Why do people think a turbo cares about engine RPM?
02-08-2012, 08:46 PM
#162
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Oh, and I just noticed:
2" bore, 4" stroke = 100.53 ci
4" bore, 2" stroke = 201.06 ci
So now I'm sure the engine with the 4" bore would outpower the 101 at any rpm.
if you wanted a 201 with a 4" stroke, it'd need a 2.828" bore.
2" bore, 4" stroke = 100.53 ci
4" bore, 2" stroke = 201.06 ci
So now I'm sure the engine with the 4" bore would outpower the 101 at any rpm.
if you wanted a 201 with a 4" stroke, it'd need a 2.828" bore.
02-08-2012, 09:32 PM
#163
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Point remains - 4" bore with 2" stroke vs 2.828" bore with 4" stroke, the longer stroke has more mechanical advantage.
02-08-2012, 09:53 PM
#164
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But less surface area = less power captured, not to mention that power has to move the piston double the distance, both inefficiencies that counteract the mechanical advantage.
Think about this: f1 engines are designed to capture as much power as possible. They use an oversquare design. Most production engines use an oversquare design. If overstroked design was more efficient, and could somehow produce more torque at lower rpm, don't you think OEM's would jump all over it? After all, the increase in low end torque would allow more economy oriented gearing, and since it'd use the same amount of air as the oversquare design, while producing more power with that same air mass, there'd be no logical reason NOT to use it. Honestly I can't think of one modern mass produced engine with an overstroked design.
Think about this: f1 engines are designed to capture as much power as possible. They use an oversquare design. Most production engines use an oversquare design. If overstroked design was more efficient, and could somehow produce more torque at lower rpm, don't you think OEM's would jump all over it? After all, the increase in low end torque would allow more economy oriented gearing, and since it'd use the same amount of air as the oversquare design, while producing more power with that same air mass, there'd be no logical reason NOT to use it. Honestly I can't think of one modern mass produced engine with an overstroked design.
02-08-2012, 10:35 PM
#165
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But less surface area = less power captured, not to mention that power has to move the piston double the distance, both inefficiencies that counteract the mechanical advantage.
Think about this: f1 engines are designed to capture as much power as possible. They use an oversquare design. Most production engines use an oversquare design. If overstroked design was more efficient, and could somehow produce more torque at lower rpm, don't you think OEM's would jump all over it? After all, the increase in low end torque would allow more economy oriented gearing, and since it'd use the same amount of air as the oversquare design, while producing more power with that same air mass, there'd be no logical reason NOT to use it. Honestly I can't think of one modern mass produced engine with an overstroked design.
Think about this: f1 engines are designed to capture as much power as possible. They use an oversquare design. Most production engines use an oversquare design. If overstroked design was more efficient, and could somehow produce more torque at lower rpm, don't you think OEM's would jump all over it? After all, the increase in low end torque would allow more economy oriented gearing, and since it'd use the same amount of air as the oversquare design, while producing more power with that same air mass, there'd be no logical reason NOT to use it. Honestly I can't think of one modern mass produced engine with an overstroked design.
You are wrong about F1 engines. They are designed for maximum power from a specific displacement. When you are limited in displacement the only other thing you can do is increase RPM. There are physical limits to how fast you can have a piston moving, so you need a shorter stroke.
What happens when you can design whatever you want, with practically unlimited funds?
The most powerful engine in the world: Wartsila-Sulzer RTA96A
960mm bore
2500mm stroke
One of the most efficient internal combustion engines ever designed.
You can lead a horse to water....
There's those fluorescents!
/argument
Last edited by Doward; 02-09-2012 at 01:25 AM.
02-08-2012, 11:58 PM
#167
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Are we out of the fluorescent bulbs yet? I'm not fond of incandescent.
You are wrong about F1 engines. They are designed for maximum power from a specific displacement. When you are limited in displacement the only other thing you can do is increase RPM. There are physical limits to how fast you can have a piston moving, so you need a shorter stroke.
You are wrong about F1 engines. They are designed for maximum power from a specific displacement. When you are limited in displacement the only other thing you can do is increase RPM. There are physical limits to how fast you can have a piston moving, so you need a shorter stroke.
On a side note, think about this; with double the stroke, it's not nearly as easy to fill the cylinder, since the air has to travel double the distance to get to the bottom. Since the piston is moving down at a greater velocity, the air must follow (at a greater velocity), otherwise VE will be affected. This might be why high spinning engines use an oversquare design; the air isn't being forced to move up and down as much, it's in effect much more stationary. This might also be why that 2300 ton monster, the RTA96-C, is so overstroked; with a mean piston speed of only 8.5 m/s, the air is capable of keeping up with the piston much more readily than an ls1 with a mean piston speed of 18.3996 m/s at only 6000 rpm, or an f1 engine nearing 25 m/s (I actually thought it'd be higher)
What happens when you can design whatever you want, with practically unlimited funds?
960mm bore
2500mm stroke
One of the most efficient internal combustion engines ever designed.
960mm bore
2500mm stroke
One of the most efficient internal combustion engines ever designed.
Also, on another side note not really related to anything, diesel ignites based on pressure and heat; therefore, when pressure and heat are right, all the diesel molecules will combust nearly simultaneously, so the size of this diesel engine won't affect burn rate. Gasoline engines on the other hand have a huge amount to do with the speed of the flame front, and quickly become rpm (or bore) limited when that flame front can't travel fast enough. this would once again point to an overstroked design being better for high rpm use (f1); but the power gains from a more complete burn are negated by the fact that the overstroked design increases mean piston speed, which means the air can't keep up with the piston as well, which means less air mass in the cylinder. the slight increase in low rpm efficiency (due to more complete burn) can't compare with the increase in air mass that an oversquare design offers at high rpm. Again, this really has little to do with our argument, except that it may prove that an oversquare design is better for high rpm use due to lower mean piston speed (less down/up air movement), and that an overstroked design burns the air mass more efficiently (smaller flame front), but simply can't pull the same air mass into the cylinders at high rpm, due to the increased mean piston speed.
Last edited by Mr. Sir; 02-09-2012 at 02:52 AM.
02-09-2012, 01:28 AM
#168
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Mr Sir, you seem to enjoy fighting the laws of physics. Go ahead and keep it up, enjoy yourself, whatever you wish. I've got work to do.
*munches bulbs*
02-09-2012, 07:23 AM
#170
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Guys,
We obviously have some switched on people here that understand air mass and its required movements to create HP & TQ....what we all chase!!!
However for the layman-LS1 346 what TT and single set up will create best TQ and HP and WHY...
forget about cams & gears ETC....
this will show all the variances in argument and help explain the difference between argements...
LS1 twin GT3582...= X(HP) & Y (TQ) @ Z (RPM)
real world result and numbers show idiots like me how to achieve targets..
We obviously have some switched on people here that understand air mass and its required movements to create HP & TQ....what we all chase!!!
However for the layman-LS1 346 what TT and single set up will create best TQ and HP and WHY...
forget about cams & gears ETC....
this will show all the variances in argument and help explain the difference between argements...
LS1 twin GT3582...= X(HP) & Y (TQ) @ Z (RPM)
real world result and numbers show idiots like me how to achieve targets..
02-09-2012, 08:44 AM
#171
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I like to follow what some of the buick GN guys preach... use the smallest available turbo to meet the desired power goal for the rpm range of your engine.
You have to look at the entire combination from intake/heads/cam/tranny/gearing/etc in order to properly size the turbo system.
The same turbo can have vastly different operating characteristics depending on pressure ratios its operating at. Stock LS1 vs Cam/heads LS1 will change the way a T76 operates, assuming no other changes but just heads/cam. Its like my friend who did a T88 on a 4.8L. Naturally thats a huge turbo for a stock 4.8 and it would be very laggy. But he had ported 317 heads and 230 deg cam, turned 7500+ rpm and by 20-23 psi, i forget the exact level but it made 702whp on pump gas. Not bad. IF you do the math, a 4.8 at 7500 rpm is in the middle of the T88 compressor map. It was still a touch laggy and a smaller turbo would have done the same goals however, but it was what was laying around at the time. Heads/cam allowed more mass flow thru the engine to fit the turbo better.
You have to look at the entire combination from intake/heads/cam/tranny/gearing/etc in order to properly size the turbo system.
The same turbo can have vastly different operating characteristics depending on pressure ratios its operating at. Stock LS1 vs Cam/heads LS1 will change the way a T76 operates, assuming no other changes but just heads/cam. Its like my friend who did a T88 on a 4.8L. Naturally thats a huge turbo for a stock 4.8 and it would be very laggy. But he had ported 317 heads and 230 deg cam, turned 7500+ rpm and by 20-23 psi, i forget the exact level but it made 702whp on pump gas. Not bad. IF you do the math, a 4.8 at 7500 rpm is in the middle of the T88 compressor map. It was still a touch laggy and a smaller turbo would have done the same goals however, but it was what was laying around at the time. Heads/cam allowed more mass flow thru the engine to fit the turbo better.
02-09-2012, 09:15 AM
#172
Straight from Borg Warner http://www.3k-warner.de/products/tur...ompressor.aspx
As you said, flow comes into play, but with a compressor you are increasing the pressure (putting more stuff in the same space) and therefore flowing more total mass per unit. That makes a centrifugal compressor a mass flow device. Unless of course you don't plan to use it as a compressor, then it can be considered a flow device because it's not changing the mass flow of the air going through it, just the flow.
Hope that makes more sense for you.
As you said, flow comes into play, but with a compressor you are increasing the pressure (putting more stuff in the same space) and therefore flowing more total mass per unit. That makes a centrifugal compressor a mass flow device. Unless of course you don't plan to use it as a compressor, then it can be considered a flow device because it's not changing the mass flow of the air going through it, just the flow.
Hope that makes more sense for you.
02-09-2012, 09:20 PM
#173
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02-09-2012, 10:56 PM
#174
not shown: the reclusive and misunderstood, TURBO BEAR
http://fc02.deviantart.net/fs18/f/20..._kokololio.jpg
http://fc02.deviantart.net/fs18/f/20..._kokololio.jpg
No one cares what you think Dave, they just care about the TURBO'S!!!
02-11-2012, 10:50 PM
#176
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Hypothetical over simplified example based on made up compressors operating as a volumetric flow device only.
Small turbo can flow 600 CFM, big turbo can flow 1800 CFM. We compound them. Big turbo is flowing 1800 CFM into the small turbo's inlet, what is the output? It would have to be 600 CFM since the small turbo is only capable of flowing 600 CFM. But we should all know that this is not what happens because compound turbo charging works.
Small turbo can flow 600 CFM, big turbo can flow 1800 CFM. We compound them. Big turbo is flowing 1800 CFM into the small turbo's inlet, what is the output? It would have to be 600 CFM since the small turbo is only capable of flowing 600 CFM. But we should all know that this is not what happens because compound turbo charging works.
Hypothetical over simplified example based on made up compressors operating as mass flow devices at normal atmospheric pressure of 14.7 PSI, I'm ignoring rpm, engine size and compressor efficiency for simplicity.
Small turbo can flow 60lbs/min at a pressure ratio of 2:1 (14.7 psi boost) when run at normal atmospheric pressure (14.7psi or 0psi boost, normal air we breath), big turbo can flow 120lbs/min at a pressure ratio of 2.5:1 (22 psi boost). Big turbo outputs 120lbs/min at 2.5:1 (22psi) into the inlet of the small turbo which then compresses that 120lbs/min by 2:1 (22psi * 2) to get 44psi. Mass flow coming from the output of the small turbo is now 240lbs/min @ 44psi boost or a pressure ratio of 4:1 or you could say the air is now 4 times denser than normal.
Small turbo can flow 60lbs/min at a pressure ratio of 2:1 (14.7 psi boost) when run at normal atmospheric pressure (14.7psi or 0psi boost, normal air we breath), big turbo can flow 120lbs/min at a pressure ratio of 2.5:1 (22 psi boost). Big turbo outputs 120lbs/min at 2.5:1 (22psi) into the inlet of the small turbo which then compresses that 120lbs/min by 2:1 (22psi * 2) to get 44psi. Mass flow coming from the output of the small turbo is now 240lbs/min @ 44psi boost or a pressure ratio of 4:1 or you could say the air is now 4 times denser than normal.
02-12-2012, 02:08 PM
#177
When a compressor is said to flow 1800cfm it is at the inlet. At a 3:1 pressure ratio the first compressor would have 1800cfm at atmospheric pressure in and 600cfm at 3 times atmospheric out. This works out.
You state that the second compressor has 120lbs/min in and 240lbs/min out. Are you suggesting that turbos can create mass? Antoine Lavoisier would disagree.
You state that the second compressor has 120lbs/min in and 240lbs/min out. Are you suggesting that turbos can create mass? Antoine Lavoisier would disagree.
02-12-2012, 06:32 PM
#178
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Small turbo can flow 60lbs/min at a pressure ratio of 2:1 (14.7 psi boost) when run at normal atmospheric pressure (14.7psi or 0psi boost, normal air we breath), big turbo can flow 120lbs/min at a pressure ratio of 2.5:1 (22 psi boost). Big turbo outputs 120lbs/min at 2.5:1 (22psi) into the inlet of the small turbo which then compresses that 120lbs/min by 2:1 (22psi * 2) to get 44psi. Mass flow coming from the output of the small turbo is now 240lbs/min @ 44psi boost or a pressure ratio of 4:1 or you could say the air is now 4 times denser than normal.
Large turbo @ 22psi = 2.5:1 (as you stated)
Small turbo @ 14.7psi = 1.4:1 (36.7 + 14.7 / 36.7)
This is assuming the small turbo wastergate is still set-up to open at 14.7psig.
