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.
Our choice of camshaft would be critical, for the sake of keeping the engine smooth and reliable, yet delivering on our power goals. We tapped the Comp Cams catalog for one of their new Xtreme Energy hydraulic cams. This new line of performance cams features a more aggressive rate of acceleration than traditional hydraulic grinds, providing more lift at a given duration than some of the more common older grinds. The cam we chose was Comp's Xtreme Energy XE268H, with a gross intake and exhaust duration of 268 degrees and 280 degrees, respectively; duration at .050-inches of 224 and 230; and .477-inch/.480-inch lift. The overlap with the cam's 110-degree lobe separation angle is 54 degrees. Comparing specs, the stock 440/383 Magnum cam carried a gross duration of 268-degree intake/284-degree exhaust; duration at .050-inches of 208 and 220; .450-inch/.458-inch lift; on 115-degree lobe centers, giving 46 degrees of overlap.
Although the stock 440 Magnum cam carries the same intake and slightly more exhaust duration, the overlap on the Comp cam is significantly higher (by 8 degrees), owing to the narrower lobe separation angle. It's not that the Comp cam has an especially narrow lobe separation angle (it doesn't), it's that at 115 degrees, the factory cam was very wide. Based upon the increased overlap, the Comp grind will be somewhat cammier, but at 54 degrees overlap, it is still well in the range of a daily-driver streetable grind in a 400-inch motor, even with an automatic trans, air, and power brakes. The most significant difference is in the duration at .050-inch. The Comp grind is so much more aggressive that it reaches .050-inches 16 degrees quicker on an intake lobe of the same duration, and on the exhaust side reaches .050-inches 10 degrees sooner on a 4-degree shorter lobe-a dramatic increase in how fast the modern Comp grind gets the valves moving. This advantage is carried right to max lift, with the Comp cam having .027-inch and .022-inch more lift on the intake and exhaust lobes respectively.
Without increasing the duration, the Comp cam profile provides a significant increase in area under the lift curve, which translates directly into airflow. Another plus for the Comp grind is that with the narrower lobe separation angle, the intake valve closing point is sooner in relation to piston position, helping with torque generation lower in the power curve. Along with the cam, we ordered Comp's lifters, timing set, steel retainers, locks, and springs. The final addition to the valvetrain was a set of Crane's proven and reasonably priced aluminum roller-tipped rockers and pushrods.
Heads Up
Because we were being conservative in the cam selection in the interest of street drivability, we would need an outstanding set of heads to deliver the flow needed to make power. While aftermarket heads can fit the bill, we planned on using production castings that we would modify ourselves to keep the costs down. With the 400 being some 40 cubes smaller than the popular RB Mopar, the airflow-to-cubic-inch picture is more favorable on the short stroke 400 than its big brother. With 9.1 percent fewer cubes to feed, a trick set of modified stock heads would be more than enough for the job. A couple of years back we did some extensive flow bench time on big block Chrysler heads, figuring out what made them move air (see "Going With the Flow," Mopar Muscle, January, February, March, 1999). Our plan was simply to apply all of the porting tricks we learned to a set of factory #346 castings we happened to have. Why the #346? We had already stolen the #452 castings that came on our 400 for another project, and we had plenty of #346s. The fact is our testing showed no real difference in flow potential between these and any of the later production heads, including the sought-after 452s, so the question really should be, why not?
The #346 head was used from 1971-1973 on B/RB engines, and was the first of the redesigned "emissions" heads. This same basic design was maintained on all later castings up until the #452 heads in the last few years of big block production. We found that any of these later heads are much easier to modify for high airflow than the earlier #906 or #915 castings. With conservative porting and the commonly available 2.14-inch/ 1.81-inch oversized valves, the "smog" heads will significantly outflow all but expertly ported early heads. The drawback of the #346 is that unlike later heads designed for unleaded fuel, they came with non-hardened valve seats. On the flip side, the merit of using later heads with factory induction hardened seats is debatable, since once the required cut is made for oversize valves, much of the hardened material is machined away, and the production heads with hardened seats are more prone to cracking. The best way to go is to have hardened seat inserts installed on the vulnerable exhaust side, which is what we had M&R Machine do on our heads.
Induction Action
With the heads and cam situation handled, the final decisions revolved around the induction. Our first choice for this application was the Holley Street Dominator single plane manifold, a proven design on low deck street engines. Unfortunately, this manifold has been discontinued. The Weiand Action Plus dual plane manifold is a direct replacement for a stock square bore manifold (minus choke well), and, if painted, can even pass for the stock piece at first glance, which is the one we decided to go with. Since the Weiand manifold is a squarebore, we also ordered a foolproof Holley 80508S 750 vacuum secondary carb. The 80508S comes with an electric choke, but being a 4160 series carb, it is not equipped with a 4150-style metering block on the secondary side, using a fixed metering plate instead of replaceable jets on the secondary side. This was remedied with a Holley #34-13S metering block conversion kit, which replaces the metering plate with a 4150-style block, allowing jet changes on the secondary side. A Mopar linkage kit is also needed, Holley #20-7. Holley also supplied one of their street performance #12-831 mechanical fuel pumps, a rebuildable type similar to the Carter high performance units of Hemi fame, but in flashy chrome.
With our good flowing cylinder heads, we reasoned that a single plane would prove to be a strong power producer. The Mopar Performance M-1 manifold is reputed to be the best single plane manifold, but the design is more race-orientated than the Holley we originally considered. The M-1 has no provisions for an exhaust crossover (we blocked ours anyway) or thermostatic choke; and unlike the moderate height of the Street Dominator, it is a significant high-rise design which may cause hood clearance problems. We were, however, eager to bolt one of our own modified big-bore Thermo-Quads to the spread bore carb flange on top of the M-1 manifold to see what it would do. To test how this manifold would work in a well set up but docile street engine, we ordered one from West Oaks Dodge. We realize that not everyone has a 55-gallon drum full of big spreadbore Thermo-Quads to fiddle with, or would even want to, so keep in mind that the carb flange on the M-1 has a dual pattern to also accept any standard squarebore carb.
Runnin' The Numbers
With the engine bolted together, it was time to hook up for some dyno numbers and see if our performance goals were reached. As stated before, the original 383 Magnum in our Challenger R/T carried a factory gross rating of 300, and a net rating of 250. The truth was probably somewhere in between, as evidenced by Mopar Performance's bracket racing bulletin, which estimates the real world output of the full range of stock 383 4-barrels at between 260-290hp (including the high compression 335hp rated version). Our 400, though the exact same physical size and weight as the 383, has an additional 17 cubic inches. More importantly, we had 10:1 compression, a nice set of ported smog heads with oversized valves, a more modern cam profile of virtually the same duration as the stock Magnum cam, an aftermarket intake very similar to the stock piece, and Mopar's electronic ignition. Nothing on this list will detract from drivability, but all of these additions will add to the power curve.
On the dyno, with the conservative two-plane manifold, our 400 produced 401hp at 5500 rpm, and 412.5 lbs/ft at 4900 rpm. As expected, the big bore/short stroke 400 loved to rev, even with the short duration cam. While big inch strokers, especially with production heads and short cams, tend to reach a peak and then the power plummets like a stone, the 400 was holding within 15hp of its peak at 6200 rpm. This combo was enough to hit the numbers to meet our goals, and produces enough power, we figure, to get our loaded 3700 lb. Challenger solidly into the 12s with an engine that idles like a dead stocker.
While we hit the targets we sought with our initial combination, we still had the M-1 single plane intake/ThermoQuad combo to try. A few bolts later, we had completed the swap, and loading up the 400 against the dyno, shattering the myth that a single plane would cost power in the low end. Torque was up to a peak of 441 lbs/ft at 4,500 rpm. That's a gain of 28.5 lbs/ft, peaking 400 rpm lower! Up top, horsepower went to 410.9 at 5,500, for a gain of about 10hp. This combo provided a much fatter power curve, but to be fair, the design goals of the two manifolds are at opposite ends of the scale. The M-1 is an unabashed race piece, while the Action Plus is a performance stock replacement unit, designed as a bolt-in upgrade while retaining stock hood clearance. Either way, this deceptive low deck has the punch for serious performance, while looking, sounding, and driving with the mild mannered ease of a stock 383.
Dyno Results
Specialized Motor Service
Clayton Engine Dyno; Depac Electronics
Standard Correction: 29.92-inch Dry Air @ 60° F
Test 1-Weiand Action plus two plane intake;Holley #50802 750 CFM Vacuum secondary carb
Test 2-Mopar Performance M-1 single plane intake; Factory 1973 440 large-bore Thermoquad carb
Hot Heads
We wanted lots of power, but smooth running with this 400. A conservative choice in cams is required to make the engine smooth, but the power end of the equation isn't going to happen by magic. A set of high flowing heads is the only way it can be done. We took a set of #346s and worked 'em with every trick in the book to do the job. The #346 heads and any of the later heads are considered by many to be "smog heads." However, we know from our testing that with the same amount of porting work, they will substantially outflow the earlier castings. The popular early heads can make big flow, but it'll take twice the grinding time, and you had better know where to cut.
The #346, like the earlier heads, were designed for leaded fuel and have soft iron seats. With unleaded, in a high load/high performance application, the exhaust seats will quickly deteriorate. The best approach for street reliability with pump gas is to have hardened inserts installed on the exhaust side. Our first stop with these heads was to M&R Machine in Glendale, California, to have inserts installed.
The head prep is where most of the money will be spent in a build, but it's also the key to making power reliably. Besides the seat inserts, bronzewall guides, quality machining, and a superior valvejob all cost money, but add performance and reliability. We did our own seats on a Serdi 100 Seat and Guide machine, going with a special 30-degree profile on the intake, which we find pays big dividends in low- to mid-lift flow-especially useful with a conservative camshaft.
Porting is another key area. We won't repeat the in-detail description of how it is done here, but we did all of the porting work at home. The plan isn't to go hog wild with the grinder, either. Paper thin, welded, and epoxied ports have no place on a reliable street engine. We took what we know about extracting flow from production Mopar big block heads, and set about modifying these castings for what ended up being a 28-percent increase in flow area under the lift curve. That's not 28 percent at peak flow, that's 28 percent average over the entire curve, with substantial increases right from very low lift on up. To learn how to do it, dig up your back issues.
We realize that not everyone can or wants to port their own heads, but Jerry Goodale of Specialized Motor Service is up on all of the tricks we used, has the Serdi machine with the special cutter to carve the 30 degree seats, and can install the required unleaded seat inserts.
