Sport Auto 718 GT4 Supertest

[ralphmusic]

Sport Auto May 2020 Issue includes the Supertest of the 718 GT4. Some highlights below:

Generally favourable as one might expect, same overall % score as the first generation.

All images CREDIT Sport-Auto May 2020

Nordschleife time with the same driver, Christian Gebhardt in 7:32, 10 seconds faster than the earlier car. Most of the time difference is in the run from Foxhole to the highest point on the circuit, Hohe Acht.

718 lap

981 lap

Odd differences, for example why new one slower/faster/slower/faster through Flugplatz through to Foxhole section? I think it just shows the variability of one run to the next even with the same experienced driver.

Weight, 718 70kg heavier than 981, of this 15kg is on the front axle and 55kg rear. I wonder how much is GPF? Both running with PCCB brakes. Also looking at the scales below, the 981 set up weights don’t too optimal, it doesn’t confirm that either were run with these settings but one assumes they would.

718 scales

981 scales

Dyno performance had 981 with 450Nm torque and 390PS, the 718 with 400Nm and 412PS. Both seem odd, I remember Jens Ehresmann telling me he had questioned the 981 torque numbers with the guy who ran the dyno tests but did not get an answer he could understand. Also for what it is worth, Jens also said that in his experience 9A1 3.4 engines dyno test results trended above the specified power and the 3.8 engine trended below. The graphs are below.

718 dyno

981 dyno

So what does it tell us?

Not how a heavier less powerful car can be 10 sec faster
Not how much difference a later tyre specification might make
Not why one should trade a 981 for a 718

All a little confusing perhaps? Anyway enough for a discussion....

 

[Twinfan]

Very confusing. Could just be a coincidence of a 'good' 981 engine vs a 'not so good' 982? The 981 figures seem very high though, and the 982 making less power than advertised is not very "Porsche GT product".

The 718 exhaust silencer amounts to around 10kg (https://www.jcr-developments.com/porsche/product/718-gt4--spyder--gts-4.0---titanium-silenced-valved-race-pipe-(inconel-rolled-tips)) but I don't know what the GPF weighs. It can't be 45kg, although some of the extra weight will be the new slightly larger engine and the start/stop technology parts. 15kg extra over the front axle? Increased cooling/larger rads perhaps? Not sure why it's 15kg though, that sounds a lot. Empty vs full fuel tank?

Andreas Preuninger claimed that like-for-like the weight difference between 981 and 982 was 30kg, so the spec of the cars must have been quite different. Aircon and radio delete in the 981? 18-way seats in the 982?

 

[Motorhead]

Thanks Ralph.

All very interesting but I should say that I don't set much store in 'Ring lap times - very long circuit, sometimes with different micro-climate conditions, traffic, etc. I'm sure that it's a useful circuit for vehicle development but unfortunately it's become the 'standard' for the marketing people. Even in the hands of an experienced pilot I suspect that there's going to be variability in the lap times - are the figures from a single lap, the best lap of 2,3,4? A same-day back-to-back 981 vs 718 comparison on a shorter race circuit with both high-speed and technical sections would be much more informative IMO.

I'm sure tyre development has played its part in improving lap times, but difficult to quantify.

I thought the increase in kerb weight of the 718 was supposed to be about 40kg, so I don't know where the extra 30kg has come from, especially the additional 15kg on the front axle, but maybe that's just in the corner weighting. The engine and ancillaries will be slightly heavier I presume, and then you've got the additional weight of the GPFs and extra front and rear aero stuff, as well as a few other bits and pieces no doubt, as weight always seems to be gained in later variants.

Slightly different scaling on the power and torque graphs doesn't help with the comparison, but the 981 torque curve and figures look a bit dubious? As above, a true back-to-back comparison on the day would be much more useful.

Jeff

 

[ralphmusic]

Some info on the weight difference the tested GT4 v 981 version:

Exhaust system inc GPF = 10.8kg

Engine inc stop/start = 7.7kg

Diffuser/undertray = 6.5kg

Other trim = 3.2kg

718 had Clubsport option = 19.9kg

718 had racing bucket seat = 9.0kg

AGM battery v 981 LiFePO4 = 16.1kg

In all 73kg. Like for like specification (just the top 4 items being different) the 718 would be 28kg heavier.

So first 3 towards the rear = 25kg + 50% of others = around 50kg?

 

[Twinfan]

Thanks Ralph, great info. The 981 having a Lithium Ion battery and no cage is the reason for the big weight difference then.

What seats did the 981 have for the 718 seats to be 9kg heavier - folding sports buckets? They don't have electric adjustment which would reduce the weight compared to all the other options.

 

[ralphmusic]

981 LWB, 718 had a driver's race seat with the big ears and passenger LWB

 

[Twinfan]

Ah, that explains it. Cheers Ralph! A very apples and oranges comparison then in some ways.

 

[AndrewCS]

Excellent information, thank you … puts one`s `butt dyno` comparisons into perspective []

More impressive for me is how the times of the standard 718 S and 718 GTS compare to these GT4s !

 

[ralphmusic]

981 LWB, 718 had a driver's race seat with the big ears and passenger LWB

Some weights for GT4 seats v the Race seat:

The FIA ​​safety seat is 26.5 kg, standard seat around 20.5 kg, carbon shell 17.5 kg, confirming the 9kg difference referenced earlier.

 

[ralphmusic]

Is the GT4 power constrained? Well sure there is a model range hierarchy, but there is something more fundamental with the 4.0L engine. It's based on the 3.0L turbo introduced to help emissions. Perhaps ordinarily one might better start with an optimised normally aspirated engine design and then turbo charge that, rather than starting with a forced induction engine.

The 4.0L GT4 engine and the 3.0L turbo engine use almost common heads and valves. PET files show part numbers for heads and valves assembly (bank 1) as follows:

3.0L 91.0 x 76.4 mm PET part# Head + valves OPB 103 063 C

4.0L 102.0 x 81.5 mm PET part# Head + valves OPB 103 063 H

The effect is that the valve sizes on the GT4 are smaller than the 981 3.8L GT4. Parts sharing has its advantages in cost but not necessarily in performance.

 

[Motorhead]

Parts sharing has its advantages in cost but not necessarily in performance.

I think you’re right Ralph. The 4.0L engine was developed quite rapidly, so using the Porsche parts-bin no doubt helped reduce both development time and cost as well as incorporating useful features like the piezo-injectors and the iron-plated cylinder bores. Keeping the same bore spacing also meant that both the n/asp and turbo engines (including the F-4) could be built on the same production line, leading to significant cost savings.

I’m surprised to note that both engines use the same heads though. The turbo engine has VarioCam Plus variable intake valve lift, whereas I was under the impression that the n/asp engine uses finger-type valve operation similar to that used on the GT3 engine to give an extended rev range, which I would have thought necessitates a different head casting, but perhaps I’m wrong?

Jeff

 

[ralphmusic]

Nothing sophisticated with the heads.

 

[Motorhead]

Ralph,

From the Porsche Newsroom site:

https://newsroom.porsche.com/en/press-kits/718-spyder-cayman-gt4/motor-und-performance.html

In view of the high engine speeds possible, rocker arms with hydraulic valve clearance compensation are responsible for operating the valves. The electronic engine management system adjusts the timing of the four VarioCam camshafts according to load requirement and engine speed, for example over an adjustment range of 30 crankshaft degrees on the outlet side. This ensures high power output and torque values across the entire engine speed range, thus improving drivability.

I would have thought that rocker arm actuation of the valves would require a very different type of arrangement compared with the cam lobes acting directly on the split inverted bucket arrangement used on the variable lift system. No details about the hydraulic valve clearance system though … via conventional inverted buckets or at the rocker arm pivot like that used on the 991 GT3?

Either way I would have thought that there will be significant differences between the head castings. Just guessing of course.

Jeff

 

[ralphmusic]

I think the part numbers suggest some commonality.

 

[Motorhead]

I’m sure there is Ralph. I would guess that the combustion chamber shape, valve position and inclination and the location of the sparking plug and injector will be the same, but the top of the head casting will need to reflect the different valve actuation mechanism. Maybe it’s a common casting but subject to different machining procedures, reflected in the C and H part number suffices?

Just my take on it though, of course. []

Jeff

 

[Motorhead]

I note that on the 992 GT3 there’s no hydraulic backlash adjustment; the clearance is set on assembly and apparently is good for 100,000+ miles. AP reckons there’s a small power gain by doing this associated with a reduction in hydraulic power losses.

Jeff

 

[TDT]

I note that on the 992 GT3 there’s no hydraulic backlash adjustment; the clearance is set on assembly and apparently is good for 100,000+ miles. AP reckons there’s a small power gain by doing this associated with a reduction in hydraulic power losses.

Jeff

This was also the arrangement in 991.2 GT3 afaik ... so not new given it’s pretty much the same unit.