Welcome back to our next series of dyno tests with our L8T To The Party crate engine project! Last month Brian Tooley Racing put this engine on their dyno to see what kind of power it could make with different camshafts and intake manifolds. The L8T crate engine comes from Chevrolet Performance as a completely stock 6.6-liter engine, straight from the production line. While GM built this as their entry-level gasoline-powered workhorse, this iron block V8 benefits from direct-injection, variable valve timing, and a healthy 400 cubic inches of displacement. So what happens when we up the game with our L8T dyno shootout with a pair of CNC-ported cylinder heads to go with the camshaft and intake changes?

Even though it was built to be used and abused in heavy-duty Silverado and Sierra pickups while running on 87-octane and being emissions-friendly, the L8T is factory-rated at 401 horsepower and 464 pound-feet of torque. One horsepower per cube out of a factory work truck engine is what makes this 6.6 appealing to hot rodders. What is more appealing is that it responds to tuning and modifications like its 5.3 and 6.2 baby brothers. At the beginning of our L8T dyno testing, ours made 500 horsepower at 5,100 rpm and 580 pound-feet of torque at 3,900 rpm with nothing more than a set of 1 7/8-inch dyno headers and a change to E85 fuel in the dyno cell.

On 93 octane, our 6.6 made 476 horsepower and 554 pound-feet of torque. Simply switching it to E85 pushed it to 500 horsepower and 580 pound-feet of torque.

At the end of our L8T camshaft shootout, the 6.6-liter crate engine cranked out 650 horsepower at 6,800 rpm and 554 pound-feet of torque at 5,400 rpm with BTR’s 230 cam and a Trinity mid-length intake manifold while still sporting the stock bottom end and cylinder heads. The cam’s duration is 230 and 24x degrees on the intake and exhaust, 0.644 inches of lift on both the intake and exhaust lobes and a lobe separation angle of 112 degrees. We suspected that some ported heads would be exactly what the BTR 230 cam wanted, and now it’s time to find out!

By the end of our camshaft shootout, this L8T was pumping out an impressive 650 horsepower at 6,800 rpm and 554 pound-feet of torque at 5,400 rpm while still sporting stock heads.

GPI Track Attack CNC Heads

Gwatney Performance Innovation has become an increasingly big name in the LS and LT world over the past few years. Doing more than developing new performance parts for these platforms, they also aren’t shy about testing them to their limit. The pair of cylinder heads they provided us for testing is from their Track Attack line. These heads are not OEM castings; they start out as USA-made bare unmachined castings from Competition Induction Designs and are then CNC machined with GPI’s porting program.

GPI advertises 346cfm on the intake at 0.700 inches of lift with their CNC-porting program. The castings are much more robust than the stockers, offering a 25-percent thicker deck surface, reshaped cooling jackets for a more solid valve spring pocket foundation, and more material around the ports that allows not only more port volume but the ability to reshape the ports overall, taking performance much further than you could just porting the OEM castings.

CID CNC LT1/LT4 Heads

Competition Induction Designs supplies their high-quality USA-made castings to a few different builders so they can put their special touches to the port design such as GPI’s Track Attack heads we discussed above. But CID is no stranger to high-performance LS and LT engines either and offers its own CNC-ported version of these LT cylinder heads. These heads are track-proven to be a serious contender, pushing Justin Keith’s sixth generation ZL1 Camaro to set multiple records in the quarter-mile.

CID’s casting allows a custom CNC port job to not only make the port larger than stock, but also reshape and contour for better port velocity.

These heads were the first aftermarket heads for the then-new Gen-V LT to hit the market a few years ago and the first six-bolt head as well. The six-bolt per cylinder design combined with the thicker deck surface and other robust features makes them very desirable for performance engine builders wanting to push the limit of the LT platform. CID advertises 351cfm on the intake at 0.700 inches of lift with their CNC port work. The best part of these redesigned castings is you get all the benefits without having to buy any special parts to make them fit. They accept factory valves, springs, and rockers, and come drilled for both LT1 and LT4 intake bolt patterns making these a true bolt-on head.

The CID castings accept factory-sized valves, springs, and rockers, and come drilled for both LT1 and LT4 intake bolt patterns making these a true bolt-on head with off the shelf parts.

It’s Head Swap Time!

It’s time to strip our L8T crate engine down to the bare short block. So off came the intake manifold, headers, valve covers, and rocker arms. The first pair of heads tested were GPI’s Track Attack LT cylinder heads. GPI offers its heads with a 55cc combustion chamber for naturally aspirated applications or a 62cc chamber volume for forced induction builds. The heads supplied were the 62cc version, which were then milled down to the stock 60cc volume to retain the factory 10.8:1 compression ratio. We fitted these heads with BTR’s dual valve springs to perfectly match the BTR 230 cam already in the block from our previous L8T dyno runs.

The GPI Track Attack heads were tightened down to the block with stock head gaskets and ARP head studs. The stock rocker arms with BTR’s trunnion upgrade were refitted after double checking pushrod length and the valve covers and headers were bolted back on. Since the goal here is a max-effort, naturally-aspirated stock short block, BTR decided to fit a CNC-ported version of their Trinity short-runner intake manifold to match the larger ports. While this may not make for a drastic change in power, it certainly matches the added airflow of the ported heads.

GPI’s Track Attack heads added 56 horsepower and 33 pound-feet of torque over the stock heads. New peak numbers came to 706 horsepower at 7,000 rpm and 587 pound-feet of torque at 5,400 rpm.

The results with GPI’s Track Attack heads were 706 horsepower at 7,000 rpm and 587 pound-feet of torque at 5,400 rpm. That’s a gain of 56 horsepower and 33 pound-feet of torque over the stock heads, which confirms our previous suspicion that the BTR 230 cam wanted more airflow. Turning a reliable truck engine into a 7,000 rpm street engine usually isn’t as easy as swapping the cam and top end and picking up over two hundred horsepower, but this is where the modern LT platform shines.

“I like the stroke and displacement of the truck engine. To be able to take a stock displacement L8T and make over 700 horsepower with a cam, head, and intake change is exciting stuff!” – Brian Tooley, Brian Tooley Racing

Next, it was CID’s turn to see what their CNC heads could do for our 6.6-liter Gen-V LT. Off came the GPI heads and back on with CID’s heads. These were installed utilizing stock head gaskets just like the test with GPI’s heads, but keep in mind the combustion chamber is a little smaller for a bump in static compression. CID offers their heads in either 64cc or 57cc chamber sizes, the pair we tested measured in at 57cc which brings our compression to a little over 11:1 from the stock 10.8:1.

A pinch of extra compression and a few extra CFM helped the CID heads edge out the highest numbers at 723 horsepower at 7,400 rpm and 598 pound-feet of torque at 5,400 rpm.

The final pull on our max effort naturally aspirated L8T dyno session with CID’s Gen-V LT cylinder heads came to 723 horsepower at 7,400 rpm and 598 pound-feet of torque at 5,400 rpm. It should be no surprise that the small bump in compression and the slightly higher flow at higher valve lifts helped it edge out the GPI heads in the higher rev range. What’s impressive is that both runs retained VVT, direct injection, and a hydraulic-roller valvetrain.

It should be noted that Chevrolet Performance advised the team at BTR that it does not recommend pushing one of these crate engines much further than 650 horsepower due to the skinny connecting rods. While they are strong enough to handle heavy loads while towing in fleet service, they’re not as strong as the LT1 rods. So while we could see how far this could go with an even bigger cam and more compression, we feel that stopping at our current power level is best.

Both CNC heads offered a big leap in performance, turning our ‘L8T To The Party’ project engine into a 700-plus horsepower, 7,500 rpm screamer!

The Gen-V LT engine platform from GM is more than proving its worth with gains like these. In the past, if you wanted a streetable and reliable 700-plus horsepower V8, it required either big cubic inches from a big block, a power-adder, or both. Now you can get that performance with the reliability and street manners of a high-performance daily driver in a small package. We’re done with the naturally aspirated L8T dyno testing, so the next time you see our 6.6 on the dyno, we will finally dig into the bottom end to build this into a four-digit horsepower screamer that can handle big boost!