My original 901 (never been rebuilt) tranny lost second gear. So I have found a loaner 901 transmission while I have mine rebuilt. I'm going to go for shorter gears. Can somone help educate me to options around shorter gears for a 901 transmission. I'm having a shop do the work but I want to educate myself to what track drivers think works best for gears. I have a 2.4 that makes something around 160 to 170 HP and use 265/45/16 tires in the rear. I will mostly be just having fun doing track days.

grant,
it depends upon the track length and your powerband.

without going to a dyno

you would need to calculate your top speed at the longest straight and then calculate that to 5th gear.

then once you have a 5th gear picked out you work backwards from there to equalize the gaps between each respective lower gear.

of course aerodynamic drag plays into it and you can have bigger gaps between the lower gears than you should have between the top gears.

finally the next factor is cost. Some gear ratios cost 50 bucks and others cost 800$. So it becomes necessary that if a certain ratio is perfect but the next closest thing is 600 cheaper you might use the compromise ratio.

in the end.
people will tell you to go msx

but that doesn't mean it is actually the fastest option.
just simple. I found my car to be faster by using lower ratio's than msx

I will have those 3 ratios available for sale soon as used.
brant

What is MSX? That is what was being reccomended to me this morning. Given I'm not going to be driving the car in competitve events and just want better than stock ratios. I think economical route will be my approach.

I agree, M,S,X. the letter codes for the ratios 3, 4, and the 5th gear ratios

http://www.pelicanparts.com/techarticles/m...anny_ratios.htm

Tire Diameter: 22.5 inches
Final Drive: 7 : 31 901 TRANSMISSION

Speed (MPH) at RPM
Name Number Ratio 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8000
A 1 11 : 34 (3.091) 10 12 15 17 20 22 24 27 29 32 34 37 39
AA 1 12 : 34 (2.833) 11 13 16 19 21 24 27 29 32 35 37 40 43
B 1 14 : 37 (2.643) 11 14 17 20 23 26 29 31 34 37 40 43 46
C 1 15 : 36 (2.400) 13 16 19 22 25 28 31 35 38 41 44 47 50
D 1 16 : 35 (2.188) 14 17 21 24 28 31 35 38 41 45 48 52 55
E 2 17 : 34 (2.000) 15 19 23 26 30 34 38 42 45 49 53 57 60
F 2 18 : 34 (1.889) 16 20 24 28 32 36 40 44 48 52 56 60 64
G 2 18 : 33 (1.833) 16 21 25 29 33 37 41 45 49 54 58 62 66
GA 2 18 : 32 (1.778) 17 21 26 30 34 38 43 47 51 55 60 64 68
H 2 19 : 32 (1.684) 18 22 27 31 36 40 45 49 54 58 63 67 72
HA 2 20 : 32 (1.600) 19 24 28 33 38 43 47 52 57 61 66 71 76
I 2,3 20 : 31 (1.550) 20 24 29 34 39 44 49 54 59 63 68 73 78
J 3 21 : 31 (1.476) 20 26 31 36 41 46 51 56 61 67 72 77 82
K 3 21 : 30 (1.429) 21 26 32 37 42 48 53 58 63 69 74 79 85
L 3 22 : 30 (1.364) 22 28 33 39 44 50 55 61 67 72 78 83 89
M 3,4,5 22 : 29 (1.318) 23 29 34 40 46 52 57 63 69 75 80 86 92
N 3,4,5 23 : 29 (1.261) 24 30 36 42 48 54 60 66 72 78 84 90 96
O 3,4,5 23 : 28 (1.217) 25 31 37 43 50 56 62 68 74 81 87 93 99
P 3,4,5 23 : 27 (1.174) 26 32 39 45 52 58 64 71 77 84 90 97 103
Q 3,4,5 24 : 27 (1.125) 27 34 40 47 54 60 67 74 81 87 94 101 107
R 4,5 25 : 27 (1.080) 28 35 42 49 56 63 70 77 84 91 98 105 112
S 4,5 25 : 26 (1.040) 29 36 44 51 58 65 73 80 87 94 102 109 116
T 4,5 26 : 26 (1.000) 30 38 45 53 60 68 76 83 91 98 106 113 121
U 4,5 26 : 25 (0.962) 31 39 47 55 63 71 79 86 94 102 110 118 126
V 4,5 27 : 25 (0.926) 33 41 49 57 65 73 82 90 98 106 114 122 131
W 4,5 27 : 24 (0.889) 34 43 51 60 68 77 85 94 102 111 119 128 136
X 4,5 28 : 24 (0.857) 35 44 53 62 71 79 88 97 106 115 123 132 141
Y 4,5 28 : 23 (0.821) 37 46 55 64 74 83 92 101 110 120 129 138 147
Z 4,5 29 : 23 (0.793) 38 48 57 67 76 86 95 105 114 124 133 143 152
ZA 4,5 29 : 22 (0.759) 40 50 60 70 80 90 100 110 120 130 139 149 159
ZD 4,5 31 : 22 (0.710) 43 53 64 75 85 96 106 117 128 138 149 160 170

Thanks!

Grant, I wish you would have rode along during that last session. Then you could have felt MSX in action.

Ride along with me at Sears this time. Or watch a video.

The concept is to put the engine in the powerband after each upshift.
It's like having "one long gear" that pulls all the way to top mph.

Let's say you shift at 6500 rpms in third ( M ) 75 mph,
when you release the clutch in 4th gear ( S ) the rpms will be at about 4900-5000 rpms, you'll be right on the power and just keep pulling to 6500 rpms at 94 mph, etc.

KT

Hi Grant,

I have a MSX box and it is great for Thunderhill, Infineon and Buttonwillow.

I use a KaQV box for Laguna Seca & Streets of Willow

I've attached my 'worksheet' that helped me select the right gears.

Click to view attachment

You want to pay attention to the 'entry point RPM' for each gear. And like others have mentioned you need to tailor to each track. For example my KaQV box is great for Laguna, but I hit redline way too early in 5th for Thunderhill so I run the MSX box there.

Hope this helps!

Greg

Grant,

Just want to acknowledge again all of the responses above. My setup is for a 2.2 litre 4.

You need to consider all advice above regarding top straightaway speed, the performance specs of your motor, the track, etc.

Since you are on the west coast, you may want to talk with Rich Bontempi... He can help you select, find the gears, and rebuild your 901!

He did my KaQV box.

Greg

I failed to mention also that your redline is going to have everything to do with your gearing.

If a ficticious car was redlining at 4000rpm its speed would be hugely different than the same car runnnig at 8000rpm.

so if you aren't working from a dyno chart then your working redline has everything to do with the gearing you choose

my new tranny is amazing because its both a long and a short box in one, and really pretty applicable at all tracks with the FJOSV ratios
brant

Wow!! That FJOSV gear set sounds like the "wonder box".

That must have cost a fortune...


KT

the trade off is that you loose first and reverse.
but who needs first and reverse for racing!
its not hard to get the car going in 2nd gear.
brant

You guys are great! Thanks again!

I have A,Ga,Ka,Q,V. I also have a twin plug 2.7 that I can run to 7800rpm. I only redline at Ca. Speedway. I have a spare box and gears to set up a MSX box but haven't gotten around to it yet. The Ka,Q,V box has worked real well for me because of my high redline.

Grant,

I agree with Drew. The KaQV box is really nice if you have the revs for your longest straight.

The cool thing about a KaQV box is that you only have to buy the Q gear.

The secret is, you 'flip' your 5th gear and move it to 3rd to make your Ka gear. Then move your fourth gear (V), to fifth. You have to buy the Q for your new fourth...

So buying one gear and making some 'moves' get you a whole new set of gears!

With this setup I'm hitting 6800 in fifth at Laguna going into turn 2 and turn 5 which is perfect for my engine!

I still have to use my MSX box at the other tracks since my four won't turn 7800 like Drew's six!

Greg

My 2.4 E engine pulls like a truck from 3500 RPM to 6500 RPM. I redline at 7000 but I really dont get much by going above 6500. I typically just use that extra 500 rpm from 6500 to redline to avoid shifting. So should I calculate from redline or peak power?

If you want to get a sense of how my little 2.4 pulls check out my 3rd gear pulls out of turn 2, the corkscrew and and 11 in my videos from the Track Masters post. I was down in very low RPM's due to losing 2nd gear. With tall 265/45/16 tires I wasnt able to get to 7000 rpm redline in 4th going down the start/finish straight. It sure would be nice to have ability to use another gear at Laguna and Infineon (never got out of 4th at Infineon either).

Theory is you set your shift point when the power drops off, sounds like 6500 for your motor. 2nd and most important attribute is pick a gear that upon 'entry point' you are at your peak torque!

From your info above, you should try a KaQV box. Here is my gear ratio sheet updated for 6500 rpm shift point and 265/45/16 Kumho 710s (25.2" diameter)

Click to view attachment

I picked up a second 901 at a swap meet (from John Seidell) and built my MSX box so now I just switch transaxles when needed. Not sure you would need to do this with your 265/45/16s... (I'm running 205/50/15 AVONs)

Hope this helps Grant!

so how are the Avon's compared to the standard Kumho Hoosyourmamas?

Sorry to do that

No really I have been watching this cause it is an update this winter for my car. But the Avon's came up your the first person I know of running them. PM me if you want.

Back to regularly scheduled programs.

Hoosyourmama?

I Love it! I need that stick on my racecar next to 4gasm!

I was a Hoosier S03 Fan than bailed out when the 04s came out. Decided to try the Avons and they are great. Try a set, I think you'll like them. Caveat Emptor, the Avon 205s are wider than the Hoosier 205s that are wider than the Kumho 205s. I guess it's an expanding universe thang....

If cost is a factor you could use KaRZ they are free, built into a stock 901 -4 cylinder tranny.

Ka is stock 5th flipped into third
R is created by flipping 4th
Z is from third flipped into 5th

Modified the spread sheet to show you what this combo looks like for your tires etc....

I am thinking of running this box for auto-x, I could get into third for all our High HP runs and keep it there. With my combo it works out to pull off idle to 5000 or so 21 mph to almost 60 if I don't bounce the rev limiter. Coming off idle is less of a problem then blowing a downshift or up-shift.

If I had a Raby Torque Monster this would not be an issue at all.

Cool thanks for the info.

Holy Crap! I remembered that the spreadsheet for calculating speed was in this thread and I just calculated 7000 RPM redline for my car in 3rd gear. Its 94 MPH in 3rd and 128 MPH for 4th! 2nd gear is good for 63 MPH. Wonder if I could grab 167 in 5th? LOL!

Trekkor, you were wondering why I was getting nervous going through turn 1 and 10 at Infineon in 4th gear? LOL! Given that I was shifting near redline out of 3rd and had plenty of time to wind out 4th I was going way too fast for my logical brain. Good thing for speedo error or I would have been psyched out.

I show a stock 3rd "N" at 84 mph

That's on 205/50's, though.

How much bigger are you tires and what is the multipler to make the gear chart accurate?

Go faster!!


KT

Try 25.2 inch tire diameter of my 265/45/16 tires. Perhaps I'm missing something - of course I didnt check formulas, just plugged in my tire diameter and 7000 RPM.

I'll buy the flipping 5th into 3rd, but can you flip 4th? Aren't the dogs on the wrong gear?

Sorry, I'm still figuring out what to do with my silly gearbox problems and I ran across this thread.

Kevin

Flipping 4th with what?

not 2nd which is part of the main shaft.

Got my interest though.

Looking for and "S" 4th.

I'd like to know about the 4th flip as well. The guy that did the 5th to 3rd flip in my transaxle told me told me that the 4th gear set won't swap on the shafts.

Mike T

I'm pretty sure it ain't possible. I went through my stash yesterday and proved to myself that while 4th and 5th gears are the same and interchangable, they cannot be "flipped" between shafts unless they're going to 3rd because the dogs are on the larger gear.

I have a 3rd N and a 4th N and they have the dogs on different gears.

Kevin,

I'm pretty certain you are correct
remember when I bought that "O" 3rd gear from you.
I had to do that because I couldn't use the 4th gear "O" in the 3rd gear slot.

brant