Thread: Highway RPM question....

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A buddy of mine is building a 36 Ford two door sedan with a 350 Chevy, 700R4 and an El Camino rear end with 2.41 gears. According to the RPM calculator, with 29" tires on the rear, it will be running 1385 rpms at 70mph. He asked me if that's too low. Will it run there or be a total dog? Will it get decent gas mileage at 1385rpms or will it dog down and keep down shifting out of overdrive?
I don't know much about the Chevy setup so I'd appreciate your thoughts.
Thanks.

At 70 mph with 2.41 rears he would be running about 1800 rpm and with 3.08 rears he would be around 2300 rpm if he is running a 1 to 1 final in the trans.

If it were me I would rather be around the 2300 rpm. Would not dog as bad when you stepped on the loud pedal.

My youngest son had a 81 Vette that ran around 1800 at 70 mph. It was fine but a Vette is lighter.

Originally Posted by Bug

At 70 mph with 2.41 rears he would be running about 1800 rpm and with 3.08 rears he would be around 2300 rpm if he is running a 1 to 1 final in the trans.

If it were me I would rather be around the 2300 rpm. Would not dog as bad when you stepped on the loud pedal.

My youngest son had a 81 Vette that ran around 1800 at 70 mph. It was fine but a Vette is lighter.

Thanks for the response Bug, but the 700R4 is an overdrive trans so the final drive is somewhere in the .68-.73 range I think. Thus, the lower 1385rpm at 70mph. I agree that it will probably dog quite a bit at that low rpm. He's trying to avoid messing with the rear end.

I read where an 81 Corvette can weigh in at around 3300 pounds. Usually, these old Fords are less than 3K, so 1800rpms at 70 would probably work great.

With those 2:41 gears he is all set up for Bonneville. With that OD transmission something in the 3:42-3:55 range would be ideal IMO.

Don

Originally Posted by Itoldyouso

With those 2:41 gears he is all set up for Bonneville. With that OD transmission something in the 3:42-3:55 range would be ideal IMO.

Don

Don, I totally agree with you. I'm hoping someone will give some details as to why this combination would not be good, perhaps some specs or something that backs up personal opinion with actual examples. If this engine is running at 1300+ rpms at 70mph in overdrive, will it continually want to downshift to 3rd because the rpm's are too low?

Not sure about the shifting up and down, but I think those low rpms might be lugging the engine somewhat. He will be running not only the overdrive ratio, but pretty tall tires and a less than aerodynamic body style. It just might overwork the engine pushing a 70 mph wind at those rpms. An engine should come into the range where it is running most efficiently, where the cam is starting to work . If he puts any kind of cam in that combo at all he will be operating below it's intended range.

None of the above is scientific, just my gut feelings.

Don

the real problem is the El Cameo rear end---he really needs a Ranchero rear with around 3.50-3.90 gears---

And if he was to happen to use a F100-F150 rear, the bolt pattern for the wheels would be right

Originally Posted by jerry clayton

the real problem is the El Cameo rear end---he really needs a Ranchero rear with around 3.50-3.90 gears---

And if he was to happen to use a F100-F150 rear, the bolt pattern for the wheels would be right

I agree the El Camino rear end is the weak link. Perhaps he'll see the light once I present him with all these responses. The F100 rear wouldn't match the front wheel bolt pattern because he converted to a Mustang II front end.

Go with a taller tire...

Originally Posted by moter

Go with a taller tire...

????????? That would only make the problem worse.

Don

The other problem with that low an rpm is if he has any kind of a stall convertor at all there won't be enough torque to keep it locked up, it'll be slipping and the trans temp will rise. Bad juju.

Drive 100mph.... Problem solved

-Chris

I would also worry about stall speed and trans temp......the mid 90s camaro had a 350 with a six speed manual and would cruze on the highway at about 1200 rpms a 60 mph....but that is a different situation all together from his.

My opinion for what it's worth.

Drive much faster ..

Let me guess that he doesn't had a external tranny cooler either.Those 700's do not like heat.