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371ci Mopar Small Block - Deadly WeaponSKMFX builds a budget iron-headed 510hp small-block Mopar From the August, 2012 issue of Popular Hot Rodding By Steve Dulcich Photography by Johnny Hunkins
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Starting with a production... Starting with a production Chrysler 360 LA-Series small-block, the bottom end was filled with basic parts that are more than up for performance use. An Eagle stock-stroke crank is spun by stock-length Scat I-beam rods, with KB Icon forged pistons providing the shove. The block received a partial fill in the water jackets to better support the cylinder walls. Maximizing bang for the buck comes down to making the most of a given combination of parts. When it’s done right, the engine will throw down power numbers that belie its modest parts list, packing a heavyweight punch in an unassuming package. This small-block Mopar from SKMFX Engines is a testament to that ideal, with righteous output from commonplace parts. This engine was built for competition in the Street Division of our annual engine building competition, the AMSOIL Engine Masters Challenge. Jesse Robinson and his teammates from SKMFX Engines assembled this bullet as a backup engine to compete in place of their big-block, which was held up by part delivery delays. The stout output from this very basic production-based combination was impressive. SKMFX started with an ordinary production “LA-Series” Mopar 360 block, and proceeded with a bottom end build that would be typical of any budget street effort. The block received basic prep, including a .060-inch overbore, line honing, and decking. Robinson also added a short fill to the water jackets: “I’m a big believer in the short fill, and I like what it does to the water and oil temperatures. The exposed cylinder ends up at about 4 inches, and it stiffens the cylinder walls considerably,” Robinson says. The block also had the main oil feeds enlarged to 9⁄32 inch, but was otherwise stock in terms of the lubrication system. The stock production main... The stock production main bearing caps were retained, though high-strength ARP main studs provide extra clamping. Jesse Robinson from SKMFX tells us the factory 3.58-inch stroke is far easier on the bottom end assembly than a long-armed stroker. Filling the block is a rotating assembly based on the stock 360’s 3.58-inch stroke. Rather than using the stock cast-iron crank, Robinson selected an Eagle 4340 steel crank. “This crankshaft is a good value for the price at under $600,” he says. “One of the major hiccups with the 360 is the external balance. A few years ago, I had a stock 360 crank internally balanced and it cost as much as this Eagle crank because it took six slugs of Mallory metal to balance it. At that point it makes sense to move to the steel.” The rods are a set of Scat I-beams in the stock 6.123-inch length. Robinson explains this choice: “I don’t like the H-beams for many applications; they are just too heavy. These Scat I-beams are just a beautiful set of connecting rods, and a great value for the money.” Filling the cylinders is a set of KB Icon pistons and Perfect Circle rings. The internal parts combination seemed to follow the theme of budget-oriented replacement parts that are proven durable in a high-output application. The KB icon pistons are wrapped... The KB icon pistons are wrapped with a conventional Perfect Circle 1⁄16-1⁄16-3⁄16 ring pack. The most exotic part of the ring package is the Napier-faced second ring, which aids in oil control. Bearings are coated Clevite pieces. Topping It For Power A similar approach followed up to the top of the engine with the EngineQuest Iron Magnum 318B cylinder heads. These heads are designed as production replacement units for the OEM Mopar Magnum heads, but are revised with the earlier “LA-Series” intake manifold bolt angle. The castings are significantly improved compared to the factory units with both durability and performance in mind. Robinson’s experience with the EngineQuest cylinder heads has been overwhelmingly positive: “I started using EQ heads on Chevrolets, using their Vortec heads on 350 and 383 Chevys, and the results were never disappointing. I started to experiment with the Dodge heads, and again they produce very good power.”  Robinson modified a stock...  Robinson modified a stock windage tray by opening large drain backs, and then bonding screens in place with epoxy. Many small-block Mopar fans question the validity of a windage tray, however, Robinson related that he has seen substantial power gains with a tray in place.  Oiling was handled by a Melling...  Oiling was handled by a Melling pump, with the pick-up relocated to the rotor cover. Robinson explains the move was primarily to ease making the custom oil pickup. The lubrication system is essentially stock, with the exception of opening up the oil passages to the mains.  A custom COMP hydraulic roller...  A custom COMP hydraulic roller cam works the valves, with more aggressive timing and lobes on the intake side. Cam specs were ground to 246/244-degree duration at .050, with .646-inch lift. A tight lobe separation angle of 106 degrees promotes torque.  A secondhand set of T&D rockers...  A secondhand set of T&D rockers were mocked-up on the EQ heads, and the geometry was optimized with custom 5.250-inch length valves measuring 2.08 inches in diameter. This arrangement bolts to the stock Magnum-style pedestals, though the fasteners are upgraded to ⅜-inch diameter rather than the OEM 5⁄16-inch dimension.  Providing for pushrod clearance...  Providing for pushrod clearance required substantial clearancing of the valve cover rails at the intake side of the head. The pushrods are Trend units, which Robinson praises for their minimal deflection.  Due to clearance issues with...  Due to clearance issues with the rockers and pushrods, standard small-block Mopar valve covers could not be used. These fabricated aluminum covers were sourced from an eBay vendor, and feature an internal rail design for increased clearance. The EngineQuest cylinder heads... The EngineQuest cylinder heads were the subject of an impressive development effort. Robinson wrote a CNC program to cut the bowls and port runners, and hand-finished the ports and chambers. Final combustion chamber volume measures 59 cc. The EQ heads on this particular engine were the subject of considerable development work at SKMFX. Robinson explains: “I got to work pretty extensively in the cylinder heads. I got away from the factory 8mm stem valves, and went to 11⁄32-inch because it opens up far more selection in off-the-shelf valves. I went with Manley Severe Duty intake valves in a 2.08-inch diameter, with a 5.250-inch length. A part of the puzzle that predicated how I built the heads was coming across a set of secondhand unused T&D rockers. I put the bedplate on the Magnum pedestals, and selected the valve length to best suit the geometry with the rockers. These EQ cylinder heads are nice to work with, having plenty of metal for porting. I put them on the CNC mill and wrote a program to open the bowl out to a venturi shape, and with this mod alone the port went to 273 cfm. I put the valve job on it and hand-blended the machine work, and then put it on the CNC again to open up the intake side of the port. After hand-finishing the port work and many trips from the cutting bench to the flow bench, I ended up with 300 cfm at .600-inch lift.” A characteristic of the Magnum-style... A characteristic of the Magnum-style heads is outstanding exhaust flow. Robinson relates that there is a danger of over-scavenging the engine. With the exhaust flow in mind, the cam was spec’d with a milder lobe and less duration on the exhaust side. On the camshaft, the custom COMP hydraulic roller cam actually uses slightly less duration on the exhaust side than on the intake. Part of this is because of the very high exhaust flow from the EQ heads. “The exhaust side has very high flow; it can get to around the 200-cfm level with very little work. I found on the other EQ head builds and the W2 head builds I’ve done in the past, that I’m over-exhausting the engines. My VE numbers will be over 100 percent from the bottom of the pulls to the top, and the power doesn’t really correspond with those kinds of efficiency numbers, so I’m led to believe that I’m blowing everything out the exhaust. I took one stab at the camshaft, and I don’t think I was too far off the mark, since on my dyno the engine was producing over 510 lb-ft of torque at the peak.” The cam features 246/244 degrees duration, using the more aggressive COMP SPTK lobes on the intake side, and the milder HRR lobes on the exhaust. Lift is .648 inch with the 1.5:1 ratio rockers, while the lobe separation angle is 106 degrees. The cam was installed at an intake centerline angle of 102 degrees. As run in the competition,... As run in the competition, the intake ports posted a peak flow of 300 cfm at .600-inch lift. A thick Teflon intake gasket was CNC cut at SKMFX to insulate the induction system from engine heat. The engine uses a Weiand X-celerator intake manifold, but Robinson tells us that he thinks there was room for improvement in the induction. “It was the only manifold I had at the time, and I figured I would just put it on if the output looks like it is within reason. I had the heads flowing around 300 cfm, and with the intake on it the intake port flow dropped to around 230 cfm. I experimented with custom runner extensions in the plenum, but they ended up hurting power everywhere. I actually cut them back out, and put in much more conservative extensions.” Topping the manifold is a Holley 750 HP. Robinson spoke highly of this unit: “What can I say about the Holley HPs? The 750 HP seems to be the one to buy if you want to have just a spectacular carburetor out of the box. I did not modify it; I just calibrated the air bleeds and jetting.” Providing the air/fuel mixture... Providing the air/fuel mixture is a Holley 750 HP-Series carb. Robinson tells us these are outstanding carburetors at this power level and displacement. The carb was run as-delivered with the exception of routine calibration changes. Taking advantage of a production oddity, the engine is fitted with a factory Chrysler Lean-Burn distributor. These distributors featured no advance curve or vacuum advance. As Robinson puts it: “Coupling the locked-out production distributor with the programmable curve of a MSD 6AL2, is a match made in heaven.” Rounding out the engine package is a pair of Schoenfeld stepped headers with 1⅝- to 1¾-inch primary tubes. Up front, a CAT crankshaft damper handles the vibes, while a custom billet water pump housing crafted at SKMFX accepts a Moroso big-block Mopar electric water pump.  A Weiand 7545 single-plane...  A Weiand 7545 single-plane intake manifold was heavily modified with porting. Robinson experimented with different runner extension configurations and found that their form had a substantial effect on the power curve.  Sandwiched between the carb...  Sandwiched between the carb and intake is a Wilson tapered spacer. Robinson finds this spacer configuration will typically show an overall improvement in mixture delivery, and thus power output.  An off-the-shelf set of Schoenfeld...  An off-the-shelf set of Schoenfeld Sprint Car headers handles the exhaust. The headers feature a stepped design, with 1⅝-inch primary tubes at the exhaust port stepping up to 1� inches. Note the crankcase evacuation system plumbed into the collectors. Interestingly, Robinson tells us on the dyno the evacuation system had a minimal influence on power. A nice custom touch on this... A nice custom touch on this engine is the SKMFX-fabricated billet electric water pump housing. The housing accepts a compact Mopar big-block electric water pump. At the AMSOIL Engine Masters Challenge, Robinson and the SKMFX team put on a very respectable performance with their budget combination, showing a peak of 515 hp in qualifying with their stock-stroke iron-headed Mopar, while running on VP100 fuel. What was even more impressive was the very fat torque curve, exhibiting over 400 lb-ft at just 2,500 rpm, with max torque swelling to 495 lb-ft at a 4,800-rpm peak. These would be decent numbers for a mildly built stroker combination, and the output is exceptional for an engine with production-style heads. As Robinson explained: “I’ve done a few stroker 408s, and you need a lot of cylinder head for that engine. Most guys don’t have that budget, and with budget cylinder heads I like the power curve of the smaller stock-stroke engines.” Thinking about how sweet an engine with this power curve would run on the street, we can’t argue with that point. The ignition system on the... The ignition system on the small-block Mopar was primarily made up of MSD components, with the exception being a rebuilt factory Chrysler Lean-Burn distributor, which has no timing advance. The MSD 6AL2 handled the ignition curve. Heads For Power The EngineQuest (EQ) cast-iron Magnum cylinder head is an iron replacement head that delivers a surprising punch at an affordable price. These heads offer an improved alternative to stock with better flowing intake and exhaust ports for enhanced horsepower and torque. The castings feature a 172cc intake runner volume, with 62cc combustion chambers. The heads are cast in Australia, using a casting technique that produces an exceptionally smooth port surface for more flow and power. Used in box-stock form, these heads offer superior quality at an affordable price with no additional machining, porting, or polishing required. The EQ Magnum cylinder heads are available for both the factory LA-series and later Magnum Mopar small-block engines. EQ’s part number CH318B head is machined with the LA-series–style intake pattern, while EQ’s part number CH318A features the vertical ’92-04 Magnum intake pattern. Both cylinder heads are built to accept the OEM Magnum-style valvetrain and rockers. For a largely production-based... For a largely production-based engine configuration, this small-block Mopar delivered healthy output, posting 515 peak horsepower at 6,000 rpm, while 495 lb-ft of twist was recorded. Besides the improved ports and flow, EQ has improved on the factory Magnum design with a thicker deck for added strength, added hardened exhaust seats for greater resistance to cracking, and redesigned the water flow system for improved thermal conductivity. With CNC-machined guides and seats, a superior port finish for increased flow, and a three-angle valve seat, EQ’s cylinder heads offer the best of both worlds—OE fit and greatly improved durability and performance. Each head is individually subjected to a rigorous 25-point inspection to ensure a quality product. For more information about EQ’s Magnum cylinder heads, phone 800-426-8771 or visit www.EngineQuest.com. At the 2011 AMSOIL Engine Masters Challenge, the SKFMX crew included Jesse Robinson, Joe Rutters, and Ron Maclean.
By The Numbers
371ci Mopar Small-Block |
| Bore: |
4.060 inches |
| Stroke: |
3.58 inches |
| Displacement: |
371 ci |
| Compression ratio: |
11.3:1 |
| Camshaft: |
COMP Cams hydraulic roller |
| Camshaft duration: |
246/ 244 degrees at .050-inch tappet rise |
| Valve lift: |
.648 inch |
| Rocker ratio: |
T&D 1.5:1 ratio |
| Piston rings: |
Perfect Circle 1⁄16-1⁄16-3⁄16 |
| Piston: |
KB Icon |
| Block: |
Mopar 360 LA, production |
| Crankshaft: |
Eagle |
| Rods: |
Scat I-beam |
| Cylinder head: |
EngineQuest Magnum |
| Intake valve diameter: |
2.08 |
| Exhaust valve diameter: |
1.60 |
| Intake manifold |
Weiand |
| Carburetor: |
Holley 750 HP |
| Header: |
Schoenfeld stepped primary |
| Ignition: |
MSD 6AL2 |
| Damper: |
CAT |
| Fuel: |
VP Racing VP100 |
| RPM |
TQ |
HP |
| 2,500 |
401.5 |
191.1 |
| 2,700 |
408.4 |
202.1 |
| 2,900 |
426.2 |
235.3 |
| 3,100 |
429.6 |
253.6 |
| 3,300 |
430.8 |
270.7 |
| 3,500 |
425.3 |
283.4 |
| 3,700 |
431.3 |
303.8 |
| 3,900 |
444.9 |
330.4 |
| 4,100 |
461.2 |
360.0 |
| 4,300 |
473.9 |
388.0 |
| 4,500 |
486.1 |
416.5 |
| 4,700 |
493.7 |
441.8 |
| 4,800 |
495.0 |
452.4 |
| 4,900 |
494.3 |
461.1 |
| 5,100 |
491.9 |
477.7 |
| 5,300 |
485.9 |
490.4 |
| 5,500 |
480.7 |
503.4 |
| 5,700 |
470.9 |
511.1 |
| 5,900 |
458.4 |
515.0 |
| 6,000 |
450.9 |
515.1 |
| 6,100 |
441.5 |
512.7 |
| 6,300 |
424.1 |
508.7 |
| 6,500 |
402.9 |
498.6 |
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