How much NA power is technically possible from a flat six?
03-11-2018, 05:23 PM
#46
03-11-2018, 06:36 PM
#47
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With a limit around 85 ft-lbs per Liter with current technology and pump fuel, you’d need
4.7 Liters, each cylinder would be 785cc, making it physically difficult with bore and stroke limitations of the engine architecture as well as very difficult to spin a motor with those dimensions to reasonable rpms needed for good power.
Flat-8 or forced induction or hybrid are better alternatives for that much or more torque...
03-11-2018, 07:03 PM
#48
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I know that many here are hoping that revs will continue to march ever upwards, but there there are some good reasons 10k rpm isn't going to happen with the current package.
Unlike anything to do with electronics (which have enabled efficiency improvements due to EFI, direct injection, variable cam timing, etc) metallurgy is a slowly evolving field.
Consider: that during the WW2 arms race piston engines pushed the state of the art like never before. One of the most impressive want the German Jumo 213 in the ME-190. In its most extreme variants it turned 3,700 rpm. Compared to pre-restriction F1 V8s and V10s which could turn 20,000 RPM in qualifying trim you'd think they were from different planets. However if you compare mean piston speed, which is a crude way to estimate the stress on the crank, rods and pistons due to G forces, you'll find a very different picture:
Jumo 213: 165mm stroke x 3,700 rpm = 20.4 m/s
Formula 1 V8: 39.8 x 20,000 rpm = 26.5 m/s
So by that measure piston speeds climbed by less than .5% per year, and while this is an imperfect comparison it does illustrate how difficult increasing piston speed is. Look at high performance production cars and motorcycles and you'll see revs all over the map. Piston speeds, on the other hand, are tightly clustered:
BMW S1000RR: 14,200 rpm, 23.6 m/s
2012 Z06 Corvette: 7,000 rpm, 23.7 m/s
2018 GT3: 9,000 rpm: 24.5 m/s
Honda S2000: 8,800 rpm, 24.6 m/s
Lexus LFA: 9,500 rpm, 25 m/s
2017 Audi R8: 8,700 rpm, 26.9 m/s (current production car record)
Compared to the Audi it would look like Porsche has considerable room to improve, however the flaw with piston speed is that it doesn't take the size of the pistons into account (a problem as a larger bore results in a heavier piston and hence more stress). "Corrected piston speed" uses bore and stroke to attempt to take that into consideration, and by that measure it goes:
2012 Z06 Corvette: 24.1
Honda S2000: 25.1
2017 Audi R8: 25.7
Lexus LFA: 26.4
2018 GT3: 27.4 (current production car record)
So at 9,000 RPM Porsche has set the high water mark. To get to 10k without reducing stroke piston speed would need to exceed F1 while corrected piston speed would be 15% beyond the nearest street car competition. I believe it could be easily achieved by giving up on longevity (as the RSRs and bikes do) and simultaneously throwing $$ at the issue (as F1 does). At the reliability/ price point Porsche is at, however, I don't see Porsche getting there in the next decade plus (and that's assuming they were trying). Even cost-no-object projects like the 918 can't significantly improve in this area because the metallurgy simply isn't available.
If you're willing to change the current package, on the other hand, they could be at 10k rpm tomorrow. Use a short 67.5mm crank stroke in the current GT3's engine and you'd be at 10k rpm from 3.3 liters with no technical development needed, so if you're just looking for the headline number there are clear paths.
Unlike anything to do with electronics (which have enabled efficiency improvements due to EFI, direct injection, variable cam timing, etc) metallurgy is a slowly evolving field.
Consider: that during the WW2 arms race piston engines pushed the state of the art like never before. One of the most impressive want the German Jumo 213 in the ME-190. In its most extreme variants it turned 3,700 rpm. Compared to pre-restriction F1 V8s and V10s which could turn 20,000 RPM in qualifying trim you'd think they were from different planets. However if you compare mean piston speed, which is a crude way to estimate the stress on the crank, rods and pistons due to G forces, you'll find a very different picture:
Jumo 213: 165mm stroke x 3,700 rpm = 20.4 m/s
Formula 1 V8: 39.8 x 20,000 rpm = 26.5 m/s
So by that measure piston speeds climbed by less than .5% per year, and while this is an imperfect comparison it does illustrate how difficult increasing piston speed is. Look at high performance production cars and motorcycles and you'll see revs all over the map. Piston speeds, on the other hand, are tightly clustered:
BMW S1000RR: 14,200 rpm, 23.6 m/s
2012 Z06 Corvette: 7,000 rpm, 23.7 m/s
2018 GT3: 9,000 rpm: 24.5 m/s
Honda S2000: 8,800 rpm, 24.6 m/s
Lexus LFA: 9,500 rpm, 25 m/s
2017 Audi R8: 8,700 rpm, 26.9 m/s (current production car record)
Compared to the Audi it would look like Porsche has considerable room to improve, however the flaw with piston speed is that it doesn't take the size of the pistons into account (a problem as a larger bore results in a heavier piston and hence more stress). "Corrected piston speed" uses bore and stroke to attempt to take that into consideration, and by that measure it goes:
2012 Z06 Corvette: 24.1
Honda S2000: 25.1
2017 Audi R8: 25.7
Lexus LFA: 26.4
2018 GT3: 27.4 (current production car record)
So at 9,000 RPM Porsche has set the high water mark. To get to 10k without reducing stroke piston speed would need to exceed F1 while corrected piston speed would be 15% beyond the nearest street car competition. I believe it could be easily achieved by giving up on longevity (as the RSRs and bikes do) and simultaneously throwing $$ at the issue (as F1 does). At the reliability/ price point Porsche is at, however, I don't see Porsche getting there in the next decade plus (and that's assuming they were trying). Even cost-no-object projects like the 918 can't significantly improve in this area because the metallurgy simply isn't available.
If you're willing to change the current package, on the other hand, they could be at 10k rpm tomorrow. Use a short 67.5mm crank stroke in the current GT3's engine and you'd be at 10k rpm from 3.3 liters with no technical development needed, so if you're just looking for the headline number there are clear paths.
03-11-2018, 08:04 PM
#49
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That would make an awesome motor for a lightweight model - something like GT4 or a little lighter. Should give ~440hp (with less torque than current GT4, but would allow very low gearing) and sound amazing...
03-11-2018, 08:15 PM
#50
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Originally Posted by Petevb
Agree, see above, though it's not an issue with the block...
Project 1 from benz will need rebuild at 20-30,000 Miles. Sounds fun. Nobody wants that in a Gt3 is both of our points
03-11-2018, 08:33 PM
#52
Drifting
Getting 400 ft-lbs in a NA Flat-6 would be very difficult without lowering redline and horsepower.
With a limit around 85 ft-lbs per Liter with current technology and pump fuel, you’d need
4.7 Liters, each cylinder would be 785cc, making it physically difficult with bore and stroke limitations of the engine architecture as well as very difficult to spin a motor with those dimensions to reasonable rpms needed for good power.
Flat-8 or forced induction or hybrid are better alternatives for that much or more torque...
03-11-2018, 08:35 PM
#53
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I'm not exactly married to the idea of a high-revving NA engine. On the surface it sounds cool to brag that hey my car revs to 9K. But... honest question: What benefit is there to an engine that revs to 9K but puts down less power and torque than a similar engine that revs a bit lower, but puts out more power/torque? Assuming all the gearing in the drivetrain is set up to match each respective setup.
03-11-2018, 08:46 PM
#54
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Thread Starter
J
03-11-2018, 08:49 PM
#55
Drifting
I'd take a GT2RS if I could afford one (and get an allocation lol).A lot of people are die-hard NA, high-revving engine fanatics. That's cool and all. I like the GT3 enough to have bought one. But I'm not one of the guys who folds his arms and says "9K NA or bust!" Many here worry that this or that car will be the last NA version. Ok, well look... if the car is powered by a drunk ferret on a treadmill I really don't care as long as it's fun to drive and performs where it matters.
So yeah, if the 992 winds up being turbo, I'm ok with it. I'm sure the car will be awesome either way.
03-11-2018, 09:20 PM
#56
In my experience the 900 hp in a 918 is a pretty great party trick. Unfortunately you really to be at the right party to use it, and I'm on the track/ autocross/ "that" stretch of road far to rarely. Meanwhile the rest of the time I felt like a shark stuck in a fishbowl- give me a Boxster Spyder 9 days out of 10. So I'm largely past wanting "more" for its own sake, and I suspect that as it becomes more accessible others will come to the same conclusion. Sure there will always be some who're playing a clubhouse game of Top Trumps and others who are chasing lap times, but for the rest of us extreme experiences can be had with much less power than is already fairly easy to buy.
03-11-2018, 09:50 PM
#57
Drifting
I've said for some time- I think we're in the twilight of the "more" era. Power and torque have been the easy part for some time, and are only getting more so. We're discussing squeaking out another 50 hp in the face of 707 hp Hellcats, "ludicrous mode" and cars like the Rimac C 2: ~1,900 hp and the ability to spin all four at will (software allowing) into triple digits. The truth is that in a few years 1000 hp simply won't be that expensive, which will force you to ask: do you really still want "more"?
In my experience the 900 hp in a 918 is a pretty great party trick. Unfortunately you really to be at the right party to use it, and I'm on the track/ autocross/ "that" stretch of road far to rarely. Meanwhile the rest of the time I felt like a shark stuck in a fishbowl- give me a Boxster Spyder 9 days out of 10. So I'm largely past wanting "more" for its own sake, and I suspect that as it becomes more accessible others will come to the same conclusion. Sure there will always be some who're playing a clubhouse game of Top Trumps and others who are chasing lap times, but for the rest of us extreme experiences can be had with much less power than is already fairly easy to buy.
In my experience the 900 hp in a 918 is a pretty great party trick. Unfortunately you really to be at the right party to use it, and I'm on the track/ autocross/ "that" stretch of road far to rarely. Meanwhile the rest of the time I felt like a shark stuck in a fishbowl- give me a Boxster Spyder 9 days out of 10. So I'm largely past wanting "more" for its own sake, and I suspect that as it becomes more accessible others will come to the same conclusion. Sure there will always be some who're playing a clubhouse game of Top Trumps and others who are chasing lap times, but for the rest of us extreme experiences can be had with much less power than is already fairly easy to buy.
03-11-2018, 11:06 PM
#58
03-12-2018, 12:00 AM
#60
Drifting