Here is a great article I found on making a 400 into a 451ci motor.
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Mopar 451. The motor that SHOULD be...
The 451 is arguably the best BB Mopar engine combination available. The 451, which is made by dropping a 440 crank into a 400 block, has almost perfect design parameters.
Deck Height: The 400 block, with a deck height of 9.980 is perfect for a 3.75 stroke since a 1.80 rod ratio yields a nice light compression height of 1.355. 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. This means that 451 has a typical piston weight of around 550 grams instead of 800+ for the 440. BTW, 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 400 block is right in between the two BBC's. The lower deck height of the 400 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 451 can weigh as much as 40 pounds less than a similiar 440 due to these differences.
R/S ratio: Okay the debate still rages but for all practical purposes, longer rods are better since they reduce side loading on the cylinder walls. A ratio of around 1.80 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. 1.80:1 is better but I'm not sure that 2:1 is worth paying extra for. The 451 allows for a 1.80:1 ratio with stock length 440 rods and it still leaves just enough room for a nice, lightweight, and strong piston.
B/S 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. A bore of 4.375 is a very nice bore size since rings are readily available. That is a 0.035 over 400 but it would be a 0.055 over 440. Less overbore means more strength and the possibility of additonal overbores. 4.375/3.75 is 1.17 which is pretty good. Better than the 1.06 of a 454 BBC but not as nice as the 1.33 of a 302.
Rotating weight: As mentioned above, the lower deck height allows for a more compact piston which in turn reduces the piston weight significantly. The longer 440 rod and longer stroke of the 440 crank also means that a 451 has lighter pistons than a stock 400. The typical piston/pin/rod assembly in a 400 weighs 1930 grams. The same assembly for a 451 using stock 440 rods weighs 1630 grams. (This can be made even lighter by using 0.990 pins) The reduction of 300 grams per cylinder means a weight reduction of 2400 grams, or 5.3 pounds from the assembly. An additional amount must be taken off of the crank counterweights to balance the motor. This amount is 1060 grams or 2.33 pounds for the above configuration. That means that the 451 rotating assembly is 7.6 pounds lighter than a 400. Pretty dramatic results when you mash on the loud petal from that kind of weight reduction. This is using fairly common parts. The 451 accomodates itself to more exotic parts due to the piston dimensions and rod lengths. That means you can save even more weight easier on a 451 than on a 400 or 440.
Easy to build: The parts are easy to come by since 400 blocks are quite common (and not very well liked so they tend to be cheap), 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. Several manufacturers produce off the shelf pistons to work with this combo so that is easy. The 440 crank needs to be turned down to the 383/400 main size and the counterweights need to be turned down to a diameter of 7.250 but that is easy crank shop stuff. The crank can have a full radii put on it so it actually turns out quite strong in the process. It is possible to bore out the mains on a 400 block to accept the stock 440 crank but that seems the hard way to go. 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. And to argue from a technical standpoint, bigger bearings cost horsepower due to increased friction.
Other close relations: The rod journals on the 440 crank 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 470 cubic inch motor. Manley is selling rods that make this combination work and it makes a really nice motor. 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 30 or 40 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. (see how it all works out kind of cool?) This is a bit more money to build but it is still very reasonable. The Manley rods are $750 and the offset grinding is usually about $100 or so.
So there are my reasons for calling it the best of the Mopar BB's. Lighter, stiffer, smaller for not much extra money. Sounds like a winner to me.
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I'd keep it cheap and just use 440 rods.
