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Classic Auto Doc: High reving engine

Engine Block

Engine Block Credit: AP

Steve Linden is a classic, antique, and collector vehicle expert who offers advice, tips, and news on all things classic car related on Long Island. Send in your questions to

I bought a 1966 Chevelle with a 350 engine and an automatic transmission. The engine and transmission are not original. The car is very fast, which is why I bought it, but I didn’t realize how fast the engine would be revving on the highway. I feel like it’s going to explode, even at only 55 MPH. Do you have any suggestions?

Sure I have an answer – don’t drive it on the highway. Actually, I have a solution, but first let me tell you what your problem is. Your car has a very “short” gear ratio in the rear end. Simply put, this gear ratio determines how many revolutions the rear axles turn for each revolution of the driveshaft. The more revolutions that the axles turn for each revolution of the driveshaft, the “shorter” the gears. The rear axles drive the rear wheels, which drive the car. These “short gears” can have a tremendously positive impact on a cars “off the line” performance, but the trade-off is that the engine will rev very high, very quickly, making it more suitable for the drag strip than the highway. Rear end gears are available in a wide variety of ratios.

You can replace them with a ratio commonly referred to as “highway gears,” but then you’ll lose the performance that you enjoy so much. If you want to retain this performance while still being able to take trips on the highway, I would suggest removing the transmission and replacing it with a later model GM transmission called a 700R4. It’s basically the same transmission that is in your car now - with one big exception. It has an overdrive gear, which will bring the RPMs down dramatically on the highway. The 700R4 transmissions are plentiful, and not particularly expensive. Any competent transmission shop can easily complete this swap, along with any minor modifications that may be required to make the overdrive functional in your car. This “shift” in transmissions will even improve your fuel mileage.

I brought my 1966 Mercedes Benz 250SE into my regular repair shop for an oil and filter change. They replaced the oil with fully synthetic oil and now I’m finding small puddles of oil on my garage floor. I brought it back to them and they checked to make sure that the oil filter and drain plug are tight, which they are. Is it just a coincidence that these leaks began right after they changed the oil?

Although it is possible that it’s a coincidence, I’ve seen this happen enough times to guess that it’s not a coincidence. Most likely your engine’s been running on conventional oil for a long time, and has built up some degree of “sludge” (that’s technical lingo) in the deep dark recesses of the engine block. As an engine ages, the seals and gaskets tend to develop microscopic fissures and cracks that will eventually begin to leak oil. Over time, this sludge will actually be drawn into these fissures and cracks, and act as a sealer, minimizing or preventing leaks.

Fully synthetic oil is so far superior to conventional oil not only in lubricating an engine, but also in removing the sludge, and I think that is exactly what happened in your case. When your repair shop placed fully synthetic oil in your engine its detergent qualities disturbed the existing sludge which was acting as a sealer and your leaks began. Although I recommend using synthetic oil in rebuilt engines (after break-in), I don’t recommend using it in engines that have been using conventional oil for many years or many miles. I would replace the synthetic oil with conventional oil, and if you’re lucky the leaks may go away over time.

Tip Of The Week
For many of us, the need for a replacement part for our classic car signifies the beginning of a search that will usually include several catalogs (both printed and on-line), specialty vendors and ebay. Often we work on our cars on weekends and are frustrated to learn that answers are not available, and parts can’t ship until Monday.

You would be amazed at how many parts are still available at your local auto parts store. This is particularly true for routine maintenance parts that we need the most often such as ignition, suspension, brake, and fuel system parts. So the next time that you need a part right away, visit or call your local auto parts store. You’ll even save the cost of shipping.

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