Hauling safety

[Soundguy]

What I don't get is why people think a big motor makes pulling anything safer. IE We had a F250 with a 300 6cyl & it had the same brakes frame & axles as the one with a 460, the only differance was the 460 would get you there alot quicker. So yes if you tow every day the big motor will pull better but the brakes, frame & springs are very close to the same on all 3/4tons of the same make, only differance is how if you need low range to get it moving or high range. but with that said everything I own has a smoker in it other than the weedeater(still working on that one any ideas are welcome).

I can think of a small difference a large engine might make in braking ability. On trucks so equipped ( like the larger diesels, when in tow mode), when you let off the gas.. the engine really starts dragging you down pretty fast.. almost like a jake brake.. I know my f250 diesel coasts to a stop MUCH faster than my dodge gasser.. and it's due to that compression braking I'm thinking..

soundguy

 

[patrick_g]

I can think of a small difference a large engine might make in braking ability. On trucks so equipped ( like the larger diesels, when in tow mode), when you let off the gas.. the engine really starts dragging you down pretty fast.. almost like a jake brake.. I know my f250 diesel coasts to a stop MUCH faster than my dodge gasser.. and it's due to that compression braking I'm thinking..

soundguy

Diesels do not do compression braking nearly as well as a gasser since the air intake is wide open all the time unlike a gasser where when you take your foot off the accelerator the air intake is closed down forcing the engine to work hard to bring in air to compress.

Jake brakes change the valve timing so the engine is compressing air with no way to get out the exhaust.

Small diesels such as in a pickup do not have jake brakes. Instead they can be fitted with exhaust brakes as an aftermarket custom installation. This is a butterfly valve in the exhaust path to extract energy from the exhaust stream and force the engine to do work to push the exhaust out. Diesels have copious exhaust flow at high RPM even with no throttle applied due to the intake being wide open.

The throttle is coupled to the control mechanism so above idle the brake is OFF.

That is the way I understand them (at least up to my '97 Dodge.)

My 2008 F-250 PowerStroke Diesel has a lot more braking effect then my Cumins 5.9 (when not using exhaust brake on Cumins) but I don't know why. It is especially noticeable when in tow/haul mode mostly, I think, due to aggressive shift pattern when in that mode.

Pat

 

[Soundguy]

I never meant to imply that a pickup truck had a jake brake.. just that my 04 diesel in haul mode.. when you let of fthe throttle, it agressivel slows down with a good bit of increase in engine nose.. reminiscent of a 'mild' jake brake'.. my gasser.. when you let of fthe gas.. rpm drops to idle and the truck continues to speed downt he highway slowing down ever so slowly.

If I had to guess.. I'd think it was something in the torque converter locking up and dragging the rpm down...??

soundguy

 

[aczlan]

My 2008 F-250 PowerStroke Diesel has a lot more braking effect then my Cumins 5.9 (when not using exhaust brake on Cumins) but I don't know why. It is especially noticeable when in tow/haul mode mostly, I think, due to aggressive shift pattern when in that mode.

I have seen similar braking differences between manufactures of the 15 passenger vans I drive, for example; moms '99 E350 w/the 7.3L Powerstroke has a good bit more engine braking than the '05 E350 with the 5.4L gas engine at the hotel where I work, however the '05 E350 has more engine braking that the '05 Chevrolet Express 3500 that we also have at the hotel.

Aaron Z

 

[patrick_g]

I never meant to imply that a pickup truck had a jake brake.. just that my 04 diesel in haul mode.. when you let of fthe throttle, it agressivel slows down with a good bit of increase in engine nose.. reminiscent of a 'mild' jake brake'.. my gasser.. when you let of fthe gas.. rpm drops to idle and the truck continues to speed downt he highway slowing down ever so slowly.

If I had to guess.. I'd think it was something in the torque converter locking up and dragging the rpm down...??

soundguy

I get a similar effect with my 2008 F-250 and I too don't know how it does it. My Dodge/Cumins free wheels in the tranny on deceleration and had to be modified to keep the torque converter locked up so the exhaust brake could work.

I hope the torque converter stays locked up but the retardation force seems to be definitely more than just a locked up converter hooked to a diesel with little fueling. Again, diesels without variable valve timing Jake brakes (or exhaust brakes) do not offer as much "compression braking" as a gasser. At least that was conventional wisdom before I drove the new F-250 which apparently acts like the Soundguy's Ford. I guess we will have to investigate this.

Oh, and Soundguy, I know you know what Jakes and exhaust brakes are and do. What we have here is a little mystery as to what is going on with our Fords.

It seems to me that the retardation is too pronounced to not be "different" from just a tranny hooked to a diesel. In tow/haul mode I like the way the tranny keeps down shifting fairly aggressively. When the 2008 was new, I looked for an exhaust brake but none of the regular suppliers had one for it yet. Maybe if I stay out of the mountains I won't need one.

Pat

 

[keeney]

On engine size and safety, I have to disagree a little.

1. If your tow vehicle is so underpowerd that it cannot maintain highway speed (perhaps on the uphills), you become a hazard on the road when everybody else wants to pass you.

2. When your under-powered vehicle overheats or otherwise breaks down, you become a road-side hazard. For some unknown reason, inattentive drivers seem to hit vehicles on the shoulder a lot more than one would think would be statistically probable. I mean how often do you drive on the shoulder? Maybe its just habit of following other vehicles and your unconcious brain just steers into the shoulder thinking its following the stopped vehicle. Anyway, any time you are on the shoulder (out of gas, flat tire, overheated tow vehicle), you are increasing the risk of an accident.

Also, the larger engine does add a slight bit more weight to the tow vehicle, giving its steering wheels more traction, more braking, etc.

In general, I think that the ratio of tow vehicle weight to trailer weight is one of the largest factors. For example, pulling a 12,000 lb trailer with a 4,000 lb mini-pickup is a tail trying to wag a dog for sure. Up the tow vehicle to a 7000 lb 3/4-ton truck and it starts to be a bit more of an even match.

- Rick

 

[patrick_g]

On engine size and safety, I have to disagree a little.

1. If your tow vehicle is so underpowerd that it cannot maintain highway speed (perhaps on the uphills), you become a hazard on the road when everybody else wants to pass you.

2. When your under-powered vehicle overheats or otherwise breaks down, you become a road-side hazard. For some unknown reason, inattentive drivers seem to hit vehicles on the shoulder a lot more than one would think would be statistically probable. I mean how often do you drive on the shoulder? Maybe its just habit of following other vehicles and your unconcious brain just steers into the shoulder thinking its following the stopped vehicle. Anyway, any time you are on the shoulder (out of gas, flat tire, overheated tow vehicle), you are increasing the risk of an accident.

Also, the larger engine does add a slight bit more weight to the tow vehicle, giving its steering wheels more traction, more braking, etc.

In general, I think that the ratio of tow vehicle weight to trailer weight is one of the largest factors. For example, pulling a 12,000 lb trailer with a 4,000 lb mini-pickup is a tail trying to wag a dog for sure. Up the tow vehicle to a 7000 lb 3/4-ton truck and it starts to be a bit more of an even match.

- Rick

Rick, Sure can't argue with the concept of dangerously underpowered. At some point you are a hazard to navigation as HP/Weight ratio goes lower and lower. Now as far as maintaining highway speed on hills is concerned. An awful lot of 18 wheelers on the highway don't maintain the speed limit on hills and aren't particularly unsafe because of it. Our interstates are often posted at 70MPH but wisely also post minimum speeds of 45. (Referring to Oklahoma) If you can maintain 45 up a hill you are not in violation. I don't think overall public safety would be enhanced by encouraging everyone pulling a trailer to drive 70MPH.

If people are so immature that they are unable to control their emotions sufficiently while driving that they commit unsafe acts out of frustration because someone is trying to tow a load safely and are not making the upper speed limit then so be it. You can't always make everything perfect for the self important few who think everyone should stay of of their way. I for one don't want everyone towing a load to try to always maintain the max posted speed just to avoid inconveniencing some impatient immature driver.

Excellent point about how roadside objects (police giving tickets, trees, etc do get hit much more often than you'd think. A traffic engineer told me that one of their primary truths is that any roadside object will eventually get hit. Someone bobbles in traffic and goes on the shoulder a bit or even more AND it happens it is when you are stopped getting a ticket or waiting for your overheated vehicle to cool.... oops.

I was taught that when the fog gets too bad to safely continue driving that I should pull well off the roadway and turn off all the vehicles lights. The reason is that a lot of bozos when driving in reduced visibility try to tailgate someone closely enough to be able to follow them instead of guiding on the highway markings (which they can't see in the fog.) The result is that if you are parked with your lights on, by the time they realize they are overtaking you not just following you down the road, it is too late to react and you are hit in the rear.

As far as the weight of a larger engine making a big difference in the stability of a truck pulling a trailer goes... It is theoretically true but I doubt you could sense the difference.

About item 2. I have seen lots of truck-trailer combinations with rather low HP to weight ratios that were in excellent repair and were no more (and probably less) likely to overheat or breakdown than many much more powerful rigs. A lower HP/weight ratio is not a reliable indicator of probability of a breakdown. If that were the case the roadside would be littered with 18 wheelers awaiting assistance/repairs.

Of course there are limits of reasonableness. At some point a vehicle would be too underpowered to be practical or safe, irrespective of its mechanical condition or ability to operate without self damage. I in no way condone such vehicles venturing out into traffic to share the roads with us. On the other hand I don't think the minimum safe HP requirement is to be able at all times go up any grade with any load to maintain the max posted speed.

I live in a rural agricultural area and see lots of trucks pulling trailers. I very very seldom see a vehicle pulled off the road with a mechanical problem. When I venture into the city (OKC) I see more stranded cars in a day than I see stranded trucks with trailers in several months.

Pat

 

[bjcsc]

Again, diesels without variable valve timing Jake brakes (or exhaust brakes) do not offer as much "compression braking" as a gasser. At least that was conventional wisdom before I drove the new F-250 which apparently acts like the Soundguy's Ford. I guess we will have to investigate this.

I agree with your last sentence. I have always found my diesels slow way faster than any gas vehicles, both cars and trucks. I don't see how it's related to the intake as you mentioned earlier. To me, it all happens in the compression stroke, not the intake. When the wheels are powering the engine, a gas car is compressing 10:1 at most and a diesel is compressing at least twice that much. My F250 is an automatic. It slows down quick and when it's doing it it gets real quiet for a while and then the engine sound picks back up. My F450 does the same thing but it's a 6 speed. Both of them slow down way faster than the F350 gas that I replaced. It's not even close...

 

[Rara Avis]

Pickups can have exhaust brakes...
GM Duramax 6.6L 04.5-05 LLY - Exhaust Brake

 

[patrick_g]

I agree with your last sentence. I have always found my diesels slow way faster than any gas vehicles, both cars and trucks. I don't see how it's related to the intake as you mentioned earlier. To me, it all happens in the compression stroke, not the intake.

The reason exhaust brakes are sold for retrofit onto diesel pickups and motorhomes (but to my knowledge not for gas engines) is because the diesels have nearly no compressive baking compared to the gas engines. The gas engine has a throttle that closes off the incoming air stream quite a bit when you go to idle. The pistons, on intake stroke, have to suck air through the orifice formed by the nearly closed air inlet. In the olden days it would have been a butterfly valve in a carburettor but the equivalent is still in use.

Diesel engines have NO CONTROL to limit intake air. The intake is wide open at all times. When you go to idle with a diesel the engine uses energy to compress the air on the compression stroke but gets most of that energy back on the power stroke (even though there is no extra energy from burning fuel) Then the exhaust valves open and the air is exhausted. There are frictional losses but otherwise there is no net loss of energy and the braking effect is small. The difference in braking effect is the gas engine sucking air in through a restriction when at idle position on the throttle. The diesel's higher compression doesn't really matter as it is getting back most of the energy put into compression at idle throttle setting.

Jake brakes (not normally found on a pickup) use variable valve timing such that the diesel engine so fitted is doing a lot of work compressing air but not getting any help back out of the compressed air. VERY EFFECTIVE braking when you make the engine into a big air compressor with no storage tank.

With an exhaust brake fitted the sequence is this: A butterfly valve in the exhaust closes (to a preset position to not float the valves with over pressure) and the exhaust has to go through an orifice/restriction which again uses the engine like an air compressor with good effect.

I have BD brand exhaust brake on my automatic Dodge/Cummins pickup and had I not had the heavy duty valve springs from the factory I would have had them put in so I could use higher pressures with the exhaust brake for better braking action. Braking action improves with RPM, of course, so you want to down shift and let the engine rev up for more effective braking (but of course not over reving.)

In mountain driving and or with heavy hauling, exhaust brakes are a terrific safety factor. Descending a grade with a heavily laden diesel and no exhaust brakes is a formula for disaster as you can overheat your friction brakes and get brake fade. Down shifting a diesel pickup on a steep downgrade when you have no exhaust brake only revs the engine and makes almost no discernible braking difference.

Now as regards the more recent Ford diesels... I don't know what is going on but they seem to have better engine braking than any other non-exhaust brake equipped diesels I have driven. Wasn't like that with my '84 Ford diesel F-250. Isn't like that with my Dodge/Cumins '97 Ram 3500. My curiosity will force me to investigate. I did search for an exhaust brake specified for the 2008 Powerstroke a few months back and did not find one.

BD Power - The Leaders in Diesel Performance
Banks Power – Exhaust Brake Systems: Why Add an Exhaust Brake to turbo diesel powered vehicles for supplemental engine exhaust braking.
Home - PacBrake - Exhaust and Engine Brakes for Automotive, Motorhome, Medium Duty and Heavy Duty Vehicles

Pat