FD2R gear ratio swap

GPS speed with new 6th speed ratio
Altering the gear ratios

The usual performance tweak for a Honda gear box is to change to a lower final drive but the FD2R already has the lowest 5.06 final drive of any Honda. So the choices left are to change 1-3 to lower ratios for more acceleration up to 120 km/h or merely swap 6th gear for a taller one to enhance cruising and fuel economy.

We felt the first two gears were low enough as we ran out of 2nd just before 100 km/h unless one over-revved. With the engine ECU reflash the rev limit is raised to 9000 rpm and besides the gear ratios already seem too low as one has to shift quickly out of 1st and indeed on to 3rd quickly or else it is difficult to avoid jerkiness in town traffic.

However on the highways one thing is very annoying, that of the buzzing engine during cruising. But it must be said the original 6th gear ratio (0.738) was chosen to achieve top speed in the FD2R which has not been officially declared but it probably lies between 240 and 250 km/h although the gearing and peak bhp arrives at 8000 rpm which corresponds to a theoretical max of 257 km/h. Problem is there is no truly flat (or safe!) stretch to verify this.

The FD2R often feels as if it needs a 7th overdrive ratio but Honda stuck with the 0.738 ratio to optimize top speed performance as 5th runs out of revs at about 220 km/h and the car can still go on. So for those of you who feel top speed is so very important, do not swap out the 6th gear.

For those who feel 220-230 km/h is plenty fast then read on. The Honda parts bin is quite an interesting place to scour around for performance enhancing bits although what we have done is sacrifice a bit of top-speed performance for a relaxed cruise along the highways.

First we had a look at the Honda cars with a 6-speed gearbox as those with just 5 gears will not fit as a direct swap although they have interesting rear ratios to offer. It turns out that the '06-'07 US Honda Civic coupe and sedan has this particular 0.659 6th gear and is also shared with the Acura TSX. So a trip to the US might be needed unless you can find a Honda dealer who is willing to ship this item, perhaps as a group buy.

Fitting is no hardship for mechanics who have opened up a Honda gearbox. The 6th gear is a direct swap. During reassembly we elected to try the Motul Synthetic manual transmission oil specially formulated for gearboxes with an integral Torsen differential such as the FD2R's. This oil has a well deserved reputation for smoothening out Honda gearboxes especially if one is having a bit of notchiness going into 3rd gear or just engaging 1st.

We found that at 100 km/h the 6th gear drops the engine revs from a buzzy 3110 rpm to a sedate 2777 rpm. And at 180 km/h it is just 5000 rpm whereas the original is at 5600 rpm(or 5000 rpm is just 160 km/h with original 6th). With the natural boominess of the FD2R exhaust between 3-4000 rpm one can choose to be at 110 km/h (3000 rpm) or 140 km/h (4000 rpm). Somehow over time the boominess is not as evident as when new and is not really a deal breaker unless one has a straight through system.

Of more interest is the usefulness of the 6th gear in town traffic and we have found that as long as quick acceleration is not needed, 6th can sustain a cruise as low as 50-55 km/h where the engine is at about 1500 rpm. As you know this engine is no paragon of low-rpm torque and those who have kept the original rear silencer box will find no trouble lugging around in 6th gear at 50 km/h. Honda has made a passive spring-loaded flow restrictor in the rear box to increase backpressure in order to enhance low rpm driveability( not that FD2R owners care too much about that) but also because of that ability to be driven sedately, lower fuel consumption and also CO2 emissions (which owners sort of care about as it effects the pocket directly).

We have also altered a standard rear silencer box by cutting out the restrictor(in the inerest of research) and making it nearly a straight through except that the connecting pipe was perforated to lower the noise. On the first day one will notice the drop in low rpm torque but also feel as if 5 or 10 bhp has been added to the very top end. By the second day the missing low rpm torque is almost a memory but the newfound power has become your favourite pastime. Also because of this there is a drop in fuel economy with all that VTEC fun. It does not make the car undriveable at low revs, just that it feels flat and less punchy especially below 2000 rpm.

At the other extreme we are sure one cannot reach the theoretical top speed of 257 km/h even with the original 6th gear and if the practical top speed is around 240-250km/h owners would be happy enough bearing in mind how dangerous and nearly impossible it is to actually hold that speed for any reasonable length of time. The new 6th gear might be good for about 230 km/h with about 6500 rpm (see Garmin GPS speed on a recent run which at 226 km/h means the speedo being about 5% optimistic would read 237 km/h).

Its real advantage is making daily life with the FD2R a bit more bearable but without hurting the usable performance at all. And for those who might worry about this mod at a track like Sepang, its is doubtful that one would be able to run out of 5th gear even on both the straights, unless by accident. The only time one can hold sustained high speeds is on the highway and while it is not as accelerative as the original 0.738 ratio, keep in mind 5th gear will max out at 220 km/h or 230 km/h if the rev limit is at 9000 rpm. Besides with the world's slickest gear shift, what is the problem of shifting 6th to 5th?

Since we are not in a habit of top speed runs (only in the interest of research) a cruising gear ratio with the ability to reduce the engine buzz and save on gas is a welcome addition. Admittedly even after buying this gear ratio set it took a while of soul searching before we decided on the actual swap mainly as we have mentioned, the loss of a top-speed gear. As it turned out we are happy for others to claim top speed bragging rights and just appreciate the rest of the sharp handling and still impressive performance during many of our N-S trips with our hearing intact.

Photos: 
GPS speed with new 6th speed ratio
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