How Much Horsepower Required to Cruise Comfortably at 70 MPH?

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mopowers said:
If it takes a given amount of power to do a given amount of work, would fuel consumption be the same as well?
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No, because the two engines will likely have significantly different volumetric efficiencies. This difference will cause the less efficient engine to use more fuel to make the same amount of power.
 
I find a lot of fuel efficiency is not just engine size but also related to vehicle weight. Normally we would say an360 will consume more fuel than a 273. If in the same vehicle & reasonably built the same.

Well, my V6 quad cab ‘03 Dakota gets worse mileage than the V8 version of the truck. The lil’bastard struggles to move the truck and requires more fuel to do the same as the V8, which just makes more torque a bit easier.

For best mileage, on any vehicle, there’s a lot of perameters to be looked at and attended to.
 
Rumblefish360 had a good point about part-throttle efficiency (post #19), which I never considered before. The darn dynos only measure full throttle performance. Only actual driving conditions will tell the tale.
 
dibbons said:
Rumblefish360 had a good point about part-throttle efficiency (post #19), which I never considered before. The darn dynos only measure full throttle performance. Only actual driving conditions will tell the tale.
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1; And this is where people will tell you to use a slightly small carb (600 instead of a 750) for best throttle response and possible mileage gains since the smaller butterflies will have a higher velocity going past them to help atomize the fuel with the air.

It is a very small change at the butterfly but this is where the little things add up.

2: And this is where the broke, home grown, old school guys say, run a TQ. Or other spreadbore carb.
 
I ran my 367 with an effective Final drive ratio of 1.97, and so 85=2000rpm, for 500 miles and it returned 32 mpgUs, with a 600VS that I had tuned specifically for the trip. Same car went 12.9/106 with a 750DP and street tires. If Ida had a TQ, I bet it wouldda done better and gone faster, without the carb swap,lol.
 
Garry in AZ said:
No, because the two engines will likely have significantly different volumetric efficiencies. This difference will cause the less efficient engine to use more fuel to make the same amount of power.
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Would the volumetric efficiency be related to the max rated hp, or is it independent of max rated hp?

This is all really interesting to me as I've been struggling with similar questions at work related to HP, load factors, and brake specific fuel consumption in ag tractors. Since you obviously know what you're talking about, do you mind if I PM you?
 
I did it with a 5HP briggs & stratton when I was a kid. Then we put an 8HP ungoverned briggs on it. It'd go over 100 then.
 
Volumetric efficiency is a measure of the volume of air the engine is ingesting compared to the volume of it's cylinders. If you measure 750cc of air going into a 750 cc cylinder, that would be 100%. But if you measure only 600cc that would be 600/750=80%.
The VE changes constantly with throttle opening and rpm and load.
It is usually measured at WOT, to help indicate what the engine is capable of.
It is possible to manipulate VE to the better in one part of the power curve, at the expense of another part of the curve..
On a given engine, the things that affect the VE to the greatest degree are the head-flow, and the cam. This is why it is often said that the power is in the heads.
Once the engine is assembled, it pretty much is what it is....... unless you choked the carb selection or restricted the exhaust.
VE is usually greatest at or very near to the torque peak. If you move the torque peak (with camming) you may move the rpm of best VE, and you may change the numbers.
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Fresh air into the carb as opposed to feeding it 400* underhood air, will change the density of the air, as will changing altitudes. These will affect the power, by varying the number of oxygen molecules being ingested,and the Effective cylinder pressure, but probably not much the VE, because a quart of air is a quart of air, regardless of it's density.
The hardest job an engine has is finding oxygen, so anything and everything you can do to make the job easier is gonna affect the power it makes.
 
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My yamaha 125 scooter has 8 HP and will go about 62.
 
My Ford Ranger has a 4 banger. My Ram 1500 quad cab has a 5.7. You’d think the Ranger would be the go to vehicle for gas mileage right? Well around town yes but on the highway, not so much. I’ve had as bad as 17 mpg out of the Ranger before with a head wind. My RAM usually gets 19-20 and has a much larger frontal area obviously. But the power to weight ratio and torque is so much higher, the engine doesn’t have to work that much to keep it at speed. That Ranger pedal stays to the floor in a head wind though. I’ve seen the same results between motorcycles that have different size engines. The “powerful” bikes could go further on a tank of fuel than the lower powered bikes because the powerful engine just isn’t working as hard to keep speed. So with what you’re talking of doing going to a 340, build it with a lot of torque. Low end torque. Everything will improve for you.
 
Who remembers those Chebby trucks that after you set the cruise to 65 mph, would slow down, kick out of lock-up, buzz back up to 65,then; slow down/buzz up endlessly, until you took it out of overdrive? Yeah,no; that wasn't their best engineering.
 
I have no fancy formulas at all, only a full year of real life experience with a 70 340 Challenger. In 1981, I was dating a girl who was going to Austin Peay State University in Clarksville, TN. That was an over 300 mile trip I was making every other weekend. I took the 3.23 Sure Grip out of the Challenger, and installed a 2.76 single legger. The 340 was factory stock, from the air cleaner to the pan with factory HP exhaust manifolds. I could cruise at 70 on the interstate at 2100- 2200 rpm. The car would get 20 mpg at that speed and rpm. That's all I got....lol. Still have the Challenger, and it has the 2.76's in it to this day! Haven't driven it in years, as it's in a million pieces awaiting resto. The girl? She became my wife, and is now the :realcrazy: ex wife....lol.
 
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