How much torque to push a stock K5 down the road? (on level ground)

[jonrpick]

Just curious...

With the selective displacement motors that can shut off cylinders when they're not needed, I was just curious as to how much torque is actually needed to maintain say, 60mph on flat ground???

I know all dyno charts are made at WOT. I'm wondering how much power drops when the carb isn't pushed into "power mode".

Any ideas? Like I said, assume a stock truck with stock weight, height, and 31" tires and 3.73 gears.

 

[Hogback Fabrication]

This post has a lot of the info you're looking for.

As for how much input torque it takes to push the truck down the road, that matters which transmission you are using (in addition to which gears, which you already provided).

I can do more math for you when you post which trans you plan on using (I'm sure you're thinking straight 6, maybe with a different trans) but in the mean time horsepower is a function of torque at a speed. The equation to calculate hp based on torque is:


horsepower = (torque*RPM)/5252

Torque is in ft.lbs.


According to the other thread, it takes 39.35 hp to keep your truck moving at 60 mph. That means the required torque is

required torque = 206,666.2/(RPM at 60 mph)

You can see that lower RPMs means more input torque and vice versa. Of course, we all know that lower RPM's generally mean better economy and that's because the lower the RPM the less power is lost to accessories and internally (power steering pump, alternator, fan, moving air through the motor, valvetrain losses, friction, etc.)

 

[jonrpick]

This post has a lot of the info you're looking for.

As for how much input torque it takes to push the truck down the road, that matters which transmission you are using (in addition to which gears, which you already provided).

I can do more math for you when you post which trans you plan on using (I'm sure you're thinking straight 6, maybe with a different trans) but in the mean time horsepower is a function of torque at a speed. The equation to calculate hp based on torque is:


horsepower = (torque*RPM)/5252

Torque is in ft.lbs.


According to the other thread, it takes 39.35 hp to keep your truck moving at 60 mph. That means the required torque is

required torque = 206,666.2/(RPM at 60 mph)

You can see that lower RPMs means more input torque and vice versa. Of course, we all know that lower RPM's generally mean better economy and that's because the lower the RPM the less power is lost to accessories and internally (power steering pump, alternator, fan, moving air through the motor, valvetrain losses, friction, etc.)

Well... RWHP would probably be better in this discussion, I suppose...

(can't believe I willingly used the term "HP"... I feel dirty)

But yeah, I guess I probably should've stated that it'd be through an OD tranny (in OD at the time of testing).

Straight six... engine type isn't really relevant to my question. Although, I do plan on doing some more research into that. I'm more or less wanting to toss around some theory and numbers to get my brain thinking...

Edit: Thanks for the link! :-)

 

[jonrpick]

Actually...

OK, so around 200 lb/ft of torque @ 60mph...

That's at the wheels or crank?

If it's at the wheels, I'd imagine it'd be less than at the crank, considering it's being reduced by 3.73:1 at the axle. Of course the tires will divide it again, but still...

31" tires... 3.73... OD (.7:1)

Edit: LOL... damn I'm tired. You said 206,000 lb/ft of torque?!?!

 

[Hogback Fabrication]

Edit: LOL... damn I'm tired. You said 206,000 lb/ft of torque?!?!

You're reading that wrong, its 206,xxx divided by engine rpm at 60 mph.

Actually...

OK, so around 200 lb/ft of torque @ 60mph...

That's at the wheels or crank?

Wheels I would imagine, that would make a lot more sense for figures like that. I don't really know, I'm not the one who came up with the numbers in that other post

If it's at the wheels, I'd imagine it'd be less than at the crank, considering it's being reduced by 3.73:1 at the axle. Of course the tires will divide it again, but still...

31" tires... 3.73... OD (.7:1)

Horsepower is horsepower, gears don't change hp at all. If you have 200 horsepower at the crank with a 1:1 drivetrain ratio, you have 200 horsepower at the wheels (ignoring frictional losses). If you add a 2:1 gearbox, you double the torque and half the speed and still end up with 200 hp.

 

[Hogback Fabrication]

With a lockup 700R4 (.7 OD), 31's and 3.73 gears, you should be spinning around 1700 RPM at 60 mph.

At 60 mph and 1700 RPM, it should take about 121.6 rear wheel ft.lbs. of torque to overcome the wind and rolling resistance of a stock K5.

 

[jonrpick]

You're reading that wrong, its 206,xxx divided by engine rpm at 60 mph.

Wheels I would imagine, that would make a lot more sense for figures like that. I don't really know, I'm not the one who came up with the numbers in that other post

Horsepower is horsepower, gears don't change hp at all. If you have 200 horsepower at the crank with a 1:1 drivetrain ratio, you have 200 horsepower at the wheels (ignoring frictional losses). If you add a 2:1 gearbox, you double the torque and half the speed and still end up with 200 hp.

Oh ok... gotcha. So my calculator says roughly 121. (206,000 / 1698) 1698 being the engine RPM @ 60mph with 3.73's and 31's.

 

[jonrpick]

With a lockup 700R4 (.7 OD), 31's and 3.73 gears, you should be spinning around 1700 RPM at 60 mph.

At 60 mph and 1700 RPM, it should take about 121.6 rear wheel ft.lbs. of torque to overcome the wind and rolling resistance of a stock K5.

You hit "submit" before I did...

Well... that's all very interesting. Theoretically, one could use a much smaller engine in a truck than a 350ci SBC if they had no real need to pull a load and had pretty flat terrain. Honestly, that power level is in 4cyl territory.

I'm wondering if a manual tranny was used (to keep the thing from hunting) and a turbo was slapped on a...let's say 3.0L motor, what the results would be in said stock K5.

Strictly speaking as a commuter vehicle, not a "truck" or OHV.

 

[Hogback Fabrication]

You hit "submit" before I did...

Well... that's all very interesting. Theoretically, one could use a much smaller engine in a truck than a 350ci SBC if they had no real need to pull a load and had pretty flat terrain. Honestly, that power level is in 4cyl territory.

I'm wondering if a manual tranny was used (to keep the thing from hunting) and a turbo was slapped on a...let's say 3.0L motor, what the results would be in said stock K5.

Strictly speaking as a commuter vehicle, not a "truck" or OHV.

There is so much to the fuel economy equation, its practically unfathomable unless your an engineer in the industry (and I certainly am not).

IMO, the 3.0L wouldn't help much because it has to be worked too hard to keep the truck moving. Boost = more fuel consumed, which means less economy. Turbos can help economy because they only help power when they are needed but I still don't think it will help much, they will be boosting every time there is a hill or when you take off from a light and the lower compression ratio will hurt when the boost isn't there. Diesels are really the engines that can take advantage of boost, a diesel will be miles ahead as far as economy.

As far as the small engine, big truck thing, look at any of the mini-trucks. Any of the Chevy, Ford, Toyota, Mazda, etc. 4 cyl. trucks are barely over 20 mpg while many of the fullsize trucks (which weigh nearly twice as much) are pretty similar in mileage (the diesels are actually better).

Don't forget about cars like the Corvette. It has a large motor (5.7, 6.0, 6.2) and still gets nearly 30 mpg on the highway. Why? Low RPM (via the .5:1 OD in the T56) and great fuel control by modern computer controls. GM has practically always been ahead of the curve on everything V8, even the new 505 hp Z06 doesn't have a gas guzzler tax. Carburetors and simpler fuel injection setups are difficult to control at lean mixtures because they can cause misfiring and overheating.

On the other end of the spectrum, my DD ('93 Probe GT) has a 2.5L V6 and super low gears (4.10's on ~24" tires). Even in 5th (OD, .8 or so) it spins ~3500 RPM at 70 and averages ~32 mpg on the highway. High RPM doesn't always mean low mileage, it depends on where the power is made and at what RPM the accompanying accessories are meant to operate at. Of course, displacement has some play in this too.



IMO, a NA L6 might get slightly better mileage than a V8 but I'm one of those people that doesn't think its worth it. L6's have more bearings journals than V8's (better bearing wear, more friction), the old L6's use non-roller cams (a seriously big source of friction, even the OEM's have all switched to roller cams primarily for efficiency reasons, both power and MPG). The V8's have easily available and cheap heads that are very efficient and make plenty of power (i.e. vortec, aftermarket, etc.). With the L6 your stuck with 40 year old heads or performance aftermarket heads that have no concern for efficiency.


Just my .02

 

[jonrpick]

I understand what you're saying... and honestly, I'm not planning to put a 3.0L motor in a truck. I'm just up (too) late and bored, LOL...

When I build a six, that's not going to be about fuel economy, although a little bump in that area would be nice, and really, in Atlanta, you lose most of your gas from stop and go. So in that case, less displacement would probably help on the city mileage, since so much time is spent idling.

Off topic...but...My goals for the six are centered around the fact that I think they're cool, not because they're necessary, or practical. I'll just be shooting for a good reliable engine that performs like a garden-variety 350 and has MPFI, with possibly a turbo for added fun when needed.

After the K5 is basically all running as it should be, I'll probably get another project, and it'll likely be a small 70's GM--preferably a Monza, but the other equivalents would do me fine (Buick Skyhawk, Pontiac Sunbird, Olds Starfire). That would be something where I'd be likely to play with a 3.0L 4cyl or small V6. That'd be my econobox.

This guy claims 36mpg highway with his...

http://detroit.en.craigslist.org/car/719788261.html

I don't know if I believe 36mpg, (thinking high 20's) but I wouldn't be entirely surprised if it hit 30. My 1980 Caprice Classic did much better than my K5 does. Probably into the mid-20's highway. I could drive for a couple of days (school-to-home and back, and hang time afterwards) on $5 of gas when it was around $1/gallon.

Edit: according to v8monza.com the V6 version would do 30mpg with a 5sp, so maybe that 4cyl version in the Craigslist ad really will do 36mpg...

Hmm...

 

[pauly383]

With a 700r4 and 3.08's and 31's , 3.08's and 32's , 3.08's and 33's , 3.73's and 33's , 3.73's and 35's , and 4.56 and 35's , it takes less than a 305 to sustain 60mph on flat ground

 

[BGKYK5]

Keep in mind that while 121 ftlbs at the wheels will keep you rolling at 60mph, you need plenty of torque in reserve for accellerating, hills, headwinds, etc. Having said that how often do you go to WOT just driving around?

For most people the answer should be almost never. In which case you could get by with a smaller engine. My DD is a gutless 2.0L neon, so I see 6500 RPM WOT every day just to stay out of people's way.

 

[BGKYK5]

Also keep in mind that 121 ftlbs of torque at the wheels is only 46 ftlbs at the flywheel (with 3.73 gears and a 0.7 OD).

 

[Hogback Fabrication]

Also keep in mind that 121 ftlbs of torque at the wheels is only 46 ftlbs at the flywheel (with 3.73 gears and a 0.7 OD).

Your not thinking about that correctly, look at my earlier posts and you'll see.

The 121 ft.lbs. was found assuming a .7 OD, 3.73 gears and 31" tires. Different gearing or tire size will change the amount of torque needed from the motor.

 

[jonrpick]

Keep in mind that while 121 ftlbs at the wheels will keep you rolling at 60mph, you need plenty of torque in reserve for accellerating, hills, headwinds, etc. Having said that how often do you go to WOT just driving around?

For most people the answer should be almost never. In which case you could get by with a smaller engine. My DD is a gutless 2.0L neon, so I see 6500 RPM WOT every day just to stay out of people's way.

Also keep in mind that 121 ftlbs of torque at the wheels is only 46 ftlbs at the flywheel (with 3.73 gears and a 0.7 OD).

I would like to see what the minimum engine is that would work in a K5, but I'm not going to try it, LOL... This is mainly just theory talk. When I get into 4cyls it'll be in a smaller car.

But I am curious. I have heard of people dropping V6's in K5's, and I know they came with a 250ci L6 (4.1L).

I think, bottom line, it takes "X" amount of power to go 60mph. "X" will vary depending on load (hills, wind, etc). It will take "Y" amount of gasoline (also a variable) to make "X" amount of power. The numbers would probably be fairly close regardless of which engine was used. So, a turbocharged 3.0L may push it down the interstate just fine, but would likely consume a similar amount of fuel as a 350ci V8.

The difference, I suspect, would be in situations where there was a lot of stop and go traffic, as well as significant idling time (anyone in Atlanta is highly experienced in this area!). Then the smaller displacement might pay off as city mileage may increase.

I guess the point is... as I stated when I started the thread, I was thinking about the selective displacement engines that shut down unneeded cylinders based on load. Why not just use a smaller motor to begin with and turbo-charge it??? Aside from longevity issues, I don't see why it wouldn't work.

Like, we know the 250ci L6 worked in a K5. I use it as an example because it's a bolt-on... What about its little brothers? Like the 230ci and 194ci L6's? Those were used in passenger cars like Chevelles (not exactly a light vehicle by modern standards). Would they push a K5 down the road? Build it reliably and slap a turbo on there for increased load situations (acceleration, hills, wind, etc).

I just wonder how it'd do... that's all.

 

[pauly383]

Like, we know the 250ci L6 worked in a K5. I use it as an example because it's a bolt-on... What about its little brothers? Like the 230ci and 194ci L6's? Those were used in passenger cars like Chevelles (not exactly a light vehicle by modern standards). Would they push a K5 down the road? Build it reliably and slap a turbo on there for increased load situations (acceleration, hills, wind, etc).

I just wonder how it'd do... that's all.

A totally optioned out big block SS 1970 Chevelle would weigh 3800 pounds +/- a little . A small block car or a car with less options weighs much less . Most sold were in the 3400-3500 range , people like saving money , thats a given in each year .

A 2003 Impala typically weighs around 3300 pounds +/- a little depending on options . And the base engine is 200 cubic inches . Still a base six cylinder .

So by modern standards its about the same anyway

 

[dyeager535]

I think the main reason turbo's are so uncommon stock is that consumers are so stupid, only on specialty vehicles designed for automobile enthusiasts, could the manufacturer trust that they would let the turbo cool down (assuming this is still required) after being run.

Variable displacement is an easier way to get there with a minimal amount of brain power required by the driver. What specific year vehicle has variable displacement on a large motor...2008 Charger for instance? I'm curious what the EPA numbers are before/after variable displacement was introduced. I drive long distances almost every week, and every time I do, I doubt that variable displacement can work effectively in this country. I have to assume that variable displacement requires fairly steady throttle input to "activate" and continue working, and the way many, I'd argue most, people drive (not using cruise control) would directly affect its effectiveness. But I suppose thats another topic.

Now what might be an interesting swap, even though you don't want to find the minimum, would be one of the small Olds diesels. I know in addition to the 350 diesel, there was a 260, there might have even been a smaller one. Since the 260/350 are the same block dimensions, and the 350's were installed in the trucks, a 260 diesel would be a fairly easy swap, IF you can find one. With an intelligent driver, modern gaskets and fasteners, it would probably be a good motor.

 

[BGKYK5]

But the 350 Diesel was a terrible motor.

My mom had one in a '85 Buick something or other.

 

[6.2Blazer]

Don't forget about cars like the Corvette. It has a large motor (5.7, 6.0, 6.2) and still gets nearly 30 mpg on the highway. Why? Low RPM (via the .5:1 OD in the T56) and great fuel control by modern computer controls. GM has practically always been ahead of the curve on everything V8, even the new 505 hp Z06 doesn't have a gas guzzler tax. Carburetors and simpler fuel injection setups are difficult to control at lean mixtures because they can cause misfiring and overheating.

The low RPM and fuel control are only a couple of small factors. The ones that really make the difference are:
1. much lighter than a truck
2. much, much better aerodynamics
3. much less rolling resistance

I guess these are all part of the reason they can get by with a lower cruising RPM because it simply takes a lot less torque to push it down the road.

 

[pauly383]

and 4. Newer Efficient Head Designs