How to compute HP

F

Fortunateson

Well-known member
I'll be getting back to my 8BA project soon. It will be bored .030 over, came with the Merc crank, running a Max1 clone cam, original Eddie Myer heads, and three '97s with th eoptionof only running the centre carb.

Will have '39 trans hooked to a '40 Ford rear end with 3.78 c&p.

So how do I calculate HP.
 
And don't believe most of the 'Desktop Dynos' . . . while they may make you feel good, they typically greatly over-estimate the HP you'll potentially be achieving. Remember, flatheads are lower-rpm Torque engines . . . not high RPM big HP generators. (At least 99.999 % of them!) - ;)
 
My SWAG horsepower calculator says that motor will pull 143 horsepower on gas with 10% alcohol, at 500' elevation, with the secondaries operating.
 
There is no way to calculate horsepower. There are way too many variables. An "estimate" would be pretty much useless. You might get fairly close by comparing your engine specifics to an engine with similar specs that has been run on a dyno.
 
I haven't done one in awhile but a road dynamometer will give you the horsepower and rpm curve for an engine and is accurate to 2%.
You need a tachometer and speedometer and in the "olden days," a stopwatch and either someone to write data down or a tape recorder. Today you could use your cell phone to record a video of the tach & speedo.
Obviously you need the car's weight too.
It's nothing more than an acceleration plot of time against "in gear" mph achieved...... and some math.
A running vehicle would also be required....
 
1948F-1Pickup said:
I haven't done one in awhile but a road dynamometer will give you the horsepower and rpm curve for an engine and is accurate to 2%.
You need a tachometer and speedometer and in the "olden days," a stopwatch and either someone to write data down or a tape recorder. Today you could use your cell phone to record a video of the tach & speedo.
Obviously you need the car's weight too.
It's nothing more than an acceleration plot of time against "in gear" mph achieved...... and some math.
A running vehicle would also be required....
Click to expand...


If done right, it sounds like that method would be more accurate than a dyno. In the way that your getting "real world" data.
 
Again, I highly suggest JWLs book as a guide in building your engine. I documents dyno runs on about any configuration you can commonly come up with.
 

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Some historians claim it’s a myth that Rolls-Royce ever described the power output of its models with a dismissive, understated word: ‘adequate’.

Well it's a bit like that. A Flathead's horsepower may be adequate for you. Some may not feel it is adequate regardless of how much hp it has. It's only human nature to want a bit more.
 
When I was in high school, we'd get out the ole' J.C. Whitney catalog and add up all of the gains claimed in the advertisements for all of the "speed equipment" that was available. Got well over 200 HP every time.:eek:
 
I agree, JWL's book is the best guide to how much horsepower can be expected from a set of hardware on the flathead engine.

The horsepower of an engine is calculated as follows:

HP=(T x N)/5252

T=torque in lb.-ft.
N=engine speed in revolutions/minute (aka RPM).

Measure the torque and engine speed on a dynamometer and the power output can be calculated.
 
I was just trying to get a ballpark before the final decision isnmade regarding my trans combination... 28 vs 29 main/input shaft. I've asked before but since the engine isn't assembled I thought someone with some experience could give me some ballpark figure. Oh well I'll see what I'll see...
 
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16/28.......15/29 will get you thru 1st and 2nd annoyingly quick, then the further annoying too big drop to 3rd. IMHO>
Charlie ny
 
I'd go for the 16/28 combination . . . as you really don't need the 15/29 combination - especially if you're not in a heavily loaded pickup.

Transmission gear selection in this case is really not related to your HP numbers - it is more about the combination of rear end gears, tires, weight and RPM ranges. If you're running a 39 trans - you really don't have a lot of choices - even if you go down to 25T or 26T Zephyr gears - which usually hurt acceleration in a normal street car.
 
Bored&Stroked said:
I'd go for the 16/28 combination . . . as you really don't need the 15/29 combination - especially if you're not in a heavily loaded pickup.

Transmission gear selection in this case is really not related to your HP numbers - it is more about the combination of rear end gears, tires, weight and RPM ranges. If you're running a 39 trans - you really don't have a lot of choices - even if you go down to 25T or 26T Zephyr gears - which usually hurt acceleration in a normal street car.
Click to expand...

I had a 47 conv for many years... a 'tank', as described by one of the forum members. And, IMO, a tank... heavy, and way underpowered, with the stock 59 flathead. It came with 15/29, and 3.54's. What a lousy performer. A slug off the line, and almost dangerous on freeway on-ramps. Bottom line, I'd suggest serious consideration for the total combination, as B&S has alluded to. In my case, and after living with the car for about ten years, I eventually made some changes to make the car more 'drivable', more hp being one.
 
tubman said:
When I was in high school, we'd get out the ole' J.C. Whitney catalog and add up all of the gains claimed in the advertisements for all of the "speed equipment" that was available. Got well over 200 HP every time.:eek:
Click to expand...


Was it the magnets on the fuel line to align the gas molecules or the water injection system?
 
tubman said:
When I was in high school, we'd get out the ole' J.C. Whitney catalog and add up all of the gains claimed in the advertisements for all of the "speed equipment" that was available. Got well over 200 HP every time.:eek:
Click to expand...

I love that one... We would use the J C Whitney catalog to enhance mpg. The problem was, if we used all the 'enhancements', we'd have trouble with the tank overflowing.
 
1948F-1Pickup said:
I haven't done one in awhile but a road dynamometer will give you the horsepower and rpm curve for an engine and is accurate to 2%.
You need a tachometer and speedometer and in the "olden days," a stopwatch and either someone to write data down or a tape recorder. Today you could use your cell phone to record a video of the tach & speedo.
Obviously you need the car's weight too.
It's nothing more than an acceleration plot of time against "in gear" mph achieved...... and some math.
A running vehicle would also be required....
Click to expand...


You got me thinking and if you enter all the data your phone could compute it for you. It knows yur speed and acceleration so by entering the gear ratio you get the rpm. This app asks for the weight and coefficient of drag which would calculate rear wheel horsepower. Ideally not done on a windy day. I assume there may be better options but here is one demo.
https://www.youtube.com/watch?v=AHacDcIQDCs
 
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