How Much Horsepower Required to Cruise Comfortably at 70 MPH?

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dibbons

dibbons

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I now cruise at 70 mph at 2500 with my stock 318 (2.71 rear end). I am planning a future 340. i don't know if the 340 will like cruising at 2500 rpm. I wonder if I should cruise in second gear (Torquefflite 1.45) with the 340, which would give me an effective rear end ratio of around 3.90 instead of the current 2.71?

In second gear I would be cruising at a lower 60 mph at 3000 rpm. To cruise at 70 mpg (in second) I would need a little over 3500 rpm, which might get a little noisy. (On the other hand, I see 3500 rpm with the 2.71 (in drive) works out to 100 mph!) According to the Camquest program dyno, 340 torque peak should be around 3500 rpm.

I wonder how much horsepower is required at 2500 rpm to comfortably cruise at 70 mph with the 2.71 rear end in a 4000 pound vehicle? is there a formula? (This is a '72 BBody but I trust you guys/gals for some answers) Thank you.
 
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dibs i am so lost
your worried a more powerful engine wont cruise as comfortable as the grocery getter engine you got now?

what exactly do you mean with "cruising"?
being able to stomp on it and go sideways?
or comfortable not overstressing the engine?

i gues it dont matter anyway, because the 340 wont cruise at the same RPM at the same speed as the 318
 
why wouldn't the 340 be the same?? 340 and 318 have the same stroke. Did I miss something?? Is there something about the 340 you didn't tell us?? running on alcohol?? Cam so large it won't idle under 1500 rpm's????

Assuming nothing wild here, the 340 will cruise every bit as nice as the 318.
 
318willrun said:
why wouldn't the 340 be the same?? 340 and 318 have the same stroke.
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i guess your right
RPM at MPH is a simple mathmetical equation based on transmission gearing, rear end gearing and tire circumference

but the 340 better have more power then the 318
so i still dont really get the question...unless hes just bragging hes getting a 340?
 
I must be over thinking this thing. I do know the factory only offered the 340 vehicles with 3.23 or lower (3.55 etc.)
 
Personally, I am more concerned about how long it takes me to attain cruising speed, not the cruising speed itself.
 
diymirage said:
i guess your right
RPM at MPH is a simple mathmetical equation based on transmission gearing, rear end gearing and tire circumference

but the 340 better have more power then the 318
so i still dont really get the question...unless hes just bragging hes getting a 340?
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yeah, I don't understand the question. How much horsepower? a slant six missing a plug wire will still do 70 mph. We'll wait for more clarification on the question
 
dibbons said:
I must be over thinking this thing. I do know the factory only offered the 340 vehicles with 3.23 or lower (3.55 etc.)
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that was due to being part of the "performance package". I had a '75 Dodge truck with a slant six that came with 3.90 gear. Not that it cruised better at 70 mph with them … LOL
 
I had a 1970 Porsche with a fuel injected 1.7 liter VW motor (factory). The 5 speed had two overdrives (4th and 5th). I could not use 5th gear on the highway unless I was cruising at least 70 MPH because at lower rpm's it lacked the necessary horsepower/torque.

So I am thinking too far ahead with the 340 switch evidently (apples to oranges?).
 
dibbons said:
I had a 1970 Porsche with a fuel injected 1.7 liter VW motor (factory). The 5 speed had two overdrives (4th and 5th). I could not use 5th gear on the highway unless I was cruising at least 70 MPH because at lower rpm's it lacked the necessary horsepower/torque.

So I am thinking too far ahead with the 340 switch evidently (apples to oranges?).
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Put an OD trans behind that 340 and have the best of all worlds.:D
You can still have low gears for the streets AND be able to cruise 70 like it's a breeze.
The 42 and 46 RH transmissions have a lower first gear anyway, and the 42 takes only minor mods to go in place of a 904.

I have yet to find a grade I couldn't easily pull at 90+ mph in OD with 273 rear gears.
In OD at 85 I'm right at 2,400 rpm's and then 2,100 when the converter locks.
You get WAY better mpg with the lower R's also.
 
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To actually move a vehicle at a constant speed & load about 65 HP for that RPM range.
 
dibbons said:
I wonder how much horsepower is required at 2500 rpm to comfortably cruise at 70 mph with the 2.71 rear end in a 4000 pound vehicle? is there a formula?
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Yes there is a formula. I had to learn this for a Bonneville project I just worked on. You need to know the vehicles coefficient of drag, (Cd) the frontal area, rolling resistance on pavement, and total vehicle weight. Once you have those values the amount of horsepower needed to achieve a given speed can be calculated. The horsepower is independent of rpm and gear ratios. This is simply the power required to do the work involved.

For example, using approximated values, if your car has a Cd of around 0.44, and a frontal area of roughly 15 square feet, a weight of 4000 pounds, it will take 12.37 HP to overcome the air resistance, and 9 HP to overcome the rolling resistance. This means that your car needs approximately 21.4 HP to maintain 65 mph on level ground, in calm air. RPM and gear ratios only come into play if you can't make the required power.
 
Put in an A518 or something, 3.90 gears, and you’d be all set for both worlds.
 
Won't the torque converter affect the RPM when you change to the 340. That being that you stick with the same torque converter.
 
Overthinking it. Unless your putting it in a go kart or a dump truck the 340 will shine
 
Technically speaking, one horsepower should be sufficient, provided you have enough torque to push through the gearing.

Jus' saying....
 
Garry in AZ said:
Yes there is a formula. I had to learn this for a Bonneville project I just worked on. You need to know the vehicles coefficient of drag, (Cd) the frontal area, rolling resistance on pavement, and total vehicle weight. Once you have those values the amount of horsepower needed to achieve a given speed can be calculated. The horsepower is independent of rpm and gear ratios. This is simply the power required to do the work involved.

For example, using approximated values, if your car has a Cd of around 0.44, and a frontal area of roughly 15 square feet, a weight of 4000 pounds, it will take 12.37 HP to overcome the air resistance, and 9 HP to overcome the rolling resistance. This means that your car needs approximately 21.4 HP to maintain 65 mph on level ground, in calm air. RPM and gear ratios only come into play if you can't make the required power.
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Oh! A Big huge like and agree for this post!
I was going to say less than 30hp. I don’t have any equations in front of me. Not even sure where to find them to be honest.

What I have found out is that the amount of power required to roll the car against the aero dynamic frontal are of the vehicle vs. the wind resistance is actually not a heck of a lot. This is also seen (not needing a lot of power) when a Prius strolls on by. While the car has a Aero dynamic adge, it still has to produce some power to roll down the Hwy at 60/70mph. And they don’t have jack for HP!

Also remember that if you dyno’d the engine, the power numbers are at wide open throttle. You don’t cruise at wide open throttle. Your carb is only opened up a little bit. Maybe, a tenth of full opening on the primary’s. So if your dyno read out said, oh, 320hp @ 3000 and your traveling @ 70mph, your not cruising at 300hp, your probably producing 40/50 hp with the closed up carb.

Are you worried about a powerful get up and go feeling? This can be addressed with more gear ratio. To high of a rpm on cruise? Add an overdrive transmission. Spinning tires to easy now? Add wider tires... LMAO...
 
Jadaharabi said:
Won't the torque converter affect the RPM when you change to the 340. That being that you stick with the same torque converter.
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same converter will not be affected cruising at 70 mph. Only way the 340 (or any engine) will affect the converter will be the amount of torque of the engine will "flex" the converter under heavy acceleration.
 
clinteg said:
Put in an A518 or something, 3.90 gears, and you’d be all set for both worlds.
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3.90 X .69 = 2.69. HWY. GEARS!
Close to the 2.76 ratios found stock in cars.
 
The power required will not change. The power supplied would. I'd imagine you need probably about 100 crankshaft hp to run a B body steady at 70mph. So the 318 might need 1/2 throttle to do that. A 340 might require less throttle, because the engine will be making more power at the same throttle as the 318.
 
If it takes a given amount of power to do a given amount of work, would fuel consumption be the same as well?
 
rumblefish360 said:
3.90 X .69 = 2.69. HWY. GEARS!
Close to the 2.76 ratios found stock in cars.
Click to expand...

Exactly! I run a 4.56 gear 2500 rpm at 75 mph. I can drive to the track, make 10 sec passes all day and drive it home.
 
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