Visit our Popular Forums

markdoane · 08-13-2004, 12:36 PM

Quote:

Originally Posted by mdeneen

Mark---Your math makes perfect sense to me. And, 20 more HP doesn't sound too far fetched as a possible gain.

I've been trying to gain some intuition on torque, and I THINK I have it. Tell me if this makes sense: Since torque is a type of applied power, if I was UNABLE to reach redline RPM going up the hill, that would indicate I am short on torque. But since I DO reach redline at 42MPH, then my torque would seem sufficient for that load. Does that make sense?
mdeneen

My intuition tell me your transmission is shot. Your 4.10 rear end tells me that torque should not be a problem. If you had a 3.21 rear end it would be a totally different story.

Does your van redline in second gear at 42 mph on level surface with no trailer?

71_safari · 08-13-2004, 12:58 PM

It is more than that. The increase in flat road horsepower alone (mostly wind drag) is 17 (from 30 to 47hp), assuming a frontal area of 72sf, and Cd of .75, so you actually need about 30 more horsepower delivered, when you include the hill, which is even higher than that due to mechanical losses. If you use 65%, which I think is a high number, you would need 45 more HP at the flywheel. I think that is a bit high, but certainly at least 40hp additional is in the ballpark.

The 77's only made about 160-175 hp so that is about 25% more - hard to do on an old truck and keep it reliable and even reasonably efficient.

john

markdoane · 08-13-2004, 01:39 PM

Quote:

Originally Posted by 71_safari

It is more than that. The increase in flat road horsepower alone (mostly wind drag) is 17 (from 30 to 47hp), assuming a frontal area of 72sf, and Cd of .75, so you actually need about 30 more horsepower delivered, when you include the hill, which is even higher than that due to mechanical losses. If you use 65%, which I think is a high number, you would need 45 more HP at the flywheel. I think that is a bit high, but certainly at least 40hp additional is in the ballpark.

The 77's only made about 160-175 hp so that is about 25% more - hard to do on an old truck and keep it reliable and even reasonably efficient.

john

John,
Very excellent analysis. I understand your comment to be that the you need 17 hp more to tow the trailer (vs. no trailer) on flat land?

I had assumed that the initial condition (42mph on grade) was with the trailer, so the effect of wind resistance on the trailer was the neglible going from 42 mph to 50 mph. That's incorrect of course, but I think the correct amount would be less than 17 hp. Can you calculate the required change in horsepower due to wind resistance between 42 mph and 50 mph?

Also, 65% efficiency was only gear train losses. Do you think it should be lower? I did not include thermal or volumetric efficiency.

dscluchfc · 08-13-2004, 02:19 PM

If I remember from 30 years ago in physics class.... wind resistance is something like the sq ft of exposed surface wind drag area times the wind speed(vehicle speed + any head wind) cubed. And I think that resistance is measured in ft lbs of pressure against the object pushing through the wind.
That means it doesn't take a huge amount of increase in speed to exponentially increase the wind drag, and thus dramatically increrase the power and torque necessary to push the tow vehicle and trailer through the air, not acccounting for the mechanical drag, thermal inefficiencies, and increased friction of tire slippage.
Reference your gas mileage. It is far better at 50 mph than it is at 75 mph.

markdoane · 08-13-2004, 04:46 PM

The formula for wind resistance is d=Cd*A*R*(v^2/2)
Hp required to overcome wind resistance at 42 mph is 14.6
Hp required at 50 mph is 20.6
The added hp to increase the speed from 42 mph to 50 mph is 6 hp,
Added to the 12.5 hp required to go up the hill at the faster speed gives a total of 18.5 hp. With 60% mechanical efficiency, the added hp would be 31 hp. Quite a stretch for a high mileage 350.

dscluchfc · 08-13-2004, 05:24 PM

Thanks, Don. You must have a handy physics book!

markdoane · 08-13-2004, 05:36 PM

Quote:

Originally Posted by dscluchfc

Thanks, Don. You must have a handy physics book!

I haven't cracked a book in years. It's all on the web.

Silver Bullet · 08-13-2004, 06:33 PM

mdeneen,
Has the tranny ever been rebuilt and which one is it, the th350 or the (I hope) TH400? Heat kills trannies and with your milage it my be due for a bit of help, torque converter included.

The motor; If you want keep it,the 350 is the cheapest of motors to rebuild. In fact, hit the PAW site (Pacific Automotive Warehouse) and get their catalog. Use the same block, heads and crank (reconditioned of course), do your own footwork on parts and you can save a bunch and build the extra ponies you need. I rebuilt my 460 this way and the cost was around $1100 but it's a Ford and I added a few goodies that took the final price up and it was a swap into a bronco that never came with that motor so some retro parts also had to be purchased.

Don; Kudos on the physics class, talk about a blast from the 30 yr (mine) past! Funny when you see them you say, WOW!, but to pull them up out of the blue would probably give me a stroke!

Oscar

mdeneen · 08-14-2004, 07:21 AM

Amazingly detailed posts! Clearly, the verdict is that getting 30Hp to 40HP more is a real stretch. I think I've been convinced to perhaps leave well enough alone, use the van for a while and look into something better next year.

I may have goofed when I said, it was "redlined uphill at 42MPH in 2nd gear." I have no tach. I was more or less reacting to the engine noise, which was substantial, and to the fact that the speed was not increasing. I really don't know what the RPM was.

A year ago, before I towed the Beast, I had a transmission cooler installed anticipating that it would be required. At the time, the fellow said he checked my transmission and that it was "aok." Now, who knows? He was a transmission shop, so I assume any opportunity he would have had to sell me a new one, he would have taken it. It SEEMS ok, but what do I know? I have no idea which model of transmission it is.

My expectation is that whenever there is a choice of what I have, it almost always turns out to be the lesser of the two choices.

At this point I think I will focus on getting the right hitch setup, brakes, controllers, shocks, tires and wheels and focus on good safe towing and just be content that it's a bit slow uphill.

I put 4 new Marathons w/new wheels on the trailer yesterday. While doing so, the tire guy gave me quite a lesson on Load Range/Pressure/Wheel Burst Strength and so on. Whereas I thought I could put Load range "D" tires on the back of my van and blow them up to 80lbs, he nixed that quick by pointing to my 15-inch wheels, "Not on those wheels you can't. You need to have 3/4-ton 16" wheels." Well, another fantasy exploded.

So, I think the next truck (next year) will be a Kenworth.
mdeneen

dscluchfc · 08-14-2004, 01:57 PM

Quote:

Originally Posted by mdeneen

So, I think the next truck (next year) will be a Kenworth.
mdeneen

I love the rig I waited 2 years to buy. Just like you I had to grind it out at 42 mph up the hills until I could do better.
If you can swing it...the GMC Duramax/Allison is wonderful as a tow rig.

71_safari · 08-14-2004, 02:58 PM

Quote:

Originally Posted by markdoane

The formula for wind resistance is d=Cd*A*R*(v^2/2)
Hp required to overcome wind resistance at 42 mph is 14.6
Hp required at 50 mph is 20.6
The added hp to increase the speed from 42 mph to 50 mph is 6 hp,
Added to the 12.5 hp required to go up the hill at the faster speed gives a total of 18.5 hp. With 60% mechanical efficiency, the added hp would be 31 hp. Quite a stretch for a high mileage 350.

Hi Don,

Travelling in Eastern Oregon and couldn't get back on until now. I went back and looked at my numbers and the 30 hp I calculated already included a factor for mechanical and rolling losses, so I did not need to add anything to that. So you are correct - around 30-31hp would be required to increase the speed going up the hill.

The flat road horsepower numbers I mentioned were for truck and trailer combined, and also included both wind resistance and mechanical/tire losses. The increase from 30 to 47hp total hp required was the difference between driving the rig at 42 or 50mph on flat ground.

Thanks, john

Don in E Texas · 10-13-2004, 01:21 PM

Good discussion on HP/Torque, etc. No one has covered going DOWN that 7% grade you have problems with going up?

Newer vehicles have anti-lock brakes, larger brakes in some cases, plus can add an exhaust brake to newer vehicles.

I agree with the post: Chev/GMC with Diesel engine and Allison transmission, if you want a vehicle to tow safely in.

... or you could get a Peterbilt, Kenworth, Freighliner, etc. Talk about making those 7% grades (up AND down)! Works for us. I have 450,000 miles to go to scheduled in-frame engine overhaul. Not in my lifetime!

Think about those hills as also going down...

don

Tarheel · 11-02-2004, 06:06 PM

I've put everything on my 92 Dodge Cummins that Banks makes. On our recent trip out west we hit some grades that went on and on and on. There were times that I was in 3rd doing 42 but usually there was a truck lane. It takes a lot of torque and a very good transmission to hold up to the loads. My clutch is rated for 500 hp and I'm putting out about 150 hp at the rear wheels. Just be careful that you don't spend alot of money just to incure new problems.