I have a V10 powered SD with the 6-speed manual transmission. I just got it a month ago. It is my first truck with a manual tranny. I abslolutely love it. My question is whether I can damage my engine/truck by downshifting rather than using the brakes. I know this is a simple question, and something that people do all the time. When you think about it, the amount of energy dissipated in the engine during engine braking is very high. Where does this energy go? Energy is heat, so what gets hot? Think how large the brake rotors are on the SD and how hot they get.

I know that this question only holds true to gas engines, as diesels don't have any airflow restrictions, unless they are fitted with a Jake or exhaust brake. As long as I don't downshift to extreme engine speeds, am I ok to use this as a regular braking method? I never exceed 4000 rpm while doing this.

Thanks guys.

 

As long as you do not overspeed the engine, using it to assist in braking should be OK. The load on the engine, while down shifting, is really not hard on the engine, high RPM's is the only thing that can hurt you.

 

Downshifting reverses the internal loads on the bottom end (pistons, cylinder walls, crankshaft and bearings) especially the thrust bearing. I try to avoid downshifting. Let the brakes take the wear not the engine. Brakes are cheaper and easier to replace.

 

excessive and impropper downshifting puts additional w&t on the clutch.

Like most things... if you let common sense prevail, you should be ok.

I learned to drive trucks in germany (MANY moons ago) when unsynchronized transmissions were still the rule. Back then I was taught to use DS-ing to control speed in the hills/mountains.

They also taught to push the clutch & put it in neutral when coming to a "normal Stop", if conditions allow.

 

drivetrain components... the load path between the energy "transfer" to the ground, and the energy "producer". everything from the tires, through the drive shaft, the trannie, the flywheel, the clutch and finally the engine. The engine dissipates its heat through the radiator. The trannie dissipates it's heat through its fluid (and the trans radiator, if available), the clutch dissipates its heat via radiation/convection to the atmosphere, or back through the other connected drive train components. This assumes they're all coupled by the transmission being engaged via the clutch. Otherwise it's all braking and dissipated via the rotors.

When you think about it, that IS a lot of energy. But that's a LOT of metal too, with mechanisms to manage the heat---assuming it's designed appropriately.

Imagine a Ranger trannie coupled to a V10 in a F250, and you let it coast DH in 3rd for a time. You'll overheat the trannie and fail something very soon.

This is where SYSTEMS engineering comes into place and you have to design appropriatly to make the SYSTEM work well together, rather than designing components in and of themselves that work well, but never testing them as a SYSTEM.

OK... I'll shut up now.

 

Matta

Your intuition about the heat dissipation is right on. The energy is dissipated as heated compressed gas ejected from the cylinder through the exhaust valves. If you think of this way....

Since each cylinder is designed to withstand enormous detonation forces and high temperature gases and forces, starving this system of fuel only serves to reduce the overall loading.

The work out of an engine ultimately is the difference between the work produced by the power stroke and the pumping work necessary to fill the cylinder with fuel, compresss it, and remove the exhaust. If you take away the power stroke, you haven't changed the direction of any loading.

None of the loading directions change during Gearing down. Only the magnitudes. During gearing down the exhaust stroke requires more effort than normal while the power stroke is starved. Each of the strokes retain the loading schemes as normal and well within the design limits of the engine. The exhaust valves still push against compressed heated gases, the cylinders still resist positive internal pressure (lower than when thrust is demanded) the pistons are pushed against the wrist pins, not pulled, the connecting rods push on the same bearing surfaces in the same directions.

The only real difference is that the crankshaft is under torque in the opposite direction, as is the entire drive train. The clutch is under a load in the opposite direction but the load is no higher than when under thrust.

That all being said, brakes are designed to be wear devices. Their entire design intent is around wear and high temperature friction.

Gearing down is fun and you get to play with the truck, but engines are for propulsion, brakes are for braking.

 

I had an 87 f250 with a 460 gasser 5 speed manuel. I drove it for 10 years and 180k miles. I always down shifted and never had a problem. The engine was still running strong when I sold it. Just watch the rpms and don't overload it. With down shifting you get a better feel of the road and better control of the truck.

 

Thanks for these explanations - everything makes good sense. Palindromelol, I am in agreement on these issues. The phrase "everything in moderation" applies here. By shifting normally and using both the brakes and engine I'm sure that I'll be all set. It was quite interesting to hear the theory behind this method. If you have anymore insight on this topic, I'd like to hear it. Thanks.

 

Yes... my point being NOT ONLY. Actually a "jake" brake takes this to the ultimate extreme, where you turn a reciprocating power producing engine to a reciprocating power ABSORBING compressor by changing the valve timing.

I guess traditionally the cams had dual lobe sets and the jake brake engagement was nothing more than engaging a solenoid which "popped" the cam shaft (translated it) such that the second set engaged turning the engine into a huge compressor.

Now the engine is absorbing the forward momentum of the vehicle and using it to compress the working fluid of the engine. Hence the gorgeous tone of a large diesel with the jake engaged.

...

 

Yes, Jake brakes are such an awesome idea - very simple and effective. Presently, they use oil pressure to actuate the exhaust valve at the end of the compression stroke rather than a change in the camshaft, but the net result is the same. Overall a very impressive system that makes a very impressive sound.

 

Downshifting should be perfectly fine. The Engine and Transmission management will not allow a forced downshift if the vehicle is traveling at a speed too fast for the engine....