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Topic Review (Newest First)
10-02-2012 11:50 AM
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.

09-30-2012 08:48 AM
against all odds i think better tires, roller lifters---quieter and less vibration and quiet well-tuned exhaust help.
09-29-2012 11:26 AM
Thumpin455 I never noticed much difference due to gearing, but I have with what engine is ahead of the trans. I have a 98 Formula with the LS1 and 4L60E pushing 2.73 gears. There is also the 70 GTO that used to have 2.93 gears and it had both the Th400 3 speed and 2004R OD behind its 455. Either one of those cars rolls along no problem at 75-80mph, the only difference is cruise RPM and of course the 455 could pass or go up hills much easier than the LS1 can at 1700 rpm.

That might be part of the thing you are noticing. The cruise RPM with OD is pretty low. Your typical carbed sbc wont be able to pull a car as easily at 1500 rpm as a later EFI engine, or the 455 that is making 450-500ftlbs right about there. Lots of things come into play when cruising a low RPM like that, not the least of which is carb signal, and your average sbc doesnt have much compared to the larger Pontiac, so it wont run quite as well that low. EFI takes all of that out of the equation and then you are left with how much torque it makes there. How much grunt it has depends on the bore, stroke, and cam timing, plus a few other variables.

So if there are other issues, like the brakes are dragging, the alignment is off, the wheel bearings need repacked/replaced, or the tires are under inflated then it will feel like the 3 speed car is quite a bit more sluggish. One of my 11-12 second cars picked up .6 in the 1/4 by simply repacking the wheel bearings, truing the front rotors and rear drums, and giving it an alignment. That kind of stuff makes a huge difference.

Its something other than the gearing that is making the difference in your mind.
09-27-2012 08:25 PM
against all odds i'm going to interject that maybe one reason i'm feeling this way is all the OD cars i've driven just happened to have well-balanced wheels while the 3spds really didn't.

Or maybe the engines in the OD cars just had more torque in general?

Really, late model gm's with 4sp OD transmissions just cruise effortlessly at 75-80+. It's just amazing.
09-27-2012 11:02 AM
AutoGear Curtis did a very good write up. Thanks for that. I will add that in his example of the T10 vs the T56; the T56 has much larger gears, and it is constant mesh which as he said, means that entire big, heavy geartrain is always turning, and paddling oil around. This comparatively takes a lot of effort, not to mention these big end-loaded gearboxes often need a pump to adequately lubricate the overdrive set.
The whole double overdrive thing can be much ado about nothing in some performance cases. When Warner Gear designed the T56 for GM, it was originally a heavier duty T5, but GM wanted a double overdrive 6speed, mostly for marketing purposes. Back in the early 90s a lot of advertising was done regarding the new 6speed as a gearbox that won a lot of road races. What they didn't mention was that the gearboxes they were using had been converted to direct drive 4 speeds, the entire OD geartrain had been dummied up.
A lot of people I do business with report that they either don't use 1st gear, or that the carbed engine lugs too much in 6th and they dont use it. Especially the road race guys. 5 speeds seems to be a good compromise of a nice ratio spread and the OD.

Now for the OE side of things. A lot of development is going on with direct drive automatics with 6, 7 or 8 speeds. And direct drive 5 speeds were around in performance transmissions for years (Doug Nash). From a design standpoint, Overdrive's build heat and noise, and large overdrive ratios may have packaging concerns and strength issues. The better answer is a numerically lower rear gear, with a deeper 1st gear. Say something like a 3.08 rear and a 3:1 1st gear. You get the strength, less noise, ease of assembly and packaging of a direct drive transmission, and the economy we all desire.

WRT the original question; I think that its your imagination, although most three speed cars came from an era and an application with smaller, lower powered engines than say a modern 350 streetrod torquey motor you typically see coupled to say a 700r4 or a T5.
09-26-2012 07:56 PM
curtis73 There is a lot to it, but the difference in friction between and OD/low axle ratio versus 1:1/high axle ratio is pretty negligible in the grand scheme of frictional losses.

Compare (for instance) a T10 with 2.64-1.77-1.35-1.00 ratios versus a T56 with 2.66, 1.78, 1.30, 1.00, .74, .50 ratios. They have basically the same ratios 1st-4th, but the T56 adds two overdrives. Highway cruise RPMs would be basically the same if the T10 was backed with a 2.37 rear and the T56 was backed with a 4.56 rear.

Its true that the T10 will provide a tiny bit less friction on the highway, but consider the much greater effort the T10 will have in the first couple gears getting the vehicle moving. Modern transmissions are also constant-mesh, meaning that regardless of which dogs are engaged on which shaft has less of an effect since all the gears are engaged regardless of where the shifter is.

Its more a function of selecting the transmission and axle ratios that best fit your vehicle's weight and torque curve. Most drivelines can sap as much as 20% of the power away as friction. The extra 1% that is consumed by an OD is often not something that affects efficiency on the highway.

In Automatic transmissions, there is also a big difference based on the design. A 700r4 places the OD planet at the front of the transmission. When OD is engaged, it overdrives the entire transmission. In a Dodge transmission like an A518, the OD is at the back. When OD is engaged, the rest of the transmission slows down. One would think that this difference would create a large disparity between efficiency, but it doesn't.

In short, frictional losses with OD are usually negligible compared to the overall inefficiencies of the driveline.
09-26-2012 04:39 AM
joe_padavano Actually, the 1:1 top gear with lower numerical rear axle should have marginally less internal friction and thus be slightly more efficient than the OD.
09-26-2012 12:05 AM
vinniekq2 Its probably your imagination.If the gearing works out close,2.61 Vs 2.73,,,then maybe the extra low first gear coupled with the lower rear gear makes the car accelerate quicker,especially not being in overdrive for very long as most of the acceleration will happen in 3rd of the 4 speed.So,when the car shifts into Od it feels like its just settling down as apposed to the long time accelerating in drive in the 3 speed from about 35 mph to 70
poor explanation but you get my idea
09-25-2012 11:03 PM
against all odds
Overdrive vs. direct drive+high gear???

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.

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