High-rpm engines demand quality parts, which is why we spec'd this engine with a good stee
While car crafters are more into horsepower than math equations, the one formula most performance enthusiasts are intimate with is: horsepower = torque x rpm 5,252.
Big engines rely on lots of displacement to make torque and horsepower, but with smaller engines, the approach is to rev it high. Push the torque higher in the rpm band, and you'll make more horsepower. As an example, 400 lb-ft at 4,000 rpm equates to 304 hp. But that same 400 lb-ft at 6,500 rpm is a smokin' 495 hp.
Among the little V-8 engines, the most popular has to be the 302ci small-block Ford. While the rest of the world is building ever-larger engines, the little Blue Oval pushrod 5.0L continues to surge in popularity. Lately, Car Craft has been playing around with different 302 Ford combinations, so this time we decided to punch up the displacement a little while still relying on rpm to deliver a big horsepower number. While 347ci strokers are OK, we decided to go with the shorter 3.25-inch arm and build a 331ci screamer.
Our goal was aggressive yet simple: build a normally aspirated pump-gas 331ci small-block Ford that could make 1.5 hp per cubic inch that wouldn't explode all over the place. You've probably already skipped down to the dyno test, so you know we came really close 500 with 496 hp at 7,400 rpm that is dead reliable.
CC contributor Tim Moore built this engine, and it's currently scheduled as the motorvation for his Dan Gurney Trans-Am-tribute '67 Cougar. Just the thought of spinnin' this small-block up to 7,500 rpm and bangin' Fourth gear coming off Turn 6 at Willow Springs sounds like a great way to spend a Saturday afternoon.
The 331ci small-block Ford is a classic stroker package that combines an excellent bore and stroke combination that can withstand the kind of rpm abuse Moore had planned. The stock 302 Ford small-block combines a 4.00-inch bore with a 3.00-inch stroke. To get to 331 inches, Scat offers a forged steel crank with a slightly longer 3.25-inch stroke that matches nicely with a 4.030-inch bore.
Heads And Cam
The one place "bigger is better" works is where the goal is lots of horsepower at high engine speeds. Since Moore is aiming for engine speeds in excess of 7,000 rpm, he was less concerned with low-speed torque. That's why he decided on a set of Air Flow Research 205cc runner wedge heads for his 331 spinner. AFR offers four different intake-port-volume cylinder heads, from the high-velocity 165cc and 185cc heads to the larger 205 and top-billing Outlaw 225cc monsters. Moore decided to go with the 205cc Outlaw castings that AFR claims will flow more than 300 cfm above 0.550 inch of valve lift on the intake. Packaged with 2.08/1.60-inch intake and exhaust valves, these heads can get with the program, especially with their much more efficient exhaust ports AFR says can generate 225-cfm-plus numbers above 0.500-inch lift. Taking nothing away from the impressive intake flow numbers, it's important to have good exhaust ports if you want to make serious horsepower. With strong exhaust port flow we can minimize the amount of additional camshaft exhaust duration necessary to make maximum horsepower.
Even for stock engine rebuilds, the smart move is to always use a stretch gauge when torqu
This is the Scat forged-steel crank and rod assembly, along with the Mahle forged pistons.
Mahle makes an excellent forged, two-valve-relief flat-top piston for the stroker 331ci Fo
Since Moore fully intended to spin the jeebers out of his 331, he searched out a '68 Mexic
Moore went with a Comp Cams mechanical roller, since there's really no reason for a hydraulic cam to be spinning more than 7,000 rpm. The advertised specs look especially big at 274/280 for a 331ci engine, but at 0.050-inch tappet lift, the numbers come in at 236/242 degrees. To be technically accurate, we also have to subtract 1/2 degree per 0.001 inch of lash that is required for clearance to compare these mechanical lifter numbers with what most enthusiasts recognize as hydraulic cam numbers. With 0.16 inch of intake lash, subtract 8 degrees from the 0.050 duration numbers, which reveals the true specs of 228 degrees of duration at 0.050-inch tappet lift. We combined good Comp mechanical roller lifters, 0.080-inch-wall-thickness Comp Hi-Tech pushrods, and a set of 1.6:1 Pro Magnum roller rockers, tied in with the ARP 7/16-inch rocker studs and AFR roller cam valvesprings-and never once experienced valve float difficulties. This immediately motivated us to consider swapping to a set of 1.7:1 roller rockers, but we ran out of time before we could take this engine to that next level.
|CAM SPECS |
|Camshaft ||Duration |
|Lift ||LASH ||Lobe |
|Comp roller (intake) ||274 ||236 ||0.602* ||0.016-inch ||110 |
|35-770-8 (exhaust) ||280 ||242 ||0.608* ||0.018-inch || |
There is a small amount of block-clearancing necessary to create room for the longer strok
These are gross valve-lift numbers that do not take into account the lash. Subtracting the lash creates a net intake lift of 0.586 inch and an exhaust lift of 0.590 inch. These numbers are based on a 1.6:1 rocker ratio.
Our first pull to check the air/fuel ratio indicated where we were headed when torque was still climbing at 5,000 rpm. We ran a Holley 750-cfm Street HP carburetor and an Edelbrock Super Victor single-plane intake manifold, and the fuel curve was incredibly steady allowing us to concentrate on optimizing the package. With peak torque at 5,600 rpm, the horsepower peaked at 7,400 rpm with 480 hp. Not only is this almost directly on top of our 1.5hp/ci goal, but it's also very repeatable with several runs at that level. Timing and jetting changes produced a few minor improvements, and then we bolted on a Wilson 1-inch open spacer that finally helped us kick the peak power up to 496 hp.
Moore also included a TFS-built main-cap girdle that will clear the Milodon oil pan and pi
With an 1,800-rpm powerband between peak torque and peak horsepower running a solid 1.5 hp/ci and plenty of durability, there's little more we could ask of this stroked-block Ford other than maybe 4 more horsepower so we could claim 500. In post-testing discussion with AFR's Tony Mamo, he suggested the 185cc Outlaw Street heads would probably have made the same peak power while also pumping up the midrange torque slightly. Here's a case where being a little more conservative might have made even more power. How can you lose on a deal like that? We also have to admit that in the car, these numbers will drop a little after fitting the engine with chassis-style heads, an accessory drive, an air cleaner, and other accouterments that must accompany a street-driven engine. But the fact remains that this is one stout small-block Ford.
Test 1 is the initial test as configured in the initial buildup of the engine, including the AFR 205cc heads, Comp mechanical roller cam, Edelbrock single-plane Victor intake, 750-cfm Holley HP carburetor, and 13/4-inch headers with open exhaust.
Test 2 added a 1-inch open spacer to the intake manifold, with all other components remaining the same.
Lubrication is important, and since this engine will also see high lateral-g turns, Moore
| ||TEST 1 ||TEST 2 |
|RPM ||TQ ||HP ||TQ ||HP |
|3,600 ||368 ||252 ||367 ||251 |
|3,800 ||375 ||271 ||371 ||268 |
|4,000 ||378 ||288 ||378 ||288 |
|4,200 ||385 ||308 ||384 ||307 |
|4,400 ||390 ||327 ||390 ||326 |
|4,600 ||395 ||345 ||393 ||344 |
|4,800 ||400 ||366 ||397 ||363 |
|5,000 ||408 ||389 ||405 ||393 |
|5,200 ||414 ||410 ||408 ||404 |
|5,400 ||416 ||428 ||412 ||423 |
|5,600 ||416 ||443 ||413 ||440 |
|5,800 ||416 ||456 ||412 ||455 |
|6,000 ||410 ||468 ||411 ||469 |
|6,200 ||406 ||479 ||407 ||480 |
|6,400 ||399 ||486 ||398 ||485 |
|6,600 ||388 ||488 ||389 ||489 |
|6,800 ||375 ||486 ||377 ||488 |
|7,000 ||366 ||488 ||366 ||488 |
|7,200 ||359 ||493 ||359 ||493 |
|7,400 ||350 ||493 ||352 ||496 |
|Peak ||416 ||493 ||413 ||496 |
|Average ||390.3 ||411.2 ||391.3 ||412.2 |
ATI supplied the balancer we used to keep everything spinning happily at our planned engin
AFR's 205cc heads came assembled with fully CNC-ported intake and exhaust ports, along wit
What is not well known is that AFR raises the exhaust ports on these heads by 0.125 inch,
|PARTS LIST |
|DESCRIPTION ||PN || SOURCE || PRICE |
|AirFlow Research 205cc heads ||1450 ||Buyafr.com ||$1,838.19 |
|ARP head studs ||154-4203 ||Summit Racing || 149.95 |
|ARP intake bolts ||454-2001 ||Summit Racing || 31.95 |
|ARP flywheel bolts ||200-2802 ||Summit Racing || 10.95 |
|ARP balancer bolt ||150-2501 ||Summit Racing || 20.95 |
|ARP oil pan stud kit ||254-1901 ||Summit Racing || 42.95 |
|ATI balancer ||918895 ||Summit Racing || 369.95 |
|Autolite spark plugs, race plugs ||AR-3923 ||Summit Racing || 21.52 |
|Clevite main bearings ||MS590H ||Summit Racing || 69.95 |
|Clevite rod bearings ||CB634H ||Summit Racing || 10.25 |
|Clevite cam bearings ||SH1321S ||Summit Racing || 21.69 |
|Comp Cams street roller cam kit ||CL35-770-8 ||Summit Racing || 599.95 |
|Comp Cams roller rockers, 1.6:1 ||1332-16 ||Summit Racing || 285.95 |
|Comp Cams timing chain set ||3135 ||Summit Racing || 69.95 |
|Comp Cams pushrods ||7937-16 ||Summit Racing || 153.39 |
|Edelbrock Super Victor intake ||29285 ||Summit Racing || 389.95 |
|Ford Mexican 302 block ||used ||Swap meet || 300.00 |
|Holley 750-cfm HP carb ||80528 ||Summit Racing || 663.95 |
|Mahle piston, 4.030-inch bore ||SBF165030F06 ||Summit Racing || 680.95 |
|Milodon oil pan ||31600 ||Summit Racing || 349.95 |
|Milodon oil pump ||18800 ||Summit Racing || 78.69 |
Sporting 2.08/1.60-inch stainless valves, the 58cc chamber made the static compression 10.
Mechanical roller cams are the kings of horsepower and also allow you to fine-tune horsepo
The induction side of things consisted of an Edelbrock Super Victor single-plane and a Hol
|Milodon oil pump pickup ||18485 ||Summit Racing || 57.39 |
|Milodon oil pump shaft ||22500 ||Summit Racing || 20.95 |
|Milodon windage tray ||32210 ||Summit Racing || 49.95 |
|Milodon install kit ||81157 ||Summit Racing || 37.95 |
|MSD billet distributor ||8598 ||Summit Racing || 268.60 |
|MSD plug wires, universal ||31189 ||Summit Racing || 80.60 |
|Scat 4340 steel, crankshaft ||43022 ||Summit Racing || 561.95 |
|Scat H-beam rod, 5.400 inches ||65400927 ||Summit Racing || 399.95 |
|TFS Ford main girdle ||51500700 ||Summit Racing || 204.95 |
|Victor MLS head gasket ||MLS-54290 ||N/A || -- |
|Victor oil pan gasket ||OS32491 ||N/A || -- |
|Victor rear main seal ||JV730R ||N/A || -- |
|Victor timing cover gasket ||JV856 ||N/A || -- |
|Victor intake gasket ||95197SP ||N/A || -- |
|Victor valve cover gasket ||VS50203 ||N/A || -- |
|Victor header gaskets ||95177SG ||N/A || -- |
|Wilson 1-inch spacer ||000010 ||Summit Racing || 89.95 |
For this test, we left the lash stock at 0.016 and 0.018 inch for the intake and exhaust.
If you look closely, the 1-inch open spacer added a little peak power, but it also cost a
AirFlow Research (AFR)
28611 W. Industry Dr.
Holley Performance Products
1801 Russellville Rd.
Bowling Green, KY 42101
ATI Performance Products
2250 Agate Ct.
Automotive Racing Products (ARP)
531 Spectrum Circle
Autotronic Controls Corp. (MSD)
Trick Flow Specialties (TFS)
1248 Southeast Ave.
4700 NE 11th Ave.
2700 California St.