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Cruise RPM with 218 and 3.91 final drive


Sam Buchanan

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While reading a manual for the industrial version of the 218 I ran across some graphs that showed 3600 rpm for rated horsepower and 2500 rpm for continuous operation. I haven't hooked up a tach on my '48 P15 so used the diameter of the L78 tires and ran the math (yes, there are online calculators...) to figure out the cruise speed at ~2500 rpm. Interestingly, the 55 mph self-imposed limit I've been using works out to 2548 rpm with the 3.91 rear end in my car. The throttle seems to be barely cracked at that speed and the car will readily accelerate if requested. But I think I'll stick with 55 mph which should keep the engine happy and contribute to the leisurely pace that seems well-suited for this vehicle.

 

Max rpm of 3600 worked out to 78 mph. In second gear, 30 mph is 2544 rpm and 3600 rpm is 43 mph.

 

How close is this to what y'all are finding as sweet spots for your Mopars with the same engine and final ratio? The car is a hoot to drive!  :)

 

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Edited by Sam Buchanan
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I have a 46 with 4:11 rear.  Ran it for years with 225 75 tires.  They calculated to 3280 rpm at 62 mph (65 indicated).  When I swapped in a freshly rebuilt engine, my machinist told me a cruising rpm of 80 to 85 percent of peak hours rpm was sustainable for extended cruising speed. Must be so, I have made dozens of trips of over 200 miles are those speeds over the past 14 years with no issues.  This does assume an engine in known good internal condition and operation within proper temp and oil pressure perameters.

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That's a good datapoint, thanks! With my 3.91, 80% of 3600 gets 2880 which is just a tic over 62 mph. Probably means I shouldn't get stressed if the ol' girl creeps up to 60 every once in awhile. The engine sure doesn't seem stressed, 55 oil pressure @ 170*F.   :)

Edited by Sam Buchanan
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2 hours ago, greg g said:

I have a 46 with 4:11 rear.  Ran it for years with 225 75 tires.  They calculated to 3280 rpm at 62 mph (65 indicated).  When I swapped in a freshly rebuilt engine, my machinist told me a cruising rpm of 80 to 85 percent of peak hours rpm was sustainable for extended cruising speed. Must be so, I have made dozens of trips of over 200 miles are those speeds over the past 14 years with no issues.  This does assume an engine in known good internal condition and operation within proper temp and oil pressure perameters.

I'm not familiar with the red term above?  What is its' meaning?  And value for our flatties.

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I think he meant “peak HP RPM” 

I agree with Greg. I’ve made many trips on the highway at sustained speeds that push my tach up around 3000-3500 without any issues. But there again, My engine is of ‘known condition’ as I rebuilt it myself over 10 years ago. Still running strong, including a 300 mile run today. 

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I use my '39 coupe with a mildly hopped up 218 and stock 3.91 rear end to commute 30 miles each way of freeway driving 3 days a week to work.  I also run a 170 degree thermostat, with average engine warmed up temp of 194.  She never heats up even when going over mountain passes in warm weather.  I cruise at 63 -67 mph and will go up to 70  - 72 to pass. Fastest i got her up to was 76, could probably could go more, however, with heavy traffic and my stock brakes it's not safe.  In 3 years I have put over 20k miles on this engine.  These things run strong and reliably if in good tune with bolt on hop ups.

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Edited by Polsonator2
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