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Chevrolet 355 Engine Build - Fleet Street

How To Make 440 LB-FT Of Torque And 425 HP From A Mild 355!

Jeff Smith Mar 26, 2003

It's easy to get wrapped up in all the excitement of big horsepower numbers. The other magazines do it all the time. In fact, they chastise us for doing stories on "wimpy" motors that barely make 400 hp as if there is some kind of horsepower ego-rama going on. That's OK. Because we feel there are plenty of enthusiasts who want to know how to assemble a great running small-block Chevy that is affordable and still makes good power. We literally had this small-block lying around, and it was a real dog. Your humble editor had assembled this engine as a basic 355 with a one-piece rear-main seal, stock cast crank, rebuilt stock rods with ARP bolts, forged flat-top Speed-Pro pistons, and a mild hydraulic-roller cam. Because this was a drone motor, it didn't need anything more than a set of stock iron heads. Out of the Ed Taylor small-block parts bin archives, we dredged up the set of iron castings from the Goodwrench Quest 350 buildup series (Part II, Oct. '99, page 25) that Todd McKenzie had lightly pocket-ported. On top, we added a simple Edelbrock Performer and a Q-jet carburetor that was probably around when Moses was a kid.

We never dyno'd this engine and it ended up as the engine under the hood of Project Moby. When it came time to bolt in the Project Twister HT 383 engine, this little small-block had to come out. Since it was taking up floor space, we decided to stick it on the dyno just to see what it would make. In the Goodwrench Quest articles, these heads on that 350 with a Comp Cams 268 Xtreme Energy flat-tappet hydraulic camshaft pulled off 336 hp at 5,300 rpm with a peak torque of 377 lb-ft at 3,700 rpm. The mild hydraulic-roller cam in our current engine matched up with a Performer intake and a Quick Fuel Technologies Pro Vac 650 vacuum-secondary carburetor. It looked,like our unassuming small-block would probably make a little less than the Goodwrench engine.

We decided to call this engine Daffy because the engine looked funny with its stock heads and painted intake manifold, and because in the cartoons, Daffy never gets any respect. With a set of 15/8-inch headers and configured much the way the Goodwrench engine was with these heads, Daffy made an honest 327 hp, but the big difference was the torque. The Goodwrench engine, with its larger, flat-tappet 268 cam, managed 257 lb-ft at 3,000 rpm, while our shorter hydraulic roller made a tire-twisting 380 lb-ft at the same engine speed--yahoo! This was encouraging, and we began to think that our Daffy nickname had been a bit hasty. This is when we left the comparisons to the Goodwrench engine behind and began to get serious about being Daffy. Clearly our little 355 was showing some muscle that we didn't think it had. It seemed appropriate to give this mild-mannered small-block a chance to show off a little with a bigger intake and carburetor. Taylor yanked the Performer intake and replaced it with a taller Edelbrock Performer RPM Air Gap and a little more aggressive 750-cfm Holley double-pumper carburetor (PN 0-4779). Gains of 6 to as much as 19 lb-ft of torque made it clear this was a good move. Peak torque moved up to 403 lb-ft at 3,400 while the horsepower jumped from 327 to 342 at 5,000 rpm. This was getting' fun! If you've followed this magazine for a few years, you know we have some favorite heads. Airflow Research (AFR) offers an outstanding small-block head with a 180cc intake port that promised excellent torque and horsepower numbers and could also keep the detonation monster away with its 68cc >> chamber. The combination of an aluminum head (that transfers heat faster than an iron head) along with a 68cc chamber promised an excellent opportunity to make power. The AFR head also generates some very good flow numbers at the low-valve-lift areas. Most street heros don't pay enough attention to low-lift flow numbers, but we feel that it is these numbers that reflect how well the head will fill a cylinder, especially at the more conservative engine speeds where street engines live. This is the domain of the 180cc AFR head. Its flow numbers at 0.300- and 0.400-inch valve lifts are excellent, while still delivering good flow numbers at 0.600-inch valve lift. Since we wanted to remain conservative on the cam timing, this AFR head seemed to be the right way to go.

Heads Up

After Taylor torqued on the AFR heads, added a set of Comp Cams 1.5 roller-tipped rockers, and buttoned up the intake and carb with new Fel-Pro gaskets, we were ready to beat on our now-aerobicized Daffy small-block. This test retained the original camshaft, but Daffy came through with both torque and horsepower. Even at 2,400 rpm, the AFR 180 heads were worth 6 lb-ft of torque, while the midrange jumped up a strong 20 lb-ft at 3,600 rpm. The horsepower was limited at the top because of the camshaft since peak power occurred at 5,400 rpm with 382 hp. Obviously it was time for a bigger cam.

The key here was to match the capability of the cylinder head with a camshaft that could extract as much power as possible without sacrificing torque down low. Besides the excellent low-lift intake flow, the big thing that really demands to be noticed is the excellent exhaust-to-intake flow relationship. The AFR 180 head has an outstanding exhaust port, which can over-scavenge the cylinder and hurt power if used with a dual-pattern camshaft. Dual-pattern cams use a longer-duration exhaust lobe that is not necessary with these heads.

When it came time to choose the cam, Comp Cams suggested going with one of the time-honored single-pattern cams from the Magnum series but in a hydraulic-roller configuration. We ended up with the 280HR10 Magnum roller, which generates 0.525-inch lift (see the "Cam Specs" sidebar) and fits perfectly with the AFR 180 head's flow curve. Ed Taylor used a two-piece front timing-chain cover from Comp to make this swap a little easier and performed the cam change on the dyno in a couple of hours. This was originally a hydraulic-roller cam engine, so we didn't have to worry about setting the camshaft endplay since the factory limiter plate does that job for us. We also reused the original roller tappets as well as the pushrods and the 1.5:1 roller rockers.

A serious break-in period isn't required on the rollers, so after a 10-minute warmup we were ready again to rock and roll. Despite the additional 16 degrees of duration (at 0.050-inch tappet lift), Daffy was in command of the torque curve over the smaller cam by 3,000 rpm, and as the engine speed continued to climb, so did the power. Longer-duration cams always drive the peak horsepower point up higher in the rpm curve, but the conservative timing figures generated only a 5,800-rpm peak horsepower point. Despite the low peak rpm, Daffy still managed an honest 425 hp while torque was broad and very flat with a peak of 440 lb-ft at 4,600 rpm. This sounds like the torque peak was a little higher than you would expect, but the engine was also making 429 lb-ft at 3,800 and never made less than 382 lb-ft even at 2,600 rpm.


With the modified iron heads, this engine made exceptional torque with almost 400 lb-ft of grunt below 4,000 rpm. Those are great numbers considering its rather tame parts combination. Once we bolted on the new AFR heads, bigger Edelbrock intake, and Comp Cams camshaft, the power increase jumped to well over 100 lb-ft of torque and 100 hp! But let's not be too quick to throw the iron heads under the bus. Frankly, if we had started with an almost stock cam and actual stock iron heads, the differential would have probably been between 135 to perhaps as much as 150 hp.

It's rare to install bigger heads and a bigger cam in any engine and see the engine increase torque below peak torque, but that's exactly what happened when we installed the AFR heads and larger Comp cam. From 3,200 rpm all the way up to 6,000 rpm, the engine never made less than 413 lb-ft of torque, and that's what helps move the car down the quarter-mile or through the intersection. For an automatic-transmission-equipped car, this midrange torque is what you feel in the seat of your pants when you mash the throttle. For fun, we plugged all the power curves into the Racing Systems Analysis Quarter Pro simulation program configured as a 3,650-pound car with 3.55 gears, a TH350 with a 2,600-stall converter, and 26-inch-tall sticky tires. The simulation reported that the Test 1 version would run roughly 13.10 at 99.6 mph. By the time we'd dialed in the Edelbrock Performer RPM Air Gap intake, bigger 750-cfm carburetor, the AFR heads, and the Comp Cams 280HR roller cam, we'd managed to push the simulation to an impressive 12.60 pass at 108 mph. That's a solid 0.50-second and 7-plus mph difference. Overall, this series of bolt-ons was a rousing success. All of these parts could qualify for easy street driver status for a '60s to '70s musclecar. If you think the cam is still a little on the big side for a daily driver even though the engine idles at 12 inches of vacuum, you could expect a one-step smaller cam to add torque in the low- and mid-range while sacrificing perhaps 10 hp at the top. Regardless of which cam you choose, you can expect those AFR 180cc heads and Edelbrock intake combination to deliver an excellent power curve that will be well worth the investment.



Comp Cams
Memphis, TN
Hedman Hedders
Whittier, CA
Holley Performance Products
Bowling Green, KY
Quick Fuel Technology
Bowling Green, KY

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