Transmission/differential question

[Young Ed]

Ok, I'm back.  I found no additional casting numbers or stamped numbers on my 3rd member (no jokes here, please).

Jacked up rear so both tires off the ground. Spun one tire, the other spun in opposite direction.  Then I chocked one tire, and spun the crankshaft 10 revolutions.  The Free tire turned approximately 5.65 revolutions.  Did it twice, same result.

 

Does that mean I have a differential gear ratio of 10/5.65 or 1.77 : 1   ?   That seems like an outrageously low ratio.  If that's true, it implies that if my engine rpms are truly high at cruising speed, my tranny must be geared with a high-high ratio.

 

Next I will buy a tach and test out engine rpm vs speed.  Unfortunately, I'm waiting to get my steering gear box back, so I cant test it until I get the steering box reinstalled in the next couple weeks.  I'll update once Ive done that.

 

Question to ponder:  so what is a max cruising rpm on a flathead I6, without fear of longterm engine damage?

3000-3200 is good. Engines were tested at the factory at 3600. I believe you've got a 3.54 as your pictures showed. If you really want to know for sure you need to pull the unit and it will be marked on the ring gear. 

[Merle Coggins]

Ok, I'm back.  I found no additional casting numbers or stamped numbers on my 3rd member (no jokes here, please).

Jacked up rear so both tires off the ground. Spun one tire, the other spun in opposite direction.  Then I chocked one tire, and spun the crankshaft 10 revolutions.  The Free tire turned approximately 5.65 revolutions.  Did it twice, same result.

 

Does that mean I have a differential gear ratio of 10/5.65 or 1.77 : 1   ?   That seems like an outrageously low ratio.  If that's true, it implies that if my engine rpms are truly high at cruising speed, my tranny must be geared with a high-high ratio.

 

Next I will buy a tach and test out engine rpm vs speed.  Unfortunately, I'm waiting to get my steering gear box back, so I cant test it until I get the steering box reinstalled in the next couple weeks.  I'll update once Ive done that.

 

Question to ponder:  so what is a max cruising rpm on a flathead I6, without fear of longterm engine damage?

 

You made a slight error in your calculations. With one wheel blocked the other wheel will turn twice as fast due to the differential action. So take your 5.65 rotations and divide it in half to get 2.825. 10/2.825=3.5398... In other words, your stamping appears correct at 3.54:1. You should be able to motor right on down the highway with those gears. I have 3.73's in my truck with 235/75R-15 tires and I can run 75+ when needed. (at 3400 RPM)

Edited August 18, 2016 by Merle Coggins

[chrismfischer]

Merle, thanks for clearing that up! Really sets my mind at ease.

Just for grins, if anyone knows what the expected 3rd gear ratio should be in my 3 speed tranny, please advise.

I want to calculate expected engine crank rpm vs speed while waiting to test it with a tach. Then I'll know what to expect once I've got steering back in and can drive it.

Incidentally, I shopped around and found Pro Steering in Portland, OR had best price to rebuild my steering gearbox. Total rebuild including new gears for about $500. They even had the gears in stock. Can't wait to see how much better it steers. After adjustment I still had 4+ inches of slack in my steering wheel despite a new front end, so it has been very tricky to keep it between the ditches.

Thanks to all who have replied. Great learning experience for me and I really appreciate your time and wisdom.

Chris

[Young Ed]

1:1 for the final gears. 

[Merle Coggins]

Yes, like Ed said... 3rd gear on a 3speed, or 4th gear on a 4 speed are always direct drive, 1:1.

(Not including later model overdrive 4 speeds)

Merle

Edited August 18, 2016 by Merle Coggins

[sstamilio]

On 8/16/2016 at 1:08 PM, HotRodTractor said:

 

2) A very popular option is to swap the rear end for a newer unit and do some upgrades along the way (better brakes). For instance I just picked up a Mercury Mountaineer rear axle with 3.73 gears, limited slip diff, disc brakes, etc... for $200. I plan on cutting off all the brackets and welding on new to make it nearly a bolt in replacement as its nearly the correct width and the right bolt pattern.

 

I'm getting ready to do both option 2 and 3 on a Pilothouse truck. Its an ugly truck I bought for parts years ago and its grown on me.... I figure it will fit in with the current trend of patina and rat rod and quick, cheap build that I can have some fun with while I start shopping for a nice truck to make a daily driver out of..... and then I'll have my entire drive train already lined out.

I noticed that the Mercury differential is a lot less $ than the Jeeps. I did some searching and found that this seems to be an upgrade some Jeepers do http://www.jeepsunlimited.com/forums/showthread.php?371713-8-8-FACTS-OPINIONS-amp-HOW-TO-S-(F-A-Q-) 

 

 

Edited September 4, 2016 by sstamilio