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Gears:
Usually referring to the ring and pinion used for the differential, which then goes into the equation for the FDR(Final Drive Ratio) of the overall drive-train(the transmission itself has different ratios in the gears used). Tire and rim size also have an effect on the FDR. The final drive ratio tells how many rotations your motor must make for each rotation of the wheels.
[video width=425 height=344 type=youtube]gIGvhvOhLHU[/video]In that vein I wish I could afford a really good aftermarket LSD such as a WavTrac.
The transmission gears have their own ratios to factor in for the FDR and overall performance. Upping this value by changing out your differential gears is usually done to give you more torque off the line, and overall lower end response, but will also make you cycle through each power band significantly faster, lowering your overall possible top speed, and of course lowering gas mileage due to having higher RPMs necessary at the same speed.
As another result, you usually lose another slice of RWHP to drive-train loss since it has to do even more surface area. Most references unless they specify “rear wheel”, “at the wheels”, or “RWHP” are giving you their at the engine’s flywheel measured horsepower rating, which usually gets eaten up by 12-19%(usually around 16%) once it gets to any wheels. More so with AWD and 4WD vehicles. And slightly more so from a manual(M/T) to an automatic transmission(A/T).
My own for example; came with a 3.31 gear set from the factory for the ring and pinion. I changed it out to 4.10. So with my manual transmission being a TR-3650:
BEFORE:
1st gear = 3.38 x 3.31 = 11.18
2nd gear = 2.00 x 3.31 = 6.62
3rd gear = 1.32 x 3.31 = 4.37
4th gear = 1.00 x 3.31 = 3.31
5th gear = 0.68 x 3.31 = 2.25AFTER:
1st gear = 3.38 x 4.10 = 13.86
2nd gear = 2.00 x 4.10 = 8.20
3rd gear = 1.32 x 4.10 = 5.41
4th gear = 1.00 x 4.10 = 4.10
5th gear = 0.68 x 4.10 = 2.79So, now my engine has to rotate almost 3 more times per revolution of my wheel than before in 1st gear. Two more revolutions per each wheel in second, and so on. But this gives that much more at command torque at the wheels than it did previously.
Combustion ratio:
This is the value that represents the ratio of the volume of the combustion chamber. Meaning at the start of the combustion stroke if it could have 1000cc of volume, then at the top only have 100, you’d have a ratio of 10:1. This then factors in with the volume rating of the engine. 1.8L, 4.6L, 5.0L, and so on, getting more power and overall efficiency from the rated volume(up to a point before detonation becomes an issue). This is one of the reasons why, for example, the 11-13 mustang GTs can get their quite high HP for the CI(cubic inch) engine size they are(302) with 5liters of volume, and still get 20+ mpg; they have an 11:1 ratio.Thank you this is very helpful. But how did you come up with these first set of numbers? I understand the second set of numbers being the new gears that you got but the first numbers came outta no where lol. : D
BEFORE:
1st gear = 3.38 x 3.31 = 11.18
2nd gear = 2.00 x 3.31 = 6.62
3rd gear = 1.32 x 3.31 = 4.37
4th gear = 1.00 x 3.31 = 3.31
5th gear = 0.68 x 3.31 = 2.25AFTER:
1st gear = 3.38 x 4.10 = 13.86
2nd gear = 2.00 x 4.10 = 8.20
3rd gear = 1.32 x 4.10 = 5.41
4th gear = 1.00 x 4.10 = 4.10
5th gear = 0.68 x 4.10 = 2.79The first numbers are from the actual transmission used for each gear. With my transmission as the example again (above) 1st gear is 3.38 on it’s own before it makes it to the differential to connect to the axle. The differential ring and pinion become the multiplier(3.31 or 4.10).
With a basic manual transmission it goes:
Flywheel>transmission>driveshaft>differential>axle>rims>tiresThere are differences of course, from front-wheel drive(FWD), RWD, 4WD, AWD with the transmissions, how many gears they have to turn, what size they are and so on. But the above gives you the usual main players in the equation. The sizes of the axle and driveshaft “can” change (and of course transmission), but are usually cost/time/effort prohibitive as they have to match in certain ways and be feasible in the space given. The differential ring and pinion is something relatively basic that can the swapped out to give an immediate performance impact’ with little extra mass/weight, cost and stress on the components.
Sometimes tricky to do as DIY due to the tight tolerances involved, having to shim correctly on different planes, but after a bit of research on your particular vehicle it’s still something that can be done with fairly minimal specialized tools in about 4-10hrs of work over a weekend. Or about 6hrs charged by a shop in labor, which as performance upgrades go, is fairly cheap.
Tires and rim sizes can also do some of this which is even easier to accomplish, but increasing them also starts exponentially increasing the force on the axle itself and increasing weight. Just like lengthening the diameter of breaking a rusted bolt free using an ever-increasing length of pipe. F=ma
Ok, I think I understand now or at least a lot better then I did before. Thank you very much! :cheer:
As for compression ratio I cover that in this video. I think the gear ratio was covered fairly well. Thanks forum.
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