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Reasonable RPM for long distance driving 361 cui Mopar
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1960desotoman
Posted 2021-01-08 11:38 AM (#607493)
Subject: Reasonable RPM for long distance driving 361 cui Mopar



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I have a 1960 DeSoto Fireflite with a bone stock Mopar 361 (DeSoto "TurboFlash"). It has 48000 miles on it and runs quite well. I am going to make a 2600 mile (round trip) road trip in it to the national DeSoto convention this year. I installed a tach, oil pressure, and water temp gauges to monitor the engine. At 2500 rpm, it runs 57 mph. At 57 it would take eternity to drive the distance. Based on the engine and its low mileage, can I push the rpm to 3000-3200? (oil pressure runs 55-58 psi at 2500 rpm, temp holds consistently at 190F) Any insight is greatly appreciated. --Ian
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ABloch
Posted 2021-01-08 12:23 PM (#607494 - in reply to #607493)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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At 110,000 original miles on my 361 I bomb around at 70-80 and it could care less.
I’m sure it could handle more but bias plys and worn suspension make it squirrelly.
3,200ish and just watch yer temperature, you’ll be fine! Have fun and take lots of pictures!
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finsruskw
Posted 2021-01-08 12:23 PM (#607495 - in reply to #607493)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar


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With the gearing in my F, I currently run around 2800 which puts me around 65 MPH
But my speedo sticks at about 60 so I have to push it over 70 to get it unstuck before it starts operating normally!
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57chizler
Posted 2021-01-08 12:34 PM (#607496 - in reply to #607493)
Subject: RE: Reasonable RPM for long distance driving 361 cui Mopar



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Welcome to the board Ian.

I have a self-imposed rpm limit of 3500 for cruising. Several of my cars have had 3.91 gears, so the engine sees that rpm often on the highway.
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LostDeere59
Posted 2021-01-08 2:05 PM (#607503 - in reply to #607496)
Subject: RE: Reasonable RPM for long distance driving 361 cui Mopar



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I had a 1960 Desoto w/the 361, and drove it on the highway fairly often.

On one trip to Carlisle (about 2 hours) my brother looked over and said "Are we really doing 75?"

"Yep. All day long."

"Cool."


Gregg
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1960desotoman
Posted 2021-01-08 3:01 PM (#607504 - in reply to #607494)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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Thanks! I've got radial whitewalls so I'm not worried about the speed. I've gone 70-80 before for short pulls just to see how it would do. The temp is remarkably stable for a stock radiator with a lot if dirt build up over the years. I've considered changing out the rear end but I've had difficulty finding anything that will fit a DeSoto.
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b5rt
Posted 2021-01-08 3:11 PM (#607505 - in reply to #607493)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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That sounds like a lot of RPM for a standard gear ratio. I'm not running a tach in mine but a previous 60 Fireflite would cruise 60 MPH very comfortably, at maybe 2200. Do you know what gear you're running? Is there a way you can double check your tach? I purchased a brand new Autometer and it was off, at higher RPM by 900!!! It explained my valve float issue.

Hope to see you at the Nationals!
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1960desotoman
Posted 2021-01-08 8:04 PM (#607518 - in reply to #607493)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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I'm not sure what the gear ratio is. As far as I know, it's stock. I know very little about the car before I bought it except that it's a numbers matching engine and transmission and it has been repainted. It is possible the tach is inaccurate. I'll look into that. I suppose I could hook up the tach to my other car which has a factory installed tach which is very accurate and compare the readings. My speedometer is not very accurate, so I've been running a GPS based one off an app on my phone. I assume it's accurate.

I'll hope to be at nationals!
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58coupe
Posted 2021-01-08 8:22 PM (#607519 - in reply to #607493)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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You should not have any trouble finding a differential for your Desoto. Any 57-61 Chrysler and Desoto, and any V-8 Plymouth or Dodge is a direct replacement. The 62 and newer removable diffs will also work but you might have to shorten your axles 1/8" The ratio of the one you have in your car is stamped on a flat spot on the bottom of the diff.

Edited by 58coupe 2021-01-08 8:24 PM
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1960desotoman
Posted 2021-01-08 8:29 PM (#607520 - in reply to #607493)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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I'll look into that. The rear end has built up some dirt over the years and I haven't had reason to clean it yet. There's my reason. As far as the tach situation- I just checked my tach against the only other tach I have -a 50 year old tach/dwell meter- and while it shows 1000 rpm at idle my dash mounted tach shows 500-600. This may be part of the problem.
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1960desotoman
Posted 2021-01-08 9:06 PM (#607521 - in reply to #607493)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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According to automobile catalogue ( https://www.automobile-catalog.com/auta_details1.php ) at 60 mph a stock DeSoto with a 2 speed auto should be turning about 2400 rpm and about 75 mph at 3000 rpm. That puts my Desoto over and b5rt's under. Odd. My theory is that ignition coil tachometers are generally not very accurate.
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22mafeja
Posted 2021-01-09 2:58 AM (#607534 - in reply to #607520)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar


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If the idle is really low but it doesn`t stall engaging a gear it is about 500rpm....if it is disturbingly high and kicks when you engage a gear it is about 1000rpm
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Powerflite
Posted 2021-01-09 11:20 AM (#607545 - in reply to #607493)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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Since you have plenty of time before you need to go, I highly recommend you swap the gears to 2.76:1. I run 2.76 gears in all of my highway cruisers and it is a real pleasure. I drive my '58 Coronet, '58 Firedome and '56 Savoy at 75mph and enjoy it. The only issue is the axles are longer on the old rears. You can shorten your axles by 1/8" to make it work, but then your old gears would no longer work. If you want the old gears to work too, you will need to swap out the center pin in the differential for one that is 1/4" shorter, or shorten it before you put it back in. Or shorten the axles & install a longer pin in your old gears to make your rear compatible with all newer gear sets. Just depends on what you want. A set of 2.76 gears are really cheap, you should be able to pick up a set for around $100. I have a couple of them, but you would be better off getting them local to you from craigslist.

Don't be like this '57 Chevy, and be stuck at low speed because of your gear ratio!
https://www.youtube.com/watch?v=JJBjEriATwY

My DeSoto keeping up with traffic at about 75mph.
https://www.youtube.com/watch?v=B5GrfgJcS7w

My Speedometer is a little slow because of the high gears. It says 70mph here, but is actually around 78mph.
https://www.youtube.com/watch?v=aMOVAXlViLo

Edited by Powerflite 2021-01-09 11:34 AM
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1960desotoman
Posted 2021-01-09 11:26 AM (#607548 - in reply to #607493)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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Thanks for the suggestions! I'll look into it. I found several rear end gear sets on Rock Auto for a 1960 Chrysler 300, but as of yet I haven't found a gear ratio that low. I'll keep my eye out for one
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57chizler
Posted 2021-01-09 2:23 PM (#607554 - in reply to #607521)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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1960desotoman - 2021-01-08 6:06 PM

According to automobile catalogue ( https://www.automobile-catalog.com/auta_details1.php ) at 60 mph a stock DeSoto with a 2 speed auto should be turning about 2400 rpm and about 75 mph at 3000 rpm. That puts my Desoto over and b5rt's under. Odd. My theory is that ignition coil tachometers are generally not very accurate.


To make sense of the numbers you have to know your axle ratio and the tire diameter. The numbers you posted in the OP are correct for 3.31 gears and a tire diameter of 27" both of which your car could have.
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1960desotoman
Posted 2021-01-09 2:30 PM (#607556 - in reply to #607493)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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The tires are 27", so that leaves the rear end ratio. I found one on eBay that I'll prob install that should bring the rpms down regardless.
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b5rt
Posted 2021-01-09 3:18 PM (#607558 - in reply to #607493)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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I'm hoping Sid joins the conversation as he's known for high speed cruising in Germany.

I wasn't thinking of a 2 speed transmission as all mine have been 3 speeds. Perhaps it was standard for a 2 speed to have a higher gear?

Edited by b5rt 2021-01-09 3:20 PM
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1960fury
Posted 2021-01-09 5:14 PM (#607566 - in reply to #607558)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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b5rt - 2021-01-09 3:18 PM

I'm hoping Sid joins the conversation as he's known for high speed cruising in Germany.

I wasn't thinking of a 2 speed transmission as all mine have been 3 speeds. Perhaps it was standard for a 2 speed to have a higher gear?


Fortunately my car has the standard 2:93 gears. I do not know how other people can life with numerical higher gears. I'd rather go below 2:93s.

My favorite cruising speed is around 70 mph, something seems right at that speed and the car seems to drive by itself, creates a pleasant humming sound, with VERY little throttle. Of course the car can go much much faster but I do not feel well driving a prolonged time above 80. My high speed driving were always only short bursts and it goes to 150+ in a heartbeat, but I rarely do that without a purpose The 60 Plymouth speedo starts again at 121mph but it is not visible, unless you add (paint, black) increments to the red background, but at 135 it stops.
On the Autobahn driving 70 mph you are a slow poke, sharing the truck lane. While I love the T-flites, I wish they had 4 or 5 gears with a longer rear or an overdrive. I think this is the most missed item for me in the FL world.

Edited by 1960fury 2021-01-10 9:44 AM
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56D500boy
Posted 2021-01-09 5:20 PM (#607567 - in reply to #607558)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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b5rt - 2021-01-09 12:18 PM
I wasn't thinking of a 2 speed transmission as all mine have been 3 speeds. Perhaps it was standard for a 2 speed to have a higher gear?


Doesn't matter, Powerflite or Torqueflite, when you are driving around in "D" and it is engaged, the ratio is 1:1 regardless of which transmission. No underdrive. No overdrive.

Engine RPM is therefore down to the rear axle ratio and the tire diameter.

For example with a 3.54:1 rear end and 235/75/15 tires, at 2500 RPM, my car should be travelling approximately 60.6 mph

At 3000 RPM, the speed should be about 72.8 mph.

According to my math.



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1960fury
Posted 2021-01-09 5:29 PM (#607568 - in reply to #607567)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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56D500boy - 2021-01-09 5:20 PM

b5rt - 2021-01-09 12:18 PM
I wasn't thinking of a 2 speed transmission as all mine have been 3 speeds. Perhaps it was standard for a 2 speed to have a higher gear?


Doesn't matter, Powerflite or Torqueflite, when you are driving around in "D" and it is engaged, the ratio is 1:1 regardless of which transmission. No underdrive. No overdrive.

Engine RPM is therefore down to the rear axle ratio and the tire diameter.

For example with a 3.54:1 rear end and 235/75/15 tires, at 2500 RPM, my car should be travelling approximately 60.6 mph

At 3000 RPM, the speed should be about 72.8 mph.

According to my math.







Well, no. The Powerflites mostly came with shorter rear axles. T-flites were almost always 2:93s, others optional.

Edited by 1960fury 2021-01-09 5:31 PM
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1960desotoman
Posted 2021-01-09 6:14 PM (#607571 - in reply to #607558)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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b5rt - 2021-01-09 1:18 PM I'm hoping Sid joins the conversation as he's known for high speed cruising in Germany. I wasn't thinking of a 2 speed transmission as all mine have been 3 speeds. Perhaps it was standard for a 2 speed to have a higher gear?

 

Yeah, my fireflite is basically the cheapest fireflite configuration possible, so it has the two speed

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56D500boy
Posted 2021-01-09 6:44 PM (#607572 - in reply to #607568)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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1960fury - 2021-01-09 2:29 PM
Well, no. The Powerflites mostly came with shorter rear axles. T-flites were almost always 2:93s, others optional.


*IF* that was the case, with a 2.93:1 rear diff, then, with my tire size example, 2500 RPM would be about 73.3 mph.
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1960desotoman
Posted 2021-01-09 6:49 PM (#607573 - in reply to #607493)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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I believe the powerflite/rear end ratio combination is different between Chrysler/Dodge/Plymouth/DeSoto because I just checked mine and its a 3.31 which, according to automobile catalogue, is stock. It seems each person on this discussion has a different idea of what the stock ratio is.
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56D500boy
Posted 2021-01-09 6:55 PM (#607574 - in reply to #607573)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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3.31:1, 235/75/15, 2800 RPM = 72.6 mph (about)

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1960fury
Posted 2021-01-09 7:28 PM (#607575 - in reply to #607573)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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1960desotoman - 2021-01-09 6:49 PM

I believe the powerflite/rear end ratio combination is different between Chrysler/Dodge/Plymouth/DeSoto because I just checked mine and its a 3.31 which, according to automobile catalogue, is stock. It seems each person on this discussion has a different idea of what the stock ratio is.


What I said (we are talking 60 Desoto) Torqueflite 2:93 and Powerflite shorter, that would be 3:31. Not sure about Canadian models, but that was true for domestic 60 Plymouth, Desoto and likely Dodge and 1959/1961 too.
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Stroller
Posted 2021-01-14 12:56 PM (#607710 - in reply to #607493)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar


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Well just remember when these cars were born the speed limits were not 55 mph. But looking and thinking back yep over heating seems to be an issue. To that if it starts to warm up, put the heater on high and roll the windows down.
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57chizler
Posted 2021-01-14 1:36 PM (#607714 - in reply to #607573)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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The Powerflite 1st gear ratio is 1.71-1, the Torqueflite is 2.45-1 so the Powerflite needs a higher numeric axle ratio to help it get rolling from a stop.
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Shep
Posted 2021-01-14 2:08 PM (#607715 - in reply to #607714)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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And a looser convertor.
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60 dart
Posted 2021-01-14 4:45 PM (#607725 - in reply to #607493)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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my thoughts are that if you are going to worry about fuel consumption in a big block mopar ,,,,,,,,,,,, just stay home --------------------------------------------------later
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1960fury
Posted 2021-01-14 5:23 PM (#607728 - in reply to #607710)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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Stroller - 2021-01-14 12:56 PM

Well just remember when these cars were born the speed limits were not 55 mph. But looking and thinking back yep over heating seems to be an issue. To that if it starts to warm up, put the heater on high and roll the windows down.


Up to 1955 at least, some states had no speed limit at all and nobody cared anyway. Just saw the film "Its a mad, mad, mad world", you can see the cars in the background zooming over a highway at the speed of light. I'd almost say, the average speed in the late 50s and early 60s was faster than today.
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Powerflite
Posted 2021-01-14 5:41 PM (#607729 - in reply to #607725)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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60 dart - 2021-01-14 1:45 PM

my thoughts are that if you are going to worry about fuel consumption in a big block mopar ,,,,,,,,,,,, just stay home --------------------------------------------------later


It's not just about fuel consumption. It's also about wear on your motor and unnecessary, grating noise in the cabin. If you want to cruise at high speed for more than an hour, you really should get a set of higher gears - especially if you have any intention of talking to your wife or kids at all during that trip. What did you say?! Hold on, I'll brake down to 50mph so I can hear you.... Most people who resist changing gears drive slow, like that '57 Chevy in my video. Over a long drive, the noise and pain isn't worth it because you can only take so much of it. Wind noise is another problem, but can be solved if you can get new window seals.
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1960desotoman
Posted 2021-01-14 8:02 PM (#607735 - in reply to #607729)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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Powerflite - 2021-01-14 3:41 PM
60 dart - 2021-01-14 1:45 PM my thoughts are that if you are going to worry about fuel consumption in a big block mopar ,,,,,,,,,,,, just stay home --------------------------------------------------later
It's not just about fuel consumption. It's also about wear on your motor and unnecessary, grating noise in the cabin. If you want to cruise at high speed for more than an hour, you really should get a set of higher gears - especially if you have any intention of talking to your wife or kids at all during that trip. What did you say?! Hold on, I'll brake down to 50mph so I can hear you.... Most people who resist changing gears drive slow, like that '57 Chevy in my video. Over a long drive, the noise and pain isn't worth it because you can only take so much of it. Wind noise is another problem, but can be solved if you can get new window seals.

 

Just got a 2.94 rear end in the mail, so I'll be good to go for high speed cruising.

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60 dart
Posted 2021-01-15 12:06 AM (#607738 - in reply to #607493)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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so 57 chevrolet ? grew up with 3 of the them . one , in fall 1968 was driven from pa to la and back , a 235 standard shift , anywhere from 65 to 85 mph . only one real problem
was noise was from the wing windows . even the straight exhaust was no problem with wild sounds . oh and lost a nut off a gen. bolt ------------------------------------------------later
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57chizler
Posted 2021-01-15 12:11 PM (#607747 - in reply to #607735)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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1960desotoman - 2021-01-14 5:02 PM
Just got a 2.94 rear end in the mail, so I'll be good to go for high speed cruising.


Take note of what I said about axle shaft lengths in the other thread and be aware you'll need to change your speedo drive pinion.
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Powerflite
Posted 2021-01-15 12:23 PM (#607749 - in reply to #607493)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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Chizler, he purchased a whole new rear end so the axle lengths are no longer an issue.
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finsruskw
Posted 2021-01-15 3:49 PM (#607757 - in reply to #607493)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar


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How about a detailed trip/performance report when you get back?

You know, miles and mileage, speeds, fuel stops, gallons etc.
Should make for some interesting reading.
I usually do this for long trips regardless what the vehicle is that I'm driving.
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1960desotoman
Posted 2021-01-15 3:52 PM (#607758 - in reply to #607757)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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finsruskw - 2021-01-15 1:49 PM How about a detailed trip/performance report when you get back? You know, miles and mileage, speeds, fuel stops, gallons etc. Should make for some interesting reading. I usually do this for long trips regardless what the vehicle is that I'm driving.

Will do

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jboymechanic
Posted 2021-01-15 4:13 PM (#607760 - in reply to #607493)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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I never knew they ever mated a PF to a BB, I would have thought for sure that having a BB automatically got you a TF trans. Personally, I cannot stand driving a PF car for the reasons discussed above. With the tall first gear you have to have a shorter rear gear ratio so you lose on both ends really. For cruising the OP will be much happier with the 2.94 gears but he's certainly going to lose some of the off-the-line pull. However, the 361 should make it more tolerable as it has plenty of torque (especially compared to a 318 or a six cylinder).
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ronbo97
Posted 2021-01-15 7:51 PM (#607766 - in reply to #607493)
Subject: RE: Reasonable RPM for long distance driving 361 cui Mopar


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1960desotoman - 2021-01-08 11:38 AM I have a 1960 DeSoto Fireflite with a bone stock Mopar 361 (DeSoto "TurboFlash"). It has 48000 miles on it and runs quite well. I am going to make a 2600 mile (round trip) road trip in it to the national DeSoto convention this year. I installed a tach, oil pressure, and water temp gauges to monitor the engine. At 2500 rpm, it runs 57 mph. At 57 it would take eternity to drive the distance. Based on the engine and its low mileage, can I push the rpm to 3000-3200? (oil pressure runs 55-58 psi at 2500 rpm, temp holds consistently at 190F) Any insight is greatly appreciated. --Ian

You are worried about something that you shouldn't be worried about. Cars are designed to be driven under a wide variety of conditions. Do you really think Chrysler Corp would sell a car that would fall apart when you drive it at 70-75 mph ? Also you have a bone stock car. Don't start messing with it. You'll be going down a rabbit hole that may include a different driveshaft, pinion seal replacement and who knows what else. You're trying to fix a problem that doesn't exist.

"If it ain't broke, don't fix it"

Ron

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Powerflite
Posted 2021-01-15 8:03 PM (#607767 - in reply to #607493)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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I've gone down that rabbit hole with every car I've purchased!
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57chizler
Posted 2021-01-16 2:17 PM (#607779 - in reply to #607766)
Subject: RE: Reasonable RPM for long distance driving 361 cui Mopar



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ronbo97 - 2021-01-15 4:51 PM
You are worried about something that you shouldn't be worried about. Cars are designed to be driven under a wide variety of conditions. Do you really think Chrysler Corp would sell a car that would fall apart when you drive it at 70-75 mph ?


I agree. Lots of cars from that era came with low axle ratios and owners cruised at the speed they wanted to. Only when tachometers became more common did drivers become cognizant of the rpm they were turning.
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hemidenis
Posted 2021-01-16 7:45 PM (#607792 - in reply to #607493)
Subject: Re: Reasonable RPM for long distance driving 361 cui Mopar



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these are the instant values taken with a GPS and a digital tachometer.

The tires are the original 9.00x14
Dif 2.93 to 1.

engine 413



(speed.jpg)



Attachments
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Attachments speed.jpg (13KB - 7 downloads)
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