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 Andrew Sivori's essentially...  Andrew Sivori's essentially stock LS1 Vette baselined a healthy 318.6 hp and 293.3 lb-ft of torque on a Dynapack chassis dyno with a wideband O2 sensor that Strictly Performance uses exclusively for its accuracy... |
 ...Since Andrew's '99 Vette...  ...Since Andrew's '99 Vette still had the stock exhaust, a wideband O2 bung was welded in just before the cats. |
 Removing the intake manifold...  Removing the intake manifold on Vettes is almost exactly the same as the F-body. Once the clamps on the intake tubing are removed and the coil covers come off, the injectors can be unplugged from the harness along with the electronic throttle... |
 ...The evap purge solenoid,...  ...The evap purge solenoid, EGR valve, and fuel line can also be disconnected before removing the 8mm bolts on the intake manifold. Robert lifts the manifold up partially then unplugs the coolant crossover, brake booster, vent for valve covers and the back brace before removing the entire intake assembly. |
The Gen III engine has proven itself to be quite a performer since its inception in 1997. In its most potent LS6 variant, it reached 405 hp thanks to improvements in cylinder head design and a lumpier cam. However, not long after the creation of the mighty LS6, the Blue Oval boys were hot on GM's heels with the supercharged Cobra. Luckily, GM fans have a new weapon in their arsenal that appropriately takes its name from the cobra's natural predator--the mongoose.
The Air Flow Research LS1 Mongoose cylinder heads are well equipped to handle any Cobra-killing setup, whether naturally aspirated, juiced, or blown. The Mongoose heads come in two varieties: 225cc for larger cubes and blown applications, and 205cc, ideally suited for stock displacement engines, or strokers that want to place an emphasis on building lots of torque down low. Both heads have enormous advantages over the typical stockers (and ported stockers) due to the fact they were designed from a clean sheet of paper, subsequently AFR's port and combustion chamber shapes are completely unique and were designed with performance in mind from their inception. AFR's unique aluminum casting also provides a .75-inch-thick head deck, reinforced rocker stud bosses, and thick wall runners designed both for durability and expandability.
A set of AFR heads, long-tube headers, and a well-matched cam has proven to make both impressive numbers at the track and the dyno. However, GMHTP was interested in finding out what was to be had from just a set of heads on a near-stock LS1. In doing so we could establish a true apples-to-apples comparison to see if the AFRs live up to its reputation.
For the test we acquired a set of the 205cc heads, which AFR designed specifically for stock bore, street-going applications. The 205cc heads advertise 300-cfm intake flow at .600-inch lift, a whopping 70-cfm improvement over stock LS1 heads. AFR's Head of R&D Tony Mamo said the improvements in flow can also be seen all across the board, in fact at .400-inch lift the AFRs outflow the stockers by 47 cfm. Since street-going vehicles will most likely be seeing lift somewhere in this range the majority of the time, the 205cc heads are ideal for applications such as this with only 346 to 396 cid. The increase in airflow, Tony says, will better fill the cylinder and increase fuel economy. In addition, the use of the near-stock-sized 205cc intake volume (stock is 200 cc) helps keep velocity to a maximum, increasing torque and throttle response, which Tony says was the goal from the outset. "We were looking to create the smallest port possible on these heads while still achieving 300 cfm, which in the end resulted in an extremely efficient cylinder head."
The efficiency of the AFR cylinder heads is also owed to a unique combustion chamber and revised exhaust ports. For comparison purposes, a stock LS6 exhaust port comes in at 78 ccs and flows around 195 cfm on AFR's test equipment. The AFR 205 exhaust flows 230 cfm, representing an 18 percent increase in flow while only displacing 6 ccs of additional volume (84 ccs total, only a 7.5 percent increase in size). Tony says this additional exhaust flow was very much needed to handle the chores of expelling all the spent gases the higher flowing intake port just brought to the table and maintaining the proper intake/exhaust ratio. Even as early as .400 lift, the AFR exhaust port shows a 36-cfm advantage over the LS6 stocker. Meanwhile, the AFR's specially designed combustion chamber with "dual quench pads" [Tony says] also increases flow and enables more thorough atomization of fuel. The fully assembled heads have the benefit of larger than stock stainless steel 2.02 intake and 1.60 exhaust valves, both with proprietary "top-secret" valve jobs that Tony assures me he spent countless hours researching for optimum results. AFR heads also come standard with premium dual 1.260 springs, 7-degree titanium retainers and valve locks, and bronze valve guides.
The guinea pig for our little experiment was Andrew Sivori's '99 Corvette convertible, the perfect daily driver for an inhabitant of San Diego. After 55,000 miles, there was not a scratch on the Vette, and barely any mods, which is surprising given Andrew's affection for power. Up until now, the Corvette sported only a Blackwing intake and Flowmaster mufflers.After having so much success with our last visit to Strictly Performance Motorsports in Van Nuys, California, we decided to use owner Robert Barth's skilled hand in wrenching and tuning the Vette. His extensive experience with Gen IIIs (Barth is known for sick LS1 head/cam packages) would make the install a snap, and given his training with HP Tuners software (by the creators themselves), we knew he would make the most of the more efficient cylinder heads. The dyno test would be conducted again at a local shop Robert uses exclusively for its high-end Dynapack chassis dyno.
Ride shotgun as we go back to Cali and see how effectively the Mongoose hunts its prey.
 The battery is disconnected...  The battery is disconnected and the ground strap for the vent system is removed, along with the 10mm bolts on the coil packs. The plug wires and the coil packs come off. Then, the serpentine belt is removed by loosening the tensioner via its 15mm bolt. |  The two 15mm bolts on the...  The two 15mm bolts on the alternator and 13mm bolt on the electric terminal must be removed to free the alternator from the passenger-side cylinder head. |  The four 8mm bolts on the...  The four 8mm bolts on the valve covers are removed to expose the rocker arm assembly, which is connected to the head via 8mm bolts. With the rocker arm assembly removed, Robert pulls out the pushrods. Keep in mind, the rocker arms and pushrods are covered in oil, so unless you let your car sit over night, they are going to be red hot. |
 A slew of 10mm bolts are removed...  A slew of 10mm bolts are removed from the dipstick, the water bypass, the exhaust manifolds... |  ...the crossover pipe, and...  ...the crossover pipe, and on the top of the heads themselves. |  Fifteen-millimeter bolts attach...  Fifteen-millimeter bolts attach the exhaust manifolds to the catalytic converters and comprise the bottom head bolts... |
 ...which is enough to free...  ...which is enough to free the passenger cylinder head from the block. |  Removing the driver-side head...  Removing the driver-side head requires removal of the power-steering pulley and pump. In order to reach the power-steering pump you will first need to remove the ABS module and pump. Four 13mm and one 18mm bolt are unscrewed from the pump bracket, and so are the three 10mm bolts that hold the module to the bracket. The module is then pulled out of the way. |  Removing the power-steering...  Removing the power-steering pulley was the most difficult part of the installation. There is a plastic tab in front of the bolt that easily comes off with pliers; however, the bolt itself is a much different story... |
 ...Robert used the pulley...  ...Robert used the pulley removal tool as recommended, but the pulley seized up and had to be destroyed (with a torch and a grinder) in order to be removed. However, the 15mm bolts on the power-steering pump and reservoir were much easier. |  The fully assembled AFR 205cc...  The fully assembled AFR 205cc heads came with dual valvesprings good for up to .600-inch lift, bronze valve guides, titanium retainers, a stainless steel 2.02 intake and 1.60 exhaust valves, and CNC porting throughout. Though we could have raised the compression with a thinner gasket, for the purposes of making this a true apples-to-apples comparison, we ordered up a factory replacement GM MLS head gasket. Since the factory head bolts are torque-to-yield, they cannot be reused, so the owner took the opportunity to upgrade to ARP head bolts. |  Tony Mamo from AFR said great...  Tony Mamo from AFR said great effort was put forth into optimizing the combustion chamber design, helping to improve the overall efficiency and flow characteristics of the cylinder head. He says the "double quench pad" helps to accomplish this by creating two fronts of air that rush into and collide with one another, helping to atomize the fuel for a more efficient and complete combustion process... |
 ...The 66cc size is slightly...  ...The 66cc size is slightly smaller than the stock LS1 (one cc) and should raise compression to about 10.4:1, slightly below a stock LS6. For best results on a naturally aspirated motor, Tony recommends using a thinner head gasket (for better "quench") and milling the heads slightly for a higher compression ratio. |  Finding the right shape for...  Finding the right shape for the intake ports was crucial in keeping a smaller diameter for increased airspeed while still improving flow. AFR's unique casting, in combination with CNC porting and extensive proprietary testing, enabled this development. Long hours were spent with a swirl meter and flow bench utilizing a specially designed 3.90-inch bore and 1.75-inch pipe for the most realistic results. |  The new GM MLS head gaskets...  The new GM MLS head gaskets were placed on top of the block before the AFR heads went on. First, the spark plugs and water-jacket plugs had to be transferred from the stock heads. Robert made sure to apply silicone sealant on the threads of the plugs since they would be going into the water jacket. |
 The 13mm ARP head bolts were...  The 13mm ARP head bolts were step-torqued in a circular pattern from the center out to 75 lb-ft, and then the stock exhaust manifolds were connected to the new heads. |  The stock pushrods and rocker...  The stock pushrods and rocker arm assembly is put into place and tightened down. |  Robert reassembles the rest...  Robert reassembles the rest of the top end and installs a new power steering pulley. Once all of the fuel, coolant, vacuum, and electrical lines are secured he changes the oil and adds some coolant. Since both coolant and oil can be found in the heads when you remove them, coolant inevitably finds its way into the oil passageway. If you are very picky you could drain all of the fluids beforehand, however Robert says it isn't necessary. |
 When we returned to the dyno...  When we returned to the dyno with the new heads the temperature had raised only 3 degrees and humidity also increased by 3 percent with very little change in barometric pressure, which helps keep consistency with the test. A few pulls were made before Robert took the helm with the HP Tuners software and the results were a respectable 345 hp and 316 lb-ft, which was a 27 hp and 24 lb-ft improvement. One concern, however, was that the pig-rich 11.18:1 air/fuel ratio was relatively unaffected. In addition to adding about 3 degrees of timing Robert went to work leaning the mixture. |  Now sporting a more respectable...  Now sporting a more respectable 12.12:1 air/fuel ratio, the Vette gained about another 10 hp and 9 lb-ft for a total of 356.0 hp and 325.1 lb-ft of torque... |  ...and that was with over...  ...and that was with over 100 degrees of intake air temperature from the multiple back-to-back pulls. That's a 37.4 hp and 31.8 lb-ft gain over stock! |