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Not sure if I'm understanding your question correctly...
Piston speed gives a good indication of the stress on the rods, pistons and crank due to acceleration. The GT3 is quite close to the practical limit of production materials. You can't simply add more material to the rods or pistons to make them stronger, because they would also get heavier and hence the stress would stay constant. Instead you need to use higher grades of light, strong materials like titanium and aluminum alloys if you want to increase stress and hence piston speed. However Porsche is using some of the best materials in the GT3 engine already. Hence the RS kept piston speed stress constant and dropped the revs.
The other option would have been to keep the stroke short and make the bore wider. This could have let them stick with 9k, but unless they changed the bore spacing the cylinder walls would have gotten very thin, leading to thermal issues. The combustion chamber would also have gotten uncomfortably large, leading to potential emissions and flame front issues.
I feel a little bit smarter everytime I read one of your posts. Thank you for that.
What's so magical about 9K rpm anyway? It bemuses me when people start waxing lyrical about 9000 over 8600 or 8200, etc. All that matters is the overall performance of the engine and in every aspect, the RS is better than a GT3. So who really cares if it's 8800 or 9000.
Location: The way to hell is paved by good intentions “Wenn ich Purist höre...entsichere ich meinen Browning” "Myths are fuel for marketing (and nowadays for flippers too,,,)" time to time is not sufficient to be a saint, you must be also an Hero
Speculative philosophy is the philosophy embodying beliefs insusceptible of proof and attempting to gain insight into the nature of the ultimate by intuitive or a priori means.
Wael
Insights or cold math calculations?
former is more powerful and for genius
Not sure if I'm understanding your question correctly...
Piston speed gives a good indication of the stress on the rods, pistons and crank due to acceleration. The GT3 is quite close to the practical limit of production materials. You can't simply add more material to the rods or pistons to make them stronger, because they would also get heavier and hence the stress would stay constant. Instead you need to use higher grades of light, strong materials like titanium and aluminum alloys if you want to increase stress and hence piston speed. However Porsche is using some of the best materials in the GT3 engine already. Hence the RS kept piston speed stress constant and dropped the revs.
The other option would have been to keep the stroke short and make the bore wider. This could have let them stick with 9k, but unless they changed the bore spacing the cylinder walls would have gotten very thin, leading to thermal issues. The combustion chamber would also have gotten uncomfortably large, leading to potential emissions and flame front issues.
Thanks for that Petevb.
But does that mean that the GT3 engine should have been at 8800, just like the RS?
In other words, why not just use a 9k redline in the RS just like they did with the GT3, and use the same materials, bore/stroke layout, etc.?
Not sure if I'm understanding your question correctly...
Piston speed gives a good indication of the stress on the rods, pistons and crank due to acceleration. The GT3 is quite close to the practical limit of production materials. You can't simply add more material to the rods or pistons to make them stronger, because they would also get heavier and hence the stress would stay constant. Instead you need to use higher grades of light, strong materials like titanium and aluminum alloys if you want to increase stress and hence piston speed. However Porsche is using some of the best materials in the GT3 engine already. Hence the RS kept piston speed stress constant and dropped the revs.
The other option would have been to keep the stroke short and make the bore wider. This could have let them stick with 9k, but unless they changed the bore spacing the cylinder walls would have gotten very thin, leading to thermal issues. The combustion chamber would also have gotten uncomfortably large, leading to potential emissions and flame front issues.
I really really enjoy every single post you contribute, thanks.
But does that mean that the GT3 engine should have been at 8800, just like the RS?
In other words, why not just use a 9k redline in the RS just like they did with the GT3, and use the same materials, bore/stroke layout, etc.?
Because they wanted more HP and torque to justify (in part) the price differential and RS "specialness", and more displacement was the only reasonable way to get it. Hence, bore/stroke had to change, etc. etc.....
Because they wanted more HP and torque to justify (in part) the price differential and RS "specialness", and more displacement was the only reasonable way to get it. Hence, bore/stroke had to change, etc. etc.....
I really really enjoy every single post you contribute, thanks.
As do many others.
Originally Posted by Mike in CA
Because they wanted more HP and torque to justify (in part) the price differential and RS "specialness", and more displacement was the only reasonable way to get it. Hence, bore/stroke had to change, etc. etc.....
Right, RS has the same specific output as the 991 GT3. And the same as the RS 4.0 (or very, very close).
**EDIT, it took some time to put the following together and the question was answered in the meantime**
Ipse, keeping same materials/bore/stroke/redline would mean the RS would have same engine, ie. 3.8 L displacement and same performance (hp, torque) as the GT3. To gain performance, they chose to increase displacement by increasing stroke. Increased stroke leads to higher piston speed. Petevb predicted the increased stroke back in January in this post:
Originally Posted by Petevb
I just noticed something interesting. I'd always expected the 991 RS 4.0 to be bored to 4.0L. It was just pointed out, however, that Porsche has a 81.5mm crank on the shelf from the 2010 9A1 base Carrera. And while it's not currently in use it's known to fit into the GT3's 9A1 block.
If one used that crank's 81.5 mm stroke in the 991 GT3, you'd get 4 Liters displacement. That would be an ideal number for homologation, as current GTE class rules state (less than) 4L. And the exactness of the displacement numbers sound too ideal to be chance to me, as Porsche likes round displacement numbers for race engines to maximize displacement for the classes, for example:
996 GT3: 100.0 x 76.4 = 3.600
997 GT3: 102.7 x 76.4 = 3.800
997 RS 4.0: 102.7 x 80.44 = 4.000
Note one dimension was always shared with the predecessor for parts commonality, while the other was always adjusted to get exactly the target displacement.
That 81.5 mm crank doesn't make a round displacement with any existing 9A1 bore (97, 89mm) etc... except the 102mm bore it's never been used with, that of the current GT3:
991 GT3: 102 x 77.5 = 3.800
991 RS?: 102 x 81.5 = 4.000
Seems like that was planned to me, and we know the crank fits the block. My bet? The 991 GT3 RS will get a 102mm bore and an 81.5mm stroke for 4.00 liters. This will make it difficult to maintain the current 9000 rpm redline, but I suspect they will pull it off.
Just thought I'd throw that out there...
The prediction on stroke was spot on.
Although, the redline has been reduced to maintain piston speed.
Again, the math from Petevb:
Originally Posted by Petevb
Mean piston speed at redline:
991 GT3 @ 9,000: 23.3 meters per second
997 RS 4.0 @8,500: 22.8 meters per second
991 GT3 RS @ 8,800: 23.9 meters per second
Because they wanted more HP and torque to justify (in part) the price differential and RS "specialness", and more displacement was the only reasonable way to get it. Hence, bore/stroke had to change, etc. etc.....
And in part because displacement = TQ = lower laptimes. Performance wise it's what the 4.0 made the 4.0.
I never needed to rev the daylights out of the 4.0 to go around the track faster. The 8500 was there to save me a shift before braking. It was the ability to take corners a gear higher that made the difference and had me see 3.8 disappear in the rear view mirror. Now wth the superfast automatic, I think the difference will be smaller with the TQ mostly there to overcome the extra drag for the extra down force and that is where most laptime will come from.
What's so magical about 9K rpm anyway? It bemuses me when people start waxing lyrical about 9000 over 8600 or 8200, etc. All that matters is the overall performance of the engine and in every aspect, the RS is better than a GT3. So who really cares if it's 8800 or 9000.
Its almost as bad as when people start talking about horsepower per liter. Thats all fine and dandy in your 458 until a 8.4 liter viper TA waxes you 6 ways to sunday.
08-26-2015, 03:37 PM
