4WD Torque Math, Oil Pressure Problem, FIxing Plastic Bumpers, Pulstar Plugs, Bleeding Brakes: MIke Allen's Weekly Online Auto Clinic

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Q: Lets say we have 250nm coming from the gearbox to a standard 2wd diff. The diff now doubles the torque to 500nm and evenly distributes the torque to both axles (i.e. 250nm per axle). With AWD system the 250nm from the gearbox is evenly (assumption) split in two giving 125mn to the front and rear diff's respectively where the diff doubles the torque to 250nm, times two diffs, and again we have a total of 500nm accelerating the car.

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My question is in the case of a locked transfer case (or a truck with two constant drive diffs)is the 250nm from the gearbox doubled by the first diff to 500nm and the same 250nm from the gearbox given to the second diff is also doubled to 500nm thus we now have a total of 1000nm accelerating the vehicle?

If true then a=f/m therefore will now accelerate twice as fast, the limiting factor being the engines max power in Kw / abilty to do the work required to accelerate the vehicle to the same top speed (Power = Force x Distance Time & Potential energy = mgh) therefor the power created by the torque and wheel speed cannot exceed the power of the engine?

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Also if the above is true then we have all seen the calculation for estimating wheel horsepower from 1/4 Mile ET, thus if I can accel my car faster by adding a second diff and still reach the same top speed haven't I just disproved this calculation?

A: Your reasoning is faulty. The differentials don't double the torque. If there's 250 nm available at the transmission output, the center or axle diffs can only split it. A locked or limited-slip differential makes no difference. There's still only 250nm to go around. So the acceleration is the same whether there's 2wd or 4wd.

Torque multiplication is different than the action of a differential. Are you confusing that with the true torque multiplication from the gear reduction from the ring and pinion gears inside the differential housing? These gears do multiply torque, but by reducing the axleshaft rotation by three or four to one from the driveshaft.

Q: In the September issue, a reader wrote in with an oil pressure problem with a 1992 Suburban. I agree with all your answers but you left one out that may be vital. I'll give my example:

I had a 1981 Jeep Pickup that I loved dearly. I had been using Pennzoil for a long time in my vehicles, and even after 100k would tear them down to find the motor very clean. The Jeep was experiencing the same problem, so I pulled the oil pan, checked all bearings and the oil pump, but everything was within factory spec, so I knew it was the gauge. A few weeks later, it threw a bearing, so I had the engine rebuilt. When I started it up again, I had weak oil pressure. I switched to Castrol, and the problems disappeared. The reader may have a bad batch of oil, you should have advised that as a try as well.

A: What viscosity was that oil? Maybe you got a mislabeled case that was a lot thinner. Say 0W-20 instead of 20-W50, that might explain the low pressure and the spun bearing.

Q: There is a five inch crack in the front bumper of my new 2008 Ford Focus. How can I repair it without having to buy a whole new bumper?

A: I covered plastic bumper repair in this Saturday Mechanic column: Plastic Bumper Repair.

Q: I recently purchased a set of Pulstar plugs and had them installed in my 3 month old 2008 Chevy Colorado, 5 cyl., hoping to improve the terrible gas mileage (12.3 mpg). Upon installation, the engine developed a miss and the shop determined that 1 coil was bad. I had them reinstall the original plugs and drove the truck immediately to the Chevy dealer. They replaced 4 of the 5 coils, which they said failed as a result of the installation of the Pulstar plugs, and charged me $310.00.
The said that any further engine problems may NOT be covered under warranty. Could the high tech plugs have caused the failure of these coils? The truck has only 2200 miles on it.

A: I'd blame the coil failures on the Pulstars, sure. Call them back and insist they pay for the coils. Let me know how far you get with that.

Q: I was trying to bleed the rear drum brakes on my 1999 Ford Windstar 3.0L, but no fluid came out of the bleeder valve of the wheel cylinder. The bleeding method I used was to open the bleeder valve and have an assistant press the brake pedal. I read on an online forum that the proportioning valve may divert the brake fluid pressure away from the rear brakes while attempting to bleed. If this is true, then how do I bleed the rear drum brakes?

A: You're on the right track, you need to press harder.

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