LSR-OFART

Z

zachary zehnacker

New member
Oct 3, 1997
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Gene,

To do the 120 MPH with a stock engine we should be ok with a stock 3.07 or
just slightly larger tires. The Olds faq says the motors are good to 5000
RPM stock, so we could push it up to 4500-4700 without too much worry.
This should be enough for 120. If we go with a motor designed for high
RPM, however, we could use the high ratio final drive and the trans
sprockets and should still have a couple of RPM to spare.

Zak

>-- [ From: Eugene Fisher * EMC.Ver #2.5.3 ] --
>
> Has any one calculated the rpm required for 120 mph??
>
>My coach ( with mod ratio xmission) is 2700 RPM at 60 MPH.
>
>I think I would have a hard time reaching 5700 RPM at 120 MPH.
>
>gene
>
>--
>Gene 76Palm Beach /Or/CA
>
>GMC Technical Information
>http://www.california.com/~eagle/
>
 
> Has any one calculated the rpm required for 120 mph??

Gene,

These are not precise but are ballpark estimates based on my coach, which
has the stock 3.07 final drive and standard size wheels & tires:

60mph= 2200rpm with 3.07 2500 with 3.50 2622 with 3.66

110mph= 4000rpm with 3.07 4580 with 3.50 4800 with 3.66

120mph= 4400rpm with 3.07 5000 with 3.50 5244 with 3.66

They do not take into account increasing amounts of torque converter
slippage at higher speed, if any, and I do not have a calibrated tach.

Can somebody check my figures, please?

I note that the stock 455 produces its 212 peak hp at 3400 rpm, which would
occur at 93mph in a coach equipped with standard 3.07 final drive.

Richard Guthart
 
Torque curve chart I obtained from Chuck Stoddard, shows peak torque
produced around 2800 vs. the 3400 rpm you say (probably came from a
brochure), for the 455 cid engine.

Paul Bartz

- -----Original Message-----
From: Richard Guthart [mailto:rguthart]
Sent: Friday, December 18, 1998 12:34 PM
To: gmcmotorhome
Subject: Re: GMC: LSR-OFART

Has any one calculated the rpm required for 120 mph??

Gene,

These are not precise but are ballpark estimates based on my coach, which
has the stock 3.07 final drive and standard size wheels & tires:
60mph= 2200rpm with 3.07 2500 with 3.50 2622 with 3.66

110mph= 4000rpm with 3.07 4580 with 3.50 4800 with 3.66

120mph= 4400rpm with 3.07 5000 with 3.50 5244 with 3.66

They do not take into account increasing amounts of torque converter
slippage at higher speed, if any, and I do not have a calibrated tach.
Can somebody check my figures, please?
I note that the stock 455 produces its 212 peak hp at 3400 rpm, which would
occur at 93mph in a coach equipped with standard 3.07 final drive.

Richard Guthart
 
Paul,

I was referring to the rpm at which peak hp occurs, not torque.

You're right, I got the figures from either a brochure, owner's manual or
factory service manual, I can't remember which. It claimed max torque of 344
ft-lbs at 2400 rpm and max power of 212 hp at 3400 rpm.

The objective would be to get the gearing so that the engine was putting out
its peak hp at about the max speed of the coach, would it not?

Richard Guthart

- ----Original Message-----
From: Bartz, Paul
To: 'gmcmotorhome'
Date: Friday, December 18, 1998 10:42 AM
Subject: RE: GMC: LSR-OFART

>Torque curve chart I obtained from Chuck Stoddard, shows peak torque
>produced around 2800 vs. the 3400 rpm you say (probably came from a
>brochure), for the 455 cid engine.
>
> Paul Bartz
 
I bookmarked a website a few months ago that has some programs for
calculating such things as gear ratios, HP, RPM, speeds, etc. based on
various items that you input. Those that can might consider going to:

http://sura1.jlab.org/~grippo/auto_noframe.html

or do a search for "Bowling's Automotive Programs."

There are likely some answers to be found there that would be of some use in
this endeavor.

Robin
73 Sequoia
Corning, NY
 
I would think that don Wheat would have the figures from the original wind
tunnel tests if they were performed. GM normally does calculations and
tests to show how slippery the body is and than can calculate the Horse
power needed at a given altitude for a given speed.

>Paul,
>
>I was referring to the rpm at which peak hp occurs, not torque.
>
>You're right, I got the figures from either a brochure, owner's manual or
>factory service manual, I can't remember which. It claimed max torque of 344
>ft-lbs at 2400 rpm and max power of 212 hp at 3400 rpm.
>
>The objective would be to get the gearing so that the engine was putting out
>its peak hp at about the max speed of the coach, would it not?
>
>Richard Guthart
>
>----Original Message-----
>From: Bartz, Paul
>To: 'gmcmotorhome'
>Date: Friday, December 18, 1998 10:42 AM
>Subject: RE: GMC: LSR-OFART
>
>
>>Torque curve chart I obtained from Chuck Stoddard, shows peak torque
>>produced around 2800 vs. the 3400 rpm you say (probably came from a
>>brochure), for the 455 cid engine.
>>
>> Paul Bartz
>
>
>
Tom & Marg Warner
Vernon Center NY
1976 palmbeach
 
Richard:

I'll check the torque curve, however I don't recall the rpm being that high
a peak torque.

Also, contrary to what most individuals think, torque and not horsepower is
the measure that you're after to provide the "twist" to propel a vehicle.
Defined: Torque results from the force exerted on a body pivoted about some
axis (crankshaft in the case of an engine). Horsepower is a unit of work or
energy.

So torque value is a more important measure vs. horsepower when determining
what it will take.

Paul Bartz

From: Richard Guthart [mailto:rguthart]
Sent: Friday, December 18, 1998 12:58 PM
Subject: Re: GMC: LSR-OFART

Paul,

I was referring to the rpm at which peak hp occurs, not torque.
You're right, I got the figures from either a brochure, owner's manual or
factory service manual, I can't remember which. It claimed max torque of 344
ft-lbs at 2400 rpm and max power of 212 hp at 3400 rpm.
The objective would be to get the gearing so that the engine was putting out
its peak hp at about the max speed of the coach, would it not?
Richard Guthart
From: Bartz, Paul
Date: Friday, December 18, 1998 10:42 AM
Subject: RE: GMC: LSR-OFART

>Torque curve chart I obtained from Chuck Stoddard, shows peak
torque
>produced around 2800 vs. the 3400 rpm you say (probably came from a
>brochure), for the 455 cid engine.
>
> Paul Bartz
 
I found the source of this data - the GMC Parts Book pub. 78Z, p. 7.

Richard Guthart

>....I got the figures from either a brochure, owner's manual or
>factory service manual, I can't remember which. It claimed max torque of
344
>ft-lbs at 2400 rpm and max power of 212 hp at 3400 rpm.
>The objective would be to get the gearing so that the engine was putting
out
>its peak hp at about the max speed of the coach, would it not?
 
Rick,

At the risk of beating this to death, I'll pass on these slippage
calculations:

My tires, 225/75R16, are a calculated diameter of 29.29" and an effective
diameter, taking into account the flattening at the contact patch, of
28.75."

When I plug in this effective diameter into the formula at
http://sura1.jlab.org/~grippo/rpmmph.html it computes that I should be
turning 2154rpm at 60 mph with my standard 3.07 final drive and the entire
gear train locked up. I measure 2200 rpm which, if accurate, would mean a
slippage of about 2%. I say if, because while the odometer portion of the
speedo checks out as right on when tested against the mileage markers on an
interstate, I have no way to know the accuracy of the tach. This was a
hand-held Sears Craftsman tach that I connected to the MSD tach output one
day.

Richard Guthart

- -----Original Message-----
From: RickStapls
To: gmcmotorhome
Date: Friday, December 18, 1998 11:02 AM
Subject: Re: GMC: LSR-OFART

>......that works out to 10% - 12% slippage
>through the torque converter, transmission, tire/road, etc. I thought that
>was high, but when I ran the same calculations on my coach (3.07 gears,
~2300
>rpm @ 60mph), I came out with about 10% slippage too

>Rick Staples
>'75 Eleganza
>Louisville, CO
>
 
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