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 Post subject: 251 Desoto Engine Build
PostPosted: Wed Oct 10, 2012 3:26 pm 

Joined: Fri Jan 27, 2012 9:23 pm
Posts: 9
After reviewing all the comments back regarding a possible swap to a Hemi engine for my 1951 desoto convertible, I was convinced that I should stick with the original powerplant, A 251 cuin 6 cylinder. I would however like to add some extra HP and dressing up a bit to this engine. If anyone can send me to a link or provide direction of a good build for this engine it would be appreciated. Dual Carbs? .030 over? headers? pertronics distributor rebuild? other?


PostPosted: Wed Oct 10, 2012 6:15 pm 

Joined: Thu Aug 12, 2010 8:22 pm
Posts: 418
Location: COS ( aka Colo Spgs )

The only dual carbed intake manifolds made for the 25 inch engine was for large
farm equipment, over three ton Heavy Haul Dodge Trucks, Marine and other such
things. This means there is no Carburator for you that has the Electrical Connections
or Switches for your M-6 Transmission in 2012.

The 1950 Flat Head Six has an peak Torque of 208@1600 RPM's. The Vehicle is
designed for a lot of 50 MPH Speed's ( sure it can do 70 for an long while ... but
the engine life will be less than you want ). The cost of after market headers and
the power gain at Hwy Speeds is not a very good $$$ to Power Enhancement.
The only possible gain is you have some bragging rights to the un-knowing That You Have Headers.

Over Bore Pistions
For an gain of Enhanced Power this too is an very low Power Gain for the cost. If
needed then cut the least possible because you will never be able to put it back

I went through this in February of this year. At that time they could not have an
operational 6 Volt Positive Ground System. They may have over come this by now.

MoPar use two widths of pulleys in 1950. Most likely you have the wide width
on your De Soto. If so the wider Pulley uses more engery to turn from the engine.

They also used two diameters on The Generator. The Large Diameter uses less
engery from the Engine. Either of these could be an two year search to find what
is needed for an tinch of power gain.

I am trying to say what can be done for an upper 1950 marketed income auto
is very little in 2012. Look for an Six Cylinder 1950 Olds and go crusing with them and you will see what you have. In the mean-time get an 1950 Road Map and
some 1950 published Magizines to lay on the seats and explain in your 1950's clothing this was the way it was.

Men wore hats when they went out in 1950. While the brims of the fedoras
and other styles were smaller than they had been in the previous decades, the
basic idea that a man was not dressed properly without a hat remained intact. Colors matched the suits: dark blue, gray, charcoal and brown.

Look at 1950 The Dealers Brochures and Advertisements of how De Soto Classed buyers dressed.

Rodger & Gabby


PostPosted: Thu Oct 11, 2012 1:10 am 

Joined: Thu Aug 26, 2010 7:00 pm
Posts: 393
The best thing I think you could do is swap the rear axle. Get a 3.20 (or something close) rear axle and let her rip!

OK, lets go wild. See if you can find a 265 crank. It has a quarter inch more throw, so more torque and more HP. Then jack the CR up a bit. Maybe 10:1. You'll have to research and see what other guys are getting away with. The carburetor won't be a limiting factor until you boost the CR and add the hot cam. Then adapt BMW M-6 fuel injection and Motronic system. It can be done!

But I'd go with the rear axle swap. This is an easy engine to work on if you are a DITY guy.

Have a nice day

PostPosted: Thu Oct 11, 2012 7:00 pm 

Joined: Thu Aug 23, 2012 10:21 pm
Posts: 213
These issues have been talked to death over in the P15-D-24 web site. Also, on the HAMB.

The flathead 6 engine will produce more power by doing the following in order of the increase:

1. Increase cubic inches (251 or 261 engine)
2. Increase the Compression ratio. MAX of 9 to 1 or you will have head gaskets go on a regular basis. Best street compression is 8 to 1 for longevity and gas. If you go past 7.5 to 1 you MUST deck the block and head or you will have sealing problems.
3. Open up the exhaust. You will see a big jump if you put a set of split headers on it. It does not need to be duel headers all the way back just down to the bottom of the flange.
4. Carburetion. A small increase will help. BUT, it must be calculated by the pumping of the engine using CR, actual cubic inches, and a very low VE ratio. At the most you will want a 2-bbl. Two very small 2-bbls will work but unless you jet them correct you will just waste gas. Use progressive 2-bbls only.

Now all the above is on the assumption that you are using a stock cam. You can get a more radical cam from He also knows more about making speed on these engines than anyone.

On my 1947 Desoto Suburban at 5000 Pounds I rebuilt the engine to the following specs:

251 Cubic inch + 20 Over.
NOS Crankshaft 0-0
about 9 to 1 CR.
New (not reground) stock cam shaft. (I wanted the ramp angle to not be so radical as you get with a regrind. Minor point but can be important at sustained high RPM's)
New head that was CC to +/- one CC on all cylinders and polished to help with carbon build up and hot spots. Plus, makes car idle real smooth.
Everything balanced well.
Connecting Rods modified to use ARP Ford 351C rod bolts.
ARP main and head studs
All new lifters and valves.
All new time chain and gears.
Stock exhaust and carburetion.

I used an original hot rod aluminum Duel Intake and it was too fussy for daily use. The single runs just as strong. If anything, I would put headers on long before messing with the carburetion.

I use fluid drive, a 3 speed stick, and BW overdrive with either 4.11 gears or 3.91 gears.

Been running strong for years. My 2 cents worth.


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