1275 cruising RPM

[drooartz]

My Morris Minor has a 1275 and Datsun 5-speed, with a 4.22 rear end and 14" wheels (same size wheels and tires as a MGB). This gives me a cruising RPM of about 3500 at 70mph, never been an issue and the engine seems very smooth at that speed.

That said, I happen to have a rebuilt 3.90 sitting on a shelf (leftover from my Bugeye project), and have been wondering about swapping it in. What I don't know is what is generally considered to be a safe/reasonable cruising RPM for one of these engines?

The car runs well and I'm not inclined to mess with it, but on the other hand freeway speeds are a bit of a necessity around here to get to some events and drives so being able to run at 70 without strain is nice.

Mostly just thinking out loud and wondering what the collected experience thinks.

 

[CARSINC]

Put the 3.90 in and cruise. There are worse things to fret about.

 

[Trevor Jessie]

My bugeye is turning 3500 rpm at 70 mph. I cruise 75-80 mph for hours on end. I do not think you should switch to a 3.9 given the weight and aerodynamics of the car.

 

[Pythias]

I seem to recall that these engines will cruise all day at 4500 to 5000 rpm. With 13' tires, a 4.22 and stock 1/1 tranny in 4th, it's 15 mph/1000 rpm or 4k rpm = 60 mph. They will most definitely do 60 ALL DAY LONG--4k rpm. Again I recall someone (Frank C) saying that that would do 75 all day which would be 5k rpm.

With 13" tires and a Datsun 5 speed and a 3.9, it's 20 mph per 1000 rpm = 3k for 60 mph. .. with 14" tires ????

The ABFMs that I attend(ed) were 50 miles, 120 miles and 270 miles from home, up and down I-5 so I set up the red thing as a freeway cruiser. Datsun 5 speed, 3.9, 175/70-13s. I tend to drive between 60 and 65 mph, but can step it up to 75 easily if I need to pass or keep up. In fact that car seems to smooth out a little at over 65 mph. .. Also using this combo I get over 35 mpg (Weber DGV Carb) on the freeway.

 

[Boink]

With my Datsun 5 speed, I felt the 3.90 diff made a HUGE difference.

 

[drooartz]

With 13" tires and a Datsun 5 speed and a 3.9, it's 20 mph per 1000 rpm = 3k for 60 mph. .. with 14" tires ????

I'm at 20mph per 1000rpm with the 4.22, Datsun 5sp, and the larger 14" wheel/tire (175/70R14), so very similar to the 3.90/13"/5-speed.

Thanks all for the thoughts. This really wasn't a concern, just a curiosity since I happen to have the 3.90 on the shelf. Given that the Morris is heavier/bigger and I live in the mountains at 6500', I'll probably just leave well enough alone.

I will say, the last long trip I took (Salt Lake City to Monticello, UT) I got 36mph at about 60mph on the way down with a group of cars, and 30mpg on the way home by myself running a fairly consistent 70mph. That's better than my modern Subaru.