E-maxx w/ Feigao 540C 8XL pinion selection
 

I'm planning on getting a Feigao 540C 8XL, BK Warrior 9920 for my E-maxx. I'm wondering what pinion gear size to use with this. I'm planning to start running with 12 cells, then upgrade to 14-18 cells or 5s-6s LiPo's in the future. I'd like to be able to hit 40mph w/ 12 cells. I currently have stock gearing (66T spur).

From my calculations it seems like a 20T pinion will match the same gear ratio as other members here, but when using the speed calculator http://www.scriptasylum.com/rc_speed/_top_speed.html the top speed goes to 33-35 mph. Should I stick with this pinion or go up a few teeth? I don't want to burn up my ESC.

First off, welcome to the forums!

If i were you, id ditch the stock slipper setup and go with a strobe/ofna setup. But if you wanna use the stock slipper, 18/66 on 12 cells would be the farthest i would go (dont want to overheat the esc). Really, id use 14 cells because you wont be too happy with 12 cells on a 8xl.

If you do use a strobe with the ofna gear, 15/51 would be a good starting point.

WELCOME:)!
if you want to use 12 cells a 7xl would be better.

Would the 7XL be too hot for the BK 9920? I read in another thread that its getting very close to the max amp limit and I don't want to upgrade to the next ESC.

Regarding the gear ratios, I'm not sure why 20/66 is too high, but 15/51 is good to start at. Here are the ratios I get, am I not factoring in something?
15/51 = 0.294
20/66 = 0.303
18/66 = 0.273

When my spur goes I'll probably go for the Robinson Racing one to keep the same gear ratios.

Depends what you run, the 7xl on 12/14 cells is good and also is good for the 9920. Sorry, i mean 14/51, dang typo. If you dont plan on running more than 14 cells id go with the 7xl, or if you start with 14 cells and go up than the 8xl would be good.

Okay, I'll probably go with the 8xl w/ 14+ cells and 18T pinion. What would be the max number of cells that I should use w/o going into meltdown?

The most cells i would do on the 8xl is 16, but for some speed runs 18 would be alright on the 8xl.

I think you did the ratios wrong.
You divide the driven gear by the pinion or drive gear to get ratio.
51/ 15 3.4 ratio. 66 / 20 is 3.3 So not much difference there.
The 66 / 18 is 3.666666

Update from Mike
 

I sent the same question to Mike and this is his reply:

His proposed 54/20 ratio is even taller than 66/20 ratio that was proportedly too high.

To clarify my email a bit, i reported a needed ballpark gear ratio to achieve 40 mph. If you run in open areas and spend your time going fast, you won't have any troubles with this gearing. If you try rock crawling with 40mph gearing, your gonna find yourself in trouble. Ideally, you want your gearing to reflect your typical driving style/terrain etc. that you spend your time in. If you spend most of your time driving in tall grass at 15-20 mph, then I wouldn't suggest a 40 mph gearing. For general bashing, gearing for low-mid 30s is typically plenty of speed and allows decent runtime. If your running in the street and just want to dust your nitro buddies, then the 20/54 gearing would get the job done for you. If you want the best of both worlds, get an extra pinion or so - it takes about 20 seconds to switch to "high speed" gearing when you need it, but in 20 seconds, you can switch back to "torque, power and runtime" gearing for lower motor/controller temps, longer runtime and general good times(gearing such as suggested in this post, for example). :)

Basically gear it to your driving style. If you bash, like Mike said, you don't need to do mach two.

will the 7xl work good on 16cells with 125b controller. Aslo with nicd cells matched of course how long of runtime do you get before you start to lose voltage and how much voltage do you lose. Can this be related to a associated 18t for example on my brothers 18t he gets 20 minutes out of it it seems to get about 15 minutes with a unnoticable voltage drop in the 16 minute we start to notice the battery is dieing but not dead like 60percent of voltage is being applied to the car. after about 18 minutes the battery is about 40 percent dead. and on the 20 minute the battery is about 20 percent of votage left and it is enougt to bearly move the car slow on only flat pavement. Should i expect the same way of battery voltage loss in a maxx with roughtly any were between 37 and 4300 mah batteries. not talking about runtime just about the time i get before the car is to slow to drive does these same nicd prinicples apply to a maxx as it would the rc 18t.

On 16 the 7xl would run a bit hot. Id try a 8xl for 16 or even a 9xl.

I ran 16 on a 10xl and a 9xl, both were good. The temps were better on the 10xl though, the 9xl seemed like it had more torque, but the speeds were comparable IMO.

what about what i just added is this true or not. thanks guys so definatly not a 7xl then remember it will have a heatsink.