Chevrolet 396 Big Block - Mean Little Rodent

Making Big Power With A Small Rat

Mike Petralia Feb 1, 2001 0 Comment(s)
Sucp_0102_01_z Chevrolet_396_big_block Carburetor 2/37

The Chevrolet 396-cid, big-block's destiny was decided long ago. After punching the block out 0.030-inch and increasing its displacement to 402-cid back in 1970, a couple years later, GM stopped offering the little big-block in cars and trucks altogether.

It probably didn't help that GM kept calling this engine a 396 big-block, further adding confusion to the already vague big-block market. Following its official retirement the 396 will forever go down in history as second fiddle to the big, bad 454. Now with newer and bigger displacement Rats like GM's new 8.1-liter (496 cid) truck engine being introduced the little old 396 gets pushed further and further into big-block obscurity.

Sucp_0102_03_z Chevrolet_396_big_block Defects 3/37

After our 31-year-old Rat was checked for defects it was align-honed and hot tanked in preparation for cylinder boring and deck machining. It gets its deck surfaces squared relative to the crankshaft's centerline to ensure every machining operation that comes afterwards will be true to the crank.

Without going into tremendous details of production figures and years of availability, it's safe to say that there were more 396-cid big-blocks installed in cars than there were 427s or 454s combined. Yet, to date the 396 is still often cast aside by the performance crowd whom usually have hailed the credo, "If you're going to build a big-block, build it as big as you can." While that's certainly a good motto to live by, it's also an expensive mantra to uphold.

A byproduct of the 396's outcast into obscurity has been the reduction in the cost of its parts. While a forged GM crank for a 454 could cost as much as $400 today, a forged 396 crank sells for about $75. And a rebuildable 396 short-block will usually only set you back less than $200. While a 454 short-block could fetch $500 easily. Sure, with cubic inches comes cubic power, but what if we showed you how to build that 396 you've been neglecting in the hopes of adding some inches to your diet and make power equal to or beyond the average street 454? We'll show you how to do it right the first time on a reasonable budget. Sorry, this isn't a totally down and dirty low-buck engine, but it certainly doesn't carry a cubic price tag either.

Sucp_0102_04_z Chevrolet_396_big_block Height_check 4/37

With the crankshaft secured by the No. 1 and 5 main caps and bearings, we installed the No. 1 rod and piston without its rings, and checked installed height. Note the position of our thumbs, which we use to "rock" the piston in its bore and determine how far below the deck the piston sits at TDC. After this, we reset the block in the mill and "deck" its surface for a "zero" piston installed height, meaning the piston is very close (+/- 0.005) to the deck at TDC.

We figure that today it takes at least 500 hp to be a street hero. Anything less and you're just paddlin' upstream while the big fish are sailing by you in their new powerboats. And you've got to make your power from the pump, since race gas is still taboo in most street car circles. So what's it take to make 500 hp with an old street 396? About $5,000 and a few gallons of 92 octane. Follow along as we detail the buildup and testing of our fearsome little Rat and maybe you'll forgive your 396's cubic-inch deficiencies and give it a new lease on life as a street brawler, instead of a dust collector.

The Dyno Dance

Our little Rat puffed out its chest and pumped up some mighty big dyno figures for us. The average power numbers from 4,000 to 6,250 rpm (where it carried over 400 lb-ft of torque) were 438 hp and 450 lb-ft of torque. While power peaked at a high 6,400 rpm, low-end torque was still plenty respectable with the motor peaking 483 lb-ft at 4,700 rpm. Here is a sampling of its best power pull with the cam four-degrees retarded and 38-degrees of ignition advance.

Sucp_0102_05_z Chevrolet_396_big_block Wonder_boy 5/37

With the deck squared and zero deck height set, our machining wonder-boy Dave Akard, who also happens to own Burbank Speed and Machine (BSM), goes to work on the cylinders. Dave first bores the block 0.060-inch over to give us a displacement of 408 cid. Dave's crew will finish the bore preparation with a Sunnen block hone.

Here's a secret piece of junkyard gold. Although all records of what vehicles they originally came equipped in have long since vanished, we know that some 366-cid truck big-blocks came with stronger connecting rods (right). These rods are the same length as stock rods, but feature a much beefier big end. We paid $80 for a set and had The Balance Shop polish and shot peen them before balancing.

Sucp_0102_06_z Chevrolet_396_big_block Connecting_rods 6/37

Since we knew this engine would see 6,500 rpm often, we choose to replace its cast crank with a forged GM crank The Balance Shop sold us this one for $75. Dave Kemblowski, owner of The Balance Shop also balanced the entire rotating assembly.

With the freshly machined crank and block back in our possession, we thoroughly washed and checked each part for machining accuracy. Even though we trust our machinist's abilities, you can never be too careful when it's your parts on the line.

Here's a tip for any 396-cid big-block. The top of the cylinder bores are plunge-cut at the factory to clear the intake valve as can be seen on the left side of this photo. We like to radius that sharp cut with a die grinder to further unshroud the intake valve in the cylinder. Also note that we scribed the head gasket's bore on the deck surface to keep us from grinding too far.

3,500 358 239
3,750 395 282
4,000 417 318
4,250 450 364
4,500 461 395
4,750 483 434
5,000 469 447
5,250 464 460
5,500 455 476
5,750 445 488
6,000 431 492
6,250 422 502
6,500 392 486
Peak HP = 504 @ 5,300 rpmPeak TQ = 483 @ 4,750 rpm

Sucp_0102_12_z Chevrolet_396_big_block Dial_gauge 10/37

We torqued each main cap and bearing in place and measured the bore diameters with a dial gauge. Then, by swapping bearing halves from one bore to another, we got perfect 0.0025-inch bearing clearances for all five mains.

Upon trial assembly we found one stripped headbolt hole. It was easily fixed with a Heli-Coil kit and this drill jig from Manley Performance Products. If you don't drill the head bolt hole straight, you'll be in a world of misery down the road and this jig solves that problem.

Since even Total Seal's gapless second rings still use a gapped top ring, we carefully checked each ring set in its bore. We used total Seal's battery powered ring grinder to set our top rings at 0.028-inch, because nitrous is in this motor's future. Our homemade ring-squaring tool is just visible in the upper right corner, sitting on the block. We fabbed it from two pieces of ABS drain pipe for about $5.

Preparing for final assembly, we completely deburred each piston's dome and face and the inner id of each valve spring using a convoluted deburring wheel from Standard Abrasives mounted on a 3/4-hp bench grinder.

One trick part we added to this block was simple hardware store drain plugs. We screwed the plugs into the block's coolant drain holes making it easy to drain water for radiator flushing and for swapping engine parts.

Sucp_0102_14_z Chevrolet_396_big_block Stripped_headbolt 11/37

We used ARP's moly assembly lubricant and a rod bolt stretch gauge to properly establish connecting rod bolt preload. Also note the ARP main cap bolts we added to the block before the first machining process even began.

Total Seal provided its slick adjustable piston ring compressor for our buildup and we installed a Comp Cams beltdrive to allow quick cam timing changes on the dyno.

Most beltdrives use shims stacked together to set endplay. Comp's drive uses a trick eccentric plate, allowing fast and accurate adjustments. We set our cam's endplay at 0.005-inch. We sealed ARP's head studs to the block with high-temp RTV silicone prior to screwing them in hand tight.

Using tools from Powerhouse, we installed our new Comp Cams street roller grind (PN 11-770-8). This cam features 236/242 duration at 0.050 and 0.647/0.654 lift and a 110-degree lobe separation angle. Even with this mild grind our baby Rat was still able to pull 16-inches of idle vacuum and pump out over 500 hp.

Sucp_0102_26_z Chevrolet_396_big_block Aluminum_heads 21/37

Using tools from Powerhouse, we installed our new Comp Cams street roller grind (PN 11-770-8). This cam features 236/242 duration at 0.050 and 0.647/0.654 lift and a 110-degree lobe separation angle. Even with this mild grind our baby Rat was still able to pull 16-inches of idle vacuum and pump out over 500 hp.

With the cam and pistons installed we bolted the oval port Brodix aluminum heads on. These lightweight castings come with 2.25/1.88-inch stainless steel valves and we added Comp Cams (PN 938) springs and valvetrain hardware with Comp's 1.7:1 aluminum roller rockers. Some of rocker arm stud holes go right through into the intake ports so those studs must be sealed with thread sealant or else oil will be sucked into the engine.

Sucp_0102_28_z Chevrolet_396_big_block Combustion_chamber 22/37

The Brodix heads feature a 119cc combustion chamber that, combined with our Speed-Pro forged pistons and Fel-Pro head gaskets gave us a final compression ratio of 10.25:1. Just within the limits of street gas with aluminum heads. We installed the heads with the unmodified chambers as shown here. You can also see the Autolite No. 53 spark plug's electrode tip just protruding into the chamber, which is just right.

Moving up to the top of our assembled long-block, we bolted on an Edelbrock Performer RPM Air Gap intake manifold and poured 30-weight motor oil in, drenching the rockers and springs at the same time.

Anytime you install steel fittings into an aluminum part you must use anti-seize. We apply it sparingly to the spark plug threads, careful not to get it past the last thread where it could be exposed into the chamber.

Sucp_0102_29_z Chevrolet_396_big_block Motor_oil 23/37

We'll be dyno testing this engine in the morning, so we primed the Moroso oil pump using a converted HEI distributor base and shaft driven by a 1/2-inch drill. Make sure to slowly rotate the crank while priming the oil pump to ensure the entire motor gets lubed.

To seal our trick GMPP cast-aluminum valve covers; we relied on Fel-Pro's new blue valve cover gaskets. These steel-cored silicone rubber coated beauties feature steel compression sleeves in the bolt holes to keep over-zealous bolt tighteners from crushing the gasket.

Speaking of the cool GMPP valve covers. They don't come with any provisions for a breather, so we welded these Moroso baffled breather tubes on. And capped them off with Moroso clamp-on breathers. We secured the covers using ARP stainless studs and hardware.

Sucp_0102_33_z Chevrolet_396_big_block Moroso_breather_tube 27/37

To finish our baby Rat we added a CSI electric water pump for dyno cooling (an Edelbrock pump will go on the street) and a CSI upper radiator hose neck. XRP supplied the -10 push-lock hose and fittings for the front-to-rear water connections in the manifold. Hindsight tells us that -10 hose is too bulky and looks out of place so next time we'll run -8 or -6.

We bolted the baby Rat on Burbank Speed and Machine's Stuska dyno and topped it off with a 750-cfm Speed Demon carb and bolted on 1 3/4-inch Hooker headers.

The engine was fired and carefully watched over by Akard himself while it broke in for 10 minutes to seat the rings and warm up.

After its initial warm-up, we lashed the valves to Comp's recommended specs (0.016-intake/0.018 exhaust) and changed the oil pouring in 7 quarts of 10W-30 Mobil 1 for the power pulls.

Sucp_0102_34_z Chevrolet_396_big_block Electric_water_pump 28/37

Burbank Speed's dyno rookie Jesse checked ignition timing for us. After several pulls we learned the engine liked 38 degrees total advance so we left it there for the whole day.

The Speed Demon idled fat, blackening the plugs' inner threads and most of the electrode. The fat idle was traced to all four mixture screws being turned out way too far. We screwed them in until they were about 1 1/4 turns from bottom and the idle cleaned up and the plugs began to color fairly well so we moved on.

We moved jets up and down in search of power and found the best numbers with 76 primary and 83 secondary jets with the 6.5 power valve left in the carb.

Comp Cams beltdrive allowed five-minute cam timing changes and we advanced and retarded the bumpstick until the best power was found at four degrees retarded. Advancing the cam made a bit more low-end torque, but we wanted horsepower so we left it retarded.

We're pretty confident that the engine would have made about 15 more top-end horsepower if we'd installed Edelbrock's Victor Jr. 454 O intake manifold. Comp Cams also makes a bigger street roller grind that might push this little Rat over the 525hp mark. Maybe we'll make some changes and return to the dyno to see how much power we really can squeeze out of this baby with off the shelf parts and no behind the scenes trickery.


Hooker Headers
Aberdeen, MS 39730
Comp Cams
Memphis, TN 38118
Chevrolet Performance Parts
Detroit, MI 48232
Manley Performance Products
Lakewood, NJ 08701
Powerhouse Products
Memphis, TN 38118
Moroso Performance Products
Guilford, CT 06437
The Balance Shop
Reseda, CA 91335
Brodix, Inc.
Mena, AR 71953
Speed-Pro Sealed Power Corporation
Muskegon, MI 49443
Burbank Speed & Machine (dyno)
Burbank, CA 91502


Project Cars
We create a street brawler from an old 396 block.
Mike Petralia Feb 1, 2001


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