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

If I have a 4000lb car and I want to go 70mph on a level grade is there a formula to get a ball park range of how much hp it takes to maintain the speed? I realize, wind resistance, rolling resistance, drivetrain friction, grade, etc are all factors but roughly speaking how could a general range be calculated?

 

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

I plugged in 25 sq ft and .44 for the CD and came up with 35 HP.

http://www.wallaceracing.com/Calculate HP For Speed.php

 

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

The aerodynamics(frontal area, coefficient of drag), wind, drivetrain losses, and rolling resistance make up almost all the power required to maintain a speed on a 0% grade(in that order I believe). When the grade changes is when weight really comes into play.

 

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

I don't know the formula,but if you want to get to 70 QUICK you will need much more HP than just cruising at 70.

Derek

 

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

im trying to figure what gears to use so when cruising i have the lowest rpms possible for mileage but enough horsepower so the motor is not straining. If I use a 700R4 with 28" tall tires and 2.56 gears on a 350 motor built for low end torque my rpm at 70 will be 1509 in 4th gear and at 55 would be 1185, is there enough hp at that rpm for the motor to work efficiently? The 700R4 has a 1st gear of 3.06 and a turbo 350 has a first gear of 2.52 so the 700R4 with 2.56 rear axle gear is equal to a turbo 350 with a 3.08 rear axle when starting from a stop sign.

 

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

Use a digital stop watch.

Bring to car up to 75 mph and put trans into neutral.

Then time how long it take to decel down to 65 mph.
Do this several times and get a good average.

Knowing the cars weight and the deceleration rate you can calculate the total force acting on the car.

Then you can figure the HP it takes to maintain 70 mph

1 hp = 550 ft.lbs/sec or 33,000 ft.lbs/min

70 mph = 6160 ft/min

Example: It take 200 lbs to maintain 70 mph.

200 x 6160/33,000 = 37.3 hp

Paul

 

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

Ok ill get that data and report back here tomorrow, im looking forward to learning how you do that.

 

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

Ahh,I see you're talking about camming and gearing then. In this case a smaller cam would be your friend,one that makes power in the lower RPM range,that makes gobs of torque right off of idle.

Chosing a cam is a trade off,a bigger cam will make more high rpm HP at the price of low rpm torque,thats why bigger cams recommend a higher stahl converters and a lower rear gears(numerically higher).

If you're building a heavy car that you want to cruise on the highway a "smaller" cam would work better for you. Your not asking about HP,you are asking about torque. Torque is what will get you down the freeway at low rpm,not HP.

To much to write to explain the above,pm me and I will send you my phone number and I'll explain the difference when you call.

Derek

 

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

To find the force needed to keep the car at 70 mph use Newton's second law. Force = Mass x Acceleration

The mass of the car is the weight/(the acceleration due to gravity) or 4000/32 = 125 slugs

The acceleration will be the time it takes to decel from 75 mph to 65 mph. 75 mph = 110 ft/sec, 65 mph = 95 ft/sec

Acceleration = (110 - 95)/T (Your stopwatch time) or 15/T

Force = 125 slugs X 15/T

Lets say it take 10 seconds to slow from 75 to 65 mph

Force = 125 x 15/10 or 187 lbs

From previous post# 6

187 lbs x (6160 ft/min)/ (33,000 ft.lbs/min) = 35 hp

35 hp doesn't look like a big deal until you consider the rpm you expect at 70 mph and this is what Derek was pointing out.

Torque = HP x 5252/RPM. Notice for a given HP, as you decrease the rpm the torque requirement is increased.

35 HP @ 1500 rpm = 122 ft.lbs of torque.

Note: The force due to air resistance is exponential with speed not linear but for 5 mph on each side of 70 mph it will get us a ball park figure and since the cars frictional forces are include the error is within reason.

Back to high school physics, ain't it great!

Paul

 

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

If your wanting to go with a 4 speed overdrive, I'd go with the 2004R NOT the 700R4, on the 700R4 the ratio spread from first to second is HORRIBLE! The 2004R is much like the T-350 trans. as far as ratios.
With the 700 trans, it will pull like a train in first, then when second hits, it will feel like it fell on its face.

 

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

EXACTLY, what Mark said. Besides the 200-4 trans is basically a bolt in. and will hold up to any power you give it. The 350 won't break that trans. you should probably go to a 342 gear. The overdrive is at .068 or 067 somewhere around there so you can use the Wallace website calculator to see what rpm you are at running 65 mph. I am at 2200 rpm with the trans convertor locked up at 65 mph. I use a 3000 stall, 373 gears.

 

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

This is great info! Thanks a lot.

 

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

Your PM box is full.

Derek

 

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

I agree go with the 2004R, and I also agree to go with a 3.42 gear... using a 2.56 gear with overdrive trans will give you a real dog at anything under 85 MPH... I had 2.56 gears in my caddy with an overdrive trans and I got better fuel mileage when I went to a 3.08 gear... This was due to higher RPM but less throttle required to accelerate and keep up my speed. The sweet spot for the RPM is about 2300-2800 for good fuel mileage on the highway... MAYBE if the engine was larger and you had it optimized for low RPM torque then you could lower that number to about 1900-2400 but with a mild 350 and a heavy car it will need some RPM or it will be a dog... No math required, been there done that...

 

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

Gary was talking about building an 350 specifically to operate at 1500 rpm cruising rpm.

It certainly can be done where the cam and head combination is designed for efficiency and lots of torque under 2000 rpm.

In this case one would need to know the minimum torque requirement for the job.

Florida roads are fairly flat.

Paul

 

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

exactly. Im trying to figue out where the power band would have to be and how much for cruising so I can choose the rear end ratio. I want to build this 350 optimized for torque because it will be much more useful and fun to me than to build it for 1/4 mile drag racing.

I have been reading that the Buick 350 has a long stroke, and is a good candidate for building low end torque, longer stroke than a 350 chevy or even a 400 chevy, the stroke is even longer than the 427 chevy. I dont see many people building them for torque though, everything seems to be quarter mile oriented.

The other thing is that not all OD trans are the same. The 700R4 has a much lower 1st gear than the 200 and effectively makes the 2.56 feel like a 3.08 compared to a TH350.

Id like to keep the original engine in my car for originality and build it for torque. Why having the original trans is not important to me, I cant tell you. I think of the engine as the heart and soul of the car I guess, the trans as a soul less replaceable component. Also, gas mileage is very important to me because then I can drive it more and further and ffrom what Im reading, highway mileage with the OD trans can be in the mid 20s.

I have recently rededicated myself to all things Buick and want to make this car all that it can be, a fun and quick B body, 0-60 acceleration runs and low rpm highway cruising, taking advantage of the Buick 350 long stroke and build it for low end torque. People talk about it, I want to do it. I want to learn eveything I can on the differences between building an engine for maximum low end torque vs building for high reving 1/4 mile hp.

 

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

This discussion is strictly about making torque below 2000 rpm and fuel efficiency.

The first obstacle to over come is low rpm intake port reversion where the intake charge is pushed back into the intake port and manifold runner.
This occurs when the intake valve opens and when the intake valve is closing.

The intake valve opens before TDC during the end of the exhaust stroke where any residual exhaust pressure can cause reversion.
The exhaust valve doesn't close till after TDC and any exhaust system back pressure will also cause reversion.
The time the intake and exhaust valve are open at the same time is called the overlap period.
Reversion with exhaust gas not only delays the start of the intake cycle but the exhaust gas content dilutes the amount of oxygen in the intake charge causing incomplete combustion.
Decreasing the overlap period and opening the intake valve later is one solution.
Using a small diameter, long tube header will create a vacuum or scavenging pulse lower in the rpm range and can also help with low rpm torque and gas mileage.

The valve closes after BDC during the compression stroke and as the piston moves upward it will push the intake charge back into the intake port and manifold runner.
Closing the intake valve sooner traps more intake charge in the cylinder for more torque.
A late opening and early closing valve timing makes for a short duration cam lobe profile.
I'm sure you already knew this.

The way to have max low end power and a wide power band is to create a very high intake port velocity.
The higher port velocity resist reversion and will allow torque to be developed at lower RPM..

This is done by port work, valve job and most importantly the intake valve opening profile.

Max piston velocity for the SBB occurs at 75* before and after TDC.
The object is to open the intake valve to the place where the the intake valve head curtain area is equal to the smallest cross-sectional area of the intake port closer to the point of max piston velocity.
This allows the piston to transfer its kinetic energy to the intake charge during the time the energy is the highest.
For a stock SBB head an intake valve lift around .380" to .390" has the same curtain area as the smallest CSA of the intake port.

If the stock valve lift is around .390" then this point will be near the intake lobe center of 109* ATDC. WAY OFF!
If you increase the lift the point moves a lot closer to 75* ATDC. Lifts above .450" are very effective.
A roller cam profile with an aggressive ramp rate is the best way to accomplish this.

Port work where airflow is increased without increasing the port CSA creates high velocity air flow.
You can also go to a larger intake valve but do not increase the diameter of the throat.
I just had a head done where going from a 1.94" intake to a 2.02 intake didn't increase flow above .400" but the low lift flows were better.(Not a Buick head)
It turns out that the longer 60* slope below the valve seat helps direct the air past the valve head at low lifts.
It would be interesting to see how a tulip shaped intake valve would affect low lift flows.

Without going into further detail, a cam with a 200* intake duration and a 210* exhaust, .450" or more lift will work wonders.
Of coarse a roller version is even better.

A 9:1 compression ratio would optimize this combination and could be done by milling the head since you don't want to modify the short block.

Hope his is helpful

Paul

 

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

And the conclusion is use the 212 cam or one smaller 342 gear and the 200r4 trans this is a no brainer. Lockup torque convertor.


The trans is a direct bolt in why make it harder than it is. You are not going to be driving around in first gear with the 700 trans so why use it. just because it has a lower 1st gear than the 200 trans.

wow.

 

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

The TA 212 cam has 18 degrees more intake duration and way too much overlap for this project.

However the 200R4 trans takes less power to turn than the 700 and will give better gas mileage.

Paul

 

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

So by the sounds of it Gary might get what he needs with just some milling of the block and a custom cam designed by Paul.... Then pair that with an overdrive trans and a new rear gear and it could be exactly what he is looking for... As Always Paul has some SERIOUSLY good advice!