What is a "safe" highway speed that won't blow my engine?

[uh6077]

Just curious what you guys think would be a safe cruising speed for my 41 Desoto flat six 3spd, no overdrive. When I bought the car I had a 1.5 hr drive home but I took mostly back roads. I used a speedo app on my phone to verify speeds and was cruising the back roads comfortably at 55, touched 80 briefly just to see what the car could do. I drive the car often and next year I plan on taking a few longer trips with it and there will be a few times when avoiding the interstate will be tough (without adding hours to the trip). 55 on the highway will get you run over by someone busy texting so what do you think is a good sustained speed that won't turn my lovely little flatty into a grenade?

[37_Roadmaster_C]

I do not know anything specific about your car or motor, but my approach would be to get a tachometer (rent or borrow). Hook it up and get a helper to go for a drive with you. Record the engine rpm at various speeds in all gears, but especially in the highest gear. Armed with this information you can check the service manual and ask your mechanic and other Desoto owners what a safe rpm would be. Use this as a guide to judge safe speeds. Just a note.... the service manual is your best friend as diferent motors have greatly different max rpm ratings and older engines are often much lower than modern engines. For example, my 37 Buick 320 has a max rpm a little over 3000 rpm while my 60 Buick 401 would be safe up toward 4800 rpm.

Also, cars seem to just feel wrong if pushed to hard. If you feel you are pushing it you probably are .

[Guest]

Just curious what you guys think would be a safe cruising speed for my 41 Desoto flat six 3spd, no overdrive. When I bought the car I had a 1.5 hr drive home but I took mostly back roads. I used a speedo app on my phone to verify speeds and was cruising the back roads comfortably at 55, touched 80 briefly just to see what the car could do. I drive the car often and next year I plan on taking a few longer trips with it and there will be a few times when avoiding the interstate will be tough (without adding hours to the trip). 55 on the highway will get you run over by someone busy texting so what do you think is a good sustained speed that won't turn my lovely little flatty into a grenade?

The answer depends on your rear axle ratio.

[hchris]

Just curious what you guys think would be a safe cruising speed for my 41 Desoto flat six 3spd, no overdrive. When I bought the car I had a 1.5 hr drive home but I took mostly back roads. I used a speedo app on my phone to verify speeds and was cruising the back roads comfortably at 55, touched 80 briefly just to see what the car could do. I drive the car often and next year I plan on taking a few longer trips with it and there will be a few times when avoiding the interstate will be tough (without adding hours to the trip). 55 on the highway will get you run over by someone busy texting so what do you think is a good sustained speed that won't turn my lovely little flatty into a grenade?

In the day your Desoto would quite comfortably cruise at 60mph, 70+ years on there are many wear and tear factors which may influence that number and without knowing the mechanical status of the car any helpful response will be pure guesswork.

Perhaps you could also ask yourself whether the tires, brakes and steering are up to that sort of cruising, particularly if you are mixing it with todays cut and thrust driving styles.

[joe_padavano]

37 Roadmaster has the answer. Road speed is irrelevant as far as your engine is concerned, it's engine RPM that matters. You can exceed a safe RPM in first gear if you over-rev it. Engine RPM at a certain speed depends on gearing and tire diameter, so one can't simply specify a safe speed without knowing the details. Knowing what the safe RPM is for your particular engine is outside my expertise, but determining the actual RPM requires a tachometer.

[cahartley]

I have Auto Meter tachs in both my '59 Chev and '65 Mustang.

They're great pieces of equipment.

I looked for 6V tachs until I read the reviews on an Auto Meter tach there >>> http://www.amazon.com/Auto-Meter-2890-Performance-Tachometer/product-reviews/B00062YWKQ

Seems it works fine on 6 volts!....... :eek:

The actual tach is there >>> http://www.amazon.com/Auto-Meter-2890-Z-Series-Black-2-5-8-4000-RPM-Mini-Tach-Low-Rev-Tachometer/dp/B00062YWKQ/ref=cm_cr_asin_lnk?ie=UTF8 and is identical to the ones I have except for the rpm scale.....mine go to 6000 and 8000 rpms.

[ply33]

Your DeSoto has the bigger 25" long block compared to the 23" long block used in Plymouth and US built Dodge cars. From what I've heard that is a beefier engine that can take more abuse than the Plymouth. For the Plymouth the RPM at max BHP was 3600 and legend has it that Chrysler engineering would run their designs at max BHP in 50 hours tests to find anything weak. If you knock a few hundred RPM off that, going about 3200 RPM which appears to be where the truck engine limiters were set at, you should be able to run indefinitely assuming your vehicle is in good mechanical shape.

For my '33 Plymouth that works out to about 62 MPH. If I had a 4.11 rear end in there it would be closer to 65 MPH.

My overall thoughts on the topic are at http://www.ply33.com/Misc/speed and there is calculator there where you can enter you tire and rear end details and see what speeds correlate to what road speeds.

Chrysler products used Lockheed brakes which need to be setup properly to give reasonable performance, I'd be verifying that you brakes are in good shape before pushing the speed too much. Get the service manual and learn to deal with these things yourself as the number of shops that understand those old brakes are not as numerous as they once were.

[uh6077]

Thanks for the replies guys. The car is mechanically very sound, engine is tight and brakes are great. Looks like I need to hook up a tach and see how the numbers unfold. This is the oldest car I have ever owned but the first flathead so I do have lots to learn.

[uh6077]

Incidentally it is a flathead 6 not a flat 6. Calling it flat stamps you as a newb. For some reason no one makes this mistake with flathead Fords, no idea why.

I was wondering how long it would take for someone to call me a "newb". No matter how many forums I join (vintage motorcycles, MG, Triumph, vintage boats) there is always someone who wants so to call a the new guy a "newb" without knowing anything about their history. Congrats on having over 6000 posts Rusty but from now on please only reply to my posts if you have something constructive to add, not just to pad you numbers. I have a few Corvairs and they are called flat 6 so I will likely keep using this term out of habit. Please excuse my fatal mistake.

[JACK M]

The newb is padding his numbers here already.

I am thinking that comment was not called for.

This is a fun forum and a little poking is OK as far as I am concerned.

[CarlLaFong]

Lighten up, Francis. I've had a number of VWs but never called my "A"s and "T"s "flat fours".

I love all of the hemming and hawing and the reluctance to commit to an answer. Tachs and complex formulas to figure out how fast to run a 41 DeSoto?!?!

The speed limit in the US in the early 40s (not counting Roosevelt's WWII 35 MPH limit) was probably 50-55. That's what American cars were designed to do with a bit more in reserve. Big Cads, Packards, Buicks etc. would do far more. I would run that car at 55-60 all day long and would not be afraid to push it to 65-70 in short bursts. No tach, no charts, no dyno, just good old common sense

[C Carl]

This is a fun forum and a little poking is OK as far as I am concerned.

Yeah , and as I suspect most of us are in our 60s and 70s , and should be a bit thick skinned by now. There is probably not one among us who would rather re-phrase some things we have said if we could. We are all fortunate to have guys like Rusty , generously sharing his vast experience , and accurately giving his time. Also very fortunate to have an intelligent car , boat and motorcycle fan branching out into older cars. Maybe he is lucky to be a bit younger than us , therefore having more miles left in him ! Seems the older i get , the less concerned I am with speed. I'll take a back seat to few regarding my youthful velocity capabilities and accomplishments , but recently bought a 200 HP Allante rather than a 300 HP Northstar powered one. 'Nuff fer me at this late date. Uh huh , this old man is getting on. So to throw out an irrefutable lesson from physics for each and every one of you , my dear Motörhead Brothers , a timely reminder : POWER REQUIREMENT GOES UP AS THE CUBE OF THE SPEED , and : FORCE EQUALS MASS TIMES ACCELERATION. Peace , Carl

[C Carl]

Oops . In re-reading the above an error needs correction : In my second sentence the word "would" should be "wouldn't" . Does kind of change the meaning , doesn't it ? If that is my most serious senior moment today , I'll be lucky. Carl

[bhambulldog]

On an uphill run. She does better downhill

[Rusty_OToole]

I was wondering how long it would take for someone to call me a "newb". No matter how many forums I join (vintage motorcycles, MG, Triumph, vintage boats) there is always someone who wants so to call a the new guy a "newb" without knowing anything about their history. Congrats on having over 6000 posts Rusty but from now on please only reply to my posts if you have something constructive to add, not just to pad you numbers. I have a few Corvairs and they are called flat 6 so I will likely keep using this term out of habit. Please excuse my fatal mistake.

So you know that a ``flat six`` is a horizontally opposed engine and a `flathead six`is an L head.

In that case I withdraw my answers to your question, as I know nothing about a DeSoto with horizontally opposed engine.

And I apologize for mistaking you for a newbie who is willing to learn.

Edited September 15, 2014 by Rusty_OToole (see edit history)

[uh6077]

Bhambulldog, I think if I tried that my next post would be "What towing company do you guys recommend". Looks like your Buick has much longer legs than the Desoto. Yes I am a bit jealous. haha.

Rusty, please refer to post number 9 above.

[bhambulldog]

posts are disappearing from this thread as fast as they are being written.

uh6007 , my Welcome to the AACA forum post has disappeared.....

So,

Welcome To the AACA forum (again)

[uh6077]

Thanks! Should have joined years ago but all my other cars had strong clubs or online groups. Lots of knowledge here. I am enjoying reading old topics and making mental notes of things to take of over the winter.

[Steve_Mack_CT]

uh6077 just a quick note - mean't to mention on some other thread where I noticed your car - NICE ride. Reminds me of my first old car, a '41 Plymouth two door sedan I wish I kept. Did not know the Desoto had a bigger mill but I was pretty impressed with the MOPAR mechanics....

[Ben Bruce aka First Born]

6077, I do not know your age or where you were raised. But in Missouri in the late '40s and '50s, 55 would have gotten one run over. 65 would have not been too fast for any thing as old as late '30s. I drove my '40 Buick 65 ,or more, most of the time. Enjoyed out running Bobs '52 Belair.

Ben

[GLong]

Define 'safe'. Safe speed to not damage the engine? First, is the engine in very good shape, by that I mean does it have normal clearances, clean oil passageways, a clean oil pan, and good oil pump? If so, we need the rear axle ratio to be more specific. But the suggestions to drive it like it would have been driven back in it's day assume a strong, good condition engine. And back in it's day, if you burnt up a rod bearing or an exhaust valve, every town had a shop that could and would repair the car. Today, smoke the engine and you will take months to find a shop and get the engine repaired. Based on how nice the car appears. I'd deduct a few mph just to keep the car preserved.

Next, 'Safe' should really refer to the rest of the car, and the environment you will drive the car in. If you try to participate in a rush-hour chicanery-demolishion derby around most major cities, I'd park any nice old car and have an early dinner. If you are going to just cruise along the interstates, 60mph will not label you as too slow, many trucks are running at 62-65mph with the cost of diesel so high.

Make sure the tires are less than 10 years old, the brakes don't pull, or have any leaks, and have good brake lines. especially the flex lines between the frame and the moving axles and wheels. Old flex lines are often ignored, until they leave you with a brake pedal on the floor. No dual braking systems in '41.

As for Rusty's comment, when I read his comment the first time, I took it as a helpful suggestion, not to be intended as criticism. Since you are here asking for advise, that implies you are a bit on the 'newb' side. If I went to a forum for your corvair club, and asked a similar question, wouldn't that imply I'm a bit of a 'newb' too? "Newb' is not to me in any way a put-down. Ask a 'newby' type question, then at least for that question, or subject you are inexperienced, need advice, and therefore appear to be 'new' on that subject. I'm a 'newby' on many, many subjects, and would proudly proclaim myself to be one when asking for helpful advise.

I'd buy, borrow or steal a tachometer and take the car for a drive. Most of the big inline 6 and 8 cylinder engine in the late 30's and early 40's would run all day long at 2500-3000 rpm.. But if, as suggested in an above message, the car feels like you are pushing it, then back off. My '33 Pierce 836 in direct is comfortable at 50-55, but with the overdrive [aftermarket, installed by myself] the car is quite comfortable at 65mph. But the 30 year old tires tend to make me drive it a bit slower most of the time.

I hope this post is helpful, not taken as criticism, and is useful for you to take your nice Desoto out on the highways.

GLong

[Oxnard Montalvo]

Hmm. Interesting thread...For a bunch of newbs.

(Don't get around like I used to.)

[Guest cben09]

The critical measure of engine / car speed,,should be piston speed,,

That is:: How far the piston travels per minute,,,

Its been going up from 1,000 ft/sec on the real old stuff to around

3,000ft/sec,,,,sometime after that the piston boss fails and then

the whole world [yours] turns to something else,,,,,,ding,,donng,,

Hope this makes sense ,,,,

AROUND 1936,,the steel back inserts were introduced and rpm

jumped,,,major landmark,,sorta like pressure oiling,,but that took

decades,,,,,,

Today's lesson,,Generation gap,,::::

Retired army vet / Afgannastan,,,age 32,,,had never heard of the

Challenger OR the disaster,,,we watched the news cast of that date

together,,,,What do they teach for US history these days???

Be sure your listning partner is on deck with your info,,,,WOW,,

Cheers,,Ben