Gear Ratios
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Gear Ratios
I'm new here and am struggling with the fuel mileage and loss of power in my 1981 F100 since swapping the worn out 1981 engine to a 1990ish FI engine using the old manifolds. The old engine made all of it's torque at 1600 RPMs and the newer one makes it at 2000 RPMs leaving the engine struggling with the 2.47 factory gears. I'm thinking 300 or 325 gears but they are more money than 350's. Maybe a trip to UPULLIT is in order. I do have a 69 429 with a C6 in my garage that would love those gears but that seemed like a lot of work. :/
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From: Virginia
What transmission are you using? A lot of those trucks used a overdrive 4 speed. If that is what you have, I would install the common 3.50 gears, the overdrive will drop it down nicely for interstate use. If you have a 3 speed automatic, you could still use the 3.50's if you only use the truck around town and no long trips.
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Transmission
What transmission are you using? A lot of those trucks used a overdrive 4 speed. If that is what you have, I would install the common 3.50 gears, the overdrive will drop it down nicely for interstate use. If you have a 3 speed automatic, you could still use the 3.50's if you only use the truck around town and no long trips.
I’m sorry, it’s a c4. I do drive on the hi way quite a bit.
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From: Angier, NC
Did some one swap that C4 into it as I did not think they came with the trany.
BTW my 81 F100 has a 4sp with the granny low andmy parts truck, also a 81 F100 had the 3sp with 4th being an OD.
I cant see that EFI motor being down on power over the old motor.
Have you gone over timing and curve?
Dave ----
BTW my 81 F100 has a 4sp with the granny low andmy parts truck, also a 81 F100 had the 3sp with 4th being an OD.
I cant see that EFI motor being down on power over the old motor.
Have you gone over timing and curve?
Dave ----
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I used the distributor from the 81 engine and timed it at 12 degree BTDC. The VIN transmission code says it's a C4 and a tranny filter kit for a C4 fits. It starts and runs great, just a bit of a pooch 
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well this old dog just learned something new. A lot of strange things with the early F100 trucks.
I take it the carb also being the motor was an EFI motor.
I did not know the early F100's came with C4's. Then again the first gen Bronco's had C4 autos with 302 motors just wonder why Ford then went to C6 with the 300 six motor.
I know you said it is a dog till the RPM is about 2000. Can you repost the rear gear ratio and the size tires you are running and at what speed the 2000 RPM happens.
There is a post about MPG and RPM and if I remember right the sweet spot RPM was around 1800.
Dave ----
I did not know the early F100's came with C4's. Then again the first gen Bronco's had C4 autos with 302 motors just wonder why Ford then went to C6 with the 300 six motor.
I know you said it is a dog till the RPM is about 2000. Can you repost the rear gear ratio and the size tires you are running and at what speed the 2000 RPM happens.
There is a post about MPG and RPM and if I remember right the sweet spot RPM was around 1800.
Dave ----
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Thanks for all the replies! The gear ratio according to the VIN is 2.47-1 and tire size is 235-75r-15 28.9 inches tall which makes the rpms about 1770 at 60 mph, it just feels like it has no power even tho it runs fine. I'm going to take a look at the timing again and see if there's something going on there. This engine is supposed to have more HP and torque than the old engine and it doesn't seem to.
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The F100 could have had either the C4 or C6 depending on configuration, but that model was dropped around 1983 or someplace thereabouts.
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From: Virginia
Thanks for all the replies! The gear ratio according to the VIN is 2.47-1 and tire size is 235-75r-15 28.9 inches tall which makes the rpms about 1770 at 60 mph, it just feels like it has no power even tho it runs fine. I'm going to take a look at the timing again and see if there's something going on there. This engine is supposed to have more HP and torque than the old engine and it doesn't seem to.
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Engine_Specifications
But, OP swapped the old manifolds, so "poof" on all or most of the extra OEM power.....
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From: Angier, NC
I can also see where EFI would also give the motor more power.
Wonder if anyone has taken a EFI motor and did testing with EFI and then a carb?
A 2.47 rear gear is not known for get up and go gear. Just be thankful you don't have over drive trany like my parts truck did.
Dave ----
Last edited by FuzzFace2; Jul 16, 2019 at 06:53 PM. Reason: to put the right 2.47 ratio in post
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From: Angier, NC
yea I am wrong and will change it in my postDave ----