HP, do we really need so much?

[samandw]

Originally Posted by dkjones96

The engine in your old truck is a 3.2L inline 6 but if you were to compare that with a newer inline 6 the power differences for the same displacement are pretty gross. My old 1988 Cressida with a 2.8L L6 made 156HP @ 5200 and 161Ft-Lbs @ 4400(figures that engine kept since its intro in 1984) and I averaged about 27mpg when I sold the car. What kind of mileage do you get? As the engines get more efficient it doesn't mean the displacement drops to make the HP numbers match.

I completely agree, my point was that we don't need nearly as much hp to be "functional" as people think. Obviously my ancient Slant 6 isn't a good example of an efficient engine. But if all I really NEED is 100 hp, that Cressida engine would only need to be 1.8L, which if it made good torque, should see significantly better mpg.

Originally Posted by dkjones96

More HP doesn't always mean worse fuel economy. The Silverado here has 105 more HP(50% more) than it used to have 20 years ago but gets better economy with .4L less displacement.

Of course, one could argue that with 105 less hp, a modern technology Silverado could get better mpg than one using 20 year old technology.

[R.I.D.E.]

The power levels of the old 198 Slant Six would be achievable with a 2 liter 4 cylinder turbo diesel engine. Give it to gayle Banks and he would make it a monster, you could probably go as low as 1.5 liter.

Add a 5 speed and some aero and you have a truck that could get what a Corolla gets now.

A Dodge Sprinter engine and tranny (Mercedes engine and 5 speed auto) would get you close. I hear the small ones get close to 30 with an atuomatic.
A manual would be better for P&G.

I worked in a Chrysler dealer body shop when that truck was 5 years old.

Old memory but I thought they offered 170 and 225 engines in 63, thought the 198 was a later motor, but those are ancient memories.

regards
gary

[samandw]

Originally Posted by R.I.D.E.

Old memory but I thought they offered 170 and 225 engines in 63, thought the 198 was a later motor, but those are ancient memories.

regards
gary

That might be right, I'm still learning about the Slant 6. All I know is that it's got the short thermostat hose indicating it's a 170/198, but the VIN tag says it's rated at 127 hp. Being as it's 45 years old though, who knows. Still haven't found the displacement stamp on the block.

[GasSavers_RoadWarrior]

If you look at the BSFC map though, the pretty colored one, you can see that you can have too little power also. For example, if you take a vehicle that is notorious for "not being able to get out of it's own way" like a 2.3HSC Tempo with the 3 speed ATX, that highway cruises at nearly 3500 RPM, where we should knwo that in frictional and pumping terms round about 3000 rpm is equivalent to around 55-60mph aerodynamically, because like aerodynamic resistance the frictional losses also square with speed. Anyway, this thing chugs along with a motor rated at 90 horsepower, and on good days, they all even manage to run in the same direction. So you're figuring, hey, I'm only using about 30 horses to roll along the highway and thrash transmission fluid, why do I need all 4 cylinders to do that? and decide that this would be the ideal vehicle to prove the superiority of DoD once and for all, so you cut the injectors to 2 of them....

Well what has happened is that you were sitting at about the equivalent of 3500, 50 in the light blue on that graph and have now shot up to about 3500, 100 with your foot flat to the floor.... and you still need that 30hp to keep you moving, and it's still sucking around .5lb/hp.... no change in FE.... so then obviously you've proved that that motor is not a good candidate for DoD or cylinder cut.

Geared different, you might jump it into the red spot.... putting a 40hp motor in that needs you to gear it to 4-5000 RPM to keep it at highway speeds might put you in the dark blue or magenta.

[monroe74]

Originally Posted by fumesucker

all else being equal, an engine tends to be most efficient near its torque peak

Good point. Thanks for explaining.

was roughly as aerodynamic as a brick

No, I think the brick was more aerodynamic.

Originally Posted by RoadWarrior

Geared different, you might jump it into the red spot

I think that's a key point. Gearing is so important. Too many cars tend to have short gearing, even in top gear, because it makes the car feel more powerful. The car creates the impression of being more powerful if it doesn't require a large throttle opening. There's a natural tendency for buyers to avoid a car that calls for a large throttle opening. But the larger throttle opening is more efficient.

Once I owned a '68 GTO and a '72 Skylark at the same time. I had too much time on my hands, so I decided to swap differentials (I also swapped the front brakes, because the GTO shipped with drums). The GTO had something like a 4.x ratio, and the Buick was something like 2.7.

The results were predictable and entertaining. The speedo error was huge.

[jcp123]

Originally Posted by samandw

I've been thinking a lot about this lately. I recently purchased a 1963 Dodge D100 for trips to the dump and lumberyard. The truck has a 198 cubic inch
Slant-6 rated at 127 Gross hp. It weighs just a tad under 4000 lbs with me in it. Since modern cars are rated in net vs. gross hp, which is generally about 80% of the gross number. That would put my truck at right around 100 hp. Last time I took it to the dump I dropped off 1/2 ton of garbage. While I wouldn't be winning any drag races, I had no problem keeping up with traffic or mantaining the speed limit. That's at 50 lbs/hp (albiet only around 35lbs/ft-lb). Compare that to a Honda Fit economy car, which weighs probably around 2650 with a driver and tank of gas, and has 109 hp and 105 ft-lbs. That's 24 lbs/hp and 25 lbs/ft-lb. To match the numbers for my loaded truck it would only need 53 hp and/or 75 ft-lbs. To match my unloaded truck it would only need 66 hp and/or 94 ft-lbs Still, you could easily make that with a properly tuned 1.2L engine, maybe even a 1.0L.

I think Americans have been fooled into thinking they "need" hp, when in reality, a slow-turning, torquey engine would be far more efficient while still producing acceptable acceleration. A 1.5L Atkinson cycle engine in that Honda would likely net a massive increase in mpg albeit at the expense of unneeded acceleration potential. Sounds like a fair trade to me as gas approaches $4/gallon.

I think you hit on something there. I'd take a low-revving torque pumper over a high revver any day. I never really cared for revving my SVT Focus, so I really wouldn't even miss the ability to do that. My Mustang now shifts @ ~1100-1200rpm when I take it easy - it's so understressed it's not even funny.

[monroe74]

Originally Posted by jcp123

My Mustang now shifts @ ~1100-1200rpm when I take it easy - it's so understressed it's not even funny.

It's good that you're at low revs. But the problem with a motor that's understressed is that this probably implies small throttle settings most of the time, and that's inherently inefficient.

[jcp123]

Well, even keeping up with the rushed pace of traffic from the light without getting a holeshot puts me around a 1600rpm shift point. If I'm seeking a holeshot from the light, it rarely revs much past 2k rpm, just because of the cam and head tuning as well as tall factory gearing. At WOT, the trans shifts at 4400rpm, about 150rpm above peak HP. 2-bbl carbs end up using so much throttle almost anybody who drives a carb'd car will tell you that 2-bbl's don't yield any extra fuel economy (more so in pickups than small cars like Mustangs), but sure sacrifice a lot of top end flow and power. I'm no engineer, to be sure, but I'm not sure that's an escapable conundrum in this car.

Correction: the 200ci I6's these cars were available with had a 1-bbl carb and were usually able to average around 20mpg in normal driving and some could even touch 30mpg on the highway. I've heard similar stories about GM's 235 I6 in the biggies. Combination of smaller motor size and less pumping losses past the butterfly valve?

[monroe74]

Originally Posted by jcp123

I'm not sure that's an escapable conundrum in this car.

I think the basic issue is common to pretty much all automatics. The problem is that it's almost impossible to combine heavy throttle and low revs. But that's generally what's needed for high efficiency.

[dieselbenz]

There is no direct correlation between peak HP and fuel economy. This is a non issue.