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Old 10-02-2012, 11:50 AM
BogiesAnnex1 BogiesAnnex1 is offline
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Originally Posted by against all odds View Post
Is it my imagination or does it seem like a 4speed overdrive transmission with, say, a 3.73 rear gear cruises more effortlessly than a 3 speed no overdrive trans, 1:1 top gear with, say a 2.73 even though the cruise rpm ends up being the same?

For example both cars will turn 2000rpm at 65 mph. If anything the car with the 3sp should be able to cruise more effortlessly because it takes more energy to drive the overdrive.

But based on my experiences, it seems that all overdrive cars cruise really easy and at higher speeds---70, 75, 80 etc. regardless of what engine they got.

Explain this one.

The entire effort is one of matching the engine's power delivery with the power consumption needed by the vehicle to run at some given speed. It really doesn't matter if the RPM/power is matched to the speed/drag is less than, equal to, or more than direct drive.

The problem that the manufacturers got into when fuel mileage standards were introduced was one of cost avoidance. They just didn't want to expand beyond their existing products to meet the fuel efficiency standards. So they opted to convince the government to tilt the test toward cruise mileage and they changed to extremely high ratio rear ends with their then conventional 3 speed automatics. While this more of less worked if all you did was run through the world at 70 mph in high gear, it was a fuel consumption disaster in stop and go traffic because there wasn't enough leverage thru the gearing to get the vehicle launched without the use of a lot of throttle opening.

The addition of a deep low provided more mechanical leverage to get moving without needing much throttle provided the stop and go solution. The point of throttle opening is that as the throttle is opened the manifold vacuum falls, this raises the absolute pressure in the cylinder regardless of compression ratio to the point of WOT. As the absolute cylinder pressure goes up the mixture needs to be richened for two reasons 1) the assumption of maximum effort demand moves the mixture to maximum torque rich; 2) as the in cylinder temps and pressure raise additional fuel must be introduced to suppress detonation by cooling the mixture through the evaporation phase change that absorbs heat. As a result mileage goes down. The solution is to give the engine better leverage so the throttle isn't opened so much as to trip the power (economizer) fuel enrichment system.

At the other end of high speed cruise the issue is similar in that while you want is to lower RPMs versus road speed but you don't want to load the engine to where the power enrichment system comes on, nor do you want to lower velocity through the venturies to where metering becomes sloppy. So you're searching for a tight band where you can run the engine at the high edge of thermal efficiency without dropping the manifold vacuum too low. This was all well understood back in the 1930ís 40ís and 50ís when overdrive 3 speeds were quite popular. But without a computer in the loop, the driver became more an operating-engineer than todayís spectator with a steering wheel. So the popularity of these early ODs waned as people found they didnít produce fuel savings for the average driver against being an expensive option. EFI makes this a lot easier since fuel metering isn't dependent upon air velocity through a venturi. The use of a computer to monitor load on the engine is also a great benefit as these are set up to keep the manifold vacuum sufficiently high to avoid the power enrichment sequence which would apply to either a carburetor or fuel injection. These modern overdrives at least where an automatic is involved will if the load on the output shaft becomes too great drop out the direct drive through the torque converter, this lets the engine speed up about 200 RPM without affecting road speed. If the overload condition persists it will command the transmission to the next lower gear to again get the RPMs up with necessarily affecting road speed. This protects the fuel mileage by keep the power enrichment off and protects the engine internals from the effects of lugging which is mighty hard on the main and rod bearings as lugging makes for high bearing loads over a long time period.

So how the engine feels at cruise relates to whether or not it has sufficient power to maintain the road speed at the RPMs it's turning without pulling in the power enrichment. So long as this condition is met the system as a whole doesn't care what the final drive ratio is. The ratios inclusive of transmission gearing, rear axle gearing, and tire diameter are nothing more than a means to that end.

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