Just an outside of the company thought...
09-16-2008, 05:06 AM
#1
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Just an outside of the company thought...
I am no engineer or anything, I hate college, I leave it up to the wife and I pay the bills for now until she is all done with school. We have been getting in rx7 cores lately for next months builds and I am studying the motors comparing the two and I have a observation I am stuck on believing tell me what you think. Now I realize that this has probably been figured out by you guys already but just curious, please discuss.
The exhaust ports are on the rotors housing and much larger obviously on the rx7(3rd gen) motors and located on the rotor housing unlike the rx8 which are located on the walls of the front/intermediate/rear housing. I am assuming this change was made due to stricter smog laws(thanks california). One of the most common issues regarding the life span of the motor is based on the carbon build up(especially the AT's) on the the side seals/springs and the apex seals/springs.
Now when you think about the motor in motion it sort of made sense to me why the rx7's ports would do a better job for blowing out carbon through the rotor housings exhaust ports and therefore those engines would last longer and actually last the life of the apex/side seals failing due to actual wear which is the way it should be,
So what do you all think? Realizing no solution came from what I said, it sort of made sense to the carbon build ups most motors come in with. There is always a little wall of solid carbon on the front/intermediate/rear housing on the edges of the exhaust slots of each where the rotor runs along, always, even if it failed for other reasons. That wall doesn't budge even with a blast of brake cleaner, we have to use a file and hope we dont scrath the housings before they reach inspection to knock the wall loose, it almost looks like it was built like that its so smooth/solid.
Just my thoughts, I dunno, off to work now. I typed more then I wanted as usual.
The exhaust ports are on the rotors housing and much larger obviously on the rx7(3rd gen) motors and located on the rotor housing unlike the rx8 which are located on the walls of the front/intermediate/rear housing. I am assuming this change was made due to stricter smog laws(thanks california). One of the most common issues regarding the life span of the motor is based on the carbon build up(especially the AT's) on the the side seals/springs and the apex seals/springs.
Now when you think about the motor in motion it sort of made sense to me why the rx7's ports would do a better job for blowing out carbon through the rotor housings exhaust ports and therefore those engines would last longer and actually last the life of the apex/side seals failing due to actual wear which is the way it should be,
So what do you all think? Realizing no solution came from what I said, it sort of made sense to the carbon build ups most motors come in with. There is always a little wall of solid carbon on the front/intermediate/rear housing on the edges of the exhaust slots of each where the rotor runs along, always, even if it failed for other reasons. That wall doesn't budge even with a blast of brake cleaner, we have to use a file and hope we dont scrath the housings before they reach inspection to knock the wall loose, it almost looks like it was built like that its so smooth/solid.
Just my thoughts, I dunno, off to work now. I typed more then I wanted as usual.
09-16-2008, 05:38 AM
#2
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Mazda first experimented with side exhaust ports many years ago and the problem then was carbon build-up also. They brought it back for the Renesis. I'm no expert, but from what I've been reading here, the main advantage is that there is no overlap with the intake cycle, greater power, lower emissions, and greater compression, I think. But how to solve the carbon problem, that still seems to be the problem. One thing that many of us do is add some FP+ to our premix. As Jax has pointed out, it contains cyclohexanone, which is supposed to emulsify carbon into a graphite lubricant. Would love to take my engine apart to see how well it's working. But then how would I get to work today.
09-16-2008, 05:58 AM
#3
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Here's a much more detailed explanation from someone who knows what they're talking about:
https://www.rx8club.com/series-i-tech-garage-22/advanced-renesis-tech-99617/#post1542277
I just thought it would be cool to get down to some of the subtle differences between the Renesis and the 13B engines. Everyone knows that the compression ratio is a little higher and that the Renesis has side exhaust ports but aside from that not too many people know much more. One thing I am going to leave out is the variable intake system.
The first thing I am going to get into is fuel economy as this seems to be a sore spot among owners. Hopefully you'll look at your engine a little differently after this. As we all know the Renesis makes much more power than any previous naturally aspirated rotary. The last naturally aspirated rotary of the same displacement was the 13B in the 89-91 RX-7. It made 160 hp. I don't care what everyone thinks their engine produces, even the low power engine can top 160!From a fuel consumption standpoint the Renesis is on average about 8% more fuel efficient than the 13B but as much as 15% in parts. Why then are RX-8 owners not getting any better mileage than the average RX-7 owner? 2 reasons. The first is that you have a 4.44:1 rear end ratio whereas the RX-7 had a 4.10:1. Lower yours to a 4.10 and you'll see a little improvement. The other reason is that the RX-8 is at least 300 lbs heavier (and in some cases depending on model comparison, much more) than the RX-7's were. Weight and gearing are holding you back from noticable improvements. You guys that complain now know what to do so do it or put up with it.
The side exhaust port led to many improvements (all of them!) over the 13B. The first of which is the fact that the Renesis no longer has any port overlap. This is the point in the rotation of the rotor where the intake and exhaust ports would simultaneously be opened at the same time. The Renesis does not have this but all of the 13B's did. The port overlap of the peripheral exhaust port rotary caused unstable combustion at low engine speeds and loads and this meant that the older rotaries had to run richer than the stoich a/f ratio in those ranges to run properly. This led to worse fuel economy than in the Renesis.
The side exhaust ports also did something else very important. It allowed the closing point to occur later, right around exhaust TDC. The exhaust closes at 3 degrees BTDC (Before Top Dead Center) to be exact. Having 2 of them per rotor made the total area 47% larger than the single peripheral exhaust port of the 13B. The 13B exhaust port closed later at 48 degrees ATDC (After TOP Dead Center)! There are 2 important things about this. The first of which is that there can now be changed intake port shapes to make them larger and still not have any port overlap. The second thing is that the 13B exhaust ports had a much longer expansion stroke. The exhaust ports opened much earlier on the old engines. Some RX-7 people seem to think that this means that it has much more time to get the exhaust out and it should do it under less total pressure. Wrong!!! A long expansion stroke is unfavorable in terms of thermal efficiency. Heat is energy that can work too and the Renesis uses it better. The proof of this was already mentioned in the fact that the Renesis has an 8-15% improvement in fuel consumption. People can argue this all they want but power levels also prove it. As some may have also noticed, this added heat from the Renesis is also murder to a cat. There are more cat failures on the RX-8 than there were on the RX-7. Now you know why!
On the intake side, the Renesis intake ports are 40% larger than those in the 13B. Again made possible by the side exhaust port layout removing port overlap. As an example of how this has happened, the primary ports of the Renesis open at 3 degrees ATDC while the 13B ports opened at 32 degrees ATDC. That's 29 degrees more of open time. The Renesis ports also close at 65 degrees ABDC (low power at 60 ABDC) while the 13B primaries closed at only 50 degrees ABDC. That's 15-20 degrees more on the closing side as well. Unlike a piston engine we can not keep total opening size the same if we change the timing. On a piston engine this would be done by simply changing the valve lift. The rotary has the piston equivalent of changing both lift and duration.
Opening the intake ports earlier had a big effect on a different aspect of the engine. That was the side seals. On the older rotaries, when people opened the intake ports too early, they risked both having a corner seal potentially fall into the intake port but also from having the side seal edge crash into the closing edge of the intake port. The Renesis has something very subtle to try to alleviate these issues. The side and corner seals are moved outward on the rotor by 2mm. I have seen people ask how hard it would be to modify a 13B rotor to work in a Renesis. This is one big issue but I'll get into the others.
A cutoff seal was also added to the Renesis rotor between the side seals and the outer most oil control ring to prevent any port overlap on the sides of the rotor. Without this ring, a small amount of exhaust gasses could actually get around the side and mix with the intake. So much for no overlap. The cutoff seal took care of this. The shape of the side seals is interesting as well. It is a wedge shape. This is to help remove any carbon from building up in the groove which would cause it to stick. Carbon is also the reason for the interesting shape of the Renesis exhaust ports as well as the overly large seal clearances in the Renesis over the 13B. And some of you thought carbon was related to synthetic oils! Shame on you! That was actually the reason why we haven't seen a side ported rotary until the Renesis. Back in the '70's Mazda did try the side exhaust port and back then they also found it to be superior. The problem was the carbon would cause seals to stick and break. You can't market that. They met the standards of the time with the peripheral exhaust port so that's what they stuck with.
The rotor shape has changed slightly as well too. It is very subtle but the 13B rotors are more "round" while the Renesis rotors are more of a true "triangle" shape. Hopefully you can figure that one out. Another very small difference and this one is real small is the rotor width. Yes there is a difference! The rotor housings are not any wider though so you don't get any added displacement. To minimize the hot gasses that could come around the sides of the rotors, the clearance on each side has been decreased an average of 18%! The 13B has more clearance than the Renesis does. Race engines add more clearance yet. This was always an issue at higher rpms over 8000 as there was the possibility that the rotors could actually come into contact with the side housings. You have a higher revving engine with less clearance! Why? Your rotors are lighter and better balanced, your bearings are better, and your eccentric shaft is lighter and stronger (more rigid). Don't sweat it.
What about compression? Well as we all know it went up from 9.0:1 on the previous RX-7 (turbo) and 9.7:1 on the last naturally aspirated rotary to 10.0:1 on the RX-8. Years ago Mazda found that there was no appreciable difference in power on a naturally aspirated rotary with compression ratios from 9.0:1 up to 11.0:1. Virtually no power difference. So why then would they do it? Emissions! Yes, emissions. I know that doesn't make much sense. Remember that although the engine has no port overlap, there is still a small space in the rotor face that some unburned air gets carried back into the intake side in. By raising the compression ratio, they made this area slightly smaller. This does a better job at minimizing the containment volume of the exhaust gasses at closing timing and reduces the need for exhaust gas recirculation. This results in improved combustion stability at low engine speeds and loads. I know many would like lower compression ratios for boosted applications but emissions comes first to Mazda. Boost is for you to work out!
I'm going to skip going over the intake system and the anti wet port jet air/fuel system.
On to apex seals... The old apex seals were 3 piece from the factory but 1 and 2 piece were available from the aftermarket and were even types that once appeared on much older engines. These seals had a total height of 8 mm. The Renesis apex seals are 4.5 mm tall and are 2 piece. That's it. A lighter apex seal can seal better with less spring pressure at higher rpms which improves efficiency and decreases wear. As a result of this decrease in wear, the Renesis does not need as much oil injection as the 13B did. The dual oil injector placement in the rotor housings also allows more even distribution of the injected oil in the engine. A downside to the lighter apex seal is that it is easier to succumb to flooding! The rounded shape of the seal tip allows gasoline to get under it. As it rotates and tries to compress the air, this added pressure will also be in the gasoline under the seal. This will exert pressure on the apex seal trying to push it into it's groove. If it does this, pressure will bleed from one chamber to the next which lowers compression. The fact that any fuel in the engine doesn't leave as easily out a side exhaust port as it does a peripheral one doesn't help matters either. That's the best explanation I can give any of you as to why your engine floods.
It's 3:30 am right now and I'm damn tired so I'll leave with one more thing and add more details to this tomorrow. Your corner seals have a coating on them which Mazda refers to as DLC. I'm too tired to go look it up right now. That makes them very hard. It has been reported that when used in Renesis rotors in older 13B engines, these seals absolutely will tear up a 13B housing. This means the Renesis housings must also have a coating on them which is much tougher. More on the engine later. I'm sure a few others who know some other small things will have contributed by then.
That's all for now.
The first thing I am going to get into is fuel economy as this seems to be a sore spot among owners. Hopefully you'll look at your engine a little differently after this. As we all know the Renesis makes much more power than any previous naturally aspirated rotary. The last naturally aspirated rotary of the same displacement was the 13B in the 89-91 RX-7. It made 160 hp. I don't care what everyone thinks their engine produces, even the low power engine can top 160!From a fuel consumption standpoint the Renesis is on average about 8% more fuel efficient than the 13B but as much as 15% in parts. Why then are RX-8 owners not getting any better mileage than the average RX-7 owner? 2 reasons. The first is that you have a 4.44:1 rear end ratio whereas the RX-7 had a 4.10:1. Lower yours to a 4.10 and you'll see a little improvement. The other reason is that the RX-8 is at least 300 lbs heavier (and in some cases depending on model comparison, much more) than the RX-7's were. Weight and gearing are holding you back from noticable improvements. You guys that complain now know what to do so do it or put up with it.
The side exhaust port led to many improvements (all of them!) over the 13B. The first of which is the fact that the Renesis no longer has any port overlap. This is the point in the rotation of the rotor where the intake and exhaust ports would simultaneously be opened at the same time. The Renesis does not have this but all of the 13B's did. The port overlap of the peripheral exhaust port rotary caused unstable combustion at low engine speeds and loads and this meant that the older rotaries had to run richer than the stoich a/f ratio in those ranges to run properly. This led to worse fuel economy than in the Renesis.
The side exhaust ports also did something else very important. It allowed the closing point to occur later, right around exhaust TDC. The exhaust closes at 3 degrees BTDC (Before Top Dead Center) to be exact. Having 2 of them per rotor made the total area 47% larger than the single peripheral exhaust port of the 13B. The 13B exhaust port closed later at 48 degrees ATDC (After TOP Dead Center)! There are 2 important things about this. The first of which is that there can now be changed intake port shapes to make them larger and still not have any port overlap. The second thing is that the 13B exhaust ports had a much longer expansion stroke. The exhaust ports opened much earlier on the old engines. Some RX-7 people seem to think that this means that it has much more time to get the exhaust out and it should do it under less total pressure. Wrong!!! A long expansion stroke is unfavorable in terms of thermal efficiency. Heat is energy that can work too and the Renesis uses it better. The proof of this was already mentioned in the fact that the Renesis has an 8-15% improvement in fuel consumption. People can argue this all they want but power levels also prove it. As some may have also noticed, this added heat from the Renesis is also murder to a cat. There are more cat failures on the RX-8 than there were on the RX-7. Now you know why!
On the intake side, the Renesis intake ports are 40% larger than those in the 13B. Again made possible by the side exhaust port layout removing port overlap. As an example of how this has happened, the primary ports of the Renesis open at 3 degrees ATDC while the 13B ports opened at 32 degrees ATDC. That's 29 degrees more of open time. The Renesis ports also close at 65 degrees ABDC (low power at 60 ABDC) while the 13B primaries closed at only 50 degrees ABDC. That's 15-20 degrees more on the closing side as well. Unlike a piston engine we can not keep total opening size the same if we change the timing. On a piston engine this would be done by simply changing the valve lift. The rotary has the piston equivalent of changing both lift and duration.
Opening the intake ports earlier had a big effect on a different aspect of the engine. That was the side seals. On the older rotaries, when people opened the intake ports too early, they risked both having a corner seal potentially fall into the intake port but also from having the side seal edge crash into the closing edge of the intake port. The Renesis has something very subtle to try to alleviate these issues. The side and corner seals are moved outward on the rotor by 2mm. I have seen people ask how hard it would be to modify a 13B rotor to work in a Renesis. This is one big issue but I'll get into the others.
A cutoff seal was also added to the Renesis rotor between the side seals and the outer most oil control ring to prevent any port overlap on the sides of the rotor. Without this ring, a small amount of exhaust gasses could actually get around the side and mix with the intake. So much for no overlap. The cutoff seal took care of this. The shape of the side seals is interesting as well. It is a wedge shape. This is to help remove any carbon from building up in the groove which would cause it to stick. Carbon is also the reason for the interesting shape of the Renesis exhaust ports as well as the overly large seal clearances in the Renesis over the 13B. And some of you thought carbon was related to synthetic oils! Shame on you! That was actually the reason why we haven't seen a side ported rotary until the Renesis. Back in the '70's Mazda did try the side exhaust port and back then they also found it to be superior. The problem was the carbon would cause seals to stick and break. You can't market that. They met the standards of the time with the peripheral exhaust port so that's what they stuck with.
The rotor shape has changed slightly as well too. It is very subtle but the 13B rotors are more "round" while the Renesis rotors are more of a true "triangle" shape. Hopefully you can figure that one out. Another very small difference and this one is real small is the rotor width. Yes there is a difference! The rotor housings are not any wider though so you don't get any added displacement. To minimize the hot gasses that could come around the sides of the rotors, the clearance on each side has been decreased an average of 18%! The 13B has more clearance than the Renesis does. Race engines add more clearance yet. This was always an issue at higher rpms over 8000 as there was the possibility that the rotors could actually come into contact with the side housings. You have a higher revving engine with less clearance! Why? Your rotors are lighter and better balanced, your bearings are better, and your eccentric shaft is lighter and stronger (more rigid). Don't sweat it.
What about compression? Well as we all know it went up from 9.0:1 on the previous RX-7 (turbo) and 9.7:1 on the last naturally aspirated rotary to 10.0:1 on the RX-8. Years ago Mazda found that there was no appreciable difference in power on a naturally aspirated rotary with compression ratios from 9.0:1 up to 11.0:1. Virtually no power difference. So why then would they do it? Emissions! Yes, emissions. I know that doesn't make much sense. Remember that although the engine has no port overlap, there is still a small space in the rotor face that some unburned air gets carried back into the intake side in. By raising the compression ratio, they made this area slightly smaller. This does a better job at minimizing the containment volume of the exhaust gasses at closing timing and reduces the need for exhaust gas recirculation. This results in improved combustion stability at low engine speeds and loads. I know many would like lower compression ratios for boosted applications but emissions comes first to Mazda. Boost is for you to work out!
I'm going to skip going over the intake system and the anti wet port jet air/fuel system.
On to apex seals... The old apex seals were 3 piece from the factory but 1 and 2 piece were available from the aftermarket and were even types that once appeared on much older engines. These seals had a total height of 8 mm. The Renesis apex seals are 4.5 mm tall and are 2 piece. That's it. A lighter apex seal can seal better with less spring pressure at higher rpms which improves efficiency and decreases wear. As a result of this decrease in wear, the Renesis does not need as much oil injection as the 13B did. The dual oil injector placement in the rotor housings also allows more even distribution of the injected oil in the engine. A downside to the lighter apex seal is that it is easier to succumb to flooding! The rounded shape of the seal tip allows gasoline to get under it. As it rotates and tries to compress the air, this added pressure will also be in the gasoline under the seal. This will exert pressure on the apex seal trying to push it into it's groove. If it does this, pressure will bleed from one chamber to the next which lowers compression. The fact that any fuel in the engine doesn't leave as easily out a side exhaust port as it does a peripheral one doesn't help matters either. That's the best explanation I can give any of you as to why your engine floods.
It's 3:30 am right now and I'm damn tired so I'll leave with one more thing and add more details to this tomorrow. Your corner seals have a coating on them which Mazda refers to as DLC. I'm too tired to go look it up right now. That makes them very hard. It has been reported that when used in Renesis rotors in older 13B engines, these seals absolutely will tear up a 13B housing. This means the Renesis housings must also have a coating on them which is much tougher. More on the engine later. I'm sure a few others who know some other small things will have contributed by then.
That's all for now.
09-16-2008, 02:16 PM
#5
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If we're busy thinking about cool ideas for efficiency and whatnot, someone help me with this idea.
Piston engines these days, especially the big ******* like a HEMI, rely on cool technologies like cylinder deactivation to bump up the efficiency. You basically pick a pair of cylinders to deactivate, and the engine stops feeding them gas and closes all of the ports for them. There's an 'air spring' effect - the piston compresses whatever exhaust was sitting in there from the last time it was running and then when it comes back down the compressed air pushes it down. The net effect is no drag on the rest of the engine.
How hard would it be to do this with a rotary?
One thing I've never been sure about is whether the ports on a rotary actually close with some sort of valve-like mechanism, or whether it's simply accomplished by the rotor being over the ports at any time when no input/output from them is desired.
If the ports actually have some kind of actuator that closes them, then it should be trivial to disable one rotor whenever the load on the engine (i.e. pedal position) is low and has been fairly static, indicating the driver doesn't need much power at the time. All you'd do is close the ports and you'd have a rotor spinning and compressing/expanding the same air over and over just like a piston would, in essence a rotating air spring.
If the ports are 'always open', stopping gas to a rotor would cause it to behave like a vacuum brake, the same way it does when you downshift and engine brake: no gas is put in and the action of sucking air into the rotor and compressing it is what slows you down.
However, you could easily mitigate the effect of that engine braking, which you wouldn't want if you were disabling a rotor for efficiency's sake, by using forced induction. If a supercharger (for simplicity of plumbing) was used, not to generate massive amounts of boost but rather to simply equalize the amount of vacuum braking 'drag' that the deactivated rotor would cause otherwise, you'd be able to run the remaining rotor(s) with no penalty.
What do you guys think? Is this even worth considering? It would certainly make having something like a three or four rotor engine a lot more workable, especially if the user was given full manual control over how many rotors were activated at what time (or if they could set up automated profiles that activated the rotors depending on driving style, i.e. sport and efficiency modes).
You could have a dual-Renesis 4-rotor engine, but run it on two rotors for city driving and even just one for a long highway cruise once you're up to speed.
Piston engines these days, especially the big ******* like a HEMI, rely on cool technologies like cylinder deactivation to bump up the efficiency. You basically pick a pair of cylinders to deactivate, and the engine stops feeding them gas and closes all of the ports for them. There's an 'air spring' effect - the piston compresses whatever exhaust was sitting in there from the last time it was running and then when it comes back down the compressed air pushes it down. The net effect is no drag on the rest of the engine.
How hard would it be to do this with a rotary?
One thing I've never been sure about is whether the ports on a rotary actually close with some sort of valve-like mechanism, or whether it's simply accomplished by the rotor being over the ports at any time when no input/output from them is desired.
If the ports actually have some kind of actuator that closes them, then it should be trivial to disable one rotor whenever the load on the engine (i.e. pedal position) is low and has been fairly static, indicating the driver doesn't need much power at the time. All you'd do is close the ports and you'd have a rotor spinning and compressing/expanding the same air over and over just like a piston would, in essence a rotating air spring.
If the ports are 'always open', stopping gas to a rotor would cause it to behave like a vacuum brake, the same way it does when you downshift and engine brake: no gas is put in and the action of sucking air into the rotor and compressing it is what slows you down.
However, you could easily mitigate the effect of that engine braking, which you wouldn't want if you were disabling a rotor for efficiency's sake, by using forced induction. If a supercharger (for simplicity of plumbing) was used, not to generate massive amounts of boost but rather to simply equalize the amount of vacuum braking 'drag' that the deactivated rotor would cause otherwise, you'd be able to run the remaining rotor(s) with no penalty.
What do you guys think? Is this even worth considering? It would certainly make having something like a three or four rotor engine a lot more workable, especially if the user was given full manual control over how many rotors were activated at what time (or if they could set up automated profiles that activated the rotors depending on driving style, i.e. sport and efficiency modes).
You could have a dual-Renesis 4-rotor engine, but run it on two rotors for city driving and even just one for a long highway cruise once you're up to speed.
09-16-2008, 03:17 PM
#7
The Local Idiot
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IMO, no. The goal of a variable displacement engine is to improve efficiency while cruising, but still produce power when it is needed/wanted. There's a much easier way to do that: forced induction. It accomplishes the same goal, and has already been tried and proven.
09-16-2008, 03:23 PM
#8
Rotary Powered Countryboy
Yeah I believe the side ports are what is causing the most carbon however I think that overall this motor is "better" then the 7 motor besides the heavier carbon build up
09-16-2008, 04:02 PM
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IMO, no. The goal of a variable displacement engine is to improve efficiency while cruising, but still produce power when it is needed/wanted. There's a much easier way to do that: forced induction. It accomplishes the same goal, and has already been tried and proven.
09-16-2008, 04:09 PM
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It doesn't improve efficiency, it lets you make more power without loosing a significant amount of efficiency. Think about a variable displacement engine as being able to add several extra pistons when needed instead of turning them off to save fuel.
EDIT: To clarify, in the example you gave instead of making a 3 rotor variable displacement engine, you'd simply use the same 2 rotor engine you had before and turbo it to get the extra power out of it, and end up with almost the exact same fuel efficiency in day to day driving.
Last edited by rotary.enthusiast; 09-16-2008 at 04:13 PM.
09-16-2008, 04:45 PM
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Oh, okay, so the 2-rotor turbo is as efficient than the 3-rotor with rotor-deactivation under normal driving, and just as powerful when under high load.
But, it still doesn't have as much torque down low, does it?
But, it still doesn't have as much torque down low, does it?
09-16-2008, 05:01 PM
#12
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Thanks for the read, sorry I started a tech talk though. I was just saying this totally based on my observation at work, I am no mechanic, I just like to figure things out that I have no background on for fun.
09-16-2008, 08:55 PM
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All depends on the FI setup. Super chargers can work over the entire RPM range, and you can get a turbo setup that spools quickly enough that the low end lag isn't noticeable. I'm not saying the two solutions are identical... there are plenty of arguments to be made for variable displacement, it's just my opinion that it isn't worth the R&D to make it work in a rotary. If they ever make full size trucks with rotaries I might revise my opinion
09-17-2008, 06:53 AM
#16
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Heck, even in car/minivan sized piston engines, variable displacement has been a variable disappointment, maybe a couple mpg better fuel economy, nothing to write home about.
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