Petevb

This thread is going sideways quickly. Some strong opinions here, let's try to stick to the facts.
There is a lot of truth to this. While there will be many situations where the throttle will be totally shut (braking hard into a hairpin, then jumping on the throttle) in most the turbos will be spinning slightly. However in the situation described above, if you want to balance the car with the throttle you're not going to have the response to do that as well as in a normally aspirated engine, regardless of revs. Most won't notice in highway driving, but at an autocross or similar even a quarter second is an eternity.

We need to keep the engine's use in mind. In a GT car the extra, effortless torque is great- easy highway passing, no need for the noise and commotion of revs. In competition car throttle response and precision is key, not just in terms of time but in terms of experience and involvement. This how you use the car will largely determine whether you like the new engines or not. For highway use or quick cruising the new engines should be great. If you autocross or track... I'd hope for a normally aspirated GTS, or look for a good used 991.1. Not in terms of speed, perhaps, but in terms of involvement (and durability, to a lesser extent).

By the way, anti-lag systems that do eliminate lag exist. They've been used in world rally for years, and the best of them turn little 4 cylinders into the equivalent of big V8s. So far, however, no one has made a street legal system that good- even the P1's "torque fill" electric assist doesn't eliminate lag.
Again, see the usage argument above. However keep in mind that we're judging engines on their merits here, not cars. If we weren't the most powerful engine would almost always win, even if it was technically inferior. To judge engines it helps to hold power roughly constant. For example: the 430 hp 9A1 from the GTS is more powerful than the 420 hp 9A2. Meanwhile it almost certainly costs Porsche less to build- most parts are similar, but the 9A2 adds turbos, intercoolers, etc.

A car built with the 9A1 X51 engine, all else equal, would have a substantial 44 fewer lbs hanging out behind the rear axle, making the car quicker overall both in a straight line and improving handling noticeably. It wouldn't have any of the dowsides of poor throttle response, nor the extra complexity and risk to durability of the turbos. Thus, fuel economy aside, it's objectively a superior package in nearly every respect, low RPM torque (with the associated lag) being the exception. However ask guys who compete and most will trade the extra torque for response any day.

I'm not one of those that will claim that fuel economy is unimportant, or that the new cars won't get better economy. However to me performance has clearly been sacrificed for improved economy in 9A2 versions we've seen so far. Tuned to 460+ hp you might be able to argue otherwise, but that's not what Porsche has delivered.

96redLT4

I think its interesting that Porsche came up with this rotary **** rather than just the old Sport Plus button. I'd like to think its not just to distract you from
noticing the pause during the spool up.
Jim

cloud9blue

- 44lbs increase of weight at the rear is highly insignificant in terms of overall weight of the car, particularly if you consider the added traction at the rear with the extra weight. Cars with sliding glass roof add just as much as weight, and despite of its weight being added to the worst place imaginable for performance (far above the the CG), I have yet to see evidence of that affect straight line performance.

- Have a look at the graphs posted in here: https://rennlist.com/forums/991/8927...yno-sheet.html

- If you look at the hp and tq curve of 9A2 vs 9A1, you can see that 9A2 makes it power very early on and held its significant power advantage of nearly 50hp from idle and all the way to 7000rpms. Overall acceleration performance is determined by the chassis/weight, tire, and total area of the rpm range used during the acceleration. Since we can assume the chassis and tire are remained the same between two cars, I don't see there is anyway 9A1, even w/ the X51 package, can be faster than a C2S 9A2. Of course, this will all be settle once we get real world performance data on the car. But you can take to look at current gen M4 (425hp) 1/4 mile and 0-60mph data in comparison to the last gen NA car (414hp), you will see how forced induction can dramatically improve straight line performance despite the moderate increase in peak hp. It is area under the hp curve, not just the peak hp figures!

Yes, we scarifies a bit of engine response here and there. But overall, it is a much more powerful engine, everywhere in the rpm range. I just don't see how you can claim otherwise...

STG

Out of respect to Petevb (who started this thread) who has so effectively made his points, I'll choose to not respond to the nonsense anymore. You were dealt enough of my responses in that other thread. Some good entertainment there.

https://rennlist.com/forums/991/8928...pse-sound.html


Reading and learning from Petevb's responses to some of the uninformed is enough satisfaction for me.

Dr. G

Being that this is a discussion about engine differences, is anyone else interested in the fact that they changed the oil pan from aluminum to plastic? I read this in the cnet article about the 991.2. The article went on to say that the pan was able to withstand the force of the entire engine assembly being dropped from a height of 3 feet. I don't know how that test translates to real world circumstances that might result in a crack or perforation of the oil pan.

Edit: Not trying to cause trouble - I actually have a deposit in for the 991.2 GT3 RS, if Porsche produces one.

cloud9blue

Mechanically, I am sure it will be just as strong as the old one. But I would worry about some careless tech stripping the thread for the oil plug during a simple oil change. Let's hope the thread is actually metal...

Petevb

Sorry, but as an engineer with experience in the field I can tell you that's simply not the case. Weight at the ends of the car means high polar moment. This makes a car harder (slower) to turn, reducing transient response and dulling handling. Meanwhile Porsche has gone to great pains to limit the weight on the back axle for a reason. They have moved the components they can forward in the car (gas tank, battery, radiators, etc) even if it hurts polar moment and overall weight in order to keep weight balance in check. An extra 44 lbs does not help the cause. A little rear weight balance is good- many race cars run between 55% and 60% of weight on the rear. A lot is bad, and the new car moves towards the edge of the sweet spot Porsche has worked hard to stay within for decades.

Why do you think Porsche uses expensive titanium mufflers on its hottest cars, ie the GT2 and GT3 RS, when that weight could far more easily be saved elsewhere? Because losing weight from the rear is a top priority, and they consider it worth serious money.

The 991.2 will be the most rear heavy normal 911 ever, and that's not a good thing. Porsche engineers have explained to me how difficult is is to get the rear of the 911 to work well compared with the Cayman, and the additional weight back there will not help matters. I suspect adding the rear wheel steering option was in some ways required to hit the handling targets. I'm sure the new car will handle great, don't get me wrong. However it would handle noticeably better with the 9A1 all else equal.
Correct. But the 9A1 car would have a 4% power to weight ratio advantage when you include the lower engine weight, meaning everywhere on the curve above 414 hp (or 308 kw) the X51 would be accelerating faster than the 9A2 does at peak. That's nearly 1000 rpm at the top of the rev range. Now consider: rev drop on the 3rd to 4th shift is only 1250 rpm, meaning for virtually all of 4th gear the 9A1 equipped car will be accelerating harder than the 9A2 car does at peak. The wide band comes into play at lower RPMs, but as soon as you start shifting the advantage swings the other way.

cloud9blue

Also an engineer here, but I respectfully disagree with everything you said in this post.

1. extra weight helps with rear end track, that's why drag racing cars have very rear bias weight distribution. And for the same reason, it is why 911 has always be able to out accelerate much more powerful cars from a dig. I suggest you look at some hard real life performance data (1/4mile in paricular). I think you are confusing things with road course, where extra rear weight might be detrimental to handling and weight transition. But you have to be at pro-level to be able to tell extra 40lbs at the rear axle on a softly sprung street car that weights 3100lbs... High polar moment? Considering the engine is nearly sitting on top of the rear wheels with two 305 size tires as the contact patch/anchoring point, the effective polar moment on the entire chassis is very minimal... It is not like the we have two 20lbs dumbbell suspensed 1 ft behind the rear bumper of the car...

2. How did we arrived at the fact 991.2 will be the most rear end heavy 911 ever??? Please tell me where you get that fact from...
Earlier 911 (as in the original 911, and 964) has much higher rear biased weight distribution, if you take a look into the archived data. And those cars rely in ancient trailing arm rear suspension which makes things even more "interesting" in a corner, and yet those cars are still being raced competitively in club events. With modern multi-link rear suspension, I don't see how this is going to an issue at all. If you train of thoughts is correct, shouldn't the much heavier Mezger engine be very detrimental to the handling of cars that have it in? But with proper rear suspension tuning, that doesn't seem to be the case in the 997 and 996.

3. We still don't know the gear ratio of the PDK and 7MT of the 991.2. But let's assume they are the same as 991.1, you would shift 500rpms earlier in the 991.2 in comparison to X51 equipped 991.1 to get the optimal acceleration. I will let yourself to do the math here, but 9A2 would still hold the power advantage in such case. It seems rather biased comparison, to not take into the significant early power advantage of 9A2 by shift 500rpm earlier to allow the engine to stay within the max. power range.

4. And if I have to nickpick, why aren't we comparing the standard 9A2 to a standard 9A1 instead. I am sure Porsche would release X51 equivalent/GTS package for the 9A2 a year or two from now. X51 was a $13k option if I remember correctly, so it isn't exactly apple to apple comparison when we are putting it against a standard 9A2.

5. I am willing to bet my money 9A2 equipped C2S is noticeably faster in 0-60 and 1/4 mile than a X51 equipped 9A1 in either a C2S or rwd GTS when the real world performance data come out.

Dodaleca

Here are the gear ratios (991.1 on left, 991.2 on right).


Up to 2nd gear the same then higher gearing (lower numerically) for the 991.2 due to the higher torque of the turbo engine. As well now manual and PDK ratios are identical. It appears that the final drive rear axle ratio is the same.


As well the 991.2 X51 rumors have been 444 BHP I believe.

Petevb

What kind? Electrical?

Rear end track? You mean grip? Sure. But a 911 is limited on launch only, after which you're carrying around 44 lbs that's simply slowing you down. At which point the math is clear- roughly a .3 lbs per hp advantage for the 9A1 X50 car vs the 9A2 car. The 9A2 might conceivably win to 60, but the kind of acceleration Carrera are designed for isn't drag racing- buy a Vette or GT-R for that. Everywhere else, off the corners, freeway roll on, etc the 9A1 takes it.

I said normal 911, not turbo, etc. Porsche says the 991.2 will have >62% weight over the rear at curb weight, and you can expect actual test results to show more.

Sorry, incorrect. Find a source, curb weight (full tank). 1989 Carrera 3.2 with a heavy G50 and the engine shifted back might be your best bet as the most rear heavy early car.

And if rear weight bias isn't an issue, why do you think Porsche put lead ballast in the front bumper of early short wheelbase cars? Why did they spend money on expensive titanium mufflers for the GT2 and GT3 RS, instead of saving more weight for less money elsewhere? Why do they locate the battery in the front of the car, adding 10 lbs of copper wiring, instead of putting it close to the starter? Why run water lines to the front (~30 lbs) instead of locating the radiators in the rear wing or fenders where they put their intercoolers?

The answer is simple: weight balance matters, and Porsche does what they can to counterbalance the heavy engine. Adding more weight doesn't help.
I don't know what math you are doing here, but you're missing the point. Let me put it another way: the 9A1 X50 car has a better power to weight ratio over ~12% of its rev range than the 9A2 car does at the best point in its range. No matter where the 9A2 car is shifting or what its gearing is, the 9A1 will be faster over that range. And since in fourth gear a 991 is only using ~16% of its rev range, it's guaranteed to be faster through at least 3/4 of the gear, even if the 9A2's power curve was entirely flat and it was making peak power 100% of the time. Which it's not.

Which do you think is cheaper, with your engineer hat on, the 9A1 X50 or the 9A2? A 9A1 X51 car will handle better, be quicker around a course (whether you believe that or not), be more reliable, and I'd argue it will be cheaper to boot. That seems a fair point of comparison, and I used it because you were suggesting that the 9A2 is better simply because it makes the car go faster. Yet power is one only facet of an engine, and one that we can control for. Taking speed out of the equation it becomes more obvious- the 9A2's weight counts against it.

If I'm the designer, fuel economy aside, I'd rather use the X51 engine- cheaper and fundamentally better in many ways- handling, response, noise. For a GT car maybe the quieter, lazier 9A2 gets the nod. For a sports car, however, zero question.

Michael_s

My head hurts from catching up on this thread......

cloud9blue

With all due respect, I think you are conveniently ignoring several key facts here.

- These are +3100 lbs street car we are talking about here, an extra 40lbs here isn't going to be no where as significant as 2300lbs early 911, especially with widen front and back stance, and most importantly, significantly lengthed wheelbase. Have a look at the engine placement of earlier generation in comparison of current gen, engines have been pushed the front significantly. Did 997 GT3 (not the RS with significant weight reduction at the rear with plexiglass rear windshield) with Mezger block handled terribly? I never drove one of course, but I have yet to see any complained about such fact. Yes, weight does add up, that's why Porsche and BMW uses some pretty clever weight saving solutions on these cars. But 44lbs in 3100lbs is only 1.4% of the overall weight.

- Yes, X51 9A1 has power advantage over 9A2 over 7300rpms. But with 9A2, you would have shifted at 7200-7300rpm already, so the engine would have returned to its optimal rpm of around 5500 rpms. Do me a favor and put your engineering hat if you are indeed one, do a rough calculation of the area under the power curve for both 9A2 and 9A1 when you would shift at the optimal rpm (7200rpm for 9A2 and 7700rpm for X51 9A1). Then tell me which is bigger... I honestly can't believe you cannot comprehend this simple point. Yes, 911 are not straight line machines, and the fact that power delivery is earlier and more linear than the 9A1 helps corner exist even more than it would be in a straight line where you would utilize the mid range of the rpm. Mind you again, these are street cars with rather high gear ratio, so you will never be able to stay in the just the top 1000rpms of the rev range like you would imagine you would be in a 9A1 once you get the car in a real world situation, whether it is a highway or your local road course. The only place I can see 911.1 have an advantage is autocross, where its more accurate engine response might allow one to better and more predictably rotate the car in a tighter than usual corner. But I doubt Porsche build this car so people can dodge cones with it in the parking lot...

- All in all, I am willing to put bet $100 that 9A2 is faster than 9A1, even with X51. Are you? I will be more than happy to paypal you the money if I indeed loses out on this bet when the real world data start to arrive next year.

As I said there is little I can do about confirmation bias over the internet, nor can you, lol. Perhaps the only way we can resolve this argument is just wait for the real world data to settle the score... As it is often the case with thread on this forum, the longer the thread gets, the more pointless it becomes... And it doesn't help to have the guys such as the one below, who must think of himself as some sort of a comedic genius on this forum, itching to bring it down to his level of playing field, again...

STG

ace37

I think you are going in slightly different directions.


cloud9blue, your posts imply primarily street use and argue that is what the car is made for. pete is clearly framing his points as targeted to rather sensitive elements of track performance usage, not street use. Your points about practical performance are not missed, but he is clearly laying out assumptions and drawing conclusions, and you appear to be primarily disputing his conclusions based on a different set of assumptions. Thus the two perspectives have no meaningful disagreement besides the assumed usage spectra. I suspect you both look for rather different things out of your cars.


By the way, I'm also an engineer, go team. I did an MS in mechanical and primarily do aircraft stress analysis. As a car nut engineer, I did pick up some academic background in automotive engineering, but it's rather subject- and context-specific.

Dodaleca

I think the difference between the models (as based on brochures Unladen DIN values for 911 Carrera S manual) is only 25 kg (55 lbs) as the 991.1S is listed at 1415 kg and the 991.2S is 1440 kg. Maximum permissible gross weight is listed as 1830 kg for 991.1 and 1900 kg for 991.2 so payload appears to have increased by 45 kg.


Some of this extra weight in the 991.2S might be from the wider tires and rims (extra 0.5" in rear rim width plus increase from 295 -> 305 tire) and the front active shutters (assembly plus actuator) so not all of it may be behind the rear axle.


The brochure time 0-100 km/hr PDK with Sport Chrono is 4.1s for 991.1S (not X51) and 3.9s for 991.2S. Those are the only comparable times that I could find easily in the PDFs I've seen to date.


I agree with the observation that the arguments appear to be based on different points of view - street vs track drivability.


BTW I'm a mechanical engineer dealing with industrial natural gas engines - reciprocating and turbine so this kind of engine data-design analysis interests me greatly!