What RPM's are acceptable at 70 MPH?

I have a 454 (.030) in my 58 Caddy. It has an RV cam and a Mighty Demon 650 carb. The heads are stock. The tranny is a 700R4. I'm at 2100 RPM's at 70 MPH. Is that considered high for that set up? I want to take advantage of the monster torque motor and change the rear gearing so I'll be in the 1500-1700 RPM range at 70. My wheels are stock size and the rear end is stock. So what RPM's are you guys turning at 70?

 

I think thats great.........a little low even.

 

unless you want to save more gas, I would leave it alone,

 

I can't think of a reason to be lower than you are!

 

In my '55 Chevy I've got 700r4, 3.08's, 235-60-15's and that's about what I turn.

 

Old rule of thumb for hot rod gearing is 20 mph per thousand RPM. At 70 most of my junk would be turning 3,500 RPM

 

Maybe Hoop can locate a Volumetric Efficiency curve, as He comes up with this sort of data, my best guess is you are just entering your better efficiency range @ 2100.

 

The Tremec in my 62 Vette has me turning 1900 at 75 mph.

BB

 

Economy is sort of what I had in mind. I know it's a BB and my options are limited. I was thinking I want the motor to be purring along like it does at 60. I didn't think to investigate what RPM is the most "efficient".

 

I personally limit RPM at 70MPH to 3000.

2100 is plenty low. I wouldn't go any lower.

Then again my Lexus at 80MPH is turning 1800....

 

Yep, that was the way we always set them up before all this overdrive crap came along. And many cars came from the factory pretty close to that as well. now everyone is spoiled and forgot (or never knew) the way it was.

 

Well you never said WTF your rear ratio is, but 2100 sounds fine.

 

I don't know what the rear end ratio is. Maybe someone could chime in. Its a stock 58 cadillac rear end. I really appreciate everyone's input. I was just thinking of taking advantage of the torque this beast makes. Maybe I should just shut up and drive. Lol.

 

Any less RPM at 70, I'd bet it would be a dog at lower speeds (MPH). If you stomp on it at 70 and dive into a lower gear, I'll bet it puts a childish grin on your face that you can't control. And this just in: 8600lb Duramax @70MPH, 2100RPM. Stomp it into passing gear and I grin too

 

You've seen me driving around town haven't you.

 

1750 would get you about 1.5-2 MPG

Losing about 5HP to friction maybe 1 to pumping.

 

From an old automotive performance book.
----------------------------------------------------
1. Gear your car to run at it's engines peak torque RPM at your desired cruising speed for a good combination of throttle response and economy.

2. Gear your car to cross the finish line at it's engines peak horsepower RPM + 5-10% for drag racing.

3. Split the difference between 1 & 2 for a street/strip car.

--- Steve ---

 

I have my Plymouth set with a Tremec TKO and a 3.23 geared Chrysler 8 3/4" rear end. 75 comes up at 2175 or so. Damned comfortable for loafing about town, and even with an aluminum headed, humpity 360 at about 420 HP, turns back about 20 mpg on the many long road runs I have done with it. I have on several occasions thought about bumping the rear gears up to more in the 3.55-3.73 range, though. The reason behind this is that it is soooo happy in the 2450-2550 range that I find myself running that RPM range allot on long trips. Unfortunately, that's right at 100.... Great for the western Arizona Autobahn chingo, just not so great where that sort of thing is a less tolerated kinda deal.

 

Sounds like a nice rpm level at 70 to me. Might be luggin that big block at lower rpm's. judt sayin. ~sololobo~

 

My setup runs 1750 at 69 mph and returns 20mpg. Unless you have more duration than will work at low rpm or your induction system has some problems, you have no reason not to lower the cruising rpm some. The operating range of the cam profile is a huge factor here.

 

Put in neutral. Jack up one rear wheel. Turn it two revolutions. count the revolutions (and fractions) of the driveshaft. That is your ratio.

i.e. 3 driveshaft revolutions is likely 3.08
3-1/2 is likely 3.43
just over 4 is likely 4.11

Look up available axles ratios for your car, that will get you close when counting the driveshaft revolutions.

 

Sounds like you have a nice economy setup. That's the main idea behind that 700R4.

 

3.07 was standard in a 58 Caddy.

Must have the optional 3.37

 

Hoop, you got a link to that calculator? I lost mine in a computer crash.

 

Enjoy!

http://wahiduddin.net/calc/calc_speed_rpm.htm

 

Put a vacuum gage in an go by what it says...Highest vacuum is where it would be best..

 

Thanks! My cars still running sweet and I check my timing more than Dennis Wilson checked jets.

 

she ain't breathing right!

 

I'm around 4000 at 70mph,so I'm the wrong guy to ask....lol


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That sounds pretty good, what you have! My '66 Dodge turns 2,900 RPM @ 80 mph with 3.23 gears and a 1:1 Third gear.