Higher top end + lower revs?
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Higher top end + lower revs?
Just bought a 78 F250 4x4 which I am restoring.
The truck still has the standard straight 6 motor and 4 speed manual gearbox.
At the moment when it gets to about 60 mph the revs are pretty high.
I am considering fitting a Cleveland 351 in there , but what do I do to get the revs down a bit and raise the top end. I would like it to be able to do 80 mph without the revs running at nearly max. Do I need to fit another gearbox or do I need to modify diff ratios?
Thanks
The truck still has the standard straight 6 motor and 4 speed manual gearbox.
At the moment when it gets to about 60 mph the revs are pretty high.
I am considering fitting a Cleveland 351 in there , but what do I do to get the revs down a bit and raise the top end. I would like it to be able to do 80 mph without the revs running at nearly max. Do I need to fit another gearbox or do I need to modify diff ratios?
Thanks
FTE Legend
Joined: Jun 2006
Posts: 31,930
Likes: 1,499
From: Ottawa, Ontario
Define "pretty high"?
You could do either or both but much depends what axle ratio you have now. I'd definitely suggest changing the trans to a newer Zf 5-speed to gain the OD ratio, that will even help the I6 cope better.
You could do either or both but much depends what axle ratio you have now. I'd definitely suggest changing the trans to a newer Zf 5-speed to gain the OD ratio, that will even help the I6 cope better.
Thread Starter
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New User
Joined: Feb 2015
Posts: 5
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Just bought a 78 F250 4x4 which I am restoring.
The truck still has the standard straight 6 motor and 4 speed manual gearbox.
At the moment when it gets to about 60 mph the revs are pretty high.
I am considering fitting a Cleveland 351 in there , but what do I do to get the revs down a bit and raise the top end. I would like it to be able to do 80 mph without the revs running at nearly max. Do I need to fit another gearbox or do I need to modify diff ratios?
Thanks
The truck still has the standard straight 6 motor and 4 speed manual gearbox.
At the moment when it gets to about 60 mph the revs are pretty high.
I am considering fitting a Cleveland 351 in there , but what do I do to get the revs down a bit and raise the top end. I would like it to be able to do 80 mph without the revs running at nearly max. Do I need to fit another gearbox or do I need to modify diff ratios?
Thanks
Its hard to say exactly but it runs high revs at 60 mph.
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New User
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I will start looking for a 5 speed transmission then. They are hard to find in South Africa though.
FTE Legend
Joined: Jun 2006
Posts: 31,930
Likes: 1,499
From: Ottawa, Ontario
Do you know what axle gearing the truck has? I'd suggest changing the gearing but it can be an expensive job with a 4wd since you have to change both axles. Installing a tech would be a good idea even if it's just temporary so you know what actual rpms are.
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This site has already helped me immensely with part numbers and other info. Just by checking the FAQ section. Thanks again.
I still have mullions of questions but I will cross those bridges as we get to them.
I still have mullions of questions but I will cross those bridges as we get to them.
Cranky Old Guy
Joined: Mar 2008
Posts: 3,562
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From: Raphine, Virginia
Don't forget to factor in the tire size you plan on using. I still like using: Gear Ratio Calculator to compare different configurations.
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New User
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Don't forget to factor in the tire size you plan on using. I still like using: Gear Ratio Calculator to compare different configurations.
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