calculating crawl ratio with an automatic

[Paul]

I have a 401/turbo400/dana 20 combo, and I want to calculate the crawl ratio. Rearend is 3.73, and I'm running 33" MTRs.I know that with a manual transmission, crawl ratio is found by multiplying the tranny's 1st gear by the t.c. low gear, and multiplying that by the axle gearing. But how do you factor in "torque multiplication" when calculating for an automatic transmission? What is torque multiplication, anyway?

[clavin001]

You can't really calculate crawl ratio with an automatic transmission. The torque converter provides variable gearing. For instance, at slow speeds and/or steep hills, more engive speed is required to impart same axle speed opposed to level driving. This means that the gear ratio changed without shifting gears. With a manual transmission, axle speed changes proportionally with engine speed unless the clutch is slipping. It will then act like a torque convertor and the calculated crawl ratio will no longer be valid.

[scotty]

the "crawlability" of an auto is comparable to a manual with roughly twice the first gear of the auto,due to the torque convertor slippage.

i.e.,if the 1st gear in an auto is 2.5 to 1,double it to 5,then multiply x low range x axle gears: 5x2.03x3.73=37.85

[clavin001]

[QUOTE]Originally posted by scotty:
[QB]the "crawlability" of an auto is comparable to a manual with roughly twice the first gear of the auto,due to the torque convertor slippage.

Scotty,
That really depends on the torque convertor. The higher the stall speed the more slippage at lower RPMs.

[lindeleasley]

Most engines don't make enough torque for the slippage to really be an issue, the x2 factor will get you into the ball park

[scotty]

Originally posted by clavin001:
Scotty,
That really depends on the torque convertor. The higher the stall speed the more slippage at lower RPMs.[/qb]thats why i said "roughly" ;)

with most factory convertors the end result is the same as the mechanical crawl ratio of a rig with approximately twice the first gear.

speaking from experience,wheeling my GW with an auto and d300 is very similar to wheeling it with a t18/2.46 spicer 18.

auto crawl:(2.94x2)x2.61x3.31=50.99

t18 crawl:6.39x2.46x3.54=55.64

i wheeled it with the same 38" tires,both ways,if that makes you feel any better that im not makin it up ;) tongue.gif

[ December 30, 2002, 10:13 PM: Message edited by: scotty ]

[clavin001]

Hey, I'm not here to get into an arguement with anyone or call anyone a liar, but I was trying to be accurate and inform. I figured their were some people that didn't really know what a torque convertor did and how it affects gear ratio. 2 times sounds about right, but is it?? How do you know? What are you using to measure the ratio? What happens to the crawl ratio if someone has a 2500rpm stall torque convertor vs a stock convertor(I know of a few people that listened to race guys when buying torque convertors for their trucks)?

[fsj1978]

I can't immagine a 2500 converter on a trail rig. UGH!!! You'd fry the trans trying to do any kind of rock crawling or slow hill climbing. The exception would be a "switch pitch or variable pitch" BOP trans like we have in our Buick. Whenever I get around to doing the Buick 455 swap (I've got a junk yard refugee sitting in the garage) it'll be getting the SP trans out of the Buick. smile.gif

[clavin001]

Yeah, I know. I tried to talk them out of it, but I don't build motors for a living so I don't know what I'm talking about. Some people just listen to people without understanding why/how it works.

[Kris]

Originally posted by Paul:
I have a 401/turbo400/dana 20 comboPaul,
So that means you got it ?
Are we going to see you this weekend ?
tongue.gif
kris.

[J20 project]

FSJ1978, Hey 1978, when you get into it on the switch pitch th400, let me know, I would like to campare notes w/ you. When I built my th400 for my 76 J20, I did just that. I pulled the guts out of a 67 Buick sp th400 and installed them into my AMC case w/ what was supposed to be a new torque convertor of the sp design. I set it up w/ a toggle switch on my console to turn it on. So far I have not realized any benefits. I expected to see a 600-1000 rpm shift on the tach while wheeling. Not so. My tranny mech didn't seem to think it would be as noticeable as that which may be proving true. He suggested that you might see it under wot.I have to wonder though if using the amc case didn't change some feature of it. At any rate, I would suggest using the buick case, it may do something different.
J20

[fsj1978]

I'm pretty sure their's an extra hole in the casing on a SP tranny but I'm not 100% sure. On our Buick just under normal accel off a light I'll see probably 300-400rpm diff between hi and lo stall speeds. If you mash the brake and go WOT with it in lo, then back off, change to hi and do it again you should see *SEVERAL* hundred rpm difference. Like say 1600 lo and 2400 hi. It sounds like yours isn't getting any pressure to the mechanism and is in lo stall.
And yes, I was going to use the Buick case to go with my Buick 455 motor when I get around to that project. The major bummer is that I've got to GUT both transmission in order to put the output shaft for the Quadratrac into the Buick trans. <sigh>

[J20 project]

Man, it seems a waste to mate that much torque onto a qtrack. Can you say chain stretch?
J20

[scotty]

Originally posted by clavin001:
Hey, I'm not here to get into an arguement with anyone or call anyone a liar, but I was trying to be accurate and inform. I figured their were some people that didn't really know what a torque convertor did and how it affects gear ratio. 2 times sounds about right, but is it?? How do you know? What are you using to measure the ratio? What happens to the crawl ratio if someone has a 2500rpm stall torque convertor vs a stock convertor(I know of a few people that listened to race guys when buying torque convertors for their trucks)?im not here to argue,either.thats why youll see these smilies: " ;) and tongue.gif " they are supposed to let you know nt to take the responses personally smile.gif

you are 100% corect that there is no way to way to completely accurately calculate a crawl ratio with an auto.

againi,this why i said "roughly". i hav eread the 2x number somewhere,on a couple different boards,cant really remember now,and believe that it is correct from the experiences ive had.

as i mentioned,wheeling with the auto felt about the same as wheeling with the t18,in situations where i needed the 1st gear final drive.

further, if i put my t18 in 2nd(roughly a 3:1 ratio) it would provide the same final drive number as the automatic if we didnt figure in the convertor(auto:3x2.6x3.31=25.91,manual=3x2.46x3.54 =26.12)

there is a BIG difference in climbing a hill in 2nd gear and 1st gear,and the auto feels more like 1st.

good enuff for ya ;) tongue.gif

[fulmetal]

I would have to go along with the 2X ratio in 1st. When I built my 727 I put a Hughes 1200 stall converter in it. They also told me the same 2X ratio for calculatoins. I love the lower stall speed, they seem to run cooler, not as much slippage and I am almost fully locked just off idle where a good portion of your wheeling will be. smile.gif

[desert_freak]

Anyone know the stall speed on the GM HD torq.conv.? I too want a LOW stall speed for my TH400 after I rebuild it with my D20 tailshaft.

P.J.