Building the 431 Mopar Stroker Engine

Referenced from: www.arengineering.com

Updated by: Mark Jackson - Jan. 19, 2005
Revision made by: Mark Jackson - Jan. 09, 2006

The Jackson Garage



Building the 431 Mopar Stroker Engine

With the 383 being the most plentiful of all of the Mopar Big Blocks, it is hard to understand why it took so long to become a popular Stroker Buildup. The engine lends itself to many of the same High Performance parts and technical data as it larger and more popular siblings, the 400 and the 440. The information below is an updated and modified version of the 451 Manifesto which can be referenced on the website:

http://www.arengineering.com/articles/articleframe.html
451 Manifesto

Block Deck Height: The 383 block, with a deck height of 9.980" is perfect for a 3.75" stroke. The 440 block is really too tall for a motor of less than 500 cubic inches since its deck height of 10.72" requires a piston which has a compression height of 2.077" to make a zero deck. The BBC has a deck height of 9.80" for the regular block and 10.2" for the "tall block". That means that the 440 is over 0.50 taller than the special Chevy tall block. The 383 block is right in between the two BBC's. The lower deck height of the 383 means less block weight. It also means the engine fits into tight engine compartments easier. It also means that the pushrods are shorter which in turn makes them lighter and more rigid. A complete 431 can weigh as much as 40 pounds less than a similar 440 due to these differences.

Rod/Stroke Ratio: The debate still rages on about Long Rod Engine Vs. Short Rod Engine. This was a major issue in the Moparts.com Forum Topic # 1390339. We will look at each Rod Length. You will see that both combinations are both 0.005 outside of the magic 1.70:1 to 1.80:1 Rod/Stroke Ratio. You have to decide which design fits you application.

Long Rod/Stroke Ratio: Some builders say that longer rods (440 Rods = 6.768”) are better since they reduce side loading on the cylinder walls. A ratio of around 1.70:1 to 1.80:1 seems to be a decent compromise between rod length, strength, weight, etc. The 454 BBC has a stock ratio of 1.53:1 and those motors run okay. The LR431 Stroker allows for a 1.805:1 ratio with stock length 440 rods and it still leaves just enough room for a nice,lightweight, and strong piston.

Short Rod/Stroke Ratio: Other builders say that the shorter rods (383 Rods = 6.358”) provide an engine with better detonation resistance. The SR431 has a Rod/Stroke Ratio of 1.695:1. As Quoted from Popular Hot Rodding magazine:

“According to two-time champion Jon Kasse the short rod yields very fast piston action at TDC and minimizes dwell time so the pistons get away from the chambers as quickly as possible. More time spent at (near) TDC increases the chance that non-homogenized portions of the mixture will ignite on their own and rattle the motor. Smaller bores are advantageous because they reduce the distance the flame front has to travel and the smaller area also offers less opportunity for unwanted secondary flame fronts to develop. The small-bore theory must not be taken to the extreme or valve shrouding becomes a larger issue.”

While it's true the piston with the shorter rod is a little heavier, the shorter rod itself is a little lighter.

Bore/Stroke Ratio: Bigger Bore to Stroke ratios tend to be good up to a point since they reduce the valve shrouding (too big on the bores and the combustion process falters). The 400 block has a STD bore of 4.340”, so it has the largest stock bore size. The 440 bore is 4.320” at STD, so that means it cannot be bored as large as a 400. The 383 bore is 4.250” at STD, which is the same size as the Ford 427 and the Chevy 427. This is a very nice bore size since rings are readily available. The 431 Stroker has a Bore/Stroke Ratio of 4.280/3.75, which is 1.14 and is pretty good. It is not as good as the 1.17 in the 451 Stroker, but better than the 1.06 of a 454 BBC. Still the Ford 302 is much better with the 1.33.

Rotating weight: As mentioned above, the lower deck height allows for a more compact piston, which in turn reduces the piston weight significantly. The following data comes from many sources, including the Mopar Engine Manual (1989), a reproduction 1968 Factory Engine Manual, and a 1969 Chiltons, to name some sources. [NOTE: The next measurements are estimates and should not be used for specification.] The typical piston/pin/rod assembly in a stock 400 weighs 1930 grams. The same assembly for a LR451 using stock 440 rods weighs 1660 grams. Were as, the SR451 using stock 383/400 rods weighs 1710 grams. The typical piston/pin/rod assembly in a stock 440 weighs 2070 grams. The typical piston/pin/rod assembly in a stock 383 weighs 1935 grams. The LR431 using stock 440 rods weighs 1650 grams. Where as, the SR431 using the stock 383/400 rod weighs 1720 grams. (This can be made even about 13 grams lighter by using 0.990 pins)

Weight Break Down and Balancing Information

383 OEM Pistons = 770 grams                  383/SR431 Pistons = 615 grams

400 OEM Pistons = 768.5 grams               383/LR431 Pistons = 500 grams

440 OEM Pistons = 857.5 grams               400/SR451 Pistons = 608 grams

383/400 OEM Rods = 810 grams              400/LR451 Pistons = 510 grams

440 “LY” Rods = 860 grams                      Piston Pin  - 1.094” = 162 grams

OEM Piston Pin = 225 grams                     Piston Pin - .0990” = 149 grams

On the LR431, the reduction of 283 grams per cylinder means a weight reduction of 2264 grams, or 4.99 pounds from the rotating assembly. The SR431 has a reduction of 215 grams per cylinder, which is a loss of 1720 grams, or 3.8 pounds from the rotating assembly. An additional amount must be taken off of the crank counterweights to balance the motor and to make it fit in the engine. [NOTE: The next measurements are estimates and should not be used for specification.] This amount should be about 1050 grams or 2.31 pounds for the LR431. The SR431 should be around 1100 grams or 2.4 pounds. However, because this engine is in the design phase, there are currently no facts that support this weight loss. But if the estimates are correct, then the SR431 - LR431 rotating assembly is 6.54lbs - 7.3lbs lighter, respectively, than a factory 383. This is using fairly common parts. Because of the success of the 451, the 431 will gain from the proven, more exotic parts. This means you can save even more weight easier on a 431, by choosing the right combination.

Balancing and Bobweight:A SR431 using Diamond Pistons, OEM rods and 1.094" pin, needs to have a bobweight around 2350 grams (2348 grams on ours). A LR431 using Diamond Pistons, 440-Source I-Beam rods and 0.990" pin has a bobweight of around 2200 grams. One interesting note was the useage of a forged crankshaft instead of a cast 440 unit. The cast crankshaft is much lighter than the forged unit. The SR431 needed over 700 grams of heavy (mallory) metal to be added to the cast crankshaft to be used in this application. I have to note that if we would have used a 440 harmonic balancer and a weighted torque convertor, the added weight would have been less. So because of the high expense in using the cast unit and the availablity of the forged units, we choose to have a forged unit cut down. Once cut, the crankshaft only needed 14 grams removed to balance. That is cutting it very close without adding material.

Engine Assembly: The parts are easy to come by since 383 blocks are plentiful, 440 cranks are easy to find in the aftermarket and not too bad swap meet stuff. Stock 440 rods work, but Manley, C&A, Eagle, Crower, etc make rods also if you want/need high strength stuff. Due to the recent interest in the 431 Stroker, pistons are beginning to become available. Several manufacturers can produce custom pistons, but Diamond Racing (www.diamondracing.net) and Ross Pistons (www.rosspistons.com) are considering producing off the shelf pistons to work with some of these combinations. There are four different piston configurations that need to be explained. Not all of these are off-the-shelf.

1.      SR431 – 4.28” Bore, 3.75” Stroke, 1.735” CH, 1.094” Pin, 6.358” Rod

2.      SR431 – 4.28” Bore, 3.75” Stroke, 1.735” CH, 0.990” Pin, 6.358” Rod

3.      LR431 – 4.28” Bore, 3.75” Stroke, 1.320” CH, 1.094” Pin, 6.768” Rod

4.      LR431 – 4.28” Bore, 3.75” Stroke, 1.320” CH, 0.990” Pin, 6.768” Rod

The 440 crankshaft has a Main Journal of 2.75”. This needs to be turned down to the 383/400 STD. main size of 2.625”. Also, the counterweights need to be turned down to a diameter of 7.250”. The crankshaft can have a full radii put on it, so it actually turns out quite strong in the process. There are aftermarket companies (www.440source.com, for example) that are selling new crankshafts that already fit this application and others. They have a 3.75", 3.915", 4.15" and a 4.25" stroker crankshaft.

In the past, one way to build the 451 was to bore out the mains on a 400 block to accept the stock 440 crankshaft. You have to machine in the tabs to hold the bearings and then you are usually stuck with an undersize crank. Besides, you still have to send the crank to the machine shop to reduce the counterweights. To argue from a technical standpoint, bigger bearings cost horsepower due to increased friction. This is no longer the recommended proceedure, but given for reference value. If a cast crankshaft is going to be used, by opening up the internal areas of the block, it is possible the install an uncut (outer diameter) cast unit. This would allow for the weight to remain on the crankshaft and it would probably internally balance.

Other Build Examples: The rod journals on the 440 crankshaft can be turned down to BBC size of 2.200”. This allows the stroke to increased or decreased by offset grinding. A max stroke of 3.90” is possible this way and that yields a 448 cubic inch engine. Manley is selling 6.700" Chevy rods that make this combination work and it makes a really nice engine. The piston is even lighter since it is 0.075” shorter and so the rotating assembly weight drops again. Also, the piston pin is changed to a 0.990” pin in this combination and that drops the weight by another 10 to 20 grams per cylinder. The smaller bearing has less frictional loss and it allows for a physically smaller rod, which means more camshaft clearance and more block clearance. Other examples come from 440-Source. Using the 383 Block: SR457" Stroker aftermarket crankshaft with a stroke of 3.915" and a Chevy 6.535" rod. LR457" Stroker aftermarket crankshaft with a stroke of 3.915" and a Mopar "LY" 6.760" rod. The 484" Stroker uses a 4.15" aftermarket crankshaft and a Mopar "LY" 6.760" rod. The 496" Stroker uses a 4.25" aftermarket crankshaft and a Chevy 6.535" rod

Final Thought: The decision on which type of 431 Stroker you want to build is based upon application and personal preference. Some say the LR431 is the way to go because that has been the combination used by most of the 451 Stroker builders. However, the SR431 may have benefits as far as high torque applications, such as trucks, towing and streetability.