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Old 07-05-2009, 11:30 AM   #18
Wavery
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The reason that I used the 60%/80% formula in my post is so that those with the high RPM DOHC engines don't use the 3000-4000RPM figure.

If one has an engine that red-lines at 7000RPM they will want to keep a minimum RPM around 4000-4500 and a max sustained RPM around 5000 when climbing hills. This may even require grabbing a lower gear on some vehicles.

The reason that I mention this is that the torque ratings on a 7000RPM DOHC engine are drastically reduced at 3000-4000RPM and the driver will probably not be able to maintain speed. That engine should be able to be run in a lower gear and allowed to turn at a substantially higher RPM without concern of damaging the engine. DOHC engines develop most of their HP and torque at the higher end of the RPM curve.

Here's an interesting article on Torque/Hp:
http://auto.howstuffworks.com/horsepower1.htm

Here's a curve chart on a 7000RPM engine that describes what I was mentioning, the thing that must be recognized here is that this chart is on a racing engine. The torque curve on your engine will be a lot less in the lower RPM range, starting at closer to zero with a smooth curve, just above the HP curve then crossing over in about the same place:


On the other hand, if one has a diesel, they develop torque quite early and it may be best to keep them at a between 2500 and 3500RPM.

It's a good idea to look up the torque/HP curve chart on your TV so that you have a better understanding how what RPM range YOUR vehicle runs most efficiently at. Every engine model is designed with a different HP/torque curve. If you drive mountains, this information could be valuable to you.

One other tip on towing engines. Be sure to use synthetic oil in your engine and transmission. Synthetic oil disperses heat far better than petrol oils. This reduces heat build up and wear & tear on your engine. It's almost like adding a larger radiator to your vehicle.

When you are pulling a grade and you see the temp gage start to climb, that means that the engine is not releasing heat at a fast enough rate. As the moving parts of your engine heat up, they expand which causes more friction and less available HP. Think of your temp gage as a HP reduction gage. This means, the hotter that your engine gets, the more fuel that you have to give it (increased throttle) which, in turn, increases the heat even more. If the engine gets hot enough, it will actually "Seize", which means it will abruptly stop and may never turn again.

When mountain driving, keep an eye on the temp gage and don't let it get anywhere near the Red zone. If the gage is close, pull over, open the hood and let the engine cool down. If you haven't lost water, it's OK to leave it at idle for a few minutes before turning it off. NEVER open the radiator cap on a hot engine EVER. If you do, make sure that you are close to hospital a burn center because you may need it. This is very serious stuff. There have been cases where people have lost their entire face and/or received 3rd degree burns over their upper body. The Steam released from that radiator can be as high as 600 degrees. When the radiator cap is removed the entire cooling system may "Explode" all over you. That's roughly 3 gallons of boiling water and a huge amount of vaporous gas (steam), all released with a second or 2. That's not something to be messed with. The radiator cap should be cool enough to remove without using a rag for burn protection. If it isn't, leave it alone.

If you are using synthetic oil in your engine and trans, a 30 minute rest may be adequate. If you are using petrol based oil, the cooling process will be far longer, up to 2 hours. Never poor water on a hot engine. Although a spray bottle that delivers a mist can be very helpful. As the mist touches the hot surface, steam will be produces and the evaporation encourages heat release. This must be done with care because the steam can cause severe burns. If you have a 12V fan, that may help a lot. Don't run your battery down. If you can, use the battery from the camper. The thing that retains heat the longest is petrol based oils.
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