It seems gasoline prices are on a constant unstoppable climb. The rate of this climb changes, and in our current national situation these changes can be abrupt, but...it seems prices are always a bit higher than they were last year, and especially higher than they were 2-5 years ago. This steady-but-gradual increase in gasoline costs is making it tougher and tougher to justify building a street machine to run exclusively on premium gas- always the highest-priced option at the local pump.
We at the car magazines are guilty of showing almost nothing but higher-octane buildups and research. We typically want to produce solid power figures and are normally willing to pay the higher price at the pump when these project engines hit the road. Often, the engines built on the pages of your favorite magazines end up living in limited-use automobiles, so the commitment to premium gasoline is not a big deal.
Times are changing, and the powers behind your favorite engine mag wanted to do something different. The overwhelming majorities of daily-driven vehicles remain bone-stock, as the factories built them, and live on a steady diet of cheaper 87-octane brew. What if a reader wanted to drive his hot rod more often than on sunny weekends, and wasn't willing to drop the larger dough for premium gas on a regular basis? Can a true performance engine be built to live on 87-octane? We think so, and we're doing the homework to find out how far it can be pushed. What we'll share is the bulk of the research we've accomplished toward getting this true street 87-octane engine going, and if you're considering something similar, we'll have a blueprint for you to follow. We based this buildup on the ubiquitous small-block Chevy because of its research-based existence, and the ease of parts and limited expense simply made sense in this vein. The techniques and ideas shown are quite universal, and should be on the list of "must-do" items for others pursuing the 87-octane dream regardless of engine make. These tricks and tips will work just as well on any domestic V-8, and probably any piston engine, for that matter. Its bad news that 87-octane may be our future, but we're trying to make lemonade from this lemon.
THE BASICS- BLOCK, CRANK, RODS, and PISTONS
We began with an '80s-era 4-bolt 350 block, with intentions toward stroking it up to the common 383-inch level. We anticipate a hard life for this particular mill, which may include racing and nitrous use down the road. We chose a complete Lunati reciprocating assembly, including the crankshaft (a good forged unit with aerodynamic counterweights and precision machining), forged connecting rods (in 6-inch lengths), and forged pistons (a flat-top design with two valve reliefs machined into their surfaces for adequate piston-to-valve clearances). Lunati sells these complete reciprocating assemblies in pre-balanced form, which saves time and money at the machine shop. They even ship with bearings. Their forged strength is not in question, and unlike 383s built using factory (400ci) cranks, these are internally balanced, which opens up parts selection dramatically. For anyone considering a Chevy 383 at any power level, it'd be hard not to recommend checking out these impressive pre-balanced kits.
To beef up our bottom end even further, we equipped the 4-bolt block with an ARP stud kit. It's important to remind readers how these studs must be installed prior to machining the mainline, and also how their torque figures may vary from a factory specification. We prefer ARP studs for their increased strength, improved bearing cap alignment, and increased grip length surface area (due to the finer threads in the torqued fastener nuts versus the coarsely-threaded bolts).
This type of bottom-end fortitude should support plenty of power, and probably is capable of more than we'll ever make with this low-octane mill. In this case, overkill is fine, since we intend to push limits and justified the extra durability by knowing we'd have our toes close to the line in the development and tuning of this engine. Should we push too hard, we know we've got the strength inside to survive.
Beyond strength, there's the issue of harmonics avoidance and the pursuit of smooth acceleration through balance. We've chosen to run a TCI "Rattler" balancer with its movable weight pucks inside, since we like the engineering of these pucks being able to move at will to correct any harmonic distortion instantly and at any rpm level. Unlike fluid-filled dampers, the Rattler design in unaffected by temperature and has instant response to varying harmonic interference. In a street-based car headed for the occasional racy jaunt, this type of engineering is a bonus. It doesn't suppress harmonics (like factory-type rubber-insulated dampers), it counteracts them with proper weight shift.
When pushing hard against an octane barrier, detonation avoidance becomes paramount and the heads are key. In addition to limiting compression (in our case, we've chosen 9.75:1), we wanted to have an efficient chamber design to make the most of what little octane we'll have. The ports need to be sized for optimal volume and velocity in the rpm ranges the engine will be running at (2,000-6,500), and our research led us to the AFR 210 (CNC-finished) aluminum units.
We were tempted to go with the zippy 190cc intake port, and also by the known-power made by the 215cc heads, but decided on the 210 to balance the aforementioned volume and velocity issues. We preferred the tried-and-true CNC-finishing to optimize the design, since the programming is proven and the port-to-port sizing is more accurate than any human hand could ever be with a grinder.
All detonation begins in the combustion chamber, so we were especially careful in designing the powerplant by choosing a good size chamber (at 76cc) and allowing the piston to sit "in the hole" by .012 to achieve our target 9.75:1 compression ratio figure. We then fortified the chamber with the addition of a thermal barrier coating, which will serve multiple purposes.
Firstly, the Calico CT-2 thermal barrier coating will insulate and isolate the chamber from the rest of the head. Research has shown this to be worth power, but immediately one would think this insulating property would bring us closer to detonation. We feel it will aid in the distribution of heat across the chamber, and by engineering an efficient cooling system, we can maintain good detonation avoidance under full load. We hope to spread the heat out over the chamber and in doing so, bring additional detonation avoidance properties along. Another advantage of the Calico CT-2 thermal barrier coating is its ability to leave a smooth surface wherever it is applied. Being a ceramic type of material, the resulting surface of the coated chamber is nice and smooth. The lack of any sharp edges in the critical chamber area adds to the detonation avoidance characteristics, and will allow us to push a little further into the land of low-octane performance.
We chose to have the coating applied directly over the CNC-finished AFR chamber without any further prep work or smoothing. We did this so our research could be easily duplicated with out-of-the-box, unmodified heads our readers could get their hands on. Also, we felt that changing the chamber as AFR finished them would probably hurt more than it would help, since so much of their research has gone into these chambers. As a final point, the coatings are .001-.002-inch thick, and this served to virtually eliminate the already-fine CNC machine finish inside the chamber. This smoothing effect is precisely what we were hoping for in addition to the thermal barrier features the coatings bring to the party.
We also coated our valves and ports. The thinking behind this is twofold, as the intake charge will be insulated from heat as it enters the combustion chamber (including the heat from the intake valve), and the hot exhaust will be escorted through a similarly insulated tunnel on the way out. Header wraps and coatings make power because they keep the heat inside the header. Hot exhaust is always trying to expand, and the insulated exhaust port aids in this quest. Once the hot exhaust gases reach the header collector, they are encouraged to expand and escape, as this is where we've designed them to do so.
By insulating the exhaust valve, we hope to minimize heat going into and coming out of it. Keeping the exhaust valve cool will also aid in detonation avoidance, since the exhaust valve and its surrounding seat area is always the hottest part of the combustion chamber.
The camshaft defines so many characteristics of the engine, its design is always critical. In wanting to maximize the performance of this 87-octane mill throughout a wide rpm band, we contacted Comp Cams and got the advice of their experts. We decided on a solid roller camshaft, since we liked the benefits of the design (rapid valve action, minimal friction) and we've got no problem checking our valve lash occasionally. While some may balk at the idea of a solid roller in a daily-driven street machine, we actually enjoy the fine-tuning and occasional lash requirements, and we want to take full advantage of the capabilities this engine can bring. A solid roller can bring us more, and we chose Comp's Xtreme Energy part number 12-772-8 (grind number XR286R). The specifications on this cam are aggressive, with 286/292 degrees of advertised duration, a quick 248/254 at .050-inch lift, and 110-degree lobe separation angle. Lift numbers check in at .576/.582-inch with a 1.5:1 Crane Gold roller rocker, and we'll experiment with different ratios of these proven Crane rockers to fine-tune the best-possible scenario with this particular cam.
The idea behind the aggressive lobe is to shed a bit of compression at the lower rpm ranges to further avoid detonation. It's honest inefficiency and overlap will shed some cylinder pressure below 2,000-2,500 rpm, and the lower pressures should be able to support the low octane gas. Once the engine picks up speed (2,500-plus rpm), the overlap will add to the efficiency instead of taking away from it, and the engine should be able to start supporting real power numbers all throughout the midrange to the 6,500 rpm (or so) redline. As you'll read, careful carb tuning and fine adjustment of the ignition curve will also be incorporated at the critical lower rpm levels to offset detonation at this most-susceptible rpm level. Careful tuning of our extremely adjustable components and parts selection focusing on precision-crafted performance hardware should become our best weapons against the curse of low-octane.