[Z06] Pushrods -vs- DOHC VTEC
09-02-2008, 12:09 AM
#21
Team Owner
Pro Mechanic
09-02-2008, 12:27 AM
#22
Burning Brakes
Member Since: Jun 2004
Location: Cedar Rapids Iowa
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I don't think there is any magic going on with the LS7 achieving good fuel economy. It is mostly the platform. In the case of the Corvette it is due to light weight, good aerodynamics, and high overdrive in 6th gear. Probably any modern engine with similar peak power placed into that platform with the same 6th gear ratio and final drive ratio would do as well. I believe the new 911 TT gets almost 26 MPG on the highway.
The beauty of the LS7 is the high power to weight ratio, relative simplicity, and the small physical dimensions.
The beauty of the LS7 is the high power to weight ratio, relative simplicity, and the small physical dimensions.
09-02-2008, 12:39 AM
#23
Le Mans Master
you do realize the engine does not produce 505 hp while it is cruising along at 28 mpg? mpg have to do with more then just the motor, there is weight, aerodynamics, gearing, etc...
the z motor is not efficient at all when running at full steam. it is only efficient when in 6th gear cruising. using very little fuel to keep it going.
the z motor is not efficient at all when running at full steam. it is only efficient when in 6th gear cruising. using very little fuel to keep it going.
09-02-2008, 07:50 AM
#24
Team Owner
you do realize the engine does not produce 505 hp while it is cruising along at 28 mpg? mpg have to do with more then just the motor, there is weight, aerodynamics, gearing, etc...
the z motor is not efficient at all when running at full steam. it is only efficient when in 6th gear cruising. using very little fuel to keep it going.
the z motor is not efficient at all when running at full steam. it is only efficient when in 6th gear cruising. using very little fuel to keep it going.
An engine is basically an air pump. There are different ways to pump a given amount of air through the engine. One is displacement, one is higher RPMs and one is forced induction. DOHC basically exists to allow the valvetrain to operate better at higher RPM. Only recently with the advent of variable valve timing have DOHC engines been able to offer similar low end torque characteristics as OHV engines with greater displacement. I realize this is a simplification as there are other factors at work.
Last edited by jschindler; 09-02-2008 at 07:54 AM.
09-02-2008, 10:41 AM
#25
would be interesting to see a 4 valve (or maybe 5 valve) per cylinder overhead cam designed engine that produced the same hp and torque with the same range of hp and torque as the 427 in the z06.
the engine would be 'cool' and you could do some interesting things by adjusting the overlap of intake to exhaust (assuming an intake cam and an exhaust cam)..
however.. the engine would be a good deal larger than the 427 and weigh more and when you went to upgrade your cam package, thats 4 cams, instead of one with a big cost differential.
probably the only answer to get that hp out of a 4 camer would be a smaller engine with turbos/supercharger adding weight and cost..
the engine would be 'cool' and you could do some interesting things by adjusting the overlap of intake to exhaust (assuming an intake cam and an exhaust cam)..
however.. the engine would be a good deal larger than the 427 and weigh more and when you went to upgrade your cam package, thats 4 cams, instead of one with a big cost differential.
probably the only answer to get that hp out of a 4 camer would be a smaller engine with turbos/supercharger adding weight and cost..
09-02-2008, 10:54 AM
#26
Drifting
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Actually it's not. One of the most overlooked (and important) aspects of a performance engine is size/weight. More importantly, hp/torque per size/weight. More importantly, just weight (size is more of a engine bay packaging deal, although a more compact motor is going to have a lower CG).
What IS virtually irrelevant is hp per liter. Yeah it's touted all the time, and it may work great in an engineering competition where the powerplant sits on a test bench. But lest we forget it sits in (in this case) a performance car and must be moved down the road, accelerated laterally, along with the car etc. Overhead cams are heavy, require larger heads, and have many more moving parts, etc.
Bottom line, a modern pushrod engine (such as an LSx) is extremely compact, has a low mass reciprocating assembly (1 cam, vs 4), and due to a cam in block design (aka very small/light heads) and a composite intake manifold that sits very low (essentially between the heads, not on top of them) it ends up having a VERY low CG in relation to it's crank centerline.
Bottom line, when you hear folks play the old "well the LS7 only makes 505 hp from 7.0L, and "xxxx" manufacturer gets 500 hp from their turbo 3.5L"....I always say, yes, please, yank that 3.5L mill, and weight all the necessary bits (turbo(s), piping, plumbing, IC's, etc) needed to make that 500 hp. Let me know what all that ends up weighing.
GM's LSx engines pack a punch for their size/weight that is VERY hard to beat.
What IS virtually irrelevant is hp per liter. Yeah it's touted all the time, and it may work great in an engineering competition where the powerplant sits on a test bench. But lest we forget it sits in (in this case) a performance car and must be moved down the road, accelerated laterally, along with the car etc. Overhead cams are heavy, require larger heads, and have many more moving parts, etc.
Bottom line, a modern pushrod engine (such as an LSx) is extremely compact, has a low mass reciprocating assembly (1 cam, vs 4), and due to a cam in block design (aka very small/light heads) and a composite intake manifold that sits very low (essentially between the heads, not on top of them) it ends up having a VERY low CG in relation to it's crank centerline.
Bottom line, when you hear folks play the old "well the LS7 only makes 505 hp from 7.0L, and "xxxx" manufacturer gets 500 hp from their turbo 3.5L"....I always say, yes, please, yank that 3.5L mill, and weight all the necessary bits (turbo(s), piping, plumbing, IC's, etc) needed to make that 500 hp. Let me know what all that ends up weighing.
GM's LSx engines pack a punch for their size/weight that is VERY hard to beat.
09-02-2008, 10:57 AM
#27
Le Mans Master
Yes, I do realize it. But take a look around. There are plenty of cars with similar profiles, weight and six speed transmissions that are NOT getting anywhere near 28mpg on the highway. That's not to say that your point is not valid - it is. But what we are talking about here is that fact that their is no inherent efficiency advantage over DOHC engines compared to the Corvette pushrod engines.
An engine is basically an air pump. There are different ways to pump a given amount of air through the engine. One is displacement, one is higher RPMs and one is forced induction. DOHC basically exists to allow the valvetrain to operate better at higher RPM. Only recently with the advent of variable valve timing have DOHC engines been able to offer similar low end torque characteristics as OHV engines with greater displacement. I realize this is a simplification as there are other factors at work.
An engine is basically an air pump. There are different ways to pump a given amount of air through the engine. One is displacement, one is higher RPMs and one is forced induction. DOHC basically exists to allow the valvetrain to operate better at higher RPM. Only recently with the advent of variable valve timing have DOHC engines been able to offer similar low end torque characteristics as OHV engines with greater displacement. I realize this is a simplification as there are other factors at work.
You have to first define what you call "efficiency". If you define it by the amount of power you get per gallon of fuel, the internal combustion engine in ALL it's forms are basically the same. The average is about 20% and hasn't changed much in over 50 years. There have been some improvements with computer management of the fuel injection systems but outside of that there is no difference in fuel consumption from an engine that is 3 liters at 10,000 rpm or a 6 liter one at 5,000 rpm. The air pumped is roughly the same in either case and the amount of fuel is typically a 12:1 ratio.
The only advantage of a small displacement engine is when it is at idle. Both large displacement and small displacement engines idle at roughly 600 rpm. Therefore one will require twice the fuel/air to keep it going. So this is why the smaller engines usually show the mpg advantage on the city driving. But highway travel and wide open throttle, with comparable power, there is no advantage. Having said that. The engines with multiple cams and valves with as much as 4 times as many moving parts have more parasitic losses. In other words, they have more "internal" drag and the car will slow down more rapidly when off throttle, thereby reducing coasting range.
Now if you define efficiency as the total packaging of the engine including accessories, drives, pulleys, blowers, turbos and exhausts, then the LS7 is one of the most efficient designs out there today. As measured by cost, physical size, weight and internal drag. And yes, in this definition of efficiency, WEIGHT DOES count.
09-02-2008, 11:33 AM
#28
Former Vendor
Sorry guys for getting into this. We should compare apples to basketballs. The S2000 is a fun little car to drive. Sure it doesn't have 427 cubic inches of raw GM power. But what are we really looking at any ways. Fuel useage, well Honda across the board will beat out every US car builder. Come on GM boys get to work
Apples to apples,
Look at the ecotec vs v-tec.
Randy
Apples to apples,
Look at the ecotec vs v-tec.
Randy
09-02-2008, 12:19 PM
#29
Le Mans Master
Sorry guys for getting into this. We should compare apples to basketballs. The S2000 is a fun little car to drive. Sure it doesn't have 427 cubic inches of raw GM power. But what are we really looking at any ways. Fuel useage, well Honda across the board will beat out every US car builder. Come on GM boys get to work
Apples to apples,
Look at the ecotec vs v-tec.
Randy
Apples to apples,
Look at the ecotec vs v-tec.
Randy
Last edited by robvuk; 09-02-2008 at 12:25 PM.
09-02-2008, 12:32 PM
#30
Le Mans Master
I just did a quick check and the Honda Accord is the ONLY vehicle Honda makes with 6 cylinders that gets more than 25 mpg on the highway.
On the other hand, Chevy makes the Corvette, the Classic, the Express, the Impala and the Malibu which all 6 cylinder models get over 25 mpg.
It's this FALSE perception that has GM in trouble. They better start touting their strengths because they'll be out of business if they wait for the general public to discover it themselves.
On the other hand, Chevy makes the Corvette, the Classic, the Express, the Impala and the Malibu which all 6 cylinder models get over 25 mpg.
It's this FALSE perception that has GM in trouble. They better start touting their strengths because they'll be out of business if they wait for the general public to discover it themselves.
Last edited by robvuk; 09-02-2008 at 12:46 PM.
09-02-2008, 01:16 PM
#31
Team Owner
Pro Mechanic
I completely agree. GM needs to re-learn how to "Brag" a little. Their marketing dept blows.
09-02-2008, 02:20 PM
#32
Former Vendor
GM's V6 platform has been rock solid for years. The 3.8 has always been a great engine, they raced the crap out of them back in the 80's and have been working on them since. The lastest is only 3.5 liters, but is a snappy engine.
My wife drives a civic 125 miles a day. It averages right around 38-40 miles a gallon. Not too bad for a 4 door, auto, she also has a heavy foot.
My civic is a newer one and is a piece of crap, I get around 34-36 miles to the gallon. Honda stopped using parts from Denso, so I have replaced too many parts already. So it's on the chopping block, good news I will make some cash selling the honda.
We both drive a ton of miles every year. She puts on about 40,000 miles, and do about 50K a year. And at 3.69 a gallon every little bit helps. 10k in fuel every year.
I heard from a old timer that half of the stuff we hear in life is BS. If the US auto makers would slow down on the negitives and start talking more about the good. Maybe things would start turning around. I think they are doing the changes a little late but it's better late then never.
Corvette and trucks GM does a great job with!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Randy
09-02-2008, 03:46 PM
#33
Le Mans Master
Well the orginal post stated that the Corvette gets worse mileage. That is the only info that I have to go off of.
GM's V6 platform has been rock solid for years. The 3.8 has always been a great engine, they raced the crap out of them back in the 80's and have been working on them since. The lastest is only 3.5 liters, but is a snappy engine.
My wife drives a civic 125 miles a day. It averages right around 38-40 miles a gallon. Not too bad for a 4 door, auto, she also has a heavy foot.
My civic is a newer one and is a piece of crap, I get around 34-36 miles to the gallon. Honda stopped using parts from Denso, so I have replaced too many parts already. So it's on the chopping block, good news I will make some cash selling the honda.
We both drive a ton of miles every year. She puts on about 40,000 miles, and do about 50K a year. And at 3.69 a gallon every little bit helps. 10k in fuel every year.
I heard from a old timer that half of the stuff we hear in life is BS. If the US auto makers would slow down on the negitives and start talking more about the good. Maybe things would start turning around. I think they are doing the changes a little late but it's better late then never.
Corvette and trucks GM does a great job with!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Randy
GM's V6 platform has been rock solid for years. The 3.8 has always been a great engine, they raced the crap out of them back in the 80's and have been working on them since. The lastest is only 3.5 liters, but is a snappy engine.
My wife drives a civic 125 miles a day. It averages right around 38-40 miles a gallon. Not too bad for a 4 door, auto, she also has a heavy foot.
My civic is a newer one and is a piece of crap, I get around 34-36 miles to the gallon. Honda stopped using parts from Denso, so I have replaced too many parts already. So it's on the chopping block, good news I will make some cash selling the honda.
We both drive a ton of miles every year. She puts on about 40,000 miles, and do about 50K a year. And at 3.69 a gallon every little bit helps. 10k in fuel every year.
I heard from a old timer that half of the stuff we hear in life is BS. If the US auto makers would slow down on the negitives and start talking more about the good. Maybe things would start turning around. I think they are doing the changes a little late but it's better late then never.
Corvette and trucks GM does a great job with!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Randy
FYI - Cobalt XFE = same as Civic.
09-02-2008, 07:10 PM
#34
Former Vendor
WOW, I didn't know about their new model. I will stop by the dealership on my way home. I test drove a two door, and can't see anything out the rear windows. But the 4 door is great, so hopefully they have the XFE in a 4 door.
I am about the most america guy in the world. Then why a Honda? We buy them used. Plus buy all my parts and tires from the same place that I would for any other car. I have spent 17 dollars at a honda dealership in 200,000 logged miles. I made a mess with a short block in my trunk and needed a new carpet peice.
Our company spends about 200,000 with GM every year. I have help convert a couple euro sport car fans into Corvette people every year.
Randy
I am about the most america guy in the world. Then why a Honda? We buy them used. Plus buy all my parts and tires from the same place that I would for any other car. I have spent 17 dollars at a honda dealership in 200,000 logged miles. I made a mess with a short block in my trunk and needed a new carpet peice.
Our company spends about 200,000 with GM every year. I have help convert a couple euro sport car fans into Corvette people every year.
Randy
09-02-2008, 07:36 PM
#35
Le Mans Master
09-02-2008, 08:46 PM
#36
Race Director
09-02-2008, 09:05 PM
#37
Le Mans Master
I don't think there is any magic going on with the LS7 achieving good fuel economy. It is mostly the platform. In the case of the Corvette it is due to light weight, good aerodynamics, and high overdrive in 6th gear. Probably any modern engine with similar peak power placed into that platform with the same 6th gear ratio and final drive ratio would do as well. I believe the new 911 TT gets almost 26 MPG on the highway.
The beauty of the LS7 is the high power to weight ratio, relative simplicity, and the small physical dimensions.
The beauty of the LS7 is the high power to weight ratio, relative simplicity, and the small physical dimensions.
09-02-2008, 09:41 PM
#38
Safety Car
Thread Starter
I was talking with a friend about this subject and he pointed out that 20 years ago DOHC was superior to OHV but the OHV technology has progressed so far with the advent of full roller valvetrains, better springs and lighter valves. This in conjunction with modern EFI has brought the OHV technology back to a position of superiority. Now we just need to get out the word.
09-03-2008, 11:45 AM
#39
Safety Car
Thread Starter
How does a Viper compare?
Dodge's 10 cylinder in Viper:
8.4 liter V10, 600-horsepower, 2valve/cylinder with VVT
600 HP, 648 Lbs total engine weight = .93 hp / Lb
71.42 hp/L
Rated 13 MPG city and 22 MPG hwy
The most recent weight figure I could find was from 1997, so I don't know what the new 8.4L weighs exactly. I'm guessing it is lighter than the older engine.
Dodge's 10 cylinder in Viper:
8.4 liter V10, 600-horsepower, 2valve/cylinder with VVT
600 HP, 648 Lbs total engine weight = .93 hp / Lb
71.42 hp/L
Rated 13 MPG city and 22 MPG hwy
The most recent weight figure I could find was from 1997, so I don't know what the new 8.4L weighs exactly. I'm guessing it is lighter than the older engine.
Last edited by TraceZ; 09-04-2008 at 10:13 AM.
09-03-2008, 12:54 PM
#40
Le Mans Master
how does a viper compare?
Dodge's 12 cylinder in viper:
8.4 liter v12, 600-horsepower, 2valve/cylinder with vvt
600 hp, 648 lbs total engine weight = .93 hp / lb
71.42 hp/l
rated 13 mpg city and 22 mpg hwy
the most recent weight figure i could find was from 1997, so i don't know what the new 8.4l weighs exactly. I'm guessing it is lighter than the older engine.
Dodge's 12 cylinder in viper:
8.4 liter v12, 600-horsepower, 2valve/cylinder with vvt
600 hp, 648 lbs total engine weight = .93 hp / lb
71.42 hp/l
rated 13 mpg city and 22 mpg hwy
the most recent weight figure i could find was from 1997, so i don't know what the new 8.4l weighs exactly. I'm guessing it is lighter than the older engine.