We hear it all the time, that cam profile, intake design or even head porting is for a naturally aspirated motor and doesn't work well on a turbo combination. Is this the reality or simply a crutch (or excuse) for poor performance? The answer is likely both, as there definitely are components that may perform better on forced induction applications, but the reality is that for all but a small handful of dedicated race motors, what works on an N/A motor carries over to the turbo (or blower) motor. Contrary to popular internet lore (before the net it was simply folklore), short-runner, big-plenum intakes are not optimized for forced induction. By the same token, cam profiles that minimize overlap will not out-power traditional cam profiles (that feature more overlap). These general statements obviously have their exceptions, but for the vast majority of enthusiasts considering the installation of forced induction on their LS motor, remember this simple rule. If it adds power on your normally aspirated motor, it will add power on your turbo (or blower) motor!
To illustrate this rather basic concept, we devised a series of tests that would also introduce a cost-effective alternative to those LS enthusiasts considering a do-it-yourself turbo kit. The most common approach of which is to flip over a set of truck manifold and weld up a simple cross-over pipe for a DIY single turbo kit. The manifolds can also be used independently for a twin-turbo kit, but that is another test for another day. The replacement for the low-buck (DIY) kit came from CX Racing. Unlike many of the eBay vendors, CX racing prides itself on providing not just a cost-effective alternative, but a quality one as well. We were able to get our hands on a full stainless single turbo kit (the hot side at least) to test on a 5.3L based, 383 LS stroker. In addition to illustrating the effectiveness of the turbo kit itself, we were able to employ it to run a cam and intake test. By comparing the cams and intake manifolds both normally aspirated and turbocharged, we could determine if the gains offered N/A carry over to the turbo configuration.
Our test mill started out life as a simple 5.3L truck motor pulled from a local wrecking yard. The block was bored to 3.902 and stuffed with a forged steel 4.0-inch stroker crank and matching 6.20-inch connecting rods from ProComp Motorsports, while Mahle supplied the forged aluminum pistons. Topping the stout short-block was a set of stock "317" 6.0L head castings. The only upgrade applied to the stock truck heads was to replace the factory valvesprings with a set of 26918 beehive springs from Comp Cams. The spring upgrade was required for our Comp cam profiles, the first of which was a mild XR265HR. Mild by most standards, the XR265HR offered a .558/.562-inch lift split, a 212/218-degree duration split and a 115-degree lobe separation angle. The build up also included a new set of lifters from Comp Cams and a new timing chain and oil pump from Sealed Power. Since combustion chamber sealing was critical on a turbo motor, we installed the stock "317" heads using a set of Fel Pro MLS head gaskets and ARP head studs. The 383 stroker was finished off with a set of 36-pound injectors, a stock truck intake and a set of long-tube headers from American Racing.
The 383 was first treated to a break-in procedure using Lucas break-in oil, then switched over to the 5W-30 synthetic using a new oil filter from K&N. After tuning with the FAST XFI/XIM management system, we were rewarded with peak numbers of 455 hp and 484 lb-ft of torque–good numbers considering the mild cam, stock heads and intake. After establishing a baseline, we swapped out the XR265HR cam in favor of a 281LRHR. The more aggressive 281LRHR13 offered .617/.624-inch lift, a longer 231/239-degree duration split (at .050) and an EFI (and turbo) friendly 113-degree lobe separation angle. The cam swap netted an additional 25 hp, elevating the peak numbers to 480 hp and 488 lb-ft of torque. The gains offered by the cam swap were most prevalent past 4,500 rpm, and there was a slight trade off in power below 4,500 rpm. The final N/A test was to replace the stock truck intake with a FAST LSXR manifold and matching 102 mm throttle body. The intake upped the peak power to 506 hp and 498 lb-ft of torque–again the gains were most prevalent past 4,500 rpm, though, with little or no trade off in power lower in the rev range.
You might think we were done at this point, but we were just getting started. Next up was the turbo system from CX Racing. In reality, the cam and intake swaps were made with and without the turbo system, so we have comparisons between the N/A and turbo motors in every configuration. Starting with the baseline configuration (XR265 cam and truck intake), the N/A 383 produced 455 hp and 484 lb-ft of torque. After adding a 76mm turbo and air-to-water intercooler (run with dyno water) to the hot-side kit from CX Racing, we ran the 383 once more with boost. With a maximum boost level of 8 psi, the CX Racing turbo kit increased the power output of the 383 to 631 hp and a thumping 680 lb-ft of torque. Torque production exceeded 650 lb-ft from our load-in point of 3,700 rpm to 5,100 rpm. The massive air-to-water (CX offers a wide variety of air-to-air coolers as well) minimized both the charge temperature and pressure drop across the core (data logged during testing). CX Racing also supplied the necessary 3.0-inch aluminum tubing, silicone couplers and T-bolt clamps to secure everything together. The configuration would obviously be different than run on the dyno, but the results would be the same.
Having established the worth of the turbo kit itself, we also had the baseline runs using the truck manifold and XR265HR cam. Swapping out the XR265HR cam for the larger 281LRHR13 produced predictable results (given those witnessed on the N/A application). Run at the same boost level with the 281 cam, the peak power numbers jumped to 661 hp and 690 lb-ft of torque. As with the N/A test, the wilder cam lost power compared to the smaller cam, but on the turbo application that cross-over point came 200 rpm earlier (the turbo motor liked the bigger cam). The final test was to replace the truck intake with the Fast LSXR intake combo. This netted another increase in power, from 661 hp to 677 hp, while the peak torque jumped from 690 lb-ft to 708 lb-ft (at a slightly higher engine speed). In every instance, the power gains mirrored those of the normally aspirated motor, but we weren't done yet. With more boost a simple twist of our Turbo Smart Boost controller away, we dialed up the boost to 10.5 psi where we were rewarded with 721 hp and 773 lb-ft of torque. This was getting near the maximum available from the turbo, but we couldn't help but wonder about running a pair of these on a larger (and more powerful) 408 stroker we had lying around!