After a year of deliberations...we are now seriously looking at using a slant six as an "upgrade" to a flathead six in our 1947 Desoto Suburban 9 passenger sedan.

We have done preliminary measurements and it appears that a Slant 6 can be placed into this engine bay without cutting the fenders or the firewall. Also, it appears that it will clear the steering box.

Our thinking it to build a slant 6 around the following mission:

1. Increase the horsepower/torque from the flathead 218 ft/lbs of torque at 1600 RPM and about 125 HP ideally into the 150 to 175 HP range with torque at the low end of the RPM range.

2. Use a pre-1965 transmission with a rear pump for down mountain compression braking.

3. Computerized fuel injection for good starting and cross mountain air/fuel adjustment. Possible supercharging if need be for power.

4. Build an engine for rugged duty, hauling around a car that loaded will be in the 5500-6000 pound range.

5. An engine that can be fixed at any shop in North America with a any necessary parts overnighted in.

6. The addition of air conditioning compressor and possible power steering compressors must be taken into account.

What I would like to ask of the folks on this forum is what year engine cores / versions would you suggest as the core for this project.

We plan on using this car extensively and one flogged it will be the basis of some serious traveling.

Sounds like a neat swap and build. Make sure you start a build thread on your progress.

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There's no such thing as too much cam....only not enough engine!

Dear James,

Welcome to the site. Sounds like a great project. Thank you for providing some clear parameters for your buildup. I will take a stab at your questions.

If you can provide an idea of the approximate budget for the engine alone, that will allow us to answer questions more thoroughly. Also, are you wanting to run pump low grade unleaded (~87 octane), or premium? What transmission and rear gear ratio do you plan to use?

1) It will not be difficult to get to the 150-175 HP range with a nicely rebuilt engine with some modest/moderate upgrades. The biggest area of focus should be the cylinder head. Since you are in SF area, I would suggest contracting Doug Dutra (Sunnyvale) to build you a new head with 1.70/1.44 oversize valves and some porting. Then, shoot for 8.5-9.5:1 compression based on your desired fuel. We can discuss camshafts, induction, and such later.

2) You get approximately the same engine braking on mountain downgrades with any year 904, so you do not need the rear pump. I have driven many cars with 65-down and 66-up trannys, and they both have nice engine braking. Using the later trans will give you more choice in cores, easier shifter use/availability, and easier parts availability. Unless you really want the pushbutton shifter (which is cool), I would stick to 68 up transmissions.

3) I have had Megasquirt I running on my two Darts since 2003 and I am about to upgrade to Megasquirt IIIX on one of them. This is a nice system and has good internet support. diyautotune.com is a good supplier. For the system components, you will mostly have to "roll your own" but some of us here can provide feedback on how to do that.

4) You will not have to worry about reliability at all if the engine is rebuilt properly and cooling and oil are looked after. Folks have towed large loads and made lots of power and such and these are tough engines.

5) I like the "overnighted in" statement - very realistic, considering this is an older engine. Should be no problem with standard parts. I drive my cars all across the country and race them along the way.

6) I suggest talking to Charlie Schmid (Charrlie_S) about AC, as he is a current expert. Power steering pumps and brackets can be bought from people on this site, or pumps can be had new/rebuilt.

At least this is a start...

All the best,

Lou Madsen

5)

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Home of Slant6-powered fun machines since 1988

James - I have never had an issue getting a part for a slant 6 in the bay area in less than 4 hours. WHen I needed an engine mount, SP had it in 3 hours, water pump was in stock at Oreillys, head gasket was in stock at napa, upper gasket set in stock in Autozone. There's really never a time I can't get a part I need here THAT DAY just so you have an idea of what you're looking at maintenance wise. I've driven my 68 dart all over the bay area with little to no issue, and all I did so far was get my head rebuilt and redid all the gaskets.

I'm also assuming that since you live in the bay area like me, you are intending to drive to lake tahoe occasionally which is why you're worried about high altitude stuff?

don't know if this is any help. My 51 Plymouth Cambridge had a slant 6 put in it. Appears hr just moved the radiator forward, clears steering box. He cut and redid the cross membed,also fabricated his own oil pan.

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51 Plymouth Cambridge with / 6

The slant six will fit into the De Soto without major mods. The huge Suburban is the car from the Happy Days TV show, most commonly used for taxis. You can get more than 150 HP from the 236 flathead six. I have a 265 crankshaft, with that crankshaft the flathead can more than match the torque of a 225. I got a 3 speed with overdrive from a '53 Dodge hemi that is a bolt-on replacement for the Tip-Toe semi-automatic - same driveshaft and shift linkage, everything should work - another choice to consider. The slow shifting semi-automatic transmission is often the deal breaker for the original drivetrain. The semi-automatic is tough, its the reason for the extended bell housing for the 331 hemi. But it shifts slow.

These Suburbans with the three row seating are now somewhat rare. I have the S11 coupe, plenty big, the extended wheelbase Suburban is huge. A 2JZ will fit as well, but that motor is OT.

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

Nice choice!

I have a 1951 S15 Sportsman Hardtop in progress.

The slant is an easy swap.....your dreams of power steering are much harder to achieve. I have 4 wheel discs and power steering in my car.....but I swapped in a late B Body suspension (78 Fury). That requires a huge amount of frame fabrication.

It is possible to add discs and R&P steering to the existing suspension but you are still left with a 1930s suspension design.

I have considered a power steering box out of a 73 to 88 chev 4x4 for the DeSoto chassis......but it usually resides ahead of the axle centreline...as opposed to the DeSoto box being behind the centerline.

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Yeah....Im the one who destroyed this rare, vintage automobile.....

Nifty swap. You get plenty of compression braking from a '66-up single-pump transmission—the same amount you get from a '65-down dual-pump transmission. The only thing you can't do with the '66-up trans is push-start the car.

Slant-6s have forged crankshafts til mid '76 production. The later cast cranks aren't as strong, but aren't weak.

You will probably find this project very illustrative for some of the mods you have in mind.
Second half of this post is worth reading.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

The Custom S11 Suburban with its 140 inch wheelbase weighs about 4,000 pounds - I think that reference weight is short of the actual heft. The 236 flathead six has a 1/8-inch greater stroke than a 225, its overall length is probably about the same as a slant six. People on the P15-D24 site can provide facts on power steering options.

A Skyview model of the S11 had a glass roof over the back seat, other than the Limousine ·the Skyview is the hardest to find S11 today. During the war when new cabs weren't produced some Skyview cabs had more than a half million miles. Before purpose built cabs like the Checker became popular the De Soto was the market leader. The 236 six was also used on 1-1/2 ton trucks.

The suspension isn't too bad overall, a Fatman Mustang II kit is available, a big car like the Suburban isn't going to handle that well.

The slant is a great old engine,don't get me wrong....but,you mention you want fuel injection,possibly supercharging,aircon and power sterering? All these options are readily available on more modern engines,as well as far more modern trans options like over drives . Why go to all the effort of modifying a car to fit another type of engine that in itself requires modification to achieve your goals? I would imagine that by the time you modify the car to accept the slant ,rebuild it to modern fuel requirements,then I assume there are some type of engineering approvals to be obtained to make it roadworthy,fit aftermarket EFI,possible blower,brackets for air,power steer etc etc it's gonna cost $$$. Sure if you want to keep it old School and do that then great....but if you want to make it an ultra reliable driver why not look at more modern engine options that will suit ALL your needs in a cheap ready to run package that can be purchased as a pull out from a wreck? Get all the looms,computers etc and you will save a ton money and have modern running gear that you can get parts for anywhere you travel?

I think Steve is correct for a car the size of the Suburban.

My Hardtop has the same interior room as my GM Suburbans,,,,,I imagine the DeSoto Sub is huge. My hardtop weighs 3700 lbs.

I put a modern suspension and modern EFI V8 im my Sportsman for just the reasons Steve outlined.

The limiting factor is the DeSoto frame ahead if the firewall. The steering box is in an awkward place and the frame piches inward to allow for sharp turning angles.
I cut off the frame at the firewall and made new rails. I did this during winter 1979. I added the 78 Fury sub frame and suspension with discs and power steering. The rear suspension is B Body leafs with aFord Explorer 8.8 w/discs.

I wont speak of the engine and trans I used cause they arnt slant.

I dont think you can easily add a V8 without a lot of frame work. I know for sure you cant add an LA v8 in the stock chassiss because the starter hits the steering box.

Her is a picture of my car in 1974:

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Yeah....Im the one who destroyed this rare, vintage automobile.....

Its not likely that you could easily find a '52-54 265 six, that crank/rod set tends to be pricey these days. The 265 has near the stroke of a 6BT, with head shaved to 8:0 and better carburation - some of those motors came with two carbs, the 265 can make torque similar to a stock 318, won't be as flexible, but it'll do okay. If a 265 crank was sourced cheap the entire build need not cost so much. It won't win races but it'll be as dependable as a rock and the zero to 30 acceleration would be better than you might think.

Tim Kingsbury's 265 in a light P15 is a better match for that combo
https://www.youtube.com/watch?v=CpyQpdc7LB8

Fatman sells a kit to mount a rack n pinion steering on that chassis, but several people said that this does not look so good, but works to relocate the steering for a V8 swap.

The steering column is angled in such a fashion as to prohibit most steering box fixes. The rack steering still needs a shaft with decent appeoach angles....thats where it would get ugly.

The front wheels are a shocking distance in front of the driver,

All these things let me to a complete frame revamp.

If I had to do it all again with the long wheelbase Suburban ...I would measure for dropping on a 15 passenger van chassis.....slanted perhaps,...

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Yeah....Im the one who destroyed this rare, vintage automobile.....

Thanks all for the responses. Let me try to add some more information.

I have been a flathead six guy for a long time. I know more about these engines than most.

In the Desoto right now I have a 251 Flathead six that has a NOS Crankshaft, NOS Head, NOS camshaft, ARP main, con-rod, and head bolts. It is about 8.5 to one. I CC the head to within +/- 1 CC.

The car has two rear ends the 3.91 on it now and a 4.11 that I had in it which broke a ring gear bolt. It is now fixed and can go back in if I wish.

I use the Fluid Drive and the Three-Speed Trans with Borg Warner Overdrive.

The car will be used to travel the USA.

Until 1949 the frames on MOPAR's angled in at the front a lot. Starting with the 1951 model year the frames got "squared out" more in the front in the anticipation of the wider V8's.

You cannot put a V8 or V6 into a 1947 Desoto unless you want to change the entire front end clip. I spent two years looking into this.

There is not a rack steering set up that can be had that is rated for a 4000 pound plus car AND that fits the dimensions for the tie rod sockets to be in the correct place (bump-steer). All the custom racks, I talked at length with the folks at places like Flaming River, are for 3500 pound or less cars,

I thought about building a fuel injected flathead with a later transmission.

That however would take a lot of ground breaking work on my part. So, I thought about the Slant Six. You folks have worked out a lot about creating more power, reliability, and usability out of that engine. So, rather than do the same for the flathead six, I am thinking that I would spend less time going the Slant Six route.

Someone asked about budget. What budget I need to make happen what I need to make happen. I want to spend nothing and get everything!

James