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Overdrive speeds


ThriftyT

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my 53 cranbrook does 70 at 2500rpm with overdrive. I normally keep it at 60-65 because there is a wild tire noise at 70. she does feel like it wants more at 70 though, maybe one day ill do a top speed test........ one day haha

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  • 3 weeks later...

Back in my sailor days I drove a 64 300. It was a three speed stick car, with 4.10 gears running N50-15's in the back.  Drove it from San Diego to Memphis for radar school and back.  Never understood people thinking they can't be driven at highway speeds.  That would have been just about 3200 rpm at 65.  Speed limit back then was 55 so I doubt I got it up past 65 for any long periods of time. 

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Where do you guys get all of these OD's.  I thought they were very rare?  I've never heard of an OD on a semi-auto transmission.  I have a 48 DeSoto with the Tip-Toe Shift trans.  I remember it did well on the freeway, and that was with a worn out engine with compression readings of 25, 55, 35, 60, 45, 30!  It made it home on the 100 mile drive, but only ran for another week!  It has a 3.73 rear end.  My 1950 Plymouth 3 on the tree, forget it.  At 55, the engine sounded like it was about to explode with no 4th gear.  

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Typical non OD standard shift transmissions both 3 and 4 speeds are 1 to 1 high gears. The old Machinist that did my engine work told me that cruising rpm that is 85

% of peak HP rpm is sustainable assuming good working condition of the engine.  So if your engine developed peak HP is 3600, 85% is around 3200 3300 rpm.

 

You can not compare the noise of these engines to what you are used too with your modern cars. To test this, roll you window down, find a gear in your dd where 3300 rpm at 60 mph. Then drive it for a half hour.  Today's cars are geared to go 65 mph at 1200 rpm, barely off idle speed, with all kinds of sound deadening, and electric fans.  2/3 of your old cars roaring at speed is fan noise.

Edited by greg g
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Piston speed is a major factor in how fast an engine can be operated safely. Our engines are long-stroke designs which drive up piston speed in comparison to modern short-stroke engines. The problem with high piston speed is not only accelerated wear in the cylinder bore but much higher stress on rod and crank bearings as the piston reverses travel at each end of the stroke. 

 

Just for grins I ran the numbers comparing my P15's 218 with my F-150's 3.5 Ecoboost V6. Smaller modern engines will have strokes even shorter than the 3.5 V6 and that is why they often have high redlines.

 

I used the piston speed calculator here:

 

218 (3.6 L)

stroke 4.375"

rpm 3600

piston speed 2625 ft/min

 

3.5 Ecoboost (3.5 L)

stroke 3.4"

rpm 4700

piston speed 2663 ft/min

 

It is evident that at 3600rpm the 218 is generating the same piston speed as the modern V6 does at 4700 rpm which is close to its 5000 rpm redline. 

 

Let's rework the numbers to where many have stated is a sustainable rpm for the 218:

 

218

rpm 3200

piston speed 2333 ft/min

 

3.5 Ecoboost

rpm 4100

piston speed 2323

 

I don't think any F-150 owner would want to run their 3.5 at 4100 rpm for an extended period of time. Yet this is the same piston speed that is advocated for our 218's by those who say 80-85% of rated rpm is safe for an extended time. 

 

Everyone is free to do as they wish but my P15 (no overdrive, 3.91)is operated between 55-60 mph on the highway in order to keep piston speed (1860 ft/min) in a range that I think is conducive to good engine life.

 

 

 

 

 

 

Edited by Sam Buchanan
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You'd do better to run moly rings to promote long engine life than anything else.  But moly rings are not to be found, alas.

 

Another consideration in regards to stress is that a longer rod spreads the time of change in piston direction over more crankshaft rotation, lessening the stress involved in the piston changing direction.  Of course a longer rod tends to limit maximum RPM.

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16 minutes ago, Sniper said:

You'd do better to run moly rings to promote long engine life than anything else.  But moly rings are not to be found, alas.

 

Another consideration in regards to stress is that a longer rod spreads the time of change in piston direction over more crankshaft rotation, lessening the stress involved in the piston changing direction.  Of course a longer rod tends to limit maximum RPM.

 

I think it would be accurate to say the length of the rod isn't a primary factor but the length of crankshaft throw will effect the acceleration/deceleration of the reciprocating mass at the ends of the stroke at a particular rpm.

Edited by Sam Buchanan
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It's a known fact that the longer the rod, with same stroke, the gentler the transition in direction, i.e. inertial loading, is. 

 

Stroke will effect piston speed but not the inertial loading.  So we are talking about two different things here.

 

Of course, nothing in our engines is designed for high speed operation.  Pistons are too heavy, too many rings that are too thick, long stroke, long rods, poor intake and exhaust port design.  But they were designed in a time that had little need for high speed operation and nowhere near the understanding of it's effects on the engine as we do today.

 

I probably wouldn't recommend exceeding 2500 fpm piston speed and that you keep it below that as much as possible.

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11 minutes ago, Sniper said:

It's a known fact that the longer the rod, with same stroke, the gentler the transition in direction, i.e. inertial loading, is. 

 

Stroke will effect piston speed but not the inertial loading.  So we are talking about two different things here.

 

 Inertia is a function of mass and velocity so I'm having a hard time visualizing your statements. But that is ok...I'm not an engineer and don't have to design engines for long service.  :)

 

I agree our flatties were optimized for a day and age of two-lane highways and they work great in that environment. A few days ago the wife and I spent some time on a rural two-lane highway and the P15 was as content as I've ever seen it at 50-55 mph. The combination of road speed, engine speed and chassis handling all came together........I think that is what that car was designed for.

Edited by Sam Buchanan
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You are correct about inertia, but a longer rod decreases the rate of change in velocity as the piston approaches TDC or BDC.  So instead of being as abrupt a change in direction as a shorter rod it is more gentle.

 

Sort of like the guy that waits to the last second to stop for a stop sign, then mats the pedal to get up to the speed limit vs the guy that gently brakes and then gently takes off, both end up at the same speed, but one is less stressful getting there.  Best analogy I can come up with and it's not 100% perfect but it works.

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1 hour ago, Sniper said:

You are correct about inertia, but a longer rod decreases the rate of change in velocity as the piston approaches TDC or BDC.  So instead of being as abrupt a change in direction as a shorter rod it is more gentle.

 

Sort of like the guy that waits to the last second to stop for a stop sign, then mats the pedal to get up to the speed limit vs the guy that gently brakes and then gently takes off, both end up at the same speed, but one is less stressful getting there.  Best analogy I can come up with and it's not 100% perfect but it works.

 

I still don't get it, and that is ok because whether or not I understand doesn't have any impact on my P15.    :)

 

But....if we have two crankshafts with identical strokes running at the same rpm, one with a rod one foot long, the other with a rod six feet long..........the piston is going to see the same speed and rate of acceleration and deceleration in both cases. Inertia is a function of speed, and speed in this case is dictated by crankshaft stroke and rpm, not rod length. There will be less angular velocity at the crank and piston pins with the longer rod and this might decrease wear on the bearings due to less total rotation, but I don't see how this effects inertial impact on the system. If the two rods are constructed the same way the longer rod will have greater mass which increases inertia on total reciprocating mass.

 

This is fun but I suspect the boredom quotient for the forum members is increasing faster than piston speed.........  ;)

Edited by Sam Buchanan
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19 minutes ago, MarkAubuchon said:

One more story,  dual carbs, exhaust, 3:55 with 15 inch wheels, runs much smoother than when I had the 4 x 16 wheels.  I like the 65 mph range

 

Just for fun......  :)

 

If your 15" tires have about the same diameter as mine, you are running 2800 rpm @ 65 mph which is a piston speed of 2042 ft/min. That is a good number and shows the advantage of a highway rear end ratio.

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3 hours ago, Sam Buchanan said:

 

I still don't get it, and that is ok because whether or not I understand doesn't have any impact on my P15.    :)

 

But....if we have two crankshafts with identical strokes running at the same rpm, one with a rod one foot long, the other with a rod six feet long..........the piston is going to see the same speed and rate of acceleration and deceleration in both cases. Inertia is a function of speed, and speed in this case is dictated by crankshaft stroke and rpm, not rod length. There will be less angular velocity at the crank and piston pins with the longer rod and this might decrease wear on the bearings due to less total rotation, but I don't see how this effects inertial impact on the system. If the two rods are constructed the same way the longer rod will have greater mass which increases inertia on total reciprocating mass.

 

This is fun but I suspect the boredom quotient for the forum members is increasing faster than piston speed.........  ;)

 

A longer rod can operate at more favorable angles, pushing the crank.  The vertical forces on the piston are slightly greater (Yay! More power) and the side forces decrease a lot. (Yay! less friction and slap and wear!) It usually allows better torque delivery than a short rod engine.

 

But you gotta draw a limit, because the engine gets way too tall. The rod gets too heavy or becomes some expensive metal. Other minor issues.

The roughly "square" engine has become a popular compromise.

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9 minutes ago, Ulu said:

 

A longer rod can operate at more favorable angles, pushing the crank.  The vertical forces on the piston are slightly greater (Yay! More power) and the side forces decrease a lot. (Yay! less friction and slap and wear!) It usually allows better torque delivery than a short rod engine.

 

But you gotta draw a limit, because the engine gets way too tall. The rod gets too heavy or becomes some expensive metal. Other minor issues.

The roughly "square" engine has become a popular compromise.

 

I'll buy that.  :)

 

The longer stroke engine has different power delivery than a short stroke......and anyone who has driven one of our old torquey cars has seen that first-hand!

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If you want to be overloaded with rod length info, google the term as it relates to drag racing.  Apparently it does have a large impact.  The gist of it is that the longer rod changes the angle of rod to cylinder, and the degree of rotation needed to start the down stroke,  which changes the piston's  'dwell' time at TDC.  That changes the burn time 

 

that's ok, I don't really get it either, but according to the drag racing HP experts...

But, it probably has minimal effect at our flat six RPM and HP level.  Except that the lower angle relative to the cylinder does lower the side thrust and tendency for the pistons to 'rock' in the bore.

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I was not going to saddle you guys with periodic functions plus trigonometry.

 

Really heavy duty engines do not have piston slap and such to deal with because they use a two piece connecting rod with an intermediate sliding block that carries the thrust loads otherwise observed at the piston skirt.
 

And of course they are the worlds tallest engines.

 

 

 

 

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Just my humble opinion.

If you watch the various bits moving you'll see that the highest piston speed is in the middle of the stroke.

At the top and bottom the piston is first slowing down then changing direction and speeding up.

Rod length equates to the angle of the rod at the point of highest piston speed. At TDC & BDC the rod is straight so the angle is greatest at the middle of the stroke.

A short rod of course has a greater angle than a long rod (with the same stroke).

Long stroke engines tend to be designed for slower speeds and higher torque. Which as we know is the prime mover of weight.

Short stroke engines tend to be designed for higher speeds (RPM) and higher horse power. Horse power gets you down the straightaway fast but torque gets you out of a corner faster. Engineers know that if you want more horse power from a given engine size you spin it faster (more power strokes).

An example would be the early Honda Grand Prix cars which had impressive top speeds but couldn't get off the starting line without stalling or spinning their wheels.

I've dealt with engines at the extremes of the short/long stroke debate. One thing I know for sure, a long stroke engine wants more cylinder bore and a short stroke engine wants more stroke. lol

 

 

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I put 32,000 miles driving speeds of 65-70 on the freeway with my 218 (is a 1950 block) and 3.9 rear end and no overdrive.  I finally spun a rod bearing and have since semi rebuilt the engine and have put 2,500 miles on it already.  I have no idea how many miles were already on the engine when I got it.  Odometer read 22,000 miles when I bought it 4 years ago.  Good compression and vacuum during those 32,000 miles.  Cylinder bores had almost no wear and I was able to use new std pistons and rings.  Ring grooves were worn wide so had to replace the pistons.  Now I take it more easy and cruise 60-63 mph, no issues so far and runs strong.  Oil pressure is a little lower then pre-rebuild but I went with larger bearing clearances: .002 mains and .0015 rods.

Edited by Polsonator2
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Because of many discussions with old timers I've concluded that Plymouth flathead sixes lose oil pressure due to worn cam bearings.

When you examine the oil passages in the block every main bearing is drilled completely through to the cam with the same size hole.

The oil actually has to turn a 90 degree corner to get to the mains and it can only do that if the cam bearings don't lose pressure and volume.

Cam bearings do not need that much volume of oil. Guys who build V8s for racing look for things like that and restrict oil flow to places where they can.

Smokey Yunick went so far as to use plexiglass valley covers to see where the oil went.

Oil flow restrictors are a manufactured item you can buy from most hot rod parts suppliers.

If I were to build a Plymouth flathead today I'd start with a 230 (no substitute for cubic inches) have the crankshaft nitrided (I've always wondered about the surface hardness of the cranks) consider custom rods and carefully install the cam bearings with the oil holes turned slightly to restrict the oil flow at least by half.

The rod bearings get their oil after its been through the main bearings. So any wear on the mains and cam bearings deplete the pressure and volume available to the rods.

The oil pumps seem to be plenty big enough so the problem doesn't originate there.

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I was always suspicious of the bypass filter oiling system. 


Perhaps this was completely miss placed.

 

I have definitely seen engines where the cam bearings were Re-drilled with smaller holes.

 

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  • 4 weeks later...

Gentlemen...I had a '48 P15 Coupe for 42 years....Stock engine, 15" wheels and I put a '52 Plymouth overdrive into it..I put 25000 miles on it and was very pleased

with it...I could   run down the road just like the newer cars... It seemed like it was just above idle at the speed limit...2300 RPM was 65 MPH...1900 RPM was 54 MPH...

You multiply your rear end ratio by .7 and this is your O.D. ratio...3.9, which was the standard '48 Plymouth rear end =2.73 with O.D....You can manipulate the governor

with a toggle switch and you have a 5 speed tranny....Those old engines have plenty of torque, unlike the new short stroke engines...That's what you call cruising...

in 2014, I was going to repaint the '48 and the kids said 'Grandpa , you shouldn't be painting that car at your age, this new paint s deadly, why don't you find one that's

already painted and ready for the road?'  So we looked on the Internet for a while and found a beautiful maroon '47 coupe, with a '53 engine and a split manifold....

So, I took the O.D. out of the '48 and installed it into the '47, and gave the '48 to my Grandson, who had grown up riding around in it...I'm 87 now, but one of the best things I ever did

was to put that O.D. in that car....Makes a world of difference, so, go for it ....

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I have run my 5000 pound 1947 Desoto for 15 years with a 251 engine, a fluid coupling, and the three speed stick with BW overdrive. I have used the 4.1 gears and the 3.9 gears. I go down the highway at 65 all the time.  On hills, I usually have to drop it out of overdrive. The car is just to heavy compared to most running the six.

 

As far as I know there is no overdrive that was made to mate with the M5/M6 transmission. I do know you can go get a gear vendors OD unit and splice it into the driveshaft. At a cost of about $6K by the time you are done. I know guys with Packard's and the like from the 1930's that have done that.

 

James

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