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post #1 of (permalink) Old 12-21-2001, 08:16 PM
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Crawl Ratio ? Auto- matic Trans

While reading a recent post on automatic ratios, I got to thinking about the Jeep I'm building in my head. I'm going to be using a TF 999 I got for free. The other post stated that the 999 has a 2.74 1st gear, but is that the number you use when figuring crawl ratio? Seems to me that because of the tourqe converter you would have to use a multiplier on that first gear number. I'm shooting for 50 to 1 using a stock D300, and 4.10 gears.

CTjeepnut

You had to do what to fit those little tires?
I'm sure glad I own a CJ!
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post #2 of (permalink) Old 12-21-2001, 08:25 PM
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Re: Crawl Ratio ? Auto- matic Trans

CTJeepnut-

The torque converter would have nothing to do with changing the movement of your 1st gear. It is only a means to transfer power from the engine to the tranny. Plug in the 2.74 with D300 number (do you have a Tera low?) and the 4.10 in the pumpkins and do the math.

Good Luck!
Kriss

post #3 of (permalink) Old 12-21-2001, 10:40 PM
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Re: Crawl Ratio ? Auto- matic Trans

The actual number won't change, but the effective crawl ratio will double due to the torque convertor. The slippage allows the engine to get into an RPM range where it is making more torque, but the speed of the tranny output is slow and controlled. Right now you would be looking at about 30:1 which is good for a slush box.

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post #4 of (permalink) Old 12-21-2001, 11:57 PM
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Re: Crawl Ratio ? Auto- matic Trans

Thanks Snow Tow, that's the word I was searching for, "effective". Phisically, I have a 30 to 1 ratio, but it would "feel" like about 60 to 1 with the auto. That's perfect for my application.
I never would have guessed that the multiplier would have been that high, I would have been happy with a 1.5 multiplier. Sweeeeeeeet

CTjeepnut

You had to do what to fit those little tires?
I'm sure glad I own a CJ!
post #5 of (permalink) Old 12-22-2001, 09:08 PM
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Re: Crawl Ratio ? Auto- matic Trans

Snowtow-

This is the first that I have heard of an "effective" crawl ratio for auto trannies. (I have been reading articles on "crawl ratios" for years.) I understand that the torque converter will slip but one turn of the main shaft of the tranny will still result in the same number of turns at the output of the x-case. I now have a 93 YJ with an auto but I have not had the opportunity to use the x-case in low.

Kriss

post #6 of (permalink) Old 12-22-2001, 11:58 PM
 
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Re: Crawl Ratio ? Auto- matic Trans

Its generaly accepted, that slush box's have an effective crawl ratio that is lower than a manual tranny. The reason being the torque converter slippage. The only problem I have seen when this equation is used to achieve the final outcome, is when the crawl ratio is not low enough, and the stall speed is set to low. In some cases I have seen trannys that will be locked up, and still unable to slow to the speed needed, in these cases, the brakes can barely hold the rig back. However if the stall is set to high, then you are slipping the torque converter all day long and creating tremendouse heat in the tranny fluid. I would suggest building your drive train to within maybe 25% of what you would run with a manual, then allow the slush box to account for the rest. I personaly think, that to double the actual numbers for a final crawl ratio, will leave you short of the final crawl ratio your shooting for. Just my .02 from what Ive seen, (and smelled). I have very limited time, behind the wheel of any formidable rock crawler rigs, equipt with an auto.

Jeff
89 YJ
Adversity is imminent, versatility is mandatory, misery is optional.
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post #7 of (permalink) Old 12-23-2001, 12:25 AM
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Re: Crawl Ratio ? Auto- matic Trans

Thanks for the replys guys.
I really don't care what the actual figures are, just as long my "effective crawl ratio" is significantly lower than my numerical crawl ratio.
It sounds like I'll be right in the range I'm shooting for, as I'm not building a rock crawler. Most of the wheeling in CT is fire roads, logging roads, some hillclimbs, not much mud or big rocks. Ya know, mostly just trail rides with the GF and dog. I never wheeled an auto before but I got a 999 for free, and I know it's strong enough for a 6 cyl., so I'm gonna use it.

CTjeepnut


CTjeepnut

You had to do what to fit those little tires?
I'm sure glad I own a CJ!
post #8 of (permalink) Old 12-23-2001, 02:03 PM
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Re: Crawl Ratio ? Auto- matic Trans

Kriss you are right that one turn of the mainshaft will equal the same turns at the xfer case. The difference with an auto is the engine is turning more revolutions than the mainshaft is because of the torque convertor. The idea behind low crawl ratios is to allow the engine to be in an RPM range where it makes the most torque and to be going slow enough to maintain control. With a manual the RPM of the engine is exactly the same as the input going into the tranny because there is a direct connection. With an auto the torque convertor allows more engine RPMs and fewer tranny input RPMs. That's why autos can get away with lower actual crawl ratios and why the term "effective" crawl ratio is used. It's not about inputs of the tranny main shaft that you should be concerned with, but engine RPM's vs tranny output. I have to agree with what H8monday said about heat, stall speeds, and the other concerns of an auto. All of it has to be weighed when looking for the right number.

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