1985 FJ60 Engine/Transmission Swap Questions

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mouchra

mouchra

10 Year
Joined
Nov 9, 2012
Threads
6
Messages
44
Location
Lake Forest, CA
Hello all,
I have a 1985 FJ 60 with a Chevy 350 short Block in it and a Turbo 400 automatic 3 speed transmission on it.
My RPMS at 70 mph are extremely high, it was initially suggested to me to switch my gear ratio in the rear end (original now 3.71 ratio?), but a counter suggestion was to change the transmission to a GM 700R4 having an "Overdrive" and maybe switch the rear gears to a 4.1 ratio. The main use of the cruiser is 2W Drive Highway and minor off road.
I would greatly appreciate the opinion of anyone that has swapped theirs or know of what is best for me to do.
Thank you.
Roger
 
Quick query, what exactly is extremely high? 4:11s will step that number even higher. Yes, stock would be 3:70s. What size tire? The closer to stock size, the higher the rpm.
 
Now you've got me curious, 33s n you're saying extremely high rpms. My p!$$ poor math is thinking rpm should be about approx 27,28 hundred at 70mph. Assuming 3:70s in the pots.
 
Are the rpm’s actually high, or just sound high? A big exhaust behind a small block is going to sound loud at 2700+ rpm, not to mention tire noise depending what they are.
 
kirvesmies said:
Now you've got me curious, 33s n you're saying extremely high rpms. My p!$$ poor math is thinking rpm should be about approx 27,28 hundred at 70mph. Assuming 3:70s in the pots.
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3022 RMPs at 80mph without the overdrive on the Turbo 400 transmission. I did the math and it appears the overdrive on a 700R should bring it down to approximately 2024 @80mph?

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Skreddy said:
Are the rpm’s actually high, or just sound high? A big exhaust behind a small block is going to sound loud at 2700+ rpm, not to mention tire noise depending what they are.
Click to expand...
3022 RMPs at 80mph without the overdrive on the Turbo 400 transmission, but you definitely have a point, the exhaust and small block. I think I will leave the gears alone for now and get the 700R4 transmission with the overdrive and see how that goes. This transmission with the overdrive should bring it down to approximately 2024 RPMs?
 
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Don't forget that TH400s do not have a lock up torque converter, either. So there is going to be considerable "slip", which will throw off the calculations. 3000+ RPM at 70 MPH wouldn't be unreasonable, I'd think.

A 700R4 does provide overdrive and lock up, which will significantly reduce your cruising RPMs. Just keep in mind that not all 700R4s are the same - there are differences in the valvebodies and governors. A 700R4 for a Corvette will not provide the right shift points for your vehicle, likewise neither will one for a diesel.

Also they are different lengths, so it won't be a direct swap.
 
Lead Head said:
Don't forget that TH400s do not have a lock up torque converter, either. So there is going to be considerable "slip", which will throw off the calculations. 3000+ RPM at 70 MPH wouldn't be unreasonable, I'd think.

A 700R4 does provide overdrive and lock up, which will significantly reduce your cruising RPMs. Just keep in mind that not all 700R4s are the same - there are differences in the valvebodies and governors. A 700R4 for a Corvette will not provide the right shift points for your vehicle, likewise neither will one for a diesel.

Also they are different lengths, so it won't be a direct swap.
Click to expand...
Thank you so much for your feedback, is there a specific 700R4 you would recommend?
 
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