Engine durability not issue with Honda’s VCM

Hey there, time traveller!
This article was published 21/10/2005 (7344 days ago), so information in it may no longer be current.

QUESTION — My husband and I are looking at the Honda Odyssey van. The model I would like has the Variable Cylinder Management system. I’ve read the information on web sites, but what I would like to know, is this a favourable buy or will we regret buying it. Have the engines shown their long term longevity? It is hard to imagine three cylinders working at top speed then all of a sudden three more step in. How can the latter function without tearing up the engine?

ANSWER — Honda’s Variable Cylinder Management (VCM) is accomplished by turning off the fuel and deactivating the valves on three of the cylinders on their V6 engine during light load conditions. To deactivate the valves, computer solenoid-controlled hydraulic oil pressure moves a synchro pin that interlocks the rockers and cam follower. Now the cam follower is still free to move as the camshaft rotates but the rocker arms are no longer linked to it and the valves don’t move. This synchro pin will move back and forth, linking or unlinking the rocker arms to control valve operation.

It seems impossible for parts that are moving at several hundred cycles per minute to be linked or unlinked in a fraction of a second by a moving pin, but that is the principle that Honda’s VTEC engines are designed on.

The VTEC technology has proved itself in racing and millions of kilometres in passenger vehicles over the years. Honda has used this technology in Formula 1 race engines, not to stop valve movement, but to change valve opening. Those engines rev to 18,000 rpm and their durability as race engines have been outstanding.

The VCM system operates the engine on three cylinders only during light loads, so there is no significant wear on the cylinders that are producing power compared to those that are deactivated. While any engine can fail, Honda durability has proved excellent. I wouldn’t worry about engine durability as an issue at all. It is more complex, but problems are almost unheard of. On the plus side, you benefit from a fuel-economy improvement.

QUESTION — A friend of mine and I have a disagreement. He has a 1999 Chevy Lumina with a 3.8-litre engine and an automatic transmission. He has been driving the car for slightly over 100,000 miles and consistently manually shifting the car for that period of time. He now has a transmission problem. I believe this was caused by him manually shifting the car rather than letting the computer automatically shift it for him. Could you please shed some light on this topic?

ANSWER — When the computer shifts the transmission, programming will momentarily decrease engine torque output so that the transmission clutches can apply with minimal slip. This reduces wear on the clutches. When the transmission is shifted manually, this torque decrease usually doesn’t occur, so the shift will be harsher and a little more wear could occur. I say “could” because a lot depends on the engine power output, engine speed and vehicle speed during the shift. If throttle opening is small and engine rpm is low, wear increase would be almost nothing. If full-throttle, high-rpm shifts are performed, wear would be much more.

Usually, the best operation for everyday driving is to let the transmission shift automatically. Use the brakes for slowing down — they are cheaper to repair than transmissions. However, you may find shifting manually is fun and this may determine how you drive. Each of us prioritizes how we spend our money.

Manual downshifting to a lower gear is the preferred choice if more engine braking is needed when going down long hills, towing or hauling heavier loads, or driving on muddy roads where you don’t want the transmission to shift to a higher gear unexpectedly. Just like a manual transmission, more performance can be achieved from an automatic by shifting manually if done by a skilled and knowledgeable driver.

As for your friend’s failed transmission, there are many other failures that could have occurred. I wouldn’t blame it on manual shifting unless I could determine there was excessive wear on the clutches. I drive both ways, depending upon conditions and the vehicle I am driving. Letting the computer shift for you may decrease wear slightly, but it is still a matter of personal preference.

Jim Kerr is an experienced mechanic, instructor of automotive technology and freelance journalist. You can e-mail questions to Jim at the address below.

kerr.jim@sasktel.net