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

He either needs shorter tires or more gear as noted. Even my diesel truck lugs at 1300 rpm and it is nearly at full torque 600 ft lbs. I'd shoot for about 1700-1800 rpm at cruise speed. This is a problem with OD's. If you have a vac gage it will tell the story very quickly. Low vac = lugging.

Everything I see says the 700R4 has a 0.70 OD ratio but some may have been different. Like Don says, he's got Bonneville gears now and he is going to be lugging at highway speeds and his mileage will suffer drastically because of that. Like bentwings says, he should shoot for something in the upper teens range for his cruise point, IMO. I assume his torque curve for the 350 is going to come up at about 2100 and be fairly flat through the normal rpm range?

Thanks for all the responses, guys!! I appreciate it! Will pass the info along and try to save him some disappointment. If he insists on using that rear end, he'd be better off without the overdrive transmission.
Thanks again for your input!

Smart move, all you can do is give him your best advice and let him make the final decisions. In something that weighs as much as his 36 I think he would be very disappointed with the performance off the line with those gears, and while 350 Chevies are great motors, they aren't exactly torque monsters.

In the old days we called rear ends with high ratio gears "Western" rear ends. The reason was that in the flat States you didn't need a lot of hill climbing torque, like you would back East, so the car manufacturers back then sold one ratio to the dealers in those flat States, and another, deeper ratio in the States with hills and mountains. A friend had a 50 Ford that came with a Western set of gears and overdrive 3 speed to boot. It was really dead on the hills, but once you got it going on the flat sections it would wind forever. I don't know if the auto makers still do that or not.

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

Go with a taller tire...

Originally Posted by moter

Go with a taller tire...

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

Don

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.

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.