Building A 427 For Today's World

Building a classic big-block for today's low-octane world

Steve Dulcich Sep 21, 2005 0 Comment(s)
0509_13z Chevrolet_L72_427_Engine Adjusting_Valves 1/24

As was the original, the Comp Nostalgia cam we selected is asolid-lifter design, requiring valve adjustment to provide the specifiedlash clearances, 0.012 inch for this particular cam. We always reset thelash hot after the engine is run in.

While the subtle changes identified so far are aimed at performance andlong-term reliability, there were other aspects of the build: Some ofthe factory specifications were backed out in favor of improved utilityand driveability in today's world. The primary factor is compressionratio. The factory-rated ratio of the L72 squeezed to a proportion of11:1, which was just right when you could pull up to the pump and askthe attendant to fill it up with ethyl, with an octane rating of100-plus waiting to splash out of the nozzle. These days, you'll grabfor the hose yourself, and here in California it's pumping from a poolof 91-octane unleaded premium. Rick says, "I want to just get in, fillit up on the road, and go--just like in the old days. I don't want totoss in a jigger of booster, mix up a concoction of higher-octane swill,or worry about where to find gas to make this thing go. I'd rather justback some of the compression ratio out, and that will cost some power,but with the other changes I should have that more than covered."

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In keeping with the objective of a stock appearance, the factory intakemanifold was retained. This original two-plane aluminum high-rise unitis a fairly good piece for an engine of this size and specification. Theaftermarket water neck is for dyno hookup only, and a stock unit will besubstituted before the engine is installed.

Piston dome and chamber volumes are the key contributors to compressionratio with a given engine combo, and here the obvious choice to dial inthe ratio was to select the appropriate piston. Rick explains, "Theseearlier 427s used closed-chamber heads that measure around 100 cc stock,and I wasn't going to consider anything but the numbers-correct heads.With the small early chamber, the trick is to use a smaller dome to cutdown on the compression ratio. For this build, I used a set of Speed-Proforged pistons (PN 2300), which have a dome volume of 16.8 cc. We foundwhen building the engine that the valve-to-piston clearance on theintake side was not enough, and had the piston's valve relief notchesfly cut 0.080-inch deeper to give a safe clearance. This reduced thedome volume another couple of cc's, down to 14. With the pistons fittingat 0.005 inch below the decks, and a 0.051-inch-thick head gasket, thefinal ratio in my engine worked out to 9.86:1. That's the truecompression ratio, and it's still high enough to make good power, but isa lot safer with today's gas and iron heads."

0509_15z Chevrolet_L72_427_Engine Carburetor 3/24

The beautifully restored factory Holley 780-cfm vacuum secondary carb isthe handiwork of Sean Murphy Induction of Huntington Beach, California.Again, the factory's choice of components lends itself well toward thegoal of performance.

The cylinder heads offered some opportunity for improved power, and alsorequired a few mods for longevity. Although porting the stock headswould be a possibility, Rick decided he wanted to keep these rarefactory castings stock and limit the flow enhancement to a good,machined, multi-angle valve job. According to Rick, "The valve job was aplace where I wanted the best workmanship possible, since machining theseats is a basic part of the rebuild. I didn't skimp here, because thereis a power difference in how well the job is done. I didn't want anyonecarving on these rare stock heads with custom porting, even though itwould have made more power. It just doesn't make sense to me to cut onsomething this rare and expensive. I did have hardened exhaust-valveseats installed when the heads were rebuilt, since the seats werehammered and the no-lead gas meant they'd always be in line for abeating. The hardened seats just add durability, and I didn't wantproblems down the road. The valves were replaced with a newhigh-performance stainless steel set (2.190/1.88 inch) from CompetitionCams. The new valves improve flow and durability compared to theoriginals. I just went to Comp Cams for the works to assemble the heads,from the valves to the springs, locks, retainers and guideplates. I knowfrom experience that this stuff is bulletproof."

0509_16z Chevrolet_L72_427_Engine Exhaust_Headers 4/24

Compared to a set of tubular exhaust headers, there was no doubt we'dgive up power potential with the stock manifolds, but they're incharacter with the restoration theme of the build. There are far worsedesigns in iron manifolds than the factory Corvette units.

The engine build wouldretain all of the major external cues that signify this as a stock earlyCorvette big-block. The factory high-rise aluminum intake manifold wouldsit between the heads, drawing air from the factory list No. 3247 Holley780-cfm vacuum secondary carburetor. To ensure that the vintage carbfunctioned like a fine artifact, Sean Murphy Induction of HuntingtonBeach, California, was called upon to fully rebuild and restore thepiece. On the opposite end of the heads, the factory iron exhaustmanifolds were retained, again to impart an appearance of originality inthis installation.

Power Test

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As configured on the dyno, the 427's exhaust manifolds would direct thespent gasses through the set of head pipes that will be installed inRick's '66 C2, though we terminated the exhaust there, running withoutmufflers.

In 1966, the stock L72 big-block was rated at 425 hp. We hada mildly revised version of this engine, essentially using all stockmajor components with about one point less compression, but with a moremodern cam profile, an upgraded valve job, and top-notch machining,assembly techniques, and replacement parts. How would the variouschanges balance out in terms of the power at the crank? Naturally, thecrew at Westech had a dyno test in mind for this in-house project, andwe were eager for the results. The engine was loaded onto Westech'sSuperFlow 901 engine dyno to determine the answer. The engine wasinstalled with a belt-driven water-pump setup, and the head pipes thatwill be installed in the car were bolted to the manifold exits. The onecompromise to originality was the installation of a modern MSDdistributor in place of the factory ignition. This substitution wasrequired since the original distributor was out for restoration andwasn't available in time for the scheduled test day. Experience hasshown there is little difference in output between these ignitions iffunctioning properly. A set of MSD plug wires was installed to directthe spark to the fresh spark plugs.

0509_18z Chevrolet_L72_427_Engine Distributor 6/24

For testing purposes, MSD ignition components were fitted in the dynoinstallation, including the distributor and wires. The factory ignitionand distributor were being restored, but we expect those pieces toprovide comparable output in a combination like this.

Since this was a new engine combination, there was more to do thansimply fire it up, pull the handle, and record the power curve. Theengine was first filled with conventional 10W-40 motor oil, and thelubrication system was thoroughly primed using a priming shaft driven bya drill motor through the distributor hole. Next, the engine wasstatically timed with the engine off, and the fuel system was checked,baselining the mixture screws at 11/2 turns out from lightly seated. Thefloat levels were checked and adjusted, with the fuel supplied by thedyno's electric pump. With the preliminaries out of the way, theignition was hit, and Rick's 427 fired instantly. With a flat-tappetcamshaft, break-in was critical to avoid cam failure. The engine wasimmediately brought up to 2,300 rpm, and the oil pressure and fuelmixture were verified on the dyno's instruments. With everything lookinggood, the timing was adjusted to establish 34 degrees of total ignitionadvance, and the engine was run for 20 minutes to complete theSuperFlow's automated break-in cycle. Dyno operator Steve Brule examinedthe running engine with a mechanic's stethoscope to listen for anyunusual internal sounds or valvetrain maladies. With everything lookingand sounding great, the engine was shut down, the valve adjustment waschecked and reset, and the oil was changed to Lucas synthetic.

0509_22z Chevrolet_L72_427_Engine Testing 10/24

Preliminary testing showed a somewhat lean mixture at wide-openthrottle, which was quickly remedied with a jet change to richen themixture.

Finally, we were ready for the testing. The engine was brought up for ashort sweep test, running from 3,500 to 4,500 rpm to get a quick gaugeof the wide-open-throttle mixture. The dyno instruments showed that theHolley carb's jetting was a little outside the zone, recording a leanmixture. The dyno pull also showed that this 427 was a truly torqueybeast, powering over 450 lb-ft of torque right from 3,500 rpm andholding nearly flat to the top at 4,500. With minor re-jetting,everything proved dialed in, so we opened up for a sweep test over abroader range, extending the dyno controls to pull to just over 6,000rpm. This time we recorded a peak power output of 451 hp at 5,900 rpm.Even with the lower compression in deference to today's pump gas, theengine was recording higher output than the factory gross rating of 425hp. The real credit goes to the Comp Cams camshaft and valvetrain, aswell as the detailed prep and assembly. The mild Rat really liked torev, holding its torque production high enough in the rpm range toregister a nice, lofty power peak. The 427s were known to rev, and thisone seemed to confirm that reputation, making horsepower right past themagic 6,000-rpm range.

0509_23z Chevrolet_L72_427_Engine Timing 11/24

Optimal timing was found to be 36 degrees total--a measure of the fulltiming with the centrifugal advance fully engaged, but with the vacuumadvance disconnected. This simulates the operating conditions atwide-open throttle.

With the recorded data making us feel secure that the air/fuel ratio wasright in the optimal range, additional tuning would be limited to makingseveral pulls in an ignition timing loop to determine the optimal totalspark timing setting. We proceeded and found the big-block to favor 36degrees of total timing, which isn't unusual for a Chevy Rat. The finalbest power figure came in at a surprisingly solid 455 hp at 5,900rpm--very stout for our 10:1 427. The unquestionably raucous power ofthis "stock" Rat makes it a worthy testimony to the 427's legacy.

Dyno Results

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When it was all said and done, the mildly altered L72 427 cranked out455 hp at 5,900 rpm, substantially above its 425hp rating, even at thelower compression ratio. The engine willingly revved to the 6,300-rpmrange in our testing, and made use of the rpm by hanging on with thepower production. Torque was stout throughout the curve, reaching a peakof 460 lb-ft, and showing muscular twist throughout the rpm range.

Superflow 901 Engine Dyno
Tested At Westech
STP Correction Factor


*Bold values are peak


Westech Performance Group
Mira Loma, CA
Lucas Oil
Corona, CA 92880
Sean Murphy Induction
Huntington Beach, CA 92647