LSR Power

[clark searle]

For you speed enthusiasts, here's some SWAG estimates of the power and axle
torque that you'll need to propel a GMC to 120 MPH. (In case there's anybody
that doesn't already know, it's a Scientific Wild As- Guess).

I think it takes about 70 HP to drive the coach at 60 MPH. Part of this is
aerodynamic drag and part is friction (tires, bearings, etc.). If you assume
225R75 16 tires and the stock 3.07 final drive ratio, 70 HP equates to an axle
torque of 267 lbs ft on each axle. If the converter slips at 5% (probably still
a reasonable guess under these conditions- and somewhat verified by Paul Bartz's
measurements of engine speed vs. MPH, which he kindly shared with me), the
engine speed would be 2225 RPM.

Extending those same assumptions out to 90 MPH, gives a required driving power
of 189 HP, axle torque of 482 lbs ft per axle and an engine speed of 3428 RPM
(assuming 7.5% converted slip).

Taking it out to 120 MPH gives a power requirement of 410 HP, axle torque of 782
lbs ft each and an engine speed of 4598 RPM (assuming a 10% converter slip).

I'll not bore the Net Gang with the details, but if anyone is interested in the
calculations, I'll be glad to share them (maybe direct is better).

Clark Searle
78 Kingsley
Mt. Pleasant, MI

 

[bdub]

>Taking it out to 120 MPH gives a power requirement of 410 HP, axle torque of
>782 lbs ft each and an engine speed of 4598 RPM (assuming a 10% converter
slip).

Wow, she's gonna be screamin!

Reckon a 455 can make 410 hp with top end enhancements like trick heads,
cam, intake, cauberation, exhaust? That would be stuff readily available
to any enthusiast without gettin too exotic.

bdub
'76 Palm Beach
In The Heart o Texas
www.web-access.net/~bmassey/
icq # 202333

 

[thomas g. warner]

The 455 will put out over 425HP using only parts from the original Toronado
engines. I have not got my factory manuals for the 1970 here but think it
was rated at 425HP. different heads, pistons and cam, dual exhausts, carb.

>>Taking it out to 120 MPH gives a power requirement of 410 HP, axle torque of
>>782 lbs ft each and an engine speed of 4598 RPM (assuming a 10% converter
>slip).
>
>
>Wow, she's gonna be screamin!
>
>Reckon a 455 can make 410 hp with top end enhancements like trick heads,
>cam, intake, cauberation, exhaust? That would be stuff readily available
>to any enthusiast without gettin too exotic.
>
>
>bdub
>'76 Palm Beach
>In The Heart o Texas
>www.web-access.net/~bmassey/
>icq # 202333
>
>
Tom & Marg Warner
Vernon Center NY
1976 palmbeach

 

[clark searle]

> In a message dated 12/21/98 10:14:41 PM Eastern Standard Time,

>
> > I'll be glad to share them (maybe direct is better).
>
> Hi Clark!
>
> Please don't do that we like to see someone put their foot in their mouth.
> We all need this information to get psyched up for the run!
>
> Scott Adohen

Ok, but remember, I had to do a lot of assuming here.

First, I assumed at 60 MPH that friction accounted for 25 HP and aerodynamic
drag accounted for 45 HP. That was pure SWAG, but was based on some earlier
experience with car data.

Second, the power to overcome aerodynamic drag goes up with the cube of speed
(2x speed = 8x power). I assumed that the friction coefficient stayed constant
with speed, therefore HP would go up in direct proportion with speed.

Third HP = Torque (lbs ft) x RPM divided by 5252 (that's not an assumption).

Fourth, I used the published revolution per mile factor for 16 inch tires (688.6
rev per mile).

At 60 MPH or 1 mile per minute, the wheels would have to rotate at 688.6 RPM.
Starting with the assumed 70 HP, that yields a combined axle torque of 533.9 lbs
ft, or 266.9 lbs ft on each driving axle. (Torque = 70 HP x 5252 divided by
688.6 RPM)

At 90 MPH, the aerodynamic power would go up by 1.5 cubed (90/60^3) or 3.375
times and friction would go up by 1.5 x 25 HP. Result: 151.9 + 37.5 = 189.4
HP. Axle speed would be 1.5 x 688.6 RPM or 1033 RPM. Combined axle torque
would be 963 lbs ft or 481.5 lbs ft per axle (189.4 HP x 5252 divided by 1033
RPM).

At 120 MPH, aerodynamic drag is up by 2 cubed (8x) and friction is 2x 25.
Result: 360 + 50 = 410 HP. Axle speed would be 2 x 688.6 or 1377 RPM. Combined
axle torque would be 1563.5 lbs ft or 781.8 lbs ft per axle (410 HP x 5252
divided by 1377 RPM).

Last, another SWAG, I assumed converter slip was 5% at 60 MPH, 7.5% at 90 MPH
and 10% at 120 MPH. I'm really on shaky ground on this one.

So, transmission output speed is axle speed times the final drive ratio (example
at 60 MPH and 3.07 ratio: 688.6 RPM x 3.07 = 2114 RPM out of transmission. That
divided by .95 (5% slip) = 2225 engine RPM.

Same for 90 MPH, 1033 RPM x 3.07 = 3171 RPM transmission output, divided by .925
(7.5% slip) = 3428 RPM engine speed.

For 120 MPH, 1377 RPM x 3.07 = 4228 RPM tranny out, divided by .9 (10% slip) =
4698 RPM engine speed.

That's what I think.

Clark Searle
78 Kingsley
Mt. Pleasant, Michigan (1 below zero as I type)