Part 2: Gearing
Now our story continues. At the very beginning of all of this, the first thing that I changed (read messed around with) in the Lancer was the gearing. Despite the engine being the center of attention, the gearing is the unsung hero working in the background. I stress this as being very important because all your engine power is useless unless you have the perfect gearing to go with it. The '79 Lancer was originally equipped with a 4-speed manual transmission and a 3.9:1 final drive, which I upgraded to a 5-speed manual transmission and (later after learning the hard way) matched to a 4.2:1 final drive.
To understand why gearing is so important, let's quickly revisit why we have a transmission in the first place. Long story short, engines operate within a rather narrow powerband so you have different gears to keep it there. Most street cars this would mean keeping the revs within that 6000 RPM range. As you tune the engine for more power this ideal range becomes even smaller as the choice of gearing even more critical.
There are three factors to consider in setting up the gearing. First would be the spacing of the transmission's gear ratios which in a way influence how wide a powerband it can handle and second would be the effective gear ratio of the final drive which would influence top speed and acceleration. The third would be tire size which I will discuss later.
We frequently hear the term close ratio gearbox. This is where the spacing of gears comes into play. The closer together the gearing of a transmission is the less the RPM will differ between shifts allowing the RPM to stay near where it is ideal. There's no hard and fast rule for this, but I would say that if your shift point would be a little past peak horse power the RPM should drop right about where peak torque is. So if for example power peakes at around 6000 RPM and the the peak of the torque is around 4000 RPM, when you shift at a little past 6000 revs RPM should drop to around 4000. This would keep the engine in it's 'sweet spot' during flat out driving. Most standard transmissions tend to have bigger spacing as you move up the gears, this is done in the interest of fuel economy. I was fortunate enough that when I was building my car to literally chance upon a close ratio 5-speed used for a Rally Car which just so happens to have the gears that work really well with my engine's powerband.
The effective gear ratio of the final drive- really a term I just made up because I don't know what it's called... is the ratio of the transmission's highest gear multiplied by the ratio of the final drive. I literally multiply the numbers together to see, ex. my original 4-speed with a 1:1 direct drive 4th gear and 3.9:1 final drive would have an effective gear ratio of 3.9. The 5-speed setup with a 0.853:1 overdriven 5th gear and the 4.2:1 final drive would have an effective gear ratio of 3.6. This is important because it has one of the biggest influences on how the car runs. Taller ratios (numerically smaller) mean that the car would be capable of a higher top speed, shorter ratios (numerically bigger) would mean the car is capable of less top speed. So as we can see the 5-speed/4.2 would be capable of more speed. How much? 5th gear at 6000 RPM would be around 170 km/h, while the 4-speed/3.9 would be at around 160 km/h in 4th gear at 6000 RPM.
Mixing and matching is possible, but I would suggest to stick with the proper combinations. Also remember that the effective gear ratio can be computed for all the gears, so a taller final drive would make each gear of the transmission taller compared to the same gear with a shorter final drive. As an example though, when I first swapped to a 5-speed, I retained the 3.9:1 final drive. This had an effective gear ratio of 3.3 and would theoretically run around 185 km/h at 6000 RPM in 5th gear. But what the numbers don't reveal was that this combination was geared so tall it took forever to accelerate. I have also run my 5-speed with a 4.6:1 final drive hoping for blistering acceleration. This had an effective gear ratio of 3.9 same as the 4-speed but with closer spaced gears and while this did give very good acceleration, I found it rather annoying on the highway and on long straights as I longed for the speed I had with the 4.2:1.
Tire size is often overlooked when considering the gear of a car. Think of it as a final-final drive. Smaller diameter tires vs bigger diameter tires have the same effect as having a shorter final drive vs. a taller one. In some cases where it's not possible to change transmissions or final drive changing tire size can have the same effect. Want better acceleration; swap to a smaller diameter tire, better top speed; taller tire.
What's annoying though with going with changing stuff is that the speedometer might not work properly. A speedometer is usually driven off the transmission's tail shaft and is calibrated assuming a particular final drive and tire size to give an accurate reading. Change one factor and it won't read right. it is possible to recalibrate this by changing the drive gear for the speedometer cable, but this is a hideously complicate task. I did this by going out to an auto supply and buying all the little plastic gears they had until I found one (thru the wonderful method of trial and error) that gave the right reading for my setup.
That pretty much covers gearing. Keep in mind that what works for one might not be the best for another and the only way to find out what works best for your particular setup is to experiment and see for yourself.
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