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Chevy Small Block Stroker - A Stroke Of Genius Part III

We Dyno Test And Tune The Hye Tech 391 Stroker

Mike Petralia Aug 1, 2001
Sucp_0108_01_z Chevy_small_block_stroker Technicians 2/10

Engine dynos make very good polygraphs. So many times the latest engine-building stud will brag to his friends or foes about the mighty power beneath his hood. Without any true dyno figures to back them up, who's going to dispute his claims?

Our latest dyno test session features a little stroker small-block that we've been following Sod Bogosian of Hye Tech Performance put together. Bogosian's customer wanted a unique small-block with a great torque curve, and he wasn't at all concerned with peak power numbers. He planned to drive this beast all over the place and liked the idea of having torque to spare at any rpm. So Bogosian designed and built a 391-cid Mouse that, in the end, made more than 400 lb-ft from 3,000 to 5,900 rpm and lost only 4 lb-ft at 6,000. This little pack mule eventually had no trouble cranking out more than 450 lb-ft of torque at a relatively low 4,200 rpm. That's enough to make any car's tires squeal with delight.

Sucp_0108_03_z Chevy_small_block_stroker Holley_carb 3/10

The new Holley Street Avenger 770-cfm vacuum secondary carb worked well right out of the box. Bob Vrbancic had to swap a 2.5 power valve for the stock 6.5 power valve because the engine made only 8-inches of vacuum at its 800-rpm idle.

Stroked To Perfection, A Recap
For just a few extra machine-shop dollars Bogosian offset-ground a 3.75-inch stroke 4340 C.A.T. Power Engine Parts crank by turning the rod journals down to the small, 2-inch diameter of the old 327-cid small-block. This added an extra 41 ci to the combination, making the little 350 think it's a big-block. Then Bogosian hung a set of full-floating 10.25:1 JE forged pistons on 5.7-inch 4340 H-beam C.A.T. rods and produced a stroker bottom end strong enough to haul the Queen Mary from Long Beach to Long Island without a trailer.

Breathing Science
The top end of Bogsian's big Mouse was designed to optimize fuel efficiency and power, while still being a smooth-running street engine with great throttle response. For his top end he choose to go the dyno-matched route and selected Holley's new Street Avenger package, which included aluminum heads, an intake manifold, a cam, a carb, and all the needed accessories to go with. That way he knew he wouldn't have to waste days on the dyno figuring out which combination would work best.

While not huge by anyone's standards, we learned that the Street Avenger heads included in the Holley package did flow a respectable number and were designed mainly to make power in real street rpm ranges. That meant that a 6,500-rpm power peak was not in our future, and the 223 cfm these heads flowed at 0.500-inch lift on the intake side meant that a huge cam would not be needed to optimize the package.

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Jet changes are usually worth some power when fine-tuning on the dyno, and Vrbancic came up with the best jetting combination of 70 primary/75 secondary which was only 2 jet sizes smaller than stock on the primary side. Holley has really done its homework on this new line of carbs.

Bogosian chose a Lunati hydraulic camshaft with slightly more duration than the recommended Street Avenger cam because he felt that the 41-extra ci this engine displaced would need a bit more breathing room. He also asked for the cam to be ground with a 112-degree lobe-separation angle to better accept the shot of nitrous that was in this engine's future. He combined the big cam with Lunati's 1.6:1 roller rockers and a double-row timing chain to be sure the valvetrain would be up to the long-term use it would see.

Polygraph Test
With the top end finished off and a set of March Performance serpentine pulleys bolted on the damper and the Weiand water pump, Bogosian hauled the motor to Vrbancic Brother's Engine and Dyno shop for a full-day's dyno bashing. Brothers George and Bob had no trouble tuning every last ounce of useable power from this street combination.

Sucp_0108_05_z Chevy_small_block_stroker Engine_run 5/10

We first ran the engine with 37 degrees total advance, but in the end it liked a little more and ran best with 39 degrees.

The first dyno pull with the 770-cfm vacuum secondary Holley carb, taken right out of its box, was promising. The Mouse belted out 430 hp and peaked with almost 440 lb-ft of torque. Twenty-nine dyno pulls and a few carb spacers later, Bob and George squeezed out 453 ponies and, ironically, exactly 453 lb-ft of torque also. But the real performance story in this engine was its ability to pump out consistent torque figures above 400 lb-ft no matter what was changed on the engine. Only when they made a few pulls up to 6,500 rpm, did the engine really fall off in power, indicating that it was definitely over after 6,000 rpm. Average power figures were terrific for an engine that will spend most of its life on the street and would work great with tall rearend gears or an overdrive transmission for long-distance cruising.

We applaud Bogosian and the Vrbancic Brothers for working so hard to create such a great street package. If your dreams include the next engine under your hood being something truly unique and ultimately reliable, try this stroker from a different point of view for a new twist on some old performance know-how.

Sucp_0108_06_z Chevy_small_block_stroker Carburetor_spacer 6/10

An engine will often respond well to different carb spacers and this Mouse was no different. We stacked two carb spacers, a four-hole and an open spacer under the Holley, and the engine picked up power all around.

Dyno Figures
It's important to remember that the owner of this engine was not looking for the ultimate fire-breathing monster under his hood. Rather, he wanted a street engine that would pull like a team of oxen and never let him down on the long haul. Sometimes, in our quest for superior power, we lose sight of what's really needed to get down the road. Sure, belting out 550 hp at 7,500 rpm is a kick in the pants, but that same engine may have trouble keeping up stoplight to stoplight with this little torque monster.

This particular dyno pull represents one of the best and was made using two plastic spacers stacked under the carb. The spacer closest to the carb was a 1-inch thick, four-hole spacer, and the one closest to the manifold was a 1-inch thick, open spacer.

Sucp_0108_08_z Chevy_small_block_stroker Dyno 7/10

Peak TQ453 at 4,200 rpm

Peak HP453 at 5,800 rpm

Average TQfrom 3,000 to 6,000 rpm = 429 lb-ft

Average HPfrom 3,000 to 6,000 rpm = 366


March Performance
Naples, FL 34110
Manley Performance Products
Lakewood, NJ 08701
JE Pistons
Huntington Beach, CA 92649
Powerhouse Products
Memphis, TN 38118
Holley Performance Products
Bowling Green, KY 42101
Jeg's High Performance Center
Columbus, OH 43211
C.A.T. Power engine parts
Baldwin Park, CA 91706



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