718 GT4RS
08-20-2020, 04:16 AM
#736
Race Director
Originally Posted by JSF101
Porsche should drag out the press release for another year just so we can have that many more wild guesses
But I suspect some of these wild guesses that you refer aren't really wild guesses at all
speculation is the lifeblood of rennlist
But I suspect some of these wild guesses that you refer aren't really wild guesses at all
speculation is the lifeblood of rennlist
Anyone remember "CONFIRMED".. yes well.
08-20-2020, 05:38 AM
#737
Although it is slightly disappointing that Lotd Lucan has not returned with the prediction of a Turbo 4 based on the GT2 RS...
08-20-2020, 01:28 PM
#739
Has not shown his face since reveal.
08-20-2020, 01:51 PM
#740
Rennlist Member
IIRC DeMan Motorsports gets this power (500 WHP) out of the 981 GT4 with mostly off the shelf Porsche parts, some machining of the heads/bore (4.5L displacement) and a software upgrade. I have not heard if they mate a PDK with is package. So Porsche could certainly create a similar package and make it reliable.
What in great baby Jesus' name are you talking about my good doctor? High displacement and blown engines and warranty claims? There is some seriously wacky information being shared here. Since when are large displacement engines at greater risk of warranty claims? Let's keep our info at least reasonably accurate because people could take these words at face value and build a lot of misconceptions.
Virtually all of the large engine replacements that I've read about in the past have been mated to a manual or a slushbox.
Alex, you are more than a gearhead than I, but physics certainly argues that the larger the cylinder the greater the challenge in keeping an engine reliable. It is certainly easier to make a reliable 8 cylinder 4.5L engine than a reliable 6 cylinder 4.5L engine because the individual cylinders are so large in the 6 cylinder engine. This is particularly an issue in an engine that revs to 9000 rpm.
As you are more of a gearhead than me, do you have examples of other engines that were reliable (including regular track work) over the same 300,000K that Porsche says the 991.2 GT3/RS engines will last? There are many mom and pop places making large engines for cars that people play around with on the weekends or they drive out to C&C events, but I'm not aware of a previous engine with more than .7L per cylinder, that revs past 8500, could be tracked regularly and still driven for 300K miles. I'd be very happy to learn otherwise.
Most of the large displacement engines either come with more than 6 cylinders, or are used briefly or just very rarely. Porsche has to make theirs reliable for years.
Last edited by Drifting; 08-20-2020 at 05:25 PM.
08-20-2020, 02:02 PM
#741
Rennlist Member
Join Date: Nov 2016
Location: Waterloo, Ontario, Canada
Posts: 1,524
Received 823 Likes
on
452 Posts
I would be very interested to know how well the PDK works on the deman 4.5L kits. It might "work", but I doubt it works as well as with the standard engine. I would be happy to know otherwise, from an unbiased source.
Alex, you are more than a gearhead than I, but physics certainly argues that the larger the cylinder the greater the challenge in keeping an engine reliable. Easier to make a reliable 8 cylinder 4.5L engine than a reliable 6 cylinder 4.5L engine because the individual cylinders are so large.
As you are more of a gearhead than me, do you have examples of other engines that were reliable over the same 300,000K that Porsche says the 991.2 GT3/RS engines will last, including regular track work? There are lots of mom and pop places making large engines that people play around with on the weekends, but I'm not aware of a previous engine with more than .66L per cylinder that could be tracked regularly and still driven for 300K miles.
Most of the large displacement engines either come with more than 6 cylinders or are used briefly or just very rarely. Porsche has to make theirs reliable for years.
Alex, you are more than a gearhead than I, but physics certainly argues that the larger the cylinder the greater the challenge in keeping an engine reliable. Easier to make a reliable 8 cylinder 4.5L engine than a reliable 6 cylinder 4.5L engine because the individual cylinders are so large.
As you are more of a gearhead than me, do you have examples of other engines that were reliable over the same 300,000K that Porsche says the 991.2 GT3/RS engines will last, including regular track work? There are lots of mom and pop places making large engines that people play around with on the weekends, but I'm not aware of a previous engine with more than .66L per cylinder that could be tracked regularly and still driven for 300K miles.
Most of the large displacement engines either come with more than 6 cylinders or are used briefly or just very rarely. Porsche has to make theirs reliable for years.
With that said, making a huge engine rev to high RPM is harder because piston speed will become excessively high for an engine with a long stroke, and a huge bore short stroke can have other issues with cylinder wall wear and combustion efficiency. This is why motorcycle engines can rev to much higher RPMs without exotic materials, and why a 9000 RPM 2.0L 4 cylidner Honda S2000 engine didn't require exotic materials and engineering the way a 4.0L 991.2 GT3 engine does. Porsche could easily build a 6.0L turbocharged flat six with torque that puts semi trucks to shame, but it won't be able to rev to 9K RPM. Alternatively, they could build an 8.0L flat 12, and keep the 9K RPM redline without doing anything too different from the current GT3 engine.
Boring out a small engine to a huge displacement can be problematic, as that makes the cylinder walls much thinner, and the block weaker. DeMan is boring out a small engine, not designing a fresh big block. I’m not saying anything bad about their 4.5L (I don’t have any data), but if they make a 5.0L from the same small block, they’ll probably run into issues due to weakening the block excessively.
Last edited by wizee; 08-20-2020 at 02:29 PM.
08-20-2020, 06:43 PM
#742
Race Director
^^^^
What he said. Bore and stroke are influencing factors on durability as engine stressors. Also if you're talking about taking an engine block and boring it out and giving it super thin liner walls, that is a factor. But increasing engine size alone has zero impact. Nothing says Porsche can't make a much larger engine.
One point of note.. back in the good old days of naturally aspirated bmw m-division... m engineers said that an optimal size was 500cc/cylinder.. I never learned why but if that were to hold true, then a 3.0 6 cyl engine would have been perfect and a 4L like Porsche's totally off.
And yet...
What he said. Bore and stroke are influencing factors on durability as engine stressors. Also if you're talking about taking an engine block and boring it out and giving it super thin liner walls, that is a factor. But increasing engine size alone has zero impact. Nothing says Porsche can't make a much larger engine.
One point of note.. back in the good old days of naturally aspirated bmw m-division... m engineers said that an optimal size was 500cc/cylinder.. I never learned why but if that were to hold true, then a 3.0 6 cyl engine would have been perfect and a 4L like Porsche's totally off.
And yet...
08-20-2020, 08:18 PM
#743
Rennlist Member
^^^^
What he said. Bore and stroke are influencing factors on durability as engine stressors. Also if you're talking about taking an engine block and boring it out and giving it super thin liner walls, that is a factor. But increasing engine size alone has zero impact. Nothing says Porsche can't make a much larger engine.
What he said. Bore and stroke are influencing factors on durability as engine stressors. Also if you're talking about taking an engine block and boring it out and giving it super thin liner walls, that is a factor. But increasing engine size alone has zero impact. Nothing says Porsche can't make a much larger engine.
However when we talk about engines specifically in the 911 family of cars which are rear mounted, the size of the engine block is also a limiting factor as there isn't much space back there compared to mid engine car and particularly a front engine car. So if you have limited engine space then increased displacement is more often achieved by boring out an engine block versus making an overall bigger engine and there are real limits as to the size of an engine you can fit rear mounted and between the wheels in a 911, unless you make an even larger 911 body and I think the 991 is plenty large enough.
Last edited by Drifting; 08-20-2020 at 08:53 PM.
08-20-2020, 08:41 PM
#744
Burning Brakes
I recall a guy showing the math once of the forces applied to a connecting rod when a piston stops and changes direction at high revs. The numbers were startling as to how high the forces were and how often they do it with good reliability. I'll see if I can find the article again.
Found some of it.
Found some of it.
4 stroke TDC exhaust.
If it was possible for boost to pressurize the cylinder well before TDC (when the intake has not yet opened) and through, during and after TDC when the ex valve is open as well, 15 psi would exert less than 200 lbs force on a 4 inch diameter (12.6 in^2) piston.
According to this online calculator ( http://2.3liter.com/Calc2.htm#PistSpeed ) a stock 1976 350 Chevy (3.48 " stroke, 5.7" long rod) piston at 5000 rpm experiences about 1600 Gs.
If a stock piston with pin and rings weighs 650 grams (1.4 lbs) or so then at 5000 rpm the rod has to exert 1600 x 1.4 = 2300 lbs to haul the piston to a stop and make it reverse direction. That force estimate is at the wrist pin centerline. The crank journal/rod big end bearing also has to haul the rest of the con rod back down the hole too, and some portions of the con rod are the load path for that feat.
2300 - 200 = 2100 lbs tension taking full credit for boost pressure.
It would take 185 psi (~ 12 bar) cylinder pressure at TDC to balance the inertia load. That is in the ball park for TDC pressure on the firing stroke, before full cylinder pressure is developed at 10-20 degrees ATDC or so.
If it was possible for boost to pressurize the cylinder well before TDC (when the intake has not yet opened) and through, during and after TDC when the ex valve is open as well, 15 psi would exert less than 200 lbs force on a 4 inch diameter (12.6 in^2) piston.
According to this online calculator ( http://2.3liter.com/Calc2.htm#PistSpeed ) a stock 1976 350 Chevy (3.48 " stroke, 5.7" long rod) piston at 5000 rpm experiences about 1600 Gs.
If a stock piston with pin and rings weighs 650 grams (1.4 lbs) or so then at 5000 rpm the rod has to exert 1600 x 1.4 = 2300 lbs to haul the piston to a stop and make it reverse direction. That force estimate is at the wrist pin centerline. The crank journal/rod big end bearing also has to haul the rest of the con rod back down the hole too, and some portions of the con rod are the load path for that feat.
2300 - 200 = 2100 lbs tension taking full credit for boost pressure.
It would take 185 psi (~ 12 bar) cylinder pressure at TDC to balance the inertia load. That is in the ball park for TDC pressure on the firing stroke, before full cylinder pressure is developed at 10-20 degrees ATDC or so.
Last edited by venom51; 08-20-2020 at 08:48 PM.
08-20-2020, 09:04 PM
#745
Rennlist Member
One point of note.. back in the good old days of naturally aspirated bmw m-division... m engineers said that an optimal size was 500cc/cylinder.. I never learned why but if that were to hold true, then a 3.0 6 cyl engine would have been perfect and a 4L like Porsche's totally off.
08-22-2020, 05:40 PM
#746
elsewhere (the other GT forum), reporting unveil 7/21
https://rennlist.com/forums/992-gt3-...-specs-17.html
so, yes Martin relax
speculate more, 11 months of waiting to go until some can claim, see "I told you so"
https://rennlist.com/forums/992-gt3-...-specs-17.html
so, yes Martin relax
speculate more, 11 months of waiting to go until some can claim, see "I told you so"
Last edited by JSF101; 08-22-2020 at 08:58 PM. Reason: added link
08-23-2020, 01:05 AM
#747
Hotspots.........................
08-24-2020, 11:03 PM
#748
Stories are now materializing on a German forum that an unofficial time of 7:10 was achieved on the Ring. Everybody is clamoring for verification of that rumor; what power, what tires, etc. and how much faster that is than the GT4 w/PDK.
The following 3 users liked this post by ExMB:
08-25-2020, 03:01 AM
#749
BTG
