There’s an old saying among hot rodders that still applies today … there’s no replacement for displacement. Just ask any Corvette owner, especially ones who own early big-block cars. Yep, there’s nothing like the seat of the pants “kick” from an early 396, or better yet an overly pushed 454. Today’s engines are making insane amounts of power with fewer cubic-inches; however, nothing will ever replace the feeling and true backside thrust gained from ground-shaking Corvette big-block power. We’re talking that deep throaty roar of a big-bore 396-, 402-, 427- or 454-inch Chevy along with seismic torque. And if you dare to venture beyond 454 ci, the sheer number of cubic-inches courting 500+ cubic-inches available from GM Performance coupled with a Scat stroker package is astounding.
The Mark II big-block Chevy first gained notice in February 1963, at Daytona when a select group of Chevy stock car racers that included Junior Johnson and Johnny Rutherford entered the high banks in cars powered by what is known today as the “Mystery Motor,” which was also tested and raced by the legendary Smokey Yunick, Ray Fox and Bubber Farr. Aside from its crankshaft, the 427ci big-block mystery mill had nothing in common with the “W” block 348 and 409ci engines it replaced. The 427 was a standalone powerhouse about to give birth to a new generation of incredible high-displacement Mark IV, V, and VI Bow Tie big-blocks.
Inside Chevrolet corporate, the Mark I “W” Series 348 and 409ci big-blocks had become long in the tooth, an indication the Bow Tie was poised for something new with potential for greater sums of power. By the time the 427 mystery motor showed up at Daytona it was known as the Mark II, replacing the “W” Series big-blocks on the superspeedway. The legendary 409ci “W” Series stump puller was available through 1965 as an RPO at the same time the Mark IV 396ci big-block was introduced. You could get the 409 in a fullsize Chevy or you could get the 396, which was also the big-displacement Z-16 option in the Chevelle. There couldn’t have been a better time to be alive and flush with cash for a new muscle car.
The Z-11 RPO Mark II 427ci big-block with its 4.3125-inch bores and 3.650-inch stroke was a true racing engine born strictly for stock car racing though some have applied it to drag racing. It was never really intended for drag racing or the street contrary to popular folklore. What made the Mark II 427 a winner was its conservative stroke and huge bores, which made it a high-revving beast on the track. There was also a short-lived 396ci racing development engine that never witnessed mass production that had the same bore size as the 409 and 427 but with a 3.390-inch stroke. It was not the same as the production 396 to follow.
The Mark II 427 had state-of-the-art cylinder heads with huge “drive-through” ports and poly-angle valves that made it revolutionary. What’s more, its wedge chambers offered excellent quench in a nice crossflow design that has endured for decades. The Mark II 427 is a standalone big-block though much of it appears identical to production Mark IV engines. The short-lived Mark III Chevy big-block was an in-house development study only and never entered production.
Chevrolet quickly vectored to the Mark IV beginning with the 1965 model year, including the 425-horse, 396ci big-block for Corvette. Although the Z-11 427ci racing engine was the first new generation big-block, the first to enter mass production was the 396 for 1965 in the big Chevys, Chevelle, and Corvette. One year later the Corvette received its greatest engine option yet: the 425-horse Mark IV 427 with a whopping 460 lb-ft of torque available at 4,000 rpm.
Forget fuel economy, political correctness and global warming. We want real displacement and spine-decalcifying torque. The persona of a powerful big-block cannot be underestimated. It excites the senses and gains all kinds of attention whether you’re cruising Main Street, cutting a canyon or navigating the interstate at speed. Yet, with all this power we are sufficiently teased. We want more. How do you turn a vintage Chevy big-block into a road-going beast without looking like that’s what you have?
When the 396 was introduced in 1965, it possessed a 4.094-inch bore with a 3.760-inch stroke shared with the production 402 and 427 to come later. There were at least seven versions of the 396 produced; the L34, L35, L37, L66, L78, L89 and the LS3. The 427, which arrived for 1966, had larger 4.251-inch bore and was available in eight different RPOs: L36, L68, L71, L72, L88, L89, LS1, and the ZL1. Horsepower ultimately went to 435. (Editor’s Note: the L88 while listed at 435 hp was reported to be actually in excess of 550 hp!)
The 402, which was fender badged as the SS 396, got a larger 4.125-inch bore for 1970-’72 and was known as the LS3, L34, or L78. The 454, which arrived in 1970, had the 427’s 4.251-inch bore with a longer 4.000-inch stroke. Translated, the 454 was the monster mash of Chevy fat-blocks at 465 horsepower and a rock-crushing 490 lb-ft of torque.
The Arab Oil Embargo and tough economic times of the ’70s and ’80s didn’t stop Chevrolet from developing improved versions of the famed big-block in its aftermarket division. The bloodline continued as the Mark V and VI, with displacements ranging from 430 to 572 ci available from the GM Performance aftermarket.
Building The BBC
Whether you’re building a stocker for cruising or a weekend road racer, the same basic rules of engine blueprinting apply. Gregg Jacobson of PHD Speedcenter in Bakersfield, California, teaches us the critical points of building a powerful, yet rugged, big-block Chevy. We’re working with a two-bolt main block (PN 3955270), which has already been machined and is ready for mock-up and final assembly.
Gregg stresses having a solid foundation to begin with, a suitable block and heads. Casting integrity is everything to your success. Cylinder bores should be measured and sonic checked for wall thickness. Keep overbore under 0.060-inch. The block and head castings should be Magnafluxed and checked for cracks and repairs. If you have a numbers-matching block that has been bored to its maximum oversize it will need to be sleeved by a qualified machine shop. (Cost is typically $100 per bore.) The cost of a block re-sleeve is well worth it to be able to keep the numbers-matching status.
Before you even get started know what you want your engine to do. And avoid changing direction once you get started. Every time you change direction it gets expensive. Gregg suggests not building more engine than you actually need. Bench racing and bragging rights lose their luster when you’re sitting at a traffic light with a temperamental high-performance engine that won’t idle. Be completely honest with yourself about what you actually need versus what your bench racing buddies think you need.
Unless you’re planning on more than 500 horsepower and comparable torque, a nodular iron crank and heavy-duty I-beam rods will work just fine. If you’re fortunate you will have a 396, 427 or 454 with a steel crank. For applications with a plan for 500 horsepower with nitrous, supercharging or turbocharging, you’re going to need a steel crank, H-beam rods and forged pistons, along with a conservative compression ratio.
With heads and induction, be thinking stock or restomod. If you want an original look with iron heads and intake, be ready to stick with that plan. If restomod, your choices are many. Pistons and combustion chambers need to work together in terms of compression ratio and what you will do with heat energy. When it comes to valve sizing, keep valve shrouding in mind. Large valves are meaningless if shrouding is the pits. Poor shrouding hinders flow. This is why you want valve sizing to be where there’s room between valves and chamber walls. Another important issue is quench. Quench is the flat surface of the combustion chamber as it relates to the flat top surface of the piston at top dead center. You want as much quench as possible while keeping detonation issues in mind. Quench is part of what allows better heat dissipation into the water jacket to prevent detonation.
Once you have the block, heads, and rotating assembly selected the next issue will be cam selection and how it relates to head and piston selection. It is suggested you consult with the professionals at your cam manufacturer of choice regarding cam selection for your engine build, offering them detailed information on what you have down under, including deck and compression height numbers along with how you intend to use the engine.
01-02. Engine builds should begin with a plan for detailed blueprinting. Gregg Jacobson of PHD Speedcenter in Bakersfield, California, is very methodical in his approach to engine blueprinting. Here, Gregg sets the two-bolt main block (PN 3955270) up for crankshaft thrust (endplay) check by installing main bearings and lubing them with assembly lube. Endplay should range from 0.005-0.007-inch for street use. Gregg stresses greater endplay for racing engines and less for the street.
03. Gregg tells us it is important to check your machinist’s work before assembly begins. Close examination of this particular block shows evidence of machine work that could use improvement. Pitting in the iron can cause poor ring and head gasket sealing. If you can see pitting, no matter how small, it needs a second pass with a hone or mill. Take the time to discuss any concerns with your machine shop.
04. Unless you’re performing a concours restoration, opt for ARP fasteners throughout your engine build. This is money well spent to get durability even if you’re building a weekend driver. Always use lubrication on bolt threads and use a torque wrench.
05. If you’re on a limited budget you can get durability from the BBC’s stock bottom end. A nodular iron (cast) crank can handle up to 500 horsepower. Stock I-beam rods can handle great sums of power if you have them shot-peened and fit them with ARP bolts. Anything above 500 horsepower calls for a steel crank and heavy-duty I-beam or brute H-beam rods. Our 427 is fitted with a factory steel crank and is ready for anything.
06. The easiest and fastest way to get power is via compression. But be aware that with compression comes a price—detonation and potential engine damage. These are JE high-compression forged pistons, hence the raised dome. Once you have established compression ratio, the rest is all about cam selection and proper tuning—ignition timing and fuel curve.
07-08. This particular 427 build at PHD Speedcenter includes Speed Pro coated and forged pistons. We’ve purchased a engine insurance policy in the form of Eagle forged I-beam 6.135-inch connecting rods (PN SIR6135P) working with press-fit Speed Pro forged aluminum 0.030-inch oversize positive dish (0.140-inch dome) pistons (PN L-2300NF-30). Dimensions are 0.9897-inch pin; 5/64-, 5/64-, and 3/16-inch ring grooves.
09. Stroking your BBC on a budget? Scat offers terrific street/strip kits with cast steel (nodular iron) crank, heavy-duty I-beam rods and forged pistons. Scat enables you to add displacement via increased stroke without breaking the bank.
10. Replacing your stock rods with affordable heavy-duty Scat I-beam cap-screw rods is a viable option for the weekend racer and canyon cutter. These are nice pieces you can get into without incurring a lot of expense.
11. H-beam rods offer unequalled strength and really are a race rod though a lot of us install them in our street mills. H-beams are necessary for blown and nitrous applications.
12. If your crank is trashed beyond repair, Scat can set you up with a drop-in replacement in cast or forged steel. If you can add displacement via stroke there’s even greater power benefit.
13-14. Never install an oil pump right out of the box. Always inspect the pressure relief valve for proper function. And always check clearances. Before you button the pump up fill the cavity to capacity with engine assembly lube to ensure a wet startup. When you install the pump, use a thread locker on the bolt threads and torque the fasteners to the proper specifications per your Chevrolet service manual.
Comp Quick Look
We’re building a streetable 427ci canyon cutter engineered to deliver great low-end torque yet ready to rev coming out of the apex. Comp Cams has set us up with a hydraulic roller cam (PN 11-433-8) with 110-degree lobe centers, 0.521/0.540-inch gross intake/exhaust valve lift (0.307/0.319-inch at lobe), and 236/242-degrees of duration at 0.050-inch lift (288/294-degees duration at 0.006-inch). The beauty of a complete cam kit like this is compatibility. Spring pressures match the cam profile because Comp Cams does the packaging for you.
Regardless of your 396/402/427/454 plan it is strongly suggested you opt for one-piece 0.080-inch wall Comp Cams pushrods. Although more expensive, one-piece thick-wall pushrods offer durability and are cheaper than a blown engine because a pushrod ball end broke and went down into the camshaft and oil pan.
A roller cam complemented with roller rockers reduces internal friction and frees up power and increases efficiency. Roller cams and rockers are more expensive going in, but are a wise investment in the long run. You will feel the difference in performance and notice the longevity. Long term, you save money.
15. We’re going all Comp Cams on this 427 Chevrolet with the hydraulic roller cam kit (PN 11-433-8) in our quest for better horsepower and torque.
16. We’re going with the 427’s original cast-iron heads with PHD Speedcenter custom port and chamber work by Gregg Jacobson. Gregg does exceptional port work along with extensive flowbench time to determine what’s most effective. Comp Cams has set us up with compatible valvesprings to get the correct spring pressure for this cam profile. Speed Pro stainless steel valves and new guides tighten things up with solid durability.
17. Gregg stresses the use of Comp Cams engine assembly lube on cam journals and lobes. The distributor/oil pump drive gear should receive moly lube for better break-in and work hardening. Use only assembly lube on cam journals. If you’re running a flat tappet cam, moly lube goes on lobes, tappets, and drive gear only.
18. It is remarkable how many of us install camshafts without checking true cam specs with a degree wheel and dial indicator. Advance valve timing to improve low-end torque. Retard valve timing for improved high-rpm power. Check valve-to-piston clearances. If ever you are stumped degreeing a cam, look to Comp Cams’ tech staff for help.
19. Here’s a cross-section of Comp Cams Pro Magnum roller rockers and pushrods. Three-piece pushrods (white arrow) offer an economic advantage if you’re on a tight budget. However, one-piece 0.080-inch wall pushrods (black arrow) yield durability and the confidence in knowing your valvetrain is as solid as it can be.
20. Use either brass or stainless steel freeze plugs (also called Welsh plugs). Never use steel. Go with the widest freeze plugs available and apply Permatex The Right Stuff around the perimeter, seating them square in the block. Some builders prefer an industrial adhesive on freeze plugs. Any press-in oil galley plugs should be replaced with screw-in plugs for durability.
21. Two-piece rear main seals don’t have to be leakers. Opt for a Fel-Pro high-performance silicone seal as shown staggering the seal ends away from main cap to block seams. Place a small dab of The Right Stuff at the seal ends before lubricating the seal lips. Seal lips must be pointed toward the crank counterweight. Crankcase pressure against the seal lip is what keeps oil inside.
22. Piston ring end gaps must always be checked even if you’re working with pre-gapped rings. Gregg suggests greater ring gaps if you’re going racing. He also stresses having a larger ring gap at the secondary ring than you have with the top ring because it allows excessive, destructive pressure to escape at the secondary ring.
23. Ring tips, once properly gapped, should be filed smooth to prevent cylinder wall scoring and ring land damage.
24. Pay very close attention to ring manufacturer’s instructions. This score mark indicates “this side up.” Not all ring manufacturers are the same so read the instructions. Ring gaps should be positioned 90 degrees apart.
25. During the block machining process lifter bores should be honed for improved oil control and smooth operation.
26. Harmonic dampers should always be replaced. Opt for one with an SFI rating like this one from Summit Racing Equipment.
27. Rocker arm studs need a dose of Loctite for security. When they go into a water jacket, use an industrial grade Teflon sealant on these threads. Permatex’s The Right Stuff is also an effective sealant.
28. Gregg gives the 427 heads a good clean-up, beginning with the chambers for reduced valve shrouding. He has his own port and bowl regiment designed to reduce turbulence and keep fuel droplets in proper suspension. On the exhaust side, Gregg reduces restriction by opening up the ports and massaging the bowl area.
29. Gregg installs hardened exhaust valve seats for use with unleaded fuels. He also gives the valve seats a nice multi-angle valve job for both improved flow and good valve-to-seat heat transfer. Unless you rarely drive your Corvette, hardened valve seats are mandatory.
30. We like these Pro Magnum 1.72:1 roller rockers from Comp Cams. Because the 390-horse 427 is fitted with hydraulic lifters there’s not much to valve adjustment unless you opt for a mechanical roller tappet cam, which calls for a thickness gauge. Turn the pushrod while slowly snugging the poly-lock adjustor. When the pushrod becomes impossible to turn with your fingertips, give the poly-lock a 1/2 turn. If you’re going racing, opt for a 1/4 or 1/3 turn and tighten the Allen set screw. Allow time for the lifter to settle.
31. Induction is one option you have when it comes to making power. Not enough credit is given the Rochester Quadrajet. The 427’s stock spread-bore manifold (right) delivers an excellent balance of horsepower and torque thanks to its well thought out dual-plane design. Where it falls short is runner size. The Edelbrock RPM Air-Gap manifold is optimum if you desire a Holley-based four-throat carburetor. The RPM Air-Gap, as its name implies, allows for a cooler runner and intake charge.
32. The 390-horse 427’s Quadrajet performs very well at 750-800 cfm. If you desire an original box-stock setup, the Q-Jet serves very well with proper tuning. With correct accelerator pump, metering rod/jet sizing and secondary air valve tuning, this atomizer delivers.
33. Holley offers a huge number of 4150-based options for your Chevy big-block. We tested both the Quadrajet induction as well as this Holley HP Series 850-cfm double-pumper. The HP and RPM Air-Gap manifold made significantly more horsepower and torque. With extensive Quadrajet tuning, you could narrow the gap.
34. Holley’s Terminator EFI is a very stealthy throttle body system that can be hidden beneath your air cleaner. It starts on demand and burns cleaner than your Quadrajet or aftermarket Holley four-hole base carburetor. You will need a Holley-based dual-plane intake manifold to accommodate the EFI.
35. When it’s time for dyno tuning keep in mind how critical ignition timing is to both performance and durability. There are all kinds of approaches to ignition timing and fuel curves. One thing remains concrete—too much timing and a lean mixture can lead to engine failure. Total ignition timing should never venture beyond 36 degrees BTDC at 3,500+ rpm. Oh sure, you can push it to 38-40 degrees BTDC. However, you’re gambling with engine life.
|Dyno Test 1|
|Rochester 750-800-cfm Quadrajet with stock 427 Dual-Plane Aluminum Manifold|
|Dyno Test 2|
|Rochester 750-800-cfm Quadrajet with stock 427 Dual-Plane Aluminum Manifold|