HP (towing ability) calculator

[Derrek]

Sent: 1/9/2005 12:42 PM

Hello everyone

There are currently two threads regarding towing ability. Ironically, I came across a calculator in Motor home magazine that can help answer the question;
"Can I tow this?"

The formula is set up to calculate the horsepower required to climb a hill.

The formula is (weight/37500) x mph x percent grade.

One thing needed before calculating this is the horsepower rating of the engine. Using the info from the link http://www.flxible.net/enginespecs.html  the 440-3 in my 1978 Apollo is rated from the factory @ 225HP.

I agree with what Denison and others have said in regards to factory horsepower specs being inflated. Tests were done on perfectly balanced engines with fans drawing the exhaust out (reverse super charging) to increase power..etc.

We also need to account for losses that would occur in the drivetrain. Doing research on the net shows that the average loss in a drivetrain using an automatic transmission is 20%

So lets say that the HP was over inflated by 10% and the drivetrain losses are 20% reducing our HP by 30% to a "real world" hp rating.

Reducing the factory rating of 225hp by 30% gives us a "real world" hp of 158.

So lets take the weight of my Apollo (16,000 lbs/37500) X 55mph x 6% grade= 140hp (88% of engine power)

Now lets drop 10 mph: (16,000/37500) x 45mph x 6% grade =115hp (72% of engine power)

Then add my 3,000 lb toad: (19,000/37500) x 45 x 6% grade = 136hp (86% of total engine power)

Doing a search for Winnebago weight, I found a message where Dave Beatty said his Winnebago weighed 8,720 unloaded with 1/2 tank of fuel.

He stated that a potential buyer would like to tow a 3,000lb Mazda Miata. This would bring the total weight to 11,720 and we will say that with passengers and still no other items the total weight is an even 12,000lbs

Looking at Allpar.com I found the hp rating for a '72 318 being 103 HP. Reduce that by 30% to get a "real world" hp of  72hp.

(12,000/37,500) x 45mph x 6% grade = 86hp (119% of engine power)

(12,000/37500) x 35mph x 6% grade = 67hp  (93% of engine power)

As you can see by the calculations, it does not appear that the potential owner could tow the Miata.

The calculations may not be 100% accurate. The 318 engine rating may be for a car engine and not a 318 used in a motor home.

Based upon my experiences with driving my motor home and looking over the calculations using the weight and horsepower of my motor home, I feel the calculator is very accurate. I usually pull a 6% grade in second gear at approx 3,000 rpm at 45 mph which would mean the engine is producing 136 hp (85% of "real world" hp) and 238 ft-lbs of torque. (hp x 5252/ rpm).

Hopefully using this formula will help members answer the question "Can I tow it?"

                                                                                   Derrek