gearing modifications explained again
The advantages of close-ratio gears with a numerically lower final drive VS factory gears, with a numerically higher final drive.

I am creating this thread as kind of a sister thread to my d16y5/d16b5 vtec-e project page. Which can be found here: http://honda-tech.com/zerothread?id=1414454&page=10

I am only going to calculate one transmission here, because all I am really trying to do is explain the difference between the two and show YOU advantages and disadvantages of each.

A lot of the stuff that I will say in here, you might not understand if you haven't read my transmission page where I have answered every question that I have ever thought of regarding gearing and transmissions and torque/horsepower, etc. In case you have missed my page and are interested in reading it, it can be found here: http://honda-tech.com/zerothread?id=1486769

The example transmission that I am going to use for all the calculations and demonstrations is a 92-95 d16z6 transmission. This is the ex transmission with a fifth gear of 0.702. Not to be confused with the Si transmission in the 92-95 hatchback si and the del sol si with a 0.750 fifth gear.

All of my calculations are going to have:
A redline at 7500.
A wheel diameter of 23" Because 205/50/15s come out to be 23.07 and I am just going to keep it simple and use 23 inches.

Stock gear ratios for this transmission are as follows:

1st = 3.250
2nd = 1.900
3rd = 1.250
4th = 0.909
5th = 0.702
FINAL = 4.25

I am now going to calculate the rpm drops of this transmission. If you take notice, all I am really doing is taking the next gear that I am shifting into, and dividing it by the gear that I am shifting out of. (2nd gear divided by 1st gear) or (3rd gear divided by 2nd gear) etc.

1.900 / 3.250 = 0.584
1.250 / 1.900 = 0.657
0.909 / 1.250 = 0.727
0.702 / 0.909 = 0.772
note: decimals have been truncated after 1/1000th

So what does this mean? Well 1st into 2nd shift for example. As you can see above that 0.584 is actually 58.4% So if one were to take 1st gear up to redline (7500) and then shifted into 2nd gear, our rpms would fall 58.4% To calculate, all we do is multipy 0.584 times 7500. Which we get 4380. Now, realize that it is a percentage and not a set number of rpm. So this means that revving to 7500 and then shift, rpms fall 3120 rpm (7500-4380 = 3120) However, if one were to rev to 10,000, revs would fall to ~5840 (that's a 4160 rpm drop) See how that rpm drop increases as we rev higher? That's because the gear ratios are taking the percentage when we shift gears. Basically, the higher the redline is, the closer that the gears need to be to each other.

Another thing, if you take note, all those calculations above had nothing to do with the final drive ratio. All the final drive ratio (or axle ratio) dictates is how engine rpms relate to wheel rpms (or miles/kilometers per hour) So in reality, when we are in 1st gear, we are in the 1st gear ratio, and the final drive ratio. To get this ratio, we simply multiply the 1st gear ratio by the final drive ratio. In this case, 3.25 * 4.25 = 13.8125 13.8125:1 Yes, that's right. It's a torque multiplier. So if you have 100hp which is like a stock d15. When you rev to 5500 rpm and dump the clutch, you are dumping 100hp * 13.8125 to the wheels! Yeah that's right, your stock d15 with a ex tranny can dump 1381.25 hp to the wheels! This is the reason that nearly any manual transmission car no matter how weak the engine is can roast em'! when you dump the clutch.

Anyways. So here's the deal. I want to see what mph my gears end at, so when I am cruising on the interstate and mr cavalier rolls up wanting to race. Now obviously, one would want to be in the lowest gear possible (like 3rd) So that they will be faster. But we don't want to downshift if it will put or engine at 12,000 rpm and then mr. cavalier will be laughing at us when we're pulling over into the break down lane. So I am now going to calculate what speed we will be at with our transmission/rpm/wheels. Yes, I know the formula for the calculations, but there's point in manually doing it, when we have computer programs to do it for us. Especially when they are embedded in convienent webpages such as this! http://www.f-body.org/gears/ Yes it's a f-body side for domestic v8s, but it still works because our final drive ratio is the same as their axle ratio.

The gears end at the following mph with the setup listed above. These number are rounded to whole numbers, so they're not accure to the 0.0001 but that's okay, so chill =)

1st = 37
2nd = 64
3rd = 97
4th = 133
5th = 172 (yes, I know that our civics can't go 172mph, but if it could, our 5th gear would prevent us from going past 172mph)


Okay, now so you know, if you haven't read my tranny page (link above and in my sig) I explain that our hondas make power in the high rpms (just before redline) So in a perfect world we would be at peak horsepower at all speeds. This is now somewhat possible with CVT transmissions. But these transmissions are very knew, and the designs are still sucky to where I wouldn't want one in a high powered car. Not to mention that they don't take a beating like our manual transmissions. Anyways...

Now, I am just going to say that we'll be trapping at 100mph in the 1/4th. I realize a stock d16z6 won't break 100mph in the 1/4th, but it's a nice round number for this example So look what happens with this setup. We get to 64 mph, and shift into 3rd. We'll take 3rd all the way up to 97mph, and then have to shift into 4th. We will use 4th to carry us from 97 to 100mph. This is very ineffecient because we will lose time while shifting into 4th.

Look at this graph that I have plotted and you can visually see the difference.



if this doesn't show up, try http://thebense.com/andrew/images/gearing.jpg

In this graph I have plotted three transmissions.

The first being the stock EX transmission. (Blue Line)
RPM DROPS
1st -> 2nd = 58.4%
2nd -> 3rd = 65.7%
3rd -> 4th = 72.7%
4th -> 5th = 77.2%

1st ends at 37mph
2nd ends at 64mph
3rd ends at 97mph
4th ends at 133mph
5th ends at 172mph

The second being the stock EX transmission with a 4.928 final drive (indicated by the red line on the graph)
RPM DROPS
1st -> 2nd = 58.4%
2nd -> 3rd = 65.7%
3rd -> 4th = 72.7%
4th -> 5th = 77.2%

1st ends at 32mph
2nd ends at 55mph
3rd ends at 83mph
4th ends at 115mph
5th ends at 148mph

As you can see, the gears still have the same rpm drop, however the mph where the gears end have dropped. This means that you will have more instances where you will be closer to the power band while accelerating.

and last, close ratio gears with the stock final drive. (Green line)
RPM DROPS
1st -> 2nd = 58.4%
2nd -> 3rd = 82.3%
3rd -> 4th = 79.8%
4th -> 5th = 80.0%

1st ends at 37mph
2nd ends at 64mph
3rd ends at 77mph
4th ends at 97mph
5th ends at 121mph

As you can see here, the gears are brought in a lot closer to each other RPM wise as well as MPH wise Also, if you take a look at the graph, you sill see that the green plot never drops below 6000 (okay, I lied, it sees 5985 rpm on a 3rd -> 4th shift from 7500rpm) But still, that keeps it much closer to the power band regardless of the final drive that one uses.

So get this. As some of you may know, I have several busted up transmissions that I have "acquired" None of them I have paid much for. Busted input shaft bearing, grinding 3rd gear, etc. People tend to donate these to me or sell them for very very cheap because who wants a busted dx transmission? Well, most people wouldn't but....

Here's where the close ratio gears shine. Because say I am road racing, and I look at a map of the track. Now I know the acceleration rate of my car, and I know that my 5th gear will end at 121mph with my 4.25 final drive. So, for smaller tracks and my "weekend warrior" I am going to be using this final drive. However say I go to a larger track, I can then use a 4.058 final drive found in a 92-00 civic DX transmission that can be found for very very cheap. I now have a top speed of 126mph. This may not matter much for most, but there's at least that option. Not only does it stop there, but also the 3.888 final drive found in 88-91 civic STDs and DXs, now my top speed is 132mph. Oh but wait, there's also a 3.722 final drive that's found in the 96-00 civic hx/cx hatchback, this brings my top speed to 138mph. And then there's the 92-95 cx/vx final drive ratio of 3.25 Which is very tall, which would bring top speed in fifth to a 158mph. A 2.95 found in the crx hf = 174mph.

Now this may not sound like anything special at first, however on a vehicle that's going at a very high speed, once you top out 4th, you need lots of power to overcome the windspeed as required power gets exponentially higher as speed increases. When shifting into 5th, this will bring the rpms closer to the powerband, allowing the vehicle to accelerate faster at the top of 5th.

Not to mention very large turbo setups where there's a very small rpm range where the engine is at full boost.

Or even since the rpm drop is tighter, the rpms fall faster from gear to gear, allowing you to shift faster without having to wait as long for the engine to slow down. This also is good for those "power shifters" because one won't be slamming the transmission because the engine won't have to drop rpm as much. Also meaning that the synchros will have to do less work.