Since there will be two TWS events this year a proper fifth gear is very important. That’s even more true if one of the events is clockwise.
For those Ford guys running T5’s, I thought I’d toss out a little information on the subject since there seemed to be a bit of confusion among the people I’ve talked to. I promised this a while back. Sorry for taking so long to get to it.
Starting with the design side will hopefully help to understand why the cool gear ratio you purchased might not be the effective gear ratio you are actually running. Or, as is sometimes the case with my discussions, it might just cause more confusion.
The T5 transmission has an input shaft that is splined to the clutch. When the clutch is engaged, this input shaft spins at engine speed. The input shaft has a gear on it that meshes with the cluster shaft. The cluster shaft is then is always spinning at a ratio of engine speed defined by how many teeth are on the input shaft and the cluster shaft. The cluster shaft has many gears of different sizes on it. Those cluster gears mesh with different gears on the output shaft of the transmission. The gears on the output shaft are free to spin until a gear is selected with the shifter. The movement of the shifter locks the selected gear to the output shaft and causes it to spin at a ratio to the cluster shaft.
The key to all that confusion is that there are two different gear meshes that determine the ratio between the input and the output shafts. (Fourth gear is a little different. For fourth the input shaft is just locked to the output shaft so no “gears” are actually used and you get the standard 1:1 ratio.)
For an example of these two gear meshes, the original T5 transmission that was available in most 5 liters has 34 teeth on the input shaft and 23 teeth on the cluster shaft. The first gear has 34 teeth on the output shaft and 15 teeth on the cluster gear. Doing a little math (34/23*34/15=3.35) we find the overall first gear ratio of 3.35:1 that the stock boxes are often referred to.
The Z version of the transmission is often referred to as a 2.95 box. It has an input ratio of 31/24 and a first ratio of 32/14. The same math trick gets you the 2.95 overall first gear ratio.
Similar math gets you the other gear ratios of the transmission including the fifth gear I’m finally getting around to talking about. The typical T5 has a fifth tooth ratio of 27/59 which gives you (after some math:34/23*27/59) a final overdrive ratio of .68.
Again, similar trickery gets you a T5Z overall fifth gear ratio of .63.
Here is where the (relevant) fun starts. The fifth gears are interchangeable between the two boxes. Given the different input ratios, the same parts used on the different boxes will net you different fifth gear ratios.
For a little example of this, lets try putting the stock fifth gears on a T5Z box. For this we have 31/24*27/59 = a .59 overdrive ratio. A T5Z fifth gear into a T5 box nets a .72 ratio.
So, enough with the math crap. How is this going to help me get the correct parts for my fifth gear conversion? Well, it won’t really, since none of the aftermarket vendors sell gears by tooth count. They sell them as gear ratios which is where the problem comes in. If you buy a .80 gear ratio you need to know which transmission that gear ratio was intended for and which box you have.
So, what if you are given a gear ratio that wasn’t meant for your transmission and you want to know what the ratio will be in your box? Well that takes more math. To go from a gear meant for a T5Z box to a regular T5 you get 24/31*34/23=1.1445. So you multiply the T5Z ratio by this factor to get to regular T5 ratios. Dividing by this factor allows you to convert the other direction.
The aftermarket tends to be centered around the T5Z. Common available ratios are .73 and .80. The math thing gets us to .83 and .91 respectively for a T5. I believe one from our group put a .80 T5Z ratio into his regular T5 box and wondered why he ran out of rpms well before the end of the front straight at TWS. I put one of the .83 ratio gears into my transmission. It worked great for a few years at CMC power levels. It lasted less than one event at AI power levels. Ooops. These are parts that came from other factory applications (including Jeep stuff) that happen to give us useful ratios in our transmissions.
The aftermarket has since come out with stronger custom gears specifically for a better racing fifth gear ratio. Not only are these gears designed to give us desirable ratios, they are fabricated with a shallower helix angle and a significantly coarser pitch. I’ve attached a comparison picture and the difference is not subtle. The redesigned gears offer greater strength at the expense of some extra noise.
My transmission is using an 18/31 tooth fifth gear ratio from G-Force (http://www.g-forcetransmissions.com/tran_gt-5.asp) which would give a .75 ratio in a T5Z gearbox and a .86 gear ratio in my regular T5. Other fifth gear ratios from G-Force include a .90 ratio fifth which when put into a regular T5 nets you a 1.03 which is no longer even an overdrive. G-Force can also upgrade all the gears in your T5 to the stronger coarse pitch gears. They even offer a dog-ring shifting setup (AIX legal only) for those who really want to trick out their T5.
Astro Performance Warehouse (http://www.astroperformance.com/index.php) also came out with the improved strength 5th gears a while back but I don’t see them on their web site. Apparently they offer a .79 fifth ratio (.90 on a T5). They also offer full upgrades for all the other gears and I’ve gotten the impression that they have much better customer support than G-Force.
Other good places for transmission parts include D&D Performance (http://www.ddperformance.com/) and Hanlon Motorsports (http://www.hanlonmotorsports.com/index.asp).
So some of the available transmission fifth gear ratios are:
.59, .63, .68, .72, .73, .75, .79, .80, .83, .86, .90, .91, and 1.03. I’m sure there are more but figuring out what they are and finding them might be tough. Actually, finding some of those would be tough at this point… The four cylinder cars of course had different input ratios and a different set of available fifth gear ratios.
Anyway, once you have decided on what ratio you want and have procured it, can you install it yourself? Yes! Yes you can. It’s really not very hard. The fifth gear on the T5 is external to the main case. This is because the base design was a four speed tranny and 5th was added in a later revision that just tacked fifth on externally. That makes it less strong but easier to get to. Very little disassembly is required to swap out fifth gear.
Hanlon offers an excellent video showing how to work on your T5. The factory manual has plenty of information to successfully do this swap. G-force has a T5 manual available online (http://www.g-forcetransmissions.com/...ice_Manual.pdf).
Basically, you pull the tranny, remove the shifter, drive out a roll pin, remove the tailshaft housing bolts, drive out another roll pin to remove the fifth gear shift fork, remove a couple of snap rings, and then slide the gears off. Reassembly is the reverse. :lol:
Some people can’t make a project that simple, though. Some typical “while we are in there” things are the tailshaft seal, the tailshaft bushing (along with a fresh driveshaft yoke), the speedometer gear ratio, fifth gear synchros, and maybe a steel input bearing retainer. The associated pilot and throwout bearings shouldn’t be overlooked either.
While you are inside the transmission, those looking to put down serious power should consider this fifth gear bearing stabilizer (http://www.gearzone.net/t5.htm). I’ve heard it helps a bit. I don’t have one myself.
To summarize the important part so you don’t have to suffer through reading that again:
T5Z ratio * 1.1445 = T5 ratio
T5 ratio / 1.1445 = T5Z ratio
Have fun upgrading and let me know if you have any questions…
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