The 317 and AFR 245 heads were run with the FAST LSXR intake designed for cathedral-port heads, while the LS3 heads received a FAST manifold designed for the rec-port configuration. The 317 and AFR 245 heads were run with the FAST LSXR intake designed for cathedral-port h Given the flow numbers and chamber volume, we assumed the AFR 245s would combine the low-speed torque of the 317s with the top-end power of the LS3 heads, but we got even more than we expected. With AFR 245s, our 408 produced 604 hp at 6,500 rpm and 556 lb-ft of torque at 5,400 rpm. Note that both peak numbers occurred higher in the rev range than with either of the two stock heads, but the seemingly high-rpm power was combined with plenty of torque. Not only did the AFRs produce the highest peak torque output (556 lb-ft), but (like the 317s) they easily exceeded the torque production of the LS3 heads below 4,000 rpm. With the exception of a merge at 4,200 rpm, the AFR 245s out-performed the LS3 heads through the entire rev range, with gains of 20 to 25 hp at the top. Truth be told, both the cam timing (skewed for rec-pot heads) and the bore size (AFR 245s were designed for 4.125-bore motors) likely hindered the 245s on this application. The airflow numbers suggest the 245 heads will support more than 700 hp, and we have seen big-bore combinations that verify this fact. The question now is: Does it make more sense to perform an LS3 head upgrade or go with a set of aftermarket cathedral-port heads? The aftermarket-head approach is a lot more expensive, but this test illustrates that the LS3 head swap isn’t the only game in town. To ensure uninterrupted airflow, both manifolds received the same 102-mm Big Mouth throttle-body from FAST. To ensure uninterrupted airflow, both manifolds received the same 102-mm Big Mouth throttl Flow Numbers*: Stock 6.0L Truck (317) vs. LS3/L92 (0821 casting) vs. AFR 317LS3AFR 245 LIFTINEXINEXINEX 0.050322433273626 0.100665375617356 0.200140102157117156112 0.300197145223161227166 0.400236171273190280201 0.500240185307205319220 0.600243191318216349230 0.700244197307222330235 *Flow Data: CF at 28 inches test depression FAST also supplied the 75-pound injectors and aluminum fuel rails used with the LSXR intake. FAST also supplied the 75-pound injectors and aluminum fuel rails used with the LSXR intak 408 Stroker Power Numbers: 6.0L Truck (317) vs. LS3/L92 (0821 casting) vs. AFR 245 317LS3AFR RPMHPTQHPTQHPTQ 3,000271475263460270472 3,200285469278455283465 3,400304470295456302467 3,600325474314459325474 3,800346478334461347480 4,000367482368483373490 4,200397496412515412515 4,400429513443529447534 4,600456522467533479546 4,800481526491538504552 5,000496521516542528554 5,200507512538543550555 5,400516502554539572556 5,600520487562527587551 5,800528478569515593537 6,000540473577505597522 6,200543460581492602510 6,400541444578475604495 6,500537434575464604488 DESCRIPTIONSOURCEPNPRICE AFR 245 headsAir Flow Research1680$1,350 (each) 6.0L truck heads (317 casting)Summit Racing12572035479.99 (each) LS3/L92 heads (0821 casting)Summit Racing12629063359.95 (each) LS3 intake rocker armsChevrolet1256916794.00 (set of 8) LS3 intakeSummit Racing12610434699.95 LS3 rocker-arm standChevrolet126009368.74 All the heads were run with the same 13⁄4-inch, long-tube headers from QTP. The headers were run with MagnaFlow performance mufflers. All the heads were run with the same 13⁄4-inch, long-tube headers from QTP. The headers we ➔Sources: Air Flow Research (AFR); Valencia, CA; 877/892-8844; AirFlowResearch.com Comp Cams; Memphis, TN; 800/999-0853; CompCams.com L&R Automotive; 562/802-0443; LNREngine.com Moroso; 203 453-6571; Moroso.com Probe Racing; 310/784-2977; ProbeIndustries.com Procomp Electronics; 909/605-1123; ProcompElectronics.com With the 317 heads installed, our 408 stroker produced 543 hp at 6,300 rpm and 526 lb-ft of torque at 4,800 rpm. With the 317 heads installed, our 408 stroker produced 543 hp at 6,300 rpm and 526 lb-ft o Up next were the stock, rectangular-port LS3 heads. The additional port volume hurt low-speed power, but there is no denying the flow potential of these heads. Up next were the stock, rectangular-port LS3 heads. The additional port volume hurt low-sp The LS3 heads required offset rocker arms on the intake valves, but all heads were run with factory 1.7:1 ratio rockers. You can use stock LS1/LS6 rocker arms for the exhaust valves. The LS3 heads required offset rocker arms on the intake valves, but all heads were run wit With LS3 heads, power jumped to 581 hp and 543 lb-ft of torque. Though the LS3 heads improved both peak power and torque, torque production was down below 4,000 rpm compared with that from the 317 heads. With LS3 heads, power jumped to 581 hp and 543 lb-ft of torque. Though the LS3 heads impro Impressed as we were with the flow numbers and power gains offered by LS3 heads, we couldn’t help but wonder how they would compare with a set of dedicated, aftermarket cathedral-port castings. AFR’s 245 heads offered not only exceptional peak flow numbers (349 cfm) but also smaller combustion chambers, which increased the static compression ratio. Impressed as we were with the flow numbers and power gains offered by LS3 heads, we couldn This final head swap of the day netted the best power numbers. The AFR heads demonstrated that you don’t need to have rectangular ports to make big power by producing 604 hp and 556 lb-ft of torque. The 245s out-powered and out-torqued the LS3 heads through the entire rev range, indicating that not only is an aftermarket cathedral-port head an alternative to an LS3 upgrade, but one that will ultimately offer even more power. Add a matching cam, and the 245s will make even more. This final head swap of the day netted the best power numbers. The AFR heads demonstrated « | 1 | 2 | View Full Article By Richard Holdener Enjoyed this Post? Subscribe to our RSS Feed, or use your favorite social media to recommend us to friends and colleagues!