (engineered model - blue) relative to the previous generation (green) - assumption 0.2l addition in capacity and the addition of more aggressive cams, porting and headers. Comparison between current GT4 and engineered model solution for 718 GT4.
The engineered model is a conservative solution, will be very robust and deliver an easy 420 crank HP and 350 RWHP. The torque curve can easily be pushed out in the rev range.
Comparison between BGB full stage 1 kit as applied to current 981 GT4 (Maha dynometer) and engineered model for 718 GT4 - kudos to BGB Motorsport, quality data and a good outcome.
a better comparison might be the full stage 3 BGB conversion with the 4.0 stroker motor plus X51 kit and plenum/tb/headers and tune.
do you have the BGB curves for the stage 3 and can you superimpose those curves on the predicted 718GT4 curves in diagram #3?
^^^ happy to do that the caveat being, the dyno data won't be from a Maha . I'll add this, my mate who provided me with the engineered model feels the model GT4 or (rather 718 GT4) is a natural 440HP and 8000 rpm car if airflow and cooling is good (and thats conservative)..
Numbers = gears
A = peak HP
B = acceleration curve and change points
I read through a lot of commentary about long gearing etc etc. My synopsis is below
(1) At 70mph in sixth gear the engine will be sitting at around 2600rpm - thus it wont be an economical cruiser
(2) At 50mph in sixth gear the engine will be sitting around 2000rpm - which will be reasonable
(3) You can not rev this engine out (realistically) on public roads - +80mph in second - suggestion if you want to do this get a BRZ/miata/s2000
(4) In a daily driving situation (4000rpm change) in first you will be at 25mph, in second 42mph, third 58mph and fourth ~70mph
(5) Basically the car can function as a daily
(6) The reality is second, third and fourth are ideally set up for track giving an optimal range of available torque and speed e.g. ~48mph through to ~142mph
Its absolutely clear Porsche designed the car as a track car that could serve as a daily. There is absolutely nothing wrong with the gearing - Porsche got it right for its intended purpose - the weak point was the engine (and third gear letting go, strut tower failure, sub optimal rear suspension).
The graph above clearly illustrates this - the blue lines represent possible torque and power curves for the proposed 4l 718 GT4 (blue lines) and the green lines the actual torque and power curves for the 981 GT4 (from Porsche).
Important - the three red dots represent the second, third and fourth gear change points (derived from the rpm vs speed and gearing graph from the panel above).
In the case of the 981 GT4, 2nd gear is initiated at the start of the meat of the torque curve and runs through to a lazy ending, third is initiated as the lazy ending starts and 4th is initiated well into the lazy ending.
In contrast, the proposed 718 GT4 engine delivers a very different experience, 2nd is initiated at the start of the meat of the torque curve (~4500 rpm) and will charge hard to 420HP, 3rd is initiated well within the meat of the torque curve (~5700rpm) and will punch hard through to the redline before the change to fourth (~6250rpm) still with plenty to deliver.
The gearing is very well positioned for a track car and clearly designed around the torque curve.
What does the above mean in numbers? (difference between 718 GT4 and 981 GT4)
On a 2-3-4 gear pull the difference in HP is a significant 104HP
and on a 3-4 gear pull the difference is a significant 75HP.
Two more graphs - firstly two sets of dyno data (Maha) for the 991 Series 2 GT3 and 981 series GT4 (sourced and digitised from BGB thread baseline runs and automotor und sport GT3 base line )
(1) Note the strength, range and depth of the GT3 torque curve and accurate representation of peak engine HP and torque
(2) Note the relatively lazy response of the GT4 engine beyond 5500 rpm (see discussion in gearing panel)
Note x axis RPM and Y axis ft lb or HP
The second graph compares the engineered model 718 GT4 4l (stroked from 3.8l) X51 power kit engine (assuming 12.5:1 compression ratio) with Maha data from the 991 Series 2 GT3 (compression ratio 13.3).
(1) The difference in compression ratio is reflected in the difference in peak torque e.g. GT3 340 ftlb vs model GT4 325ftlb
(2) The model GT4 torque curve is displaced to the left of the GT3 curve delivering more in the mid-range
(3) The GT3 torque curve is displace to the right of the GT4 curve delivering more at the top end.
(4) Both engines pull hard to 420HP and the GT3 continues on to 500HP (493BHP) and 9000RPM
(5) The current gearing in the 981GT4 will work well with model 718 GT4 engine
(6) The BGB powerkit engine power delivery is very consistent with the independently derived model (but pushed out to higher redline and peak power).
These data indicate the 4l X51 power plant, proposed for the 718GT4, will provide formidable performance. This assertion is strongly supported by data delivered through BGB motorsports 981 GT4 development program. The addition of PDK will only enhance the performance further.
What does the above mean in numbers? (difference between 718 GT4 and 981 GT4)
On a 2-3-4 gear pull the difference in HP is a significant 104HP
and on a 3-4 gear pull the difference is a significant 75HP.
How do you figure this? At no point is there even a 75 HP difference in the power vs vehicle speed curves.
How do you figure this? At no point is there even a 75 HP difference in the power vs vehicle speed curves.
Its the cumulative difference e.g. 2nd to redline difference plus third to redline difference plus fourth to redline difference (as calculated from the areas under the curve - using the gear change points from the rpm - mph - gear figure).
Also this illustrates the difference in how the 718 will feel relative to the 981 - the former will pull harder to the redline than the later. Look at the gear change points on the curve and the shape of the curves.
Overall it will feel like a significant improvement (thats one of the reasons why I went through the numbers because at first blush it looks like we'll be getting a warmed over engine - its in fact both good and strong in its delivery).
Another point to note is the 981 GT4 had to fit into the line up of the day e.g. Base Carrera 350HP, GT4 385HP, Carrera S 400HP and Carrera GTS 430HP whereas the 718 GT4 will be released in the 992.1 cycle e.g. Base Carrera 400HP 718GT4 420-430HP, Carrera S 450HP and Carrera GTS 475HP. The apparent hierarchy has been preserved.
Same with the GT Cars - 718 GT4 420-430HP, 991.1 GT3 475HP, 991.1 GT3 RS 500HP, 991.2 GT3 500HP and 991.2 GT3 RS 520 HP - anyhow this matters to the marketing and sales department and those that trade badges - to those of us that enjoying racing of various types we know its more about the driver and tires.
Final point - I know a very good and very reliable source is saying no PDK - all I can say is I'm up to dealer number six saying there will be a PDK in the non CS GT4 (FWIW).
06-10-2018, 07:22 AM
718 GT4 possible power curves and known gains
. I'll add this, my mate who provided me with the engineered model feels the model GT4 or (rather 718 GT4) is a natural 440HP and 8000 rpm car if airflow and cooling is good (and thats conservative)..
