installing mitchell overdrive

K

klrh20frank

Member
I'm going to install a Mitchell overdrive in a model a.

thr ring pinion ratio is 3.78. what are the recomendations for OD ratio. Main driving is in Kansas with few tours to the mountains.
thanks
frank
 
The 26% ratio is designed to work best with a 378, A Snyder head is also a big plus, my choice is a 5.5, but a 6.1 will also work.

Tom Endy
 
To figure the overall ratio, divide the rear end ratio by the overdrive ratio. So with a 3.78 rear end and a 26% overdrive, divide 3.78 by 1.26. The result is 3.00 to 1. This an excellent overall ratio for the flats. For the few mountain tours you will find second overdrive very useful. It helps to have a torquey engine, one with a higher compression head.
 
nkaminar said:
To figure the overall ratio, divide the rear end ratio by the overdrive ratio. So with a 3.78 rear end and a 26% overdrive, divide 3.78 by 1.26. The result is 3.00 to 1. This an excellent overall ratio for the flats. For the few mountain tours you will find second overdrive very useful. It helps to have a torquey engine, one with a higher compression head.
Click to expand...

I agree on all counts and I especially agree with the higher compression head. I run a 5.5 head on one of my cars and a 6:1 on another. I find the 5.5 not quite as torquey but easier to drive. The 6:1 is more fussy about ignition timing.
 
I don’t want to hijack this thread but Synchro, you got my attention about the difference between the 2 heads, I had no idea that a .5 difference would be particularly noticeable. I have a 6.1 head from Snyders that I have not installed yet. Maybe we should communicate by pm and not crowd out the existing conversation. I also have some info I recently learned about the Marvel carburetor we discussed as well.
 
jeepguy1948 said:
I don’t want to hijack this thread but Synchro, you got my attention about the difference between the 2 heads, I had no idea that a .5 difference would be particularly noticeable. I have a 6.1 head from Snyders that I have not installed yet. Maybe we should communicate by pm and not crowd out the existing conversation. I also have some info I recently learned about the Marvel carburetor we discussed as well.
Click to expand...

Why not start a new thread so all can learn and benefit.
 
thanks for the suggestions. Sounds like i'm going with the 3.78 with 0.26 OD and a 5.1 Head

thanks again especially to Tom for his help in Vancouver installing the ring and Pinion in my other car.
 
Regarding the difference between the 5.5 and 6.0 head: The combustion chamber design has a lot to do with how fast the fuel/air burns and thus the ignition timing. In my opinion, the 6.0 combustion chamber is a better design and will provide better breathing and fuel/air burning. Adjusting the timing for driving conditions is part of driving a Model A. If you want you can run a distributor with automatic ignition advance but you will have to have the right advance curve. For me it is better to be able to manually control the advance. It is more fun and gives me more control.
 
nkaminar said:
Regarding the difference between the 5.5 and 6.0 head: The combustion chamber design has a lot to do with how fast the fuel/air burns and thus the ignition timing. In my opinion, the 6.0 combustion chamber is a better design and will provide better breathing and fuel/air burning. Adjusting the timing for driving conditions is part of driving a Model A. If you want you can run a distributor with automatic ignition advance but you will have to have the right advance curve. For me it is better to be able to manually control the advance. It is more fun and gives me more control.
Click to expand...

Yes, it gets to be a habit, like double-clutching.
 
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