Packard Forum Index » Pre-War (1899-1942)

As a former airline pilot, I admit to being particularly obsessive with measurements, indicators, readings, etc.

So, it bugs me not to be able to directly measure the engine RPM and, worse still, to not have any sense of "how much is too much". In aviation, we have green (normal) operating ranges, yellow (a range in which operations are time limited), and a red line (never exceed).

What would those values correspond to on the 245cu in Packard 6 cylinder engine?

The owner's manual references a power output of 100BHP at 3600rpm....so, is that the "redline"?

I have worked out the following speed/RPM relationship using my 4.3:1 final rear end ratio and a tire radius of 13.875" (measured as the tire sits under the weight of the car, thus slightly compressed) with the transmission in high gear:

45MPH @ 2344RPM
50MPH @ 2600RPM
55MPH @ 2864RPM
60MPH @ 3125RPM
69MPH @ 3600RPM (redline?)

From my experience driving the car,the engine noise increases significantly around 55MPH and, while I do have more throttle available on a level stretch, it just "feels" like I'm pushing it to go beyond that. So, does 2800-2900RPM sound to you guys like a reasonable maximum for continuous driving or am I being too conservative?

It would be great to have some real numbers relating to this flathead engine where I could know what the practical limits are.

Any input is much appreciated!

Green, yellow and red operating zones are determined by many interacting engineering factors. Here's a relevent chart from a British car magazine in 1955 about the 1955 Clipper they were testing.



The "Power Available" line is road wheel horsepower, The "Power Required" line is drag force to be overcome (aerodynamic drag and tire and other frictions). Where the two intersect is the maximum top which can be achieved -- in the example about 116MPH. One can see that if the Clipper were geared differently, it could have a higher top speed, but not by a lot -- maybe 120MPH.

The yellow and red operating zones you describe are due to many engine factors. The power output that eventually shows up as road wheel horsepower is primarily determined by the engine's air flow capability (carb, intake & exhaust manifolds, valve size, head design, port efficiency, camshaft, etc.), plus fuel delivery capability. The RPM range is mostly determined by the stresses of the reciprocating assembly and materials used, but also the oiling system's ability to supply oil to the bearings and other moving parts. Both factors obviously increase as RPM increases, the former as roughly the square of the RPM and the latter as a linear function.

As to these considerations as it applies to the 245CID Packard 6-cylinder, I have no idea.

Hope this helps.

Craig

Your gut is probably going to be your best source of guidance here. If you're looking to maximize the life of your engine, staying at or below the speed at which it feels like you're pushing it is the best course of action. Truthfully, that's all anyone really had to go on back when the cars were new, and given how long your car has been in service, any number of things could have lowered or raised the safe operating range. Anecdotally, I can say it's been my experience that almost any of these cars from the mid-30's onward will comfortably maintain 50 MPH all day long.

And, in fact, there are probably other practical limits that come into play as well around that speed. Consider the bias ply tires, the limit of your brakes, your fuel consumption, the ability to maintain steering control, the capacity of the cooling system, and your tolerance for noise.

A few points of anecdotal reference:

-When we took a 1936 Standard 8 Phaeton to Pasadena for the Rose Parade, we drove it on the freeway and just cruised along at 50 most of the way. For me, 55 felt like I was pushing it just a bit (especially after a guy cut me off), but 50 felt comfortable. More than that and the steering also got a little scary. Different tire size but a similar final drive ratio to the six you reference.

-We had a 1940 120 sedan on the PAC All-Western Regions tour last year. 60-65 was no problem, but 70 felt like a stretch to that driver.

-A properly tuned Model A Ford will run at 45 MPH all day long, and at that speed your distance and time are only limited by your capacity to tolerate noise, vibration, and harshness. Faster and the engine temp will start to climb.

Any engine by any mfg'er prior to the horse power revolution of 1955 is a 2500 RPM engine for a safe SUSTAINE LONG DISTANCE cruising speed. 2800 rpm at the max.

Anything over 2800 rpm becomes a major strain on the internal engine parts that will greatly shorten the life of the engine or break parts.


For short bursts of speed such as in drag racing or passing slower moving vehicles then probably 3200 rpm is ok. But i sure as hell would not try to hold it at RPM range above 2800 rpm.

The ADVERTISED 3600 rpm is strictly a mfg'ers claim to fame. The mfg'er has literaly 100's of engines to run on a TEST STAND. So if the mfg'er blows up 10 engines before they get the results they want then they don't care.


Let me put it another way:
If u have 8 or 10 engines setting around and 3 or 4 mechanics that can swap an engine in 2 or 3 hours then by all means get out there and try to turn the engine 3600-4000 rpm all day long. Have your cell phone and roll back following u around the track to tow u in when the engine lets loose.

Otherwise, 2500-2800 rpm is about a safe sustained cruisinggggg engine speed.

There were no Interstate hi ways prior to the LATE 1950's Which means that it would be a rare place in the world where one could run 70 mph non-stop for hours on end.

As for "...any real numbers related to this engine....", you won't find any such numbers.

Go to eBay. Search on "Tiny Tach" Literaly 100's of them at about $7 to $25 each. REAL SIMPLE TO INSTALL. I have one on a 56 Cushman. U'll have a tachometer to check engine rpm.

Note that it is a rather slow changing readout on the tach. It is didgital. But for slow changing engine rpm or close to sustained engine rpm it works just fine.

LEt me put it another way:
Unless u plan to sit and rev the engine alot over 2 or 3 seconds then the Tiny Tach works just fine.

Also, i have ran 12v tachs on 6 v systems just fine.

I wondered about the safe sustained cruising speed myself. After doing piston speed calculations and talking to many folks I decided that 2500 RPM would be my self determined safe sustained RPM.
I also have a 4:36 rear end and 28" dia tires BUT I also have an R11 overdrive. Using a tach and a GPS 2500 RPM gives me 66 MPH. Still in my 38 120 driving at 66 starts to get "interesting" but doable.

As you can see by the shape of the power curve, the power peak is not necessarily the redline.

Rather obviously in addition to engine condition and gearing a lot depends of engine design. For example with 5" stroke engines and poured babbitt bearings you want to be more conservative with sustained RPMS than you would with a shorter stoke and thin-wall insert rod bearings. My '34 has 17" tires and a 4.69 rear axle; I like to keep it just under 2000 rpm for a sustained speed which corresponds to about 45-48 mph though I don't mind going to 50 or 55 for short periods if conditions mandate it. The engine with poured babbitt bearings did go about 80,000+ miles before I got my first lightest hint of rod knock so obviously those conditions were not abusive to the engine.

There is anecdotal "evidence" and then there are computer programs. I agree that anecdotal is probably a good guide on older Packards (pre V-8).

I have computer derived information on my Panther that I will throw into the discussion on this thread. The first is cruising speed:

Rear gear ratio: 3.07:1
Tire diameter: 26.2in (Goodyear Eagle RS-A P225/50R17)
Desired all-day cruising speed: 75MPH.
Engine RPM: ~3,000

This is exactly what I was looking for: a freeway cruiser than I can drive anywhere, anytime at expected Interstate continuous speeds (particularly in NV, AZ, NM, TX, etc.).

Of course the Packard V-8 has a relatively over-square bore/stroke (3.5in stroke). My Panther engine has lightweight forged pistons and tapered piston pins to further reduce the recip weight.

Just for grins, I plugged the Panther numbers into a good top speed calculator I have and it comes out 145MPH+. I'll never drive it anywhere near that fast even in bursts, but it's fun to know. I'm buying "V" speed rated tires (149MPH) because of this.

Craig