Built to idle like a stocker,...
Built to idle like a stocker, but pulling down over 400hp and winding willingly to well over 6,000 rpm, our 400 lowdeck will get you where you want to go in a hurry.
Last month we built the bottom end of our street 400 Mopar mill. Better known as a low compression smog chugger, or the basis of radical stroker combos, we were intrigued with the idea of building a stock-stroke street performance lowdeck 400. After all, with its 4.342-inch x 3.375-inch bore/stroke combo, the 400 makes for some interesting comparisons versus some of the more tried and true big block Mopars. What if you could take a 383 Road Runner mill and bore it out over a tenth of an inch, or .122-inch? It would be one radical 383 if it was possible, but we know that with the available bore material this is just a pipe dream. That bore also equals a stock 400 block with a basic .030-inch overbore ...hmmm. What about a 440, de-stroked by 31/48-inch to make a high rpm screamer? Wanna know what it would do? Again, it's basically the same package as the stock factory-issue 400.
True, as issued by the factory, the 400 never got a chance to show its colors as a high performance mill. Introduced in 1972, the era of high compression was by then replaced by the emissions control era. The 400 was hit especially hard in the compression ratio department, with a rated ratio of 8.2:1-though the actual measured ratio was dippin' into the sevens. Add a host of smog equipment and it's no wonder the 400 never got noticed.
In last month's issue, we took care of the low compression problem, compliments of KB Pistons' KB-240 hypereutectic pistons. These are the first reasonably priced off-the-shelf performance pistons for the 400. We added the commonly available 383 forged crank to replace the 400's factory cast piece, and assembled the shortblock with quality components from Summit Racing, including a set of Total Seal moly rings. The stuff that comes next will ultimately determine the temperament and output of our finished engine.
Buildin' To Drive
We've built a wide variety of engine combos over the last few years, some radical, some a bit more tame, but all streetable. What's the plan for this 400? Think "'71 Challenger R/T, factory 383 auto/air car, loaded to the gills with two body buck tags full of options." The original 383 Magnum came through with a factory gross rated 300hp (250hp SAE net); with the original 3.23:1 rear it was no drag racer, but an almost unsurpassed road car. We wanted our 400 replacement engine to deliver an output of 1hp per cubic inch, but do it with the same level of civility and smoothness of the stock 383 Magnum. The easiest and quickest path to making big power was now out. We just didn't want to live with a big lumpy cam, completely changing the personality of our Challenger, but we still wanted a big power edge over the stock 383. Oh, and let's not forget about comparable fuel economy, plus long term reliability all at a reasonable cost.
Making this combination a reality meant keeping the camshaft short-no longer than the stock Magnum cam of the original 383. Instead of a big cam, we would depend on a good set of heads to let the air flow in. With a short cam for street drivability, we were banking on the 400's short stroke/big bore combined with excellent head flow to provide good higher rpm torque-to extend the powerband, delivering the power increases we were after with solid power numbers up top.
 Putting the lid on the bottom...  Putting the lid on the bottom end, we went with an original Mopar windage tray (available from Mopar Performance, part #P4120998) and the stock Hp pan. Note the factory's use of effective horizontal baffling in the stock sump. |  Competition Cams' new Xtreme...  Competition Cams' new Xtreme energy XE268H cam was a key component in making big power numbers while maintaining smooth operation. We went with Comp's K-Kit, which includes the cam, lifters, springs, valve seals, retainers, locks, and timing set. Be sure to specify the correct type of locks for the valves used. Our Milodon valves use single-groove locks, while production valves can be single, dual, or triple groove. |  The cam was installed and...  The cam was installed and degreed-in to an installed centerline of 108, rather than the recommended 106, to aid breathing at higher rpms given the cam's conservative specs. |
 With the cam in, the front...  With the cam in, the front end was buttoned up using the stock timing cover, damper, and water pump housing. The new oil pump was a Melling high volume unit from Summit. We found in testing that the bypass spring in the pump was too short, not allowing the valve to seat, bleeding oil pressure. A couple of washers behind the spring fixed it. The water pump is a new GNB aluminum unit. |  The assembled heads went on...  The assembled heads went on next, using ARP bolts from Summit Racing. The heads would be the key component in making our 400 turn in powerful performance. The heads were bolted on using the .040-inch thick FelPro gaskets from our Summit sourced kit, giving us a true 10:1 compression ratio with zero deck on the pistons. |  The valvetrain was set up...  The valvetrain was set up using Crane's excellent aluminum roller tipped 1.5:1 ratio rocker and shaft kit. This kit is complete, minus the hold downs-use the stock pieces, available new from Mopar Performance, part #P4529744. Preload was set in sequence to 11/42 turn in from zero lash on the adjusters. |