GM is advertising a new engine for the Suburban that has "Active Fuel Management™ technology, which allows for seamless movement from eight cylinders to four cylinders with no reduction in available power". Two questions for a GM technical guru:
1. Does this really make a difference in fuel consumption, or is it just a scam to sell Suburban's to environmental whacko's?
2. If it's for real, can our engines be retro-fit to run on 4 and 8 cylinders?
GM is advertising a new engine for the Suburban that has "Active Fuel Management™ technology, which allows for seamless movement from eight cylinders to four cylinders with no reduction in available power". Two questions for a GM technical guru:
1. Does this really make a difference in fuel consumption, or is it just a scam to sell Suburban's to environmental whacko's?
2. If it's for real, can our engines be retro-fit to run on 4 and 8 cylinders?
TIA,
Texster68
Its called Displacement On Demand "DOD" for short. Yes of course you will get better fuel economy BUT mostly on the Highway. DOD is nothing new Cadillac in 1981 had a crude version called V8-6-4 . It was a miserable failure due to the lack of real "computer" controls to manage the system. Today such a system is a no brainier to get better highway economy from a big V8. Chrysler was first to market the new computerized version of displacement on demand last year. I hear all of the Cadillacs V8-6-4s woes are gone in the latest designs. Its also a way for a company like GM to get a better Corporate average fuel economy rating. Retrofit DOD? I highly doubt it would be cost effective.
My SSR: BUFFY = Big Ugly Fat Friendly Yankee - 2004 Vin# 8120
I have DOD on my 2005 Envoy Denali. (In fact, it is the same motor & tranny setup as the SSR, but it also has DOD!) I wanted something to tow my enclosed Harley trailer with no troubles going up hills.
I will tell you, it IS seamless! You can't tell when it's 4, 6 or 8 cylinders. But hit the throttle and all 8 will kick in within microseconds.
I wasn't sure about it, but no complaints so far.
Oh yeah, on the road, 21 to 23 MPG consistently
__________________
Having fun, driving "Miss Daisy"
I had a rental Impala SS a few months ago...this is the one with the 300bhp small block. Here is what Chevy says about it;
A 303-hp small block V8 is standard in SS and features Active Fuel Management that uses eight cylinders when you want more power, four cylinders when you need better mileage — delivering an EPA estimated, petrol-pinching 28 MPG highway.
It seemed to work great, I got good gas mileage except when I was giving it a lot of stick, at times there was a very slight lag, almost like an old turbo car, but it was only noticeable now and then.
Unfortunately, DoD is only available on the 5.3L and not on the 6.0 in the Suburban. If I bought a Suburban, and I've already looked at them, I'd be getting the 6.0. I looked at the EPA fuel mileage estimates on the two and there is a notable difference. Several mpg if I recall correctly.
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GM is advertising a new engine for the Suburban that has "Active Fuel Management™ technology, which allows for seamless movement from eight cylinders to four cylinders with no reduction in available power". Two questions for a GM technical guru:
1. Does this really make a difference in fuel consumption, or is it just a scam to sell Suburban's to environmental whacko's?
2. If it's for real, can our engines be retro-fit to run on 4 and 8 cylinders?
TIA,
Texster68
A retrofit is not really a practical alternative - not to say it can't be done. The piece parts for the engine should be interchangeable but the big issue will be the computer and software is different for the DOD as compared to the SSR. Reverse engineering that would be alot of fun for someone with nothing else to do..
Retrofit is not an option without wholesale swapping of components. There are pleasibility hardware items like the exhaust and engine mounts that make the transition seamless. The fuel economy is real and we made a specific decision not to implement this technology on the SSR, because it didn't seem to fit our theme. There are expensive hardware components in the engine. The Corvette doesn't have it now, but maybe it will come in the future.
Retrofit is not an option without wholesale swapping of components. There are pleasibility hardware items like the exhaust and engine mounts that make the transition seamless. The fuel economy is real and we made a specific decision not to implement this technology on the SSR, because it didn't seem to fit our theme. There are expensive hardware components in the engine. The Corvette doesn't have it now, but maybe it will come in the future.
The Corvette doesn't have it now, but maybe it will come in the future.
I doubt that the Corvette will see it anytime in the future for the same reason the SSR didn't get it and the fact that it gets more mileage on the highway due to it's gearing. The SSR looses 8 mpg as compared to the Vette with a similar motor. 27 mpg for highway is not bad at all for a sports car...........
Does the DOD system do anything for mileage? Compare a 2004 with a 2007Yukon with the 5.3, 22 mpg with and 18 without. A 4 mpg jump isn't bad IMHO.
DragonU2: The reason the Corvette gets such great milage is NOT primarily gearing.
While the stultifying gearing certainly contributes a bit, it's two other factors that make the BIG differences in fuel mileage:
1. It has a VERY small frontal surface area
2. It has a very low (good) coefficient of drag.
When I had my two most recent Corvettes, a 1995 Convertible 6-speed and a 2002 Z06, I drove both mostly in 5th on the highway, because 1300 to 1400 rpm at 60 mph is insane and mechanically harder on the engine than a higher rpm. My fuel mileage was nearly as good in 5th as in 6th.
The SSR has BOTH a large frontal surface area and a mediocre coefficient of drag.
More importantly, if you actually express concern about fuel mileage on your SSR, the typical non-gearhead human being will laugh to the point of exhaustion, and then might say "Ok, tell me again with a straight face that fuel mileage and other practical issues mean anything to anyone who buys an SSR". And, they'd be right.
It's remarkably practical for a "street rod", but that's where the practicality stops.
DragonU2: The reason the Corvette gets such great milage is NOT primarily gearing.
While the stultifying gearing certainly contributes a bit, it's two other factors that make the BIG differences in fuel mileage:
1. It has a VERY small frontal surface area
2. It has a very low (good) coefficient of drag.
When I had my two most recent Corvettes, a 1995 Convertible 6-speed and a 2002 Z06, I drove both mostly in 5th on the highway, because 1300 to 1400 rpm at 60 mph is insane and mechanically harder on the engine than a higher rpm. My fuel mileage was nearly as good in 5th as in 6th.
The SSR has BOTH a large frontal surface area and a mediocre coefficient of drag.
More importantly, if you actually express concern about fuel mileage on your SSR, the typical non-gearhead human being will laugh to the point of exhaustion, and then might say "Ok, tell me again with a straight face that fuel mileage and other practical issues mean anything to anyone who buys an SSR". And, they'd be right.
It's remarkably practical for a "street rod", but that's where the practicality stops.
Jim G
Can you please explain why "1300 to 1400 rpm at 60 mph is insane and mechanically harder on the engine than a higher rpm"? I had an '00 Z28 with the LSI/M6 and always used 6th on the highway. Got high 20's if I recall correctly.
While the stultifying gearing certainly contributes a bit, it's two other factors that make the BIG differences in fuel mileage:
1. It has a VERY small frontal surface area
2. It has a very low (good) coefficient of drag.
When I had my two most recent Corvettes, a 1995 Convertible 6-speed and a 2002 Z06, I drove both mostly in 5th on the highway, because 1300 to 1400 rpm at 60 mph is insane and mechanically harder on the engine than a higher rpm. My fuel mileage was nearly as good in 5th as in 6th.
The SSR has BOTH a large frontal surface area and a mediocre coefficient of drag.
More importantly, if you actually express concern about fuel mileage on your SSR, the typical non-gearhead human being will laugh to the point of exhaustion, and then might say "Ok, tell me again with a straight face that fuel mileage and other practical issues mean anything to anyone who buys an SSR". And, they'd be right.
It's remarkably practical for a "street rod", but that's where the practicality stops.
Jim G
Interesting ........ while I too agree that drag will account for quite a bit of the difference I do not agree that it has a "BIG" affect which sounds like "most. If it were I would think the numbers would be different. I know alot goes into the final outcome and I have looked at various formulas and info for the affect of drag on mpg but I can't agree with 8-10 mpg being mostly drag related.
Drag SSR - .42 Vette - .26 06 Yukon Denali AWD - .36
Weight SSR - 4701 Vette - 3200 06 Yukon Denali AWD - 5538
Final Drive SSR - 3.73 Vette - 3.42 06 Yukon Denali AWD - 3.73
HWY mpg SSR - 19 Vette - 29 06 Yukon Denali AWD - 17
Also, I am a bit confused on why anyone would drive in 5th instead of 6th due to engine loading. If driving in 6th actually caused a negative issue with engine reliability I think we would see more issues with engine replacements on cars with "highway" gears and there wouldn't be 6000 lb SUV's motoring down the road on 4 cylinders with the same gearing.
Just my .02 but I think the bottom line is that they made the SSR more of a "fun" factor vehicle with some towing in mind and therefore we get the mpg that is associated with that fun and practical idea. No I didn't buy my SSR for gas mileage, but if I could get 10 mpg more on the highway I sure would enjoy it!
The best way I can explain why low rpm cruising is bad for an engine is to use a little physics.
Power is defined mathemtically as Force x distance per unit of time. If you examine the units used, you see that:
Power = Force x speed
The power required to drive down the road = (resistance force) x speed
That power is supplied by the engine.
At 6000 rpm or so, an LS2 engine in an 05 SSR makes about 320 to 330 rear wheel horsepower. But at 1300 rpm, it makes only somewhere between 50 and 80 rear wheel hp (dyno curves don't even go that low for reasons explained below).
So, at 60 mph, in 6th gear, you are asking that engine to overcome air drag, fricitonal drag, air cinditoning load, hills, headwinds, and all other loads when its output is only a fraction of its peak output, because it is WAY below its power band.
You are "breaking its back". This is precisely why dyno operators will not run an high performance engine at low rpm.
By using a numerically higher axle ratio, and thus more engine rpm, you are allowing the engine to operate closer to the rpm range in which it is mechanically happy.
Here's an analogy that may also help.
Imagine a man shoveling sand from a sand pile into a concrete mixer. He needs to move 150 pounds of sand per minute to keep up with the mixer's needs. He can do that by shoveling 15 pounds per shoveful times 10 shovelfuls per minute. He can also do it by shoveling 50 pounds per shovelful times 3 shovelfuls per minute. Most men will prefer to do in 15 pound shovelfuls, because it is a better balance of muscle and speed. A few men can shovel 50 pound shovelfuls, but most men would strain their back muscles and consequently be unable to shovel ANY sand after a short time.
And by the way, it IS air drag that is THE big factor at steady state highway speed. In fact, it is almost ALL the power being consumed, except the heat going into the tranny, abit of rolling friction, and the power being used to run air conditioning and other accessories. I do calulations and computer modeling daily for a couple of web based businesses I run, and so I learned a long time ago where and why the power is actually consumed.
Also by the way, once you get off a LEVEL road, or into non-highway driving, the most important factor becomes the SSR's WEIGHT. I've not only proved that in my computer modeling, but also re-leanr it vividly every day when I commute back and forth to work. In the morning, I get to cruise at steady speeds for much of the time, except for a few traffic lights. In the evening, I have to do a lot of acclerating and decelerating as I deal with heavy traffic. Per the DIC, the mileage returning home (13 to 14 mpg) is always miserable compared to the mileage going in (16 to 18 mpg). The difference is all the extra acceleration of 5000 lbs.
Try your own test on a 10 to 20 mile trip. Reset the average fuel mielage at the start of the trip. Acclerate up to highway speed. Look at your DIC right after doing that acceleration. It will shock you. Then watch that rolling average on the DIC as you put on miles. You will see the average MPG climb steadily as you cruise at steady speed. But, after coming to a stop ata light and then re-accelerating to highway speed, you will be shocked at how that ONE accleration burst costs your MPG figure.
And, do a search on "4.56 gear" to see the past rigrous testing I ahve done on fuel mileage before and after changing gearing. The change from 3.73 to 4.56 gearing, a 22% increase in gear ratio, cost me only 7.5% in fuel mileage.
The best way I can explain why low rpm cruising is bad for an engine is to use a little physics.
Power is defined mathemtically as Force x distance per unit of time. If you examine the units used, you see that:
Power = Force x speed
The power required to drive down the road = (resistance force) x speed
That power is supplied by the engine.
At 6000 rpm or so, an LS2 engine in an 05 SSR makes about 320 to 330 rear wheel horsepower. But at 1300 rpm, it makes only somewhere between 50 and 80 rear wheel hp (dyno curves don't even go that low for reasons explained below).
So, at 60 mph, in 6th gear, you are asking that engine to overcome air drag, fricitonal drag, air cinditoning load, hills, headwinds, and all other loads when its output is only a fraction of its peak output, because it is WAY below its power band.
You are "breaking its back". This is precisely why dyno operators will not run an high performance engine at low rpm.
By using a numerically higher axle ratio, and thus more engine rpm, you are allowing the engine to operate closer to the rpm range in which it is mechanically happy.
Here's an analogy that may also help.
Imagine a man shoveling sand from a sand pile into a concrete mixer. He needs to move 150 pounds of sand per minute to keep up with the mixer's needs. He can do that by shoveling 15 pounds per shoveful times 10 shovelfuls per minute. He can also do it by shoveling 50 pounds per shovelful times 3 shovelfuls per minute. Most men will prefer to do in 15 pound shovelfuls, because it is a better balance of muscle and speed. A few men can shovel 50 pound shovelfuls, but most men would strain their back muscles and consequently be unable to shovel ANY sand after a short time.
And by the way, it IS air drag that is THE big factor at steady state highway speed. In fact, it is almost ALL the power being consumed, except the heat going into the tranny, abit of rolling friction, and the power being used to run air conditioning and other accessories. I do calulations and computer modeling daily for a couple of web based businesses I run, and so I learned a long time ago where and why the power is actually consumed.
Also by the way, once you get off a LEVEL road, or into non-highway driving, the most important factor becomes the SSR's WEIGHT. I've not only proved that in my computer modeling, but also re-leanr it vividly every day when I commute back and forth to work. In the morning, I get to cruise at steady speeds for much of the time, except for a few traffic lights. In the evening, I have to do a lot of acclerating and decelerating as I deal with heavy traffic. Per the DIC, the mileage returning home (13 to 14 mpg) is always miserable compared to the mileage going in (16 to 18 mpg). The difference is all the extra acceleration of 5000 lbs.
Try your own test on a 10 to 20 mile trip. Reset the average fuel mielage at the start of the trip. Acclerate up to highway speed. Look at your DIC right after doing that acceleration. It will shock you. Then watch that rolling average on the DIC as you put on miles. You will see the average MPG climb steadily as you cruise at steady speed. But, after coming to a stop ata light and then re-accelerating to highway speed, you will be shocked at how that ONE accleration burst costs your MPG figure.
And, do a search on "4.56 gear" to see the past rigrous testing I ahve done on fuel mileage before and after changing gearing. The change from 3.73 to 4.56 gearing, a 22% increase in gear ratio, cost me only 7.5% in fuel mileage.
Jim G
Perhaps it is my ignorance, but I still don't get how running at 1300 or 1400 rpm is doing any damage merely because it is not near it's peak hp numbers. It is also operating more slowly, which I would think means less wear and tear, and of course, it DOES return better mpg in the case of the LSI or LT1 in 6th at 60 mph than at 5th at 60mph, indicating to me at least that the motor is doing less work, and so again, less wear and tear is occurring. If the motor was fighting so hard to overcome all those forces you mentioned, why is it consuming less fuel?
Samuel: You did miss the point. One last try: To get the same work done with lower rpm, you have to apply more force PER revolution. And, you are applying those bigger forces in an rpm range that the engine is not happy at. I'm sorry I can't explain it better.
The next time you are going uphill in 6th gear, into a headwind, LISTEN to your engine tone, and try applying more throttle pedal and see what happens in terms of both RESPONSE and SOUND. Then, repeat the experiment in 5th gear.
If that doesn't make it clear, just go on driving and enjoying your SSR, and don't worry about it. But, promise me you'll never install nitrous without a window switch.
I hear the Camaro will have a 6.0L LS2 with about 415 HP The standard C6 Corvette will get the 6.2L motor in the area of 440 HP. Corvette will get the 6.2L in model year 2008. The Z06 version of the 6.2L will be about 100 HP over the standard 440 HP Corvette.
While the load per rev on the engine is easier at the middle RPMs, you are not considering the rotational load which is harder on the engine. Engines are just happier at low RPM, which could possibly explain why diesels can last 100's of K miles.
Also, the best highway point to run an engine is at its peak voumetric effeciency, which is a function of all the components -- cam, headers and the like. Run the engine at this RPM and the air pumping is the most effecient, which normally requires the lest amount of gas. I believe this is part of the original gear ratio calculation from the factory.
