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The Effects Of Gear Changes |
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Written by Devin Durham
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Tuesday, 15 January 2008 |
There are a number of things that a person can do to their car in order to modify the performance of that car. To some people performance might mean better gas mileage. To some it might mean better towing capacity. To most of us on this site it means "Get me to that 1/4 mile mark as fast as possible" so we are going to explore what effects gear changes have on how a car performs (by our definition) along with the help of the guys over at Midwest Car Care.
 Open diff with installed carrier assembly
Gears, both the ones in a transmission and the ones in a differential, are a means of leveraging the engine's power and are usually expressed as a ratio such as 3.73:1. The higher the ratio, the more the engine is being leveraged towards torque, the lower the ratio the more the engine is being leveraged towards speed. Because most gasoline/diesel engines have particular RPM ranges where they deliver the most power, a transmission with multiple gear ratios is used to help deliver power to the wheels in the most efficient way possible for any given speed the vehicle happens to be travelling.  Don't forget to get a speedo gear!
Think of it this way: If you have ever ridden a bicycle with multiple speeds, you know that if you are in first you can start off easily and you can climb nearly any hill, but to go more than a few miles an hour you have to switch to a higher gear or you will sit and windmill your legs furiously. By the same token it is nearly impossible to pedal a bike from a standing start in 10th gear, but you can achieve much greater speeds in that gear.
Differential gear changes change the way every gear operates because it is at the end of the line... the transmission drives the differential. Increasing the ratio of the differential makes every gear in the transmission produce more torque but less speed, and decreasing the ratio of the differential makes every gear in the transmission produce less torque but more speed.
As a general rule car manufacturers take the lower gear ratio route, because at hiway speeds your engine is turning less RPM and makes better gas mileage. Like all things, this is a tradeoff... that same lower gear ratio causes your car to accelerate slower. So you will find that many performance buffs will increase the ratio of the differential, trading off fuel economy for acceleration.
But wait! I thought you said that lower ratios go faster! Yes, that's true, but most races aren't top speed races, and to truly race at top speeds you would need miles of clear land, like the salt flats of Bonneville. A higher ratio does decrease your top speed but you will get to that top speed much faster.
 The sort of thing you find lying around a speed shop
Alright, so taking all that into account, why bother? Well the most interesting thing about gear changes is that while it can make a substantial difference in how the car launches and performs, your engine isn't being impacted much at all. No power adders, no tricks, no potentially damaging additions... just simple power to the ground in a more efficient way. In terms of bang for the buck, it is one ot the most major changes that you can make to a vehicle while at the same time not negatively impacting its reliability.
The Install
 MIdwest Car Care in Lebanon, MO
3.73:1 was the selected ratio. After having done a lot of reading, and also weighing the fact that this car sees quite a bit of daily driving, this ratio was chosen over the typical 410. It was thought that the 373 would have a lesser impact on MPG while at the same time keep the car in a more pleasing RPM range while on the highway, yet still giving a substantial acceleration boost over the factory 308 gears.
Josh at Midwest Care Car had done this exact job on a Mark VIII in the past, and dove into the work immediately. First the rear wheels were removed, then the brakes and the discs. After that the axles were pulled from the differential unit, and finally the diff removed from the car. Since this car is 14 years old and had 113,000 miles on it, some fighting was to be expected.
 Bad brakes. No soup for you!
Another more troubling problem was found when the diff was disassembled: One of the axle bearings had begun to lock and had spun some in the case. If this would have locked up and burned up on the track, it would have made for an interesting and frustrating day to be sure.
A word of advice: When having work done to a vehicle, go ahead and replace any and all wear items that you may encounter. Don't re-use old bearings, etc. It doesn't cost that much to do the job correctly and it will save you a ton of headache and money in the long run
 It is important to have the right tools: in this case a brass hammer
Installation is pretty much reversing the removal process and went without a hitch. And in just a few hours the Mark VIII had a new set of gears, and hopefully a stronger presence at the starting line.
Real World Results
The buttometer told us immediately that the gears were making a difference. The car seemed lighter and more nimble. Stomping on the gas from a standing start still spun the tires, but it spun both tires which grabbed quickly instead of one side taking all the torque and spinning forever. Also the car was now launching hard enough to set off the seat belt locks, something it didn't do before. First gear seems to pass by like a shot.
Engine RPMs were definately higher when both city driving and highway driving, but the overdrive helps with that considerably. By no means were the RPMs uncomforably high.
Shifting
In the past the Mark VIII would shift out of first and into second at just under 60 mph, and into third at just under 100 mph at full throttle. This would mean a shift into third just immediately before the 1/4 mile mark, probably affecting overall times. With the new gears, it shifts into second at 40mph, and into 3rd at around 75 mph. This gives a shift well before the finish line, and the car pretty much runs out of acceleration about 100 feet from the line. This is just about as close to finding a perfect balance between top speed and acceleration as you are going to get. This tag shows that this diff is factory sealed
Upshifts from first to second at full throttle now cause the car to break free slightly on public roads, which despite being a rude way to drive is a lot of fun. Downshifts into first from about 30 mph are brutal, causing the tires to break free and then grab again and then immediately break free again as the car hits second. Not overly smart but it is fun to waggle your ass like that at the idiot that just tried to cut you off.
RPMs
RPMs were recorded at 1900 RPM at 70 MPH on the highway before the change, and 2250 RPM on the highway afterwards.
Economy
Like any other performance addition, generally everything is a tradeoff between economy and power. In this case highway economy was observed as 27.7 mpg before the change, and 25.0 afterwards. City economy is harder to track, since although this car is a daily driver the distance it drives every day to and from work is so short that the motor actually doesn't even have time to properly warm up. Casual observation seems to indicate that city economy wasn't impacted though, which could be a result of the car getting through the gears faster. It is possible that the MPG increased slightly.
Strip performance
The previous best sixty foot time was a very high 2.2, for all intents and purposes you could call it a 2.3. With the new gears the Mark laid out a flat 2.0, a very respectable time for a 4000 pound luxo-barge.
The Mark has a 100 shot of nitrous that is usually used at the strip, but for purposes of testing several runs were made without the nitrous to compare at a baseline how much of a difference the gears made. On motor only, the best run ever recorded in the quarter mile was a 14.8. Three runs with the new gears produced a 14.5, a gain of 3/10ths of a second.
On the bottle the best recorded run was a 13.44. Three runs with the new gears produced a new all time fastest pass, a 13.08. That's 3/10ths of a second, nearly 4/10ths.
Speed
Top speed in both the pre gear and post gear runs were surprisingly close, with a 105.68 mph top speed before and 105.86 afterwards. That doesn't really accurately reflect the changes though, since before the gear change the car had just entered third as it crossed the line, and after the change it hits third well before it. Getting to that 105 earlier down the track is what produced the 4/10ths difference in times.
Final words
 Track-Lock unit
Economy goes down, but if economy is your number one priority you probably shouldn't be into fast cars anyway. Going too high of a ratio can make for an annoying highway ride, but a conservative gear change can add very nice acceleration while at the same time minimally impacting both economy and comfort.
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Last Updated ( Monday, 25 February 2008 )
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