What is the formula to calculate how much HP is needed to maintain a speed of 70 mph?

Re: What is the formula to calculate how much HP is needed to maintain a speed of 70

Gary

Have you had a chance to time your car from 75 to 65 MPH?

Paul

 

Re: What is the formula to calculate how much HP is needed to maintain a speed of 70

tire pressure 35 psi, level grade, no noticeable wind, brought the car to 75 mph, started stop watch and shifted to neutral at the same time, the behemoth LeSabre slowed to 65 mph in 9.22 seconds.

 

Re: What is the formula to calculate how much HP is needed to maintain a speed of 70

updated

 

Re: What is the formula to calculate how much HP is needed to maintain a speed of 70

If the LeSabre weighs 4000 lbs with you in it.

Total force on the car to maintain 70 mph = 125 x 15/9.22 = 203 lbs.

Hp = 203 x 6160/33,000 = 37.9 hp

The amount of torque needed to maintain 70 mph at 1500 rpm = 37.9 x 5252/1500 = 132 ft.lbs

See post #9 for details.

A 350 will easily make that kind of torque at 1500 rpm.
The goal is to make the engine operate efficiently at 1500 rpm so you can get good gas mileage.

Post # 17 has most of the details to do just that.

Paul

 

Re: What is the formula to calculate how much HP is needed to maintain a speed of 70

nice thanks. Math is powerful I wish I was better at it. so if i only need 132lbs then I should be able to build an engine optimized for torque that has no problem efficiently sipping along at 1500 rpm at 75mph. According to my calculations 2.56 gears with 700 R4 in 4th gear (.7) RPM would be 1614. I would love to get 25mpg and have enough torque to get it up to 75mph quickly. 2nd gear doesnt look like it would be a dog for an engine built for low rpm torque. Ill post my chart here later tonight.

 

Re: What is the formula to calculate how much HP is needed to maintain a speed of 70

Post #17 holds a lot of info.

Low rpm engine efficiency is mainly a function of valve overlap. The less time the intake and exhaust valve are open together the more efficient.
The exhaust system also has an effect.

Low rpm torque is also affected by overlap but primarily is a function of the intake valve closing point after BDC.
The earlier the intake valve closes, the more low end torque is produced.

Here's the catch.

You can decrease the valve overlap by increasing the lobe separation angle (LSA) which will give you better low rpm efficiency for great gas mileage
but increasing the LSA moves the intake valve closing point later which decreases low rpm torque.

So we look at very short duration lobe profiles which helps with both overlap and intake valve closing.
The short duration profiles moves the torque curve lower and decreases horsepower since HP is a function of torque X rpm.

However for a given .050" duration If the lobe lift is increased the mid lift durations are increased (.200" lobe lift duration spec) and the power band width is increased.
The other "high lift" benefit is getting the intake valve open sooner to take advantage of the 75* ATDC max piston velocity point.

Very short duration and high lift combinations are where roller cams come into play.

Lets say you shift at 4000 rpm going into second gear (Disregarding converter slip).

The 700R4 tranny will drop 1870 rpm while the 200R4 drops 1709 rpm.
Only a 161 rpm difference and the converter will take care of the difference.

Since you will be running a lock-up converter it doesn't have to be very tight.
You can run a higher stall if you want to.
If you use a 2400 stall that's what the rpm will be after each shift (unlocked) no matter what tranny you decide to use.
There will be a ton of torque available at 2400 rpm.

Paul

 

Re: What is the formula to calculate how much HP is needed to maintain a speed of 70

here are some comparisons 700R4 2.56 gear, 200 with 3.56

 

Re: What is the formula to calculate how much HP is needed to maintain a speed of 70

You should have put in the same gear ratio between the 200 trans and the 700 trans so you could see the rpm drops between 3rd gear and a drop to 2nd gear at say 40 mph or 55 mph to see where the engine will be at these rpm's. Will the engine be at the correct rpm at these levels with the cam you use, or lets say at the correct power band with the cam you use.

Yes the torque convertor will only drop to its stall speed but how long will it take for the rpm to start to climb between the two trans. Will you be in the power band at this point.

To me the 200 trans will be way easier of an install and with a 342 gear you will be in the right combo to move your heavy car. 2200 stall and a cam or two below the 212 if you are looking for the more torque in the lower rpm range, and of course I would use lockup
TQ as this drops the rpm 2-300 rpm. You can also lockup the TQ in 3rd gear at 40 mph to 55mph as this will help drop the rpm on roads that have a lower mph.

If the rpm drop or gain between the two trans is only 160 rpm then I would use the trans that is easier to install and that would really be the 200 trans. I am corrected here as I see in the graph the 700 trans will keep a slightly higher rpm between the two trans in 2nd gear.

I am at 2200 rpm locked up at 65 mph 373 gear 28" tall tire, 3000 stall.

I think you are just trying to justify using that low first gear of the 700 trans when you are really asking about driving at 70 mph and the 200 trans has a lower 4th gear than the 700 trans. The 200 will win this the rpm will be lower here at the 70 mph locked up in 4th gear.

Which one will be lighter and which one takes the most HP to operate. This will make a difference.

Don't forget, you will need an adapter to install the 700 trans to the Buick motor and will add to the weight.

You will also need the driveshaft changed adding to the total cost of the project.

Winner, 200 trans! Its just more cost friendly and easier!

 

Re: What is the formula to calculate how much HP is needed to maintain a speed of 70

you make some good points. I like the lower first gear of the 700 because then I can use higher gears in the rear end and ultimately have a lower rpm and better gas mileage despite the 200 higher final drive which is .03 higher, a negligible amount. Your best points are cost and ease of installation. I will post a graph of the 200 apples to apples later on tonight i have some family responsiblities at the moment and cant talk about buicks right now.

 

Re: What is the formula to calculate how much HP is needed to maintain a speed of 70

If you run anything less than a 3.08 the Trans will be downshifting on every small hill or any head wind. You won't like the 700. It'll make your car feel like a late 80'a truck.