Big Block M-Body Swap
For cheap, durable, stump-pulling, ground pounding brute power, a Chrysler big block engine combination is hard to beat. Introduced in 1958 with modest 350ci and 361ci displacements, Chrysler's bulletproof big block design has proven its worth through the years. Along the way, Chrysler big blocks grew in size to the pinnacle factory 440ci displacement, and earned a well deserved reputation for delivering awesome power with simple modifications. Types of Chrysler Big Blocks
Chrysler big block engines came in two basic types:
- "B" engines are low deck big blocks with a deck height of 9.980". Deck height is the distance from the crankshaft centerline to the deck surface of the block with the head off. All B engines have a 3.38" stroke crank. The most common B engine displacements available now are the 383 and 400. The factory B engine displacements are:
- 350 - 1958 only
- 361 - 1958 to 1966 (cars) and later in trucks
- 383 - 1961 to 1971 (cars) and later in trucks
- 400 - 1972 to 1978
- "RB" (or "raised B" engines are tall deck big blocks with a deck height of 10.725". All RB engines have a 3.75" stroke crank. They share the same basic design as the B engines, except that the blocks are physically taller and wider. The most common "RB" displacement found now is the 440. The factory RB engine displacements are:
- 383 - 1959 to 1960. This version of the 383 is not to be confused with the shorter stroke low deck version. It was only available through 1960, and it is very rare now.
- 413 - 1960 to 1966 (cars) and later in trucks
- 426 - 1962 to 1965
- 440 - 1966 to 1977
"B" and "RB" heads will interchange between block types, but the intakes will not due to the greater distance between the heads on the taller RB blocks. Most other components will interchange except for the cranks, which have different journal sizes.
Although M-bodies never received big block engines from the factory, there has been considerable interest among M-body owners in swapping in a big block. Indeed, several of Farley's Forum members have done just that.Case for the Big Block Swap
- Dippy Doug Da Wagon Guy: "The answer to the question 'how can I make 500+ horsepower for the smallest cash investment' is ALWAYS gonna be a big block. It's stupid-easy to make over five hundred HP for under two grand cash with a stock crank, rods, pistons (maybe) & heads..a general freshening, intake, carb, cam, & headers & some mild head work will get ya there. Show me a small block that can pull that trick off without mucho investments in expensive chunks of metal, and I'll buy the dang thing.
- S-Type: "While the stretched (stroker) smallblock may actually outperform a stock 440, It will not do it for long (unfortunatly, we all know what happens when you stretch things). It also will cost a lot more to do it. Less money in a big block will make more power... longer. The problem with a big smallblock is, when your done spending the money on a big motor, you are strapped with the small valves and ports of the original design. Yes, the crank, rods and pistons are lighter. It may rev like crazy and make some good power, but no - it won't match the torque. I'm not belittling the A engine, just pointing out the economy of building a big block that can live a long time, as opposed to trying to make the same power stretching the limits of the smaller motor. You start with a lot more power with the B and continue to make more with each similar step. The B engine's larger parts will simply survive longer under higher loads than the smaller parts of the A engine, that is dependability. When I invest $8,000 in a motor, I want to use it for a looooong time. I think Doug would use an analogy similar to this. Get yourself a rubber band powered airplane. Get two rubber bands, one twice as thick as the other. Start at 30 turns apiece and see how they perform. Keep cranking it up 10 more turns every test until they fail. How did they perform? I return to the bench...
- S-Type: "I have done several big block swaps with A, B and M bodys, there are NO serious issues with any. It just takes a little work. The 360 is an easy swap, but even with mods, won't touch a stock big block for torque. The weight issue is a non-starter as my big block car is lighter than a smallblock M of the same body as Doug said. You won't get the punch and longevity from the smallblock dollar for dollar. You can beat this (SB vs BB debate) to death but it won't change it. The 440 is an 800lb gorilla, right out of the box. Try making 480 ft. lbs. @ 2800 RPM with a smallblock."
- Dippy Doug: I'm a huge fan of the B/RB engine series..they make mountains of torque right off idle, and they'll make very serious horsepower without needing to rev the snot out of 'em to do it. Obviously, you can build a small-block to make just as much HP & torque..but it'll cost a bundle to do so and it'll be a time bomb. I like my stuff simple & cheap..my philosophy is that if you can run the same quarter mile times with an engine that's barely breaking a sweat for less money, DO IT!
Below, excerpted from several posts by Farley's members, is what is needed to swap a big block into an M-body. Note that this swap requires considerable planning, knowledge of parts interchangeability, junkyard scrounging and possibly fabrication skills. It is NOT for the faint of heart, but it is a very worthwhile and rewarding swap if you want all out raw blistering power with longevity and without a lot of radical engine modifications. This page is a work in progress, so please let us know if you have any updates or corrections. What You Will Need:
Front End Weight Considerations:
- Engine: A 383, 400 or 440 big block engine. If you're going to go though with this swap, you might as well go with at least a 400. Low deck B engines will allow a bit more room in the engine compartment than high deck RB engines.
- Oil Pan: Use a production C-body or R-body center sump oil pan. The ideal choice is a pan from a police package car with the acceleration baffles in the pan. You can also use the Moroso deep sump oil pan, which fits well. The oil pan must be a center sump pan. Van (front sump) and truck (rear sump) oil pans will not fit.
- Transmission: A 727 transmission with a big block bellhousing pattern. 727s were standard equipment in police M-bodies until around 1983, so they will fit. However, a small block 727 has a different bellhousing bolt pattern, and it will not fit a big block engine. Your 904 transmission also WILL NOT fit a big block.
- Drive shaft: If you use an 8.25" rear axle, you will need a driveshaft from a 727 equipped M-body with the same wheelbase as yours. If you also swap in an 8.75" rear, the rear section of a two piece driveshaft from a late 70's Dodge ClubCab long box pickup will bolt right in. Another option is to take your driveshaft and measurements to a truck repair shop and have it cut and rebalanced. Your old 904 drive shaft WILL NOT WORK.
- Engine mounts: Proper mounts will depend on the original engine that your M-body has. Slant 6 cars and V8 cars have different K-members, and require a different mounting configuration for a big block:
- Slant 6 cars: Shumacher sells special mounts to adapt a big block to the Slant-6 k-member. Slant 6 cars also have taller motor mount/steering bracket supports welded to the driver's side of the k-member that may need to be trimmed for clearance. The big gusset for the steering box will probably need the most work.
- V8 cars: On V8 cars, you can use the factory 1974-up big block motor mounts. Part numbers are:
- Left - 3642815
- Right - 3642814
- Wiring: For wiring, try to find a 1983 or earlier slant six M-body wiring harness from the firewall forward. The slant sixes and big blocks use the same harness, that keeps everything neat as a pin. It isn't a big deal as the 318 harness can be made to fit easy enough. The six harness just fits perfect and there are no mods needed, thats all.
- Transmission kickdown linkage (Throttle control): Use kickdown linkage from 1971 or earlier transmission. You can't use the single rod kickdown linkage. Try to find a two piece w/bellcrank (you'll need to tweak this) or buy the LOKAR cable kit.
- Shift Selector Linkage: Your existing shifter linkage from your car will work
- Exhaust: Your options are:
- Manifolds: Get "Magnum" style high performance exhaust manifolds from 1969 to 1971 HP Mopars. Casting numbers are Left - 2843992 and Right - 2806900. They show up on eBay from time to time.
- Headers: Hedman 78030 headers for B & E bodies. You will have to cut and reroute the rear driver's side tube (#7)only.
- Headers: Shumacher Tri-Y headers are made especially for this swap, but they are expensive.
- Dual Exhaust System: You will need to upgrade your exhaust system to duals.Strangling a big block with the stock single exhaust system, or even a 3" single system would be a waste after doing all of the work to do the swap, so a free breathing dual exhaust system is a must.
- Cooling: You have two options for the lower radiator outlet:
- Passenger side outlet (stock position). Get the newer style 1973-up water pump with the lower outlet on the passenger side of the car. You can then use your original radiator, go with the 1987 Dodge Ram Pickup 360/AC three row radiator, or even upgrade to a 1973-up big block 5-row radiator.
- Driver's side outlet (non-stock position). Get the older style pre-1973 water pump. Member S-type used a 1970 C-body used the early pump with a radiator from a 1970 Chrysler New Yorker donor. It is a single row out of a non-A/C car and it has never given him any trouble. He did have the radiator shop put a tilt on the lower neck.
- If you want to keep A/C and power steering, you will have to modify the stock A/C and P/S brackets to use them.
- Power Brake Booster: You will need to swap in a smaller power brake booster/master cylinder (found on various A-B-C body Mopars) to clear the driver's side valve cover. A power brake booster for a 1978 Cordoba with a 400 engine should work. Big block boosters are smaller in diameter but are longer and stick out farther farther from the firewall. It will come close to the cruise control if you have it, and you will have to extend your the brake lines some. A K-car master cylinder and booster will also work.
- Speedomater cable: The stock speedometer cable from your old 904 transmisison will fit right into the big block 727 transmission.
- Rear Axle: With the torque of a big block, you will need to upgrade your rear axle to at least an 8.75" unit. The first time you really get on the throttle with the 8.25" rear, you'll leave 200 yards of rear end parts behind you on the pavement.
- Brakes: You should upgrade your braking system with the largest drums, rotors, and calipers you can find, WITH a proportioning valve. Most of the brake bias will be in the front (where all the weight is) so set the proportioning valve to give you a little more rear braking. See 11.75" Disc Brake Upgrade. Larger 11" cop car rear brake drums and backing plates are also a worthwhile swap.
- Shocks and Sway Bars: Get the best you can afford on shocks and a rear sway bar (if not so equipped). This is mandatory if you want the car to go around corners at all
- Tires and Wheels: You will want something that'll be able to give you more traction from a standing start. Use at least 8" wide rims on the rear.
- K-member: Some swap posts mention that the motor mount/steering bracket supports welded to the k-member may need to be trimmed for clearance, with the big gusset for the steering box needing the most work. Other posts, such as Marshall's Danny Greenberg post and another post on Moparts by CrAzYMoPaRGuY make no mention of any clearancing. In both of the instances where no clearancing was done, stock 1974-up spool-type factory big block motor mounts were used. Member S-type concluded that this was probably due to the Slant-6 K-members being different than V8 K-members. His car was originally a Slant 6 car, so he had to cut the web in front of where the steering box mounts to clear the oil filter and trim the rear of the K for oil pan clearance. He further advised that whether or not you need to trim anything depends on the K member, and they varied a lot as they were stampings that were thrown together and welded.
- Danny Greenberg: "One of these cars with a big block would be a super sleeper, one capable of squashing most anything on the road in a straight line. Because of the extra weight in the front, the handling dynamics will be completely different so don't expect to be able to corner in the car like you once did when it had the 318 under the hood, UNLESS you're gonna' be spending some major green to upgrade the suspension.
- S-Type: "As far as the weight on the nose, it just isn't as bad as everyone says. I got rid of the P/S and P/B to save some weight, and although the car is about 300 pounds heavier than the six was, it has the same front rear weight bias within 10-15 pounds as the six car. My slant six car weighs in @ 3240lbs and the bias is 57.2% on the front. The 440 car is @ 3,540lbs and the bias is 57.7%. I would think that moving the battery to the trunk would give the BB car a better bias than the six car. So that means yes, my BB car has a much better bias than a smallblock with air/ps/pb. I think the 727 transmission, 8 3/4" rear axle and six leaf springs helped to balance it. It is a coupe and weighs in at 3,540lbs and yes, it can hat right up and smokes the tires at will. When your in park and wing the motor to 2-3 grand, it will raise the left fender an inch or two, you know you've got some serious torque then. A 4 door car will have better bias numbers because a coupe just has about a square foot of window. The two extra doors adding more weight in the rear would make it even more evenly balanced.
- Dippy Doug: "With an aluminum intake, headers, and no A/C, a big-block weighs LESS than a stock small block that's equipped with cast iron intake & exhaust and a big honkin' compressor.
A Few Big Block Diplomat Specs:
- Although the power will be great, the offset in fuel costs will not. M-body fuel tanks range between 18 and 19 gallons at their largest. If you're getting 10 MPG, that's less than 200 miles per tankful of mostly high-test gas, which will be required if the engine has any compression at all. Bottom line - there are no mileage considerations with a big block. As you drive, you will see the needle on the fuel gauge slowly creep towards the "E".
- Because of the poor mileage and the current price of gasoline, this swap is NOT recommended for a daily driver.
- Danny Greenberg - 440 Diplomat 4-door: Danny's car is a 1989 Dodge Diplomat with a 440 4 barrel V8 taken from an early '70's Fury patrol car. With a few minor mods, the 440 dynoed at 480 HP, with 318 at the rear wheels [good number!]. The present rear end in his car is from a Plymouth RoadRunner, narrowed to fit under the car. He runs a set of 9" police style wheels in back with the stockers in front, with "V" rated sticky Yokohama tires on all four corners. Danny's Diplomat looks bone stock from the outside except for "440" badging under the Diplomat markings on the front fenders, the larger tires and wheels. It also has a custom A/C system, power steering. power brakes, a thumping stereo system, and custom leather seats with rawhide inserts. The complete swap cost him about $2000.00 for parts (the most expensive were the hi-po manifolds, which he tracked down in a small junk yard in Torrance California) and the rebuild of the 440 and rear end.
- S-type - 440 Diplomat 2-door: S-types car is a 1980 Diplomat Couple S-Type which was originally equipped with a Slant 6 engine. S-type dropped in a complete 440 4 barrel V8 engine, 727 transmission and the stock exhaust manifolds from a very low mileage 1970 Chrysler New Yorker 4 door donor car. Modifications to the engine include a Mopar Performance distributor (modified from a 340), Electronic Ignition with an Orange ignition box, and an Edelbrock 600 CFM carburetor. The driveshaft is a 4" Mopar Performance piece (no taper) and internally balanced. The rear housing is from a 1968 B-body, and the differential is the stock '489' casting (tapered pinion) unit from the Chrysler donor with 2.76 gears, Sure-Grip and a pinion snubber. S-type said that the car launches pretty hard despite the 2.76 gear ratio, and with the tall gears, he can upshift to third at 90 MPH!! The rear springs were upgraded to 6 leaf by keeping the Diplomat main leaf and adding five C body leaves. Polyurethane bushings were also added to the spring eyes. The rear brakes are from a station wagon, so they are big. The tires are 235/75R/15 Goodrich Lifesavers mounted on 15" X 7" cop car rims with dog dish hubcaps. The car was originally equipped without A/C and the power steering and power brakes were tossed. The master cylinder, linkage and plate are from a Volare. The complete car weighs in at only 3540 lbs. Future upgrades include a 360 degree Mopar M1 intake manifold, a hotter cam, headers and a 750 CFM Holley carburetor. S-type also has a 1980 Diplomat drag car (his first 1980 coupe - called the 'Green Hornet' which will have a 12.5:1 compression big block, a .533" lift cam, 4.57 gears and other assorted goodies. It should weigh in at 2800 lbs!!
- Compiled posts by Farley's members Woodvark, Dippy Doug Da Wagon Guy, S-Type, Thunderstruck, 79Sleeper
- Compiled posts by Moparts members CrAzYMoPaRGuY and Glen440
- FMJ Big Block Swap
- "How to build High Performance Chrysler Engines" By Frank Adkins
- Special thanks to Danny Greenberg for the additional information that he sent to Marshall for this topic! Also thanks to Marshall for sharing the original information on the old forum.
- Also thanks to member S-Type for helping to unravel the mystery of the two different mounting methods that have been floating around the web for so long.