Rpm

[Tams50]

Hello, I have a stock 50 coronet with a gyromatic tranny. I have no tach.

 What is a safe road speed and what rpm would that be?

[OUTFXD]

I have a 46 with a manual trans and a flat six.  I have gotten her to do 60.  But I started getting nervous about how fast the engine was spinning.  The flat sixes top at a little under 3000RPM.  But they are massively over square so have tons of torque to make up for it.

[desoto1939]

First question what is the condition of the engine, trans, rear and the brakes and tires.

 

All of these componets need to be in good shape prior to thinking about any specific speed to travel down the highway.

 

I own a 39 Desoto witht he flat head stock 6 with 3 on the column and the 4:10 rear.  I have 25k on the rebu8ilt engine, new bias ply tires and have redone my brake system with the standard lockheed brakes.

 

I just drove my 39 to attend the National desoto car show out in Altoona, PA. I drove the car out to Altoona via the Pa turnpike at a average speed of 50-53 MPH. I di dnot have any issues with truckers and other cars and the spped limit is 70MPH for these types of vehicles.  I averaged 16-18 mpg.

 

Everyone was respectful of the old car.

 

So my best suggestion is to make sure the car is road worth and get to know your car and find the sweet spot where the cars like to be at when going down the highway or on busy roads.  Knowing your car is the most important factor not how fast it can go. You have to remember that you are dealing with a car that is 72 years old. Just think how your own body will be like at 72 years of age.  You have to think about what you are driving and anticipate for issues and the driving conditions.  So when you get inthe car to take it for a drive start the car and then sit there for a minute and remind your self about what vehicle you are driving and that you have to act more defensive with this antique versus a modern car.  I have owned my 39 Desoto for 35 years and i always remind myself of these fact upon taking it out on the road.  You have to respect the other drivers if you want them to respect your old vehcile.

 

Rich Hartung

Desoto1939@aol.com

[Tams50]

Thanks for the replies. The old girl was original with 82k on the clock .  After about 3k of touring she blew. Everything but the steering components (next job) are new. I just don't have a clue what she's spinning at 50 mph.

[Sniper]

Plenty of RPM calculators out there that will tell you if you know the tire size and rear gear ratio.

 

For example, my 51 has a P255/50-R17 rear tires, right at 27" tall.

 

3.90 rear gear

 

Engine speed at 70 mph is just under 3400 rpm.  I have had it up to 65, 3143 rpm.

[Doug&Deb]

I keep my 52 Coronet below 60 mph but I’ve had it up to 75 once or twice. I avoid highways whenever possible. I’m comfortable cruising around 45-50 on a two lane road. On the highway I stay at 55 and let people pass. I agree with Rich. Be aware of what you’re driving and be defensive.

[Loren]

In college I had a 49 Special Deluxe Club Coupe. My Dad said keep it to 55 mph and it will last forever. I used to drive it from San Diego to L. A. often and it just kept going.

My current 49 has a 3.73 rear axle and 205/75/15 tires all pretty standard out of the box.

At 55 it sounded a little busy so I got an old Garmin GPS to check the speeds. The speedo was reading 55 when the GPS said 60! So I had the speedo serviced and was confirmed to read 5 mph slow.

Along the way I installed an Overdrive transmission and that really quieted things down. I have read in these pages that the cruise speed should be about 80% of peak power rpm. The flathead sixes are listed at 3,600 rpm for peak power. So somewhere around 2,780 sounds about right.

I found a neat little tachometer on eBay that is wired for 6 volt positive ground so I have that now too.

Unfortunately the Coupe developed a coat of rust over the hood and front fenders that alarmed me enough to start a painting project, so I haven’t done much road testing lately. ( more on that in a separate post )

I think my car is capable of pulling the 3.54 gears I have on my shelf and I have some 235/75/15 tires I’d like to try as well.

With the Overdrive I think a 70 mph cruise speed is possible. Before the Overdrive I kept it to 55-60.

So that has been my process thus far.

[JonathanC]

I created a simple calculator for this in Google sheets based on available data about vintage tire sizes. This was reinventing the wheel to some extent but I like to do stuff like that). With the original size tires (7.10x15) and rear end (3.9:1), in direct top gear your Coronet would turn around 2160 rpm at 50 mph, 2590 rpm at 60 mph, and 3020 rpm at 70 mph. If the mechanicals are in good shape the car should have no problem sustaining that. Maximum power in these engines was generated around 3600 rpm, so you probably wouldn't want to have the engine run at sustained speeds near or above that peak. Whether driving conditions are safe to do those speeds for other reasons is a different matter of course.

 

My '41 Chrysler runs at 60-65 mph on the highway quite comfortably. It has a very slightly taller 3.54:1 rear end and 6.00x16 tires (which are about the same overall diameter as the 7.10x15s, however mine are Coker classic radials which I think are an asset for real-world driving). It does sound busier than a modern car that is revving well below 2000 rpm at that speed of course but that doesn't mean it's overstressed. Some of our highways have a 110 km/h speed limit (about 68 mph), that is probably about as fast as I'd want to drive for any length of time under favourable traffic and road conditions.

 

Chrysler engines were well built and engineered with quality features like full-pressure lubrication, four rings per piston, and superfinished surfaces, which is what makes them good for use today in my opinion. It's the cars that aren't as well-engineered and run on shorter rear gears (partial splash lubrication and rear ends in the 4.11-4.56:1 range were not uncommon back then) that would make me worry about sustained speeds of 60+.

Edited August 24, 2022 by JonathanC

[Adam H P15 D30]

For those that want to try more a drastic change, I installed a 3.0 rear axle with 26" tires.  I consistently run 70MPH which is about 2800 RPM.  Slow speed performance is still acceptable for a lighter vehicle.  Some of the heavier Desoto and Chrysler models, not so sure.

[desoto1939]

I also used the RPM calculator for my 39 Desoto and I like to run between 50-53 mph. Based on the information that I could provide based on 6.00x16 tires with average height of 27 inches and a 4:10 rear my RPM's came out as follows:  50 -2550 , 51- 2602, 52 - 2653, 53 - 2704, 54 - 2755, 55 - 2803  and 60 -3061.

 

Not sure if this all correct but just a base point for me.

 

My car has the seat spot around the 50 MPH mark and cruised nicely at the speed. As I stated before I drove from valley Forge Pa to Altoona to attend the National desoto car show. Averaged based on my speedometer 50 MPH and got 16-18 mpg.  Also had no issues with the traffic traveling at 70+ mph on the other fast lane and the truckers were very kind to me.

If you treat these old ladies with respect, they will always give you a reliable tour.

Rich Hartung

Desoto1939@aol.com

[Booger]

On 8/23/2022 at 5:29 PM, Sniper said:

Plenty of RPM calculators out there that will tell you if you know the tire size and rear gear ratio.

 

For example, my 51 has a P255/50-R17 rear tires, right at 27" tall.

 

3.90 rear gear

 

Engine speed at 70 mph is just under 3400 rpm.  I have had it up to 65, 3143 rpm.

RPM calculator is a nice tool. I wouldnt  go beyond 3K just to play safe

[TodFitch]

My RPM calculator is on a page giving my thoughts about how fast to drive.

[Ranger]

Why not just install a tach so you always know what the engine is doing. Fairly inexpensive and easy to install.

[Sniper]

9 hours ago, Ranger said:

Why not just install a tach so you always know what the engine is doing. Fairly inexpensive and easy to install.

 

Not exactly inexpensive but here you go

 

https://www.westach.com/product-page/3ct43-6v

[greg g]

Once again I will relay what my old-school machisnist  ( he worked for Don Gartletts when his swamp rats were Ford flathead powered) told me when I asked about sustainable  cruising rpm for these long stroke engines.  He was talking about fresh rebuilds and moderate mileage engines in good general condition  with operating Perameters within factory spec with reasonable wear.  His rule of thumb was 80 to 85 percent of factory HP  rpm. Or the 2900 to 3300 neighborhood.   Get out you slide rules and run your numbers with 3600 rpm for most flathead Chrysler 6 cylinder applications.   Hope this gives you some peace of mind.  It's worked for me on dozens of long mileage runs, 200 plus miles.

[Loren]

The tachometer I bought on eBay was $149 with $6.99 shipping.

It comes fully assembled with housing and bracket. Three wires so it is easy for non-electricians to install.

You'll need a hose clamp to install it on the steering column.

0 to 6,000 rpm which puts your cruise speed nearly straight up. ( on race cars you twist the instruments so that all the needles are straight up when normal )

It's an Autometer manufactured item modified by the seller to your particular needs.

If you think you'd find a tach useful this one works.

The old time racers believed that the more instruments you had in a race car the more marginal it was.

The last one I built had a temp gauge, an alternator light and nothing more.

[Sniper]

25 minutes ago, Loren said:

The old time racers believed that the more instruments you had in a race car the more marginal it was.

 

Lots of pilots would disagree.  Racers race for a few seconds and likely need nothing to distract them.  Pilots fly for hours, I drive for hours, I need to know how things are going.

[Los_Control]

There is a person floating around the internet @Tubmanthat has created a kit for a tach.

The idea is you can use any 12 volt tach & add the kit to it and create a 6volt +ground working tach.

 

I'm Just wondering if @Loren tach the tubman did not have a hand in it ..... very few start up vendors out there for these old cars.

[Loren]

The fellow’s eBay handle is rswitmore….so I couldn’t say if its him.

 

Instrumentation is up to the builder of course and everybody has their own preferences.

Aircraft are some of the most complex devices known to man. I keep thinking of the B 36 with 6 huge propeller engines and 4 jet engines. Can you imagine the rack of gauges the flight engineer had to monitor?!

I saw a post about the flight crew required at the dawn of the jet age, 2 pilots, a navigator and a flight engineer. Now days the planes are bigger and are flown by only the two pilots.

In the case of the cars I built they were the toughest of endurance racers for the Baja 1000 Ensenada to La Paz race.

Every wire, every hose in fact every part had to be considered a point of failure.

The thought pattern was, “what if this thing fails?” Could the car continue? If it can why are we carrying the dead weight? Then it moved to what does this thingy contribute to finishing?

In the case of the gauges it became “if we build the engine right we should not have to monitor things to keep it alive except temperature.”

Experience told us the engine was the most reliable part of the car. So one by one we eliminated stuff and the dashboard looked like a modern car (without the speedo) with idiot lights, then we got rid of the idiot lights.

In my current project the GPS and the little tachometer are there to test some ideas. Once they are no longer needed for that duty, I will likely remove them.

I will have knowledge of what the actual road speed is vs indicated, what the engine rpm is at various road speeds in all gears and tire sizes for this car. After all that is documented I won’t need the extras so why clutter that beautiful dash with non-period junk?

This is not to say I don’t have cars with lots of gauges because I do. It’s just not my vision for this one. So its not only the builder who gets a say, it is also the car.

[soth122003]

Hey Lauren, My dad went to school in the Air Force for the B-36. Got to Loring AFB Maine just as they phased them out. So they put him on the replacement, the B-52. Any way he told me that they had 2 Flight Engineers assigned to the B-36, 10 engines and all the associated gages that go with them, 5-6 gauges for the jets and about 5 for the prop engines. Then add in fuel gauges and transfer for fuel etc..

 

Joe Lee

 

 

Edited August 27, 2022 by soth122003

[Loren]

Have to see Jimmy Stewart in “Strategic Air Command” again! (1953)

I stopped at one of the Air Force Museums in Nebraska and they had a B 36 on display. They are huge!

Under one wing they had an A 26 (aka B 26 after 1948) which was the plane my Uncle piloted in Korea. That was a treat too.

I have seen Howard Hughes Spruce Goose when it was in Long Beach and I still think the B 36 is pretty impressive.

I would have loved to see one flying with “6 turning and 4 burning”!

[kencombs]

B36 was a monster.  There was a cargo version, xc99.  One built.  No longer exists.  But I got to tour it at Kelly field  San Antonio back in the 60 s.   Pistons only, no jets on that one.   The wing was huge. There was a passage all the way out to the outboard engines and a full engine gauge set on the airframe side of each.  Some components could actually be accessed in flight by the crew.    Too bad it was scrapped.