Agent 87, Part I

In the February, 2001 issue, we brought you the “411 HP on 87 Octane” story. This was based on a basic 350ci small-block that used high-quality parts to make this kind of power. Hye Tech Performance built the engine using a CAT steel 5140 crank, good forged Speed-Pro pistons, and aluminum TFS 23-degree cylinder heads. This engine responded with great power on 87-octane fuel especially with its 9.4:1 compression ratio. While this was a great example of how to make good power, we felt we could make similar power with a much less expensive engine combination. What began as a notes-on-a-burger-stained-napkin idea evolved into the Miserly Mouse.

The Miserly Mouse is very similar to the small-block described above, but with a few subtle differences. Based on the success of that combination, we looked at where we could save some money and yet still produce similar power. The key to this exercise comes down to matching cylinder heads and cam timing. To make decent power, you have to have a good set of cylinder heads. One half of the Miserly Mouse’s one-two punch is a set of the GM Performance Parts iron Vortec cylinder heads. CHP’s Goodwrench Quest series (September ’99 through July 2000) showed us that these inexpensive production iron heads can make great power at a very reasonable price of around $450 per pair complete and ready to bolt on.

The other half of this equation was a bit more challenging. The concept of building a performance engine that makes decent power on 87-octane fuel requires a special camshaft. The basis for this 87-octane effort revolves around a slightly lower 8.75:1 compression ratio (because of the iron heads) along with an emphasis on improving combustion efficiency.

If you read the story “How to Whip Detonation” in the February ’01 issue, you’ll remember that chamber design and pistons can combine with a tight quench area to create an “active” combustion space that can help reduce the tendency toward detonation. Since detonation kills power and can destroy an engine, we want to avoid it at all costs. But savvy engine builders will tell you that you can get away with higher compression if you build the engine correctly. This is why we chose the Vortec iron heads. Not only do the heads have an excellent reputation for airflow right out of the box, but the Vortec heads also enjoy the benefit of an outstanding combustion chamber design. This heart-shaped chamber creates turbulence in the combustion space to prevent detonation.

When we conspired with Ed Taylor to build this engine, we had three goals in mind. The first was to be conservative with a compression ratio under 9:1 to suppress detonation. The second was to optimize combustion efficiency by minimizing the piston-to-head clearance. This minimal clearance enhances turbulence in the chamber that should suppress detonation. The third was to use a dished piston, but to choose a D-shaped cup instead of a full dish. The flat portion of the piston matches the flat portion of the combustion chamber to create a quench, or squish area between the chamber and the piston that creates turbulence.

Savvy engine builders know that minimizing the piston-to-head clearance in this quench area to less than 0.040-inch dramatically increases the amount of turbulence created in the combustion space between the piston and the chamber. Increasing this activity minimizes the engine’s sensitivity to detonation. As a result, Taylor chose Speed-Pro forged pistons with a D-shaped 21cc dish. Add this to the Vortec head’s 64cc chamber and we end up with the equivalent of an 85cc combustion chamber size with a flat top piston. Then Taylor mocked up the engine with all eight pistons, measured the piston deck height, and then milled the block to create as close to a zero deck height as possible. He then added a Fel-Pro composition head gasket with a compressed thickness of 0.041-inch that created an average piston-to-head clearance for all eight pistons of 0.044-inch. In discussions with race engine builders, it is possible to build a street engine with a slightly tighter piston-to-head clearance, but this requires a very tight piston-to-wall clearance to prevent piston rock at TDC. We decided to stick with our 0.044-inch clearance.

The Engine

Since we intended to build a budget-based engine, we could have used a stock Chevy crank, but for just a little more money, we picked up a cast Scat crank to match our resized stock rods. Jim Grubbs Motorsports (JGM) performed the machine work on our stock block including a 0.030-inch bore, torque-plate honing, connecting rod resize with ARP bolts, and an align-hone to accommodate the ARP studs for the two-bolt main caps. You should always align hone the mains anytime studs are used since the studs change the load distribution on the main caps that will affect main bearing clearance. Speed-Pro also supplied the rod, main, and cam bearings along with the 5/64-inch ring package.

Once JGM performed the final deck operation on the block to establish the proper deck height, Taylor spent a few hours assembling the short-block. This led us to the camshaft. Since we have had excellent luck with the Xtreme Energy 268 camshaft from Comp Cams, we didn’t have to burn the midnight oil to come up with that decision. The cam specs are listed in the Cam Specs sidebar, but with the later closing intake valve, the engine we figured the engine will not see detonation, even with 36 to 38 degrees of total ignition timing.

The final assembly included a Mellings oil pump, a Hamburger oil pan from TD Performance, and a two-piece timing chain cover from Comp Cams that will allow us to make cam timing changes if necessary. TD Performance also supplied a set of centerbolt valve covers, an aluminum water pump, and an SFI-approved harmonic balancer. Pertronix supplied an HEI distributor and plug wires, while Made For You supplied the wire looms. Finally, Taylor bolted on the Edelbrock Vortec Performer RPM intake and a Road Demon carburetor to handle the induction duties while a set of Hedman Elite 15/8-inch headers supplied the exhaust ducting.

The Test

Ken Duttweiler once again offered his dyno cell to flog our Agent 87 engine. After warming up the engine and performing a short break-in period, Taylor pulled the handle on Duttweiler’s dyno and started with the tuning process. Taylor tried several jetting configurations on the Road Demon 625-cfm carburetor but the power seemed best at the stock jetting numbers. We also started at 36 degrees of total timing, and after several pulls also discovered that the engine preferred 36 degrees.

After several pulls, Taylor went back to the stock timing and jetting combination and made several pulls to confirm the power numbers. The combination of the Xtreme Energy cam, Vortec heads, and a tight quench resulted in a great power curve that we pulled clear down to 2,400 rpm. The 355 managed over 400 lb-ft of torque at 3,300 with a peak torque of a stout 427 lb-ft at 4,000 and then delivered 390 hp at 6,000 rpm. This is an outstanding 2,000-rpm power band between peak torque and peak horsepower. The engine also was still pulling 375 hp at 6,300 rpm. What this means is that the engine offers a wide power band that has the potential to run low 13s in a 3,600-pound Chevy with easy regularity. Remember now, this is on 87-octane pump gas.

We also did a little quick money-saving math. If the price differential between 87 octane and high-octane 92 is 25 cents per gallon and you use an average of 20 gallons of gas per week, that’s a savings of $260 per year just on fuel. That’s not a ton of money, but it’s enough to allow you to save enough money in gas to pay for a nitrous system! So if you’re thinking about building a mild street engine, this might be the way to get your horsepower and save money too. It doesn’t get any better than that!

Agent 87, Part II

Agent 87, Part III