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Diff Ratio Question????


fred

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Hi here's a question I have probably asked before on this forum and some others. I have a 1948 Chrysler C38S Royal Club Coupe, built in Windsor Ontario. My engine is from a 1951 Canadian built Dodge (25" Block) 218 or 230 CI, 1951 3 spd trans and dry clutch. The diff is of unkown origin, so is what came as original equipment, don't know if it was a 3spd and fluid drive or M5 and fluid drive, as well not sure what diff came with the car. 3 spd should be a 3.90 and the M5 should be a 3.54. After much investigation I can't find a stamp on my diff, the marking the wheel test and driveshaft revealed a 4.10 diff, I think. This diff is equipped with a sway bar, covered leaf springs and 11 inch brakes. This really has me scratching my head.

My plan is to swap to a 3.54 I have or may have in a 1947 Windsor parts car, would this be advantageous, I plan on cruising at 55 to 65 mph, most of the time it would be the bottom number. Because this engine is a little different than the 251, stroke and bore, can it handle these speeds and rpm with a 4.10 diff or should I switch to the 3.54. I live in flat country and the highway systems around here have plenty of 2 lane highways to take your time on.........Thanx Fred

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According to my Canadian Service Data Book your car came with the 3 7/16 X 4 1/2 engine 250 cu in 114 HP and manual transmission. The rear axle came with a 39 tooth ring gear and an 11 tooth pinion. This gives a ratio of 3.54:1.

The 51 Dodge came with a 3 3/8 X 4 1/16 engine (model D39, D40) or a 3 3/8 X 4 1/4 (D42). You should be able to tell what engine you have by the last 3 digits of the serial number. It is stamped on the block on the left side, at the front, between the 1st and 2nd cylinder.

Since you have a smaller displacement lower powered engine you really need a lower gear ratio than stock in order to keep the same accelleration and hill climbing power. These cars were not known for being overpowered in the first place.

By going the other way you will slow the engine down at hiway speeds no doubt. But you will also cut your power for accellerating and hill climbing.

I wouldn't do it. Unless you feel you have power to burn right now. I expect if you made the change accelleration would be very sluggish and at hiway speeds the car would slow down on every grade.

You should have no problem cruising at 55 to 65 with your present gears. Those were normal hiway speeds when your car was built. It may sound noisier than today's cars but that is to be expected.

I wouldn't change anything. You are apt to do more harm than good.

If anything I would be looking for a 265 engine from a 1952 - 54 Chrysler Windsor. One of those would have the power to handle a lower gear. If you could find an overdrive transmission that would be even better.

But for all the driving you are likely to do with that car I would leave it alone.

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Rusty, it's indeed a D40, with 98 HP it is 16 less than the 251, good enough reason to keep it coupled to a dry clutch 3spd trans and a 4.10 diff. My main concern was to be able to cruise on the highway at 55 to 60 mph. Some old mopar guys have told me this type of set-up was best run at 50 to 55 mph, and could be run as such all day with no problems, but if you ran at 60 to 65, you could maybe burn up the engine, not sure if this is fact or fiction. My engine is not rebuilt, has compression of about 105 except #5 cyl which is 98, this was taken last summer, after then engine had sat for at least 30 years without regular use and had not even been started in about 5 years. The oil pressure is 40 lbs at an idle when warm, I use Shell Rotella 15-40 oil. I have no idea how many miles are on this engine, or if it has been rebuilt. I do know that it ran real well in the car it was taken from. I also own a 251, fluid dirve M5 trans and 3.54 diff from a 47 Chrysler windsor, condition not known. I don't think I will return to the original type of set-up , as the dry clutch and 3 spd trans are very simple, ( iam not a purist), I also own a flat head 6 from a 1955 Fargo 1/2 ton truck, (would like to know it's displacement, maybe it's a 265), this engine is also running but tired. Any thought's on this from you or anyone else on this forum welcomed......................Fred AKA Rockwood on P15 D24

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You are the best judge of whether you have power to burn or not. But I don't think you need to change gearing.

Some of the old engines had a short life when the new 4 lane hiways came out in the late 50s and early 60s. Chevrolets dropped like flies and some Hudsons suffered premature wear. Both these makes used splash oiled engines not made for high speed use. Chrysler products always had full pressure oiling and stood up better.

I'm not going to say you can drive as fast as you want. After all that is a 50 year old car. But when it was a current model no one would have thought twice about a long trip at 60 or 65 MPH.

Before I changed gearing I would be taking a close look at that 251. If it is in decent shape (not too much cylinder wear) you might be able to overhaul it with new rings, bearings, gaskets, a valve job and possibly a new oil pump. That engine with the standard transmission would keep up with traffic better than the Dodge engine.

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I would like to swap in the 251 I have someday, but the engine will definately need an overhaul, in fact I think it's seized at the moment, I don't have it home yet so I couldn't go out and check. What about going back with everything the trans and fluid drive included, and of course that would depend on the condition of those items. I still like the idea of a standard with dry clutch, the 251, with dual carbs and exhaust, and a 3.54 rearend, that would allow me to highway cruise at 70 to 75 mph and I am sure with the 251, my off the line performance should be fine and hills should be okay also. Where I live it's very flat for 75 miles in either direction, so hills are not an issue, but high speed traffic can be, when I venture into the city, which is Winnipeg Canada.....Thanx Fred

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You should read this article.

http://www.allpar.com/cars/desoto/suburban-1951.html

It is about an 8 passenger DeSoto suburban with the flathead 6. At one point the owner rebuilt the engine with a long stroke 4 3/4" crankshaft to increase displacement from 251 to 265 cu in. I don't know what kind of gearing is in this whale but I guarantee it is lower than anything you have, and the engine has a longer stroke, yet he thinks nothing of driving it at 70 thru the California desert, into Death Valley, over the mountains to Yellowstone Park, towing a trailer no less.

So I don't think you have much to worry about. The only note of caution is, ANY engine if it is in poor shape is more likely to fail at hi speed than if you baby it. If your engine is in good shape you have nothing to worry about but if the bearings are shot, you have no oil pressure, the pistons are slapping and the valves are burnt I take no responsibility if it blows sky high.

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Thanx Rusty, as I mentioned before, the engine has good oil pressure,always over 40 pounds, decent compression, runs at about 170 when warm. Not sure how many miles are on it, how much it can take and how long it will last. I did the compression test last year after it had not been run for several years. Thanx for sending me the link, very interesting read. But what is your opinion regarding my current set-up using a dry clutch and standard 3 spd trans. I contemplate returning to a fluid drive and M5 semi-auto hyraulic trans every now and then, but not sure it would be worth the effort and the standard and reg clutch is very simple and reliable..............Fred

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Maybe I can answer part of your question by telling you that I have a 1951 DeSoto Custom with the 236 cu in 116HP six and Fluid Drive and I would trade it for a standard 3 speed any day. And I like the Fluid Drive, and know how to keep it in repair. But there is no doubt I would take a standard over a Fluid Drive.

I went back and reread the DeSoto Suburban article, he has 4:11 gears in that car, it weighs 4800 pounds, when he tows the trailer with a full load of passengers and baggage his GVW is over 8000 pounds. Yet he cruises at up to 3600 RPM which is 70 to 75 MPH.

The only difference is I think the big limo models came with 8.20X15 tires like the New Yorker and Imperial. Yours would have had 7.60 15s. This would have a slight effect on gearing.

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