HP needed to go 65MPH? - Fuelly Forums
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Old 05-09-2008, 09:16 AM   #1
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HP needed to go 65MPH?

I am a newbe to your web site and this is my first question. I am wondering if we have any engineers on here who can give me an approxmate HP needed to propel a 2500lb car on a flat out highway at 65 MPH? I know their are many variables that will affect the answer such as aero and roll resistance among others, but if I could just get a ball park figure. I am asking this because I am thinking of building a project with a lightened up small car such as a toyota tercell or honda civic and would like to put in as small of an engine as possible (maybe a briggs or small motorcycle engine) that would let me drive highway speeds. My daily commute to work consist of 25 miles interstate and 5 miles street driving. I may even install an electric motor to help with inital acceleration if needed and then let the small gas engine take it at higher speeds. My second idea is to install a larger engine in a very light weight car and run a very high gear and final drive ratio to run the engine basically at almost idle at highway speeds? Any thoughts, ideas, suggestions??? Thanks

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Old 05-09-2008, 09:41 AM   #2
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Have a play with this...

I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice
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Old 05-09-2008, 10:04 AM   #3
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remember too that if you use a small briggs + stratton or something like that, it still has to be street legal (if inspections are required for your state). I know that some states don't require inspections.

I have also heard that it only takes about 10 hp to keep you at 65mph. (I figure that there are some limitations on that one) someone on here was playing around with the idea of adding a small motor and 5th wheel so that they could cut the engine on the vehicle and just use that motor once they were up to speed. not sure who it was that was saying that but a project like that would take a lot.

also remember that the geo metro had a 3cyl that only produced 63hp (varied slightly by year) it also weighed about 1800 lbs. that may be a start.
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Old 05-09-2008, 10:52 PM   #4
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Originally Posted by KLP View Post
My second idea is to install a larger engine in a very light weight car and run a very high gear and final drive ratio to run the engine basically at almost idle at highway speeds? Any thoughts, ideas, suggestions??? Thanks
If you are doing that, a better idea would be to install a small engine, but still run a high gear and final drive ratio to run the engine nearly at idle . The bigger the engine, the more wasteful it is (for many reasons, such as that if there is a bigger compressed volume in the cylinder at idle... then you need to add more fuel to that even at idle. A 3-cylinder geo metro / suzuki swift / pontiac firefly? engine is ideal - only 3 cylinders so less mechanical drag, and the engine is small.

If you are going to build a whole new car, the way to go would be to get a geo metro / suzuki swift 1.0 manual, and get a spare 1.0 manual gearbox from a breakers yard. Get a machine shop / engineering firm to make a different final drive on the gearbox so that the revs are much lower, and fit that gearbox. Then, fit a full undertray, and as many aero mods as you dare.

Later down the line you could then fit an electric motor, but I think a 1.0 metro with modded gearbox and aero mods would be a very good start for hypermiling in the US. Also fitting low rolling-resistance tyres would help a lot.

P.S. If it took 10 horsepower to drive along at 60mph, a 10 horsepower would 'never' reach 60 in theory... a 10.5 horsepower engine would probably take 2-3 minutes to reach 60! (since, once you got to 55, it might be taking 9.7 horsepower for wind resistance... so... from at 55mph you only have 0.8 horsepower from a 10.5 horsepower engine to accelerate with!. For a highway capable car, I wouldn't go below about 25 horsepower in a metro sized car with aero mods.

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Old 05-10-2008, 06:47 AM   #5
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I have a watt meter on my comuti-car, it's a 1,400 pound car that is rather brick shaped, so I figured that if one horse power under ideal conditions is 750 watts, then with all the losses that I could have then 1,000 watts is more accurate, cruising down the road at 35mpg takes 6,000 watts, hit a slight slope and it jumps to 10,000 watts, going up the hill to get to my house it slows down to 20mph and is drawing 20,000 watts for almost two minutes, granted this is all at lower speeds, but if air drag is squared as speed increases.
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Old 05-10-2008, 07:05 AM   #6
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some highway access ramps go up hill. I wouldn't want to try merging up a hill into highway traffic with 25 horsepower.
three stripes the charm!

Car mods are overrated. Just gotta adjust that nut behind the wheel for best mpg.

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Old 05-10-2008, 07:10 AM   #7
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Figure out how fast you are burning gasoline at 60mph and then calculate 9kw per gallon. If you are burning 1 gallon an hour then you are using 9kw which would translate to 60mpg . . . 30mpg would be 2gph or 18kw /750 watts per Horse Power you end up with 24HP at 60mph.
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Old 05-10-2008, 05:21 PM   #8
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Totally depends on total aero drag.

My project is an old Karmann Ghia and a 600 CC Kubota diesel rated at 16.5 HP. It will do 12 sustained at 3 k rpm on 1 gph.

I may just got with a 3 wheeler 2 in front and 1 rear, with a belt drive and a harley tranny, or my hydraulic infinitely variable tranny and an accumulator to give me one ungodly burst of acceleration.

In Va a 3 wheeler is classified as a motorcycle, the body would be radically aeromodded.


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