Our combo this month was a GMPP crate engine producing 290 hp at 5,100 rpm and 326 lb-ft of torque at 3,750 from the factory.
With its 8.5:1 compression, four-bolt block, and small street cam, this piece is a true 87-octane-burning mill that'll put new life back into any Chevrolet. While the engine would serve as a great mule for the average commuter, we're about to showcase the untapped potential waiting to be released.
The PlanOur engine design started by addressing several key areas of the engine where GM knowingly sacrificed potential power for a smooth idle, low emissions, and low-octane operation. The factory cylinder heads may be ideal for a daily driver, but they are definitely not meant to make big power. Our plans called for a set of GMPP's newest Fast Burn 210cc aluminum cylinder heads. Their ports offer superior airflow characteristics with vastly improved intake and exhaust runners, while the new Fast Burn combustion chambers ensure that the incoming air/fuel mixture will burn efficiently. These heads alone are so good we decided to bolt them onto our crate engine using an Edelbrock Air Gap Performer RPM and a 650-cfm Holley HP carburetor with mechanical secondaries. Along with these tried-and-true induction pieces we also used an MSD billet distributor, a 6AL ignition box, and high-performance 8.5mm wires to make sure our cylinders stayed lit during the upper-rpm flog. The factory heads feature 76cc combustion chambers, which were hurting power with an 87-octane-compromising 8.5:1 compression ratio. The new GMPP heads feature smaller, 62cc combustion chambers that would raise the compression of our engine to a 91-octane-friendly 9.9:1. This alone would be worth nearly a 3-percent power gain, not to mention how much more power would come from the Fast Burn chamber design and improved port layout.
The GMPP crate engine arrived accessory-free, requiring several supporting pieces, including an oil filter and bracket, cylinder-head water plugs, spark plugs, and a harmonic balancer. The only dilemma we ran into while putting our engine on the dyno was that the Edelbrock intake manifold had some interference issues with the stamped-steel valve covers that came on our engine. We could have milled the factory valve covers to fit, but with our tight schedule, swapped them out for a set of Comp Cams aluminum tall-body valve covers and opted to utilize an electric water pump and 131/44-inch dyno headers with no other accessories. We also added 5 quarts of 20W50 Lucas oil and a bottle of Comp Cams break-in lube to ensure our new hydraulic flat-tappet camshaft would break in properly. It's important to note that GMPP does not break its engines in before they arrive, which makes it critical for the end user to do so with hydraulic flat-tappet applications.
On The Dyno
With the new powerplant fired up, we immediately varied the rpm between 1,500 and 3,200 for 20 minutes to make sure the cam, lifters, and piston rings were seated properly. Our first few dyno pulls were made half way through the engine's rpm range to tune for optimum timing and jetting, where we established a total timing advance of 38 degrees with Holley's box-stock HP jetting. Once all the engine vitals were stable we made a series of baseline pulls to come up with a repeatable 315 hp at 5,100 rpm and 351 lb-ft at 3,700 rpm. Both GM and our dyno numbers occurred at the same peak points, showing us that our lack of accessories and use of high-performance dyno headers were perhaps why our baseline numbers were a bit higher than the GMPP ratings. It just goes to show that GMPP doesn't fudge its power numbers to sell engines.
With solid baseline runs under our belt we installed the new GMPP heads by themselves out of the box. While there was a recommended camshaft selection in our engine design to go along with the intake and spark-enhancing pieces, we wanted to see how good the heads were on their own. An added feature of the Fast Burns is that their heads are tapped with two sets of valve cover and intake-mounting holes to allow for use with Vortec or conventional small-block patterns.
After a series of test pulls, we achieved an optimum total timing advance curve of 36 degrees with the box-stock Holley HP jetting. The more efficient combustion chambers of the new heads explained why our engine required less total timing. As for the factory jetting from the Holley HP carburetor, it would take a story in itself to explain why this carburetor continued to deliver optimum air/fuel ratios out of the box; in short, as more air is drawn through the carburetor more fuel is delivered proportionately to accommodate it.
Like kids in a toy store, we couldn't wait to see the potential power our new heads had to offer, so we poured in some 91-octane and began testing. This time around we were absolutely amazed at the power increase our heads had made with the stock cam still in place: peak readings of 359 hp at 5,500 rpm and 378 lb-ft at 4,000 rpm. We knew the improved ports, chambers, and valve size of the new heads would make more power, but without additional valve timing the gains were impressive: 44 hp and 27 lb-ft.
With limited dyno time left in the day, we swapped out the stamped-steel 1.5:1 rockers for a set of Comp Cams 1.6:1 Magnum-style roller pieces. This added nearly 0.300 inch of lift to the intake and exhaust valves, while increasing the duration of the valve by nearly 2 degrees. All carburetor jetting and timing variables remained the same as we pulled our engine to a new peak reading of 371 hp at 5,900 rpm and 382 lb-ft of torque at 4,600 rpm. The additional lift and duration made only a small difference in the torque curve but raised the peak rpm of the engine by nearly 400, to deliver an additional 12 hp up top. While the rocker swap delivered a relatively small kick in the pants compared to the heads, we knew there would be more power with a better cam grind.
We were now on a mission for ultimate power, and up next was a new bumpstick from Comp Cams. The grind of choice featured 0.470/0.470-inch intake and exhaust lifts with a 1.5:1 rocker ratio and 224/224 degrees of duration at 0.050 inch lift on 110 LSA. The hydraulic flat-tappet piece is a mild choice, picked to help improve street power without affecting the engine's idle vacuum. Once the fuel lines were hooked back up we started the engine and repeated the break-in procedure. The timing remained at 36 degrees total, and the jetting was still good, showing air/fuel readings of 12.8:1 at peak torque. After a quick look over all the vitals, we ran the engine across its power curve. With new readings of 385 hp at 5,600 rpm and 407 lb-ft at 4,200 rpm, we were pleased to see additional improvements to the tune of 6 hp and 29 lb-ft over the factory cam with 1.5:1 rockers. What's interesting is that our new single-pattern cam increased the horsepower at a lower peak rpm point while also significantly improving the torque across its entire power curve.
The Final Word
Adding more camshaft duration would have raised the peak rpm point up into the 6,000-rpm range and perhaps made 400-plus horsepower, but idle vacuum would have been sacrificed along with some torque. Our factory long-block with either stock or high-performance GMPP heads delivered 13 inches of idle vacuum at 800 rpm, and our new camshaft delivered more power while sacrificing only a half inch of idle vacuum at the same rpm. We set out to build a solid street engine using readily available performance parts and increased our total power by 70 hp and 58 lb-ft. It just goes to show that with the right parts and help from some of today's leading high-performance manufactures, ultimate street power is only a phone call away.