It was tempting to subtitle this article "Reaching for 500 hp theold-fashioned way."
After all, messing around with an engine's cylinderheads, cam-shaft, valvetrain, intake, and exhaust is about astraditional a hot-rodding approach as you can get. But other than thefact that they use pushrod technology, is there really anything"old-fashioned" about 21st Century Corvette engines?
No, we thought not. That's why we went for the "natural" term, to setthis quest for power apart from the forced-induction setups you'll readabout elsewhere in this issue.
We teased you with news of this buildup in the July Currents. For thoseof you who missed that tip-off, here's the deal--Al Carreon and the crewat the LA Performance Division (or LAPD, not to be confused with SoCal'smen in blue), had on their hands a like-new '04 Z06 with just 800 mileson the clock. They decided to take apart the minty-fresh LS6 and golooking for 500 naturally aspirated horsepower, within the parameters ofleaving the block and reciprocating assembly alone.
"We're not just looking for peak power, though," Al explained. "We wantdriveability, too. We're exploring what can be done with an LS1-basedengine."
It's not tough to make 500 horses when you're pushing lots of air andfuel via a turbo or supercharger, or if you've poked and stroked theengine to big-block proportions. From 346 cubes, though, and using justatmospheric pressure, it's a challenge. Not an insurmount-able one, buta challenge nonetheless. As we reported in July, the shop had reached483 hp and 430 lb-ft of torque with its first buildup attempt, but thecrew was looking to make more changes to find those additional ponies.
As Al put it, "If you're willing to accept 'just' 430 to 450 hp, wecould knock that out and do it for about half the price. But going fromthere to 500 horses means paying close attention to every little detailand using some exotic parts. When you're getting close to 500 hp, youcan expect to pay about a grand for every 10 hp."
What's the bottom-line cost of the engine profiled here? We'll save thatpiece of good news for later. Now, let's get into making power the LAPDway.
LAPD chose a pair of Air Flow Research's (AFR) 205cc "Mongoose" LS1aluminum cylinder heads to go on the stock block. To get enough air intothe engine to work with the big cam he had in mind, Al sent the heads toTotal Engine Airflow to have the intake runners opened up to about 228cc's. (He admitted he would have started with AFR's 225cc heads had theybeen available when he began the buildup, but the 205s flowed just fineafter the porting job.) Inside the heads are 2.020-inchintake/1.600-inch exhaust valves that are actuated by a set of HarlandSharp 1.7:1-ratio adjustable rocker arms getting the nudge from CompCams 7/100 pushrods.
To clear the taller rocker arms, LAPD swapped the stock valve covers fora set of aluminum Katech covers that the shop had powdercoated red.Katech coil relocation brackets were also installed to give the shopbetter access to the cover bolts.
When Al first began the power search, he left the heads with theiras-delivered 66cc combustion chambers. Combined with the stock pistons,they gave the engine an 11.3:1 compression ratio--a healthy increaseover the stock 10.5:1. Yet, when Al and the guys hit that 483hp plateau,they determined a bump in c.r. would help their case. So, they milledthe heads to shave the combustion chambers down to 59 cc's and then usedthin Cometic 0.035 head gaskets, the combination of which raised thec.r. to 12:1. "We'll just put in some Torco [octane boost] with the91-octane pump gas," Al reasoned.
To hit the LAPD's target number, the engine was going to have to carry abig stick, and at first Al and company thought they had the hot setup--asolid-roller cam with 0.624/ 0.649 inch of gross lift and an advertisedduration of 294/300 degrees. It was with this cam that the crew saw the483 horses. Not good enough.
To Al's reasoning, there was a problem with the cam's lobe profile.Though it was ramping up very quickly, "there wasn't enough advertisedduration," Al figured. "If you have the proper amount of duration, youwon't hurt the engine's driveability, even with a big cam. The durationallows the engine to breathe."
So a change was in order. Out came the solid roller, and in went anUltradyne hydraulic roller. It didn't have as much gross lift--"only"0.612 inch on both sides--and the ramp rate wasn't as radical. But theadvertised duration went up to 299 degrees on the intake side and 307for the exhaust, a happy place, it turns out, for this motor.
There was another advantage to switching cams. Although solid camstypically have a more docile idle, "they're way harder on the springs,"said Al. The hydraulic cam has less seat pressure, "so it doesn't beaton the seats, valves, and stems," he said.