Saying an LS/VTEC conversion is fairly common is an understatement. The hybrid conversion is so popular the trend has quickly caught on in Japan, Indonesia, Australia, Philippines and other foreign countries. Today, it's more common to find an LS/VTEC conversion than a GS-R engine under the hood of many high-output Hondas.
By combining the larger displacement of the LS bottom-end and the high-lift VTEC head, the enthusiast tuner can create a high-revving, high-output engine for a fraction of the cost of a GS-R engine. The one thing that must be kept in mind when performing the conversion is maintaining a proper air/fuel ratio to reap full horsepower benefits from the transplant.
Whether installing a VTEC head onto a vehicle equipped with a non-VTEC engine or adding an LS bottom-end to a B16A-equipped vehicle; either way a Frankenstein is a Frankenstein. For the purpose of our test, we found a 1990 Acura Integra equipped with a Japan-spec B16A engine. The Acura was equipped with the basic bolt-ons, a DC Sports two-piece header, AEM cold-air intake, Tanabe exhaust, APEXi V-AFC and Hayame adjustable cam gears. The combination netted 145.3 hp and 111.5 lb-ft of torque at the wheels.
With a MoTec meter hooked up, we saw air/fuel ratios from 12.6:1 to 12.8:1 at wide-open throttle. By combing the local junkyards, we found a tired LS bottom-end, which we didn't mind since the owner was planning on rebuilding it to increase the longevity of the engine. The bottom-end was rebuilt with flat-top pistons from JE Pistons to maintain the same 10.0:1 compression as the B16A bottom-end.
After installing the LS block, we performed the proper break-in procedure (1,500 miles of easy driving) and we were off to the dyno. We performed a baseline again with the LS/VTEC conversion and were immediately amazed at the low horsepower output (135.9 hp and 104.3 lb-ft to the wheels). However, looking at the MoTec meter, we found the culprit. Under wide-open throttle, the air/fuel ratio was going dead lean with 14.0:1 to 15.9:1 at the top-end.
Since we didn't change a single thing on the top-end, running the same cams, intake, injectors and header, we knew the engine would run lean. Since the displacement increased from 1.6 liters to 1.8 liters--an increase of about 12 percent--the car sould and did run lean. Already prepared for this, we ordered a set of Venom 310cc injectors. The factory B16A injectors are rated at about 240cc. The 310cc injectors would be an increase of almost 30 percent. Although the injectors were on the overkill side, we knew we could trim the fuel pressure and also adjust the APEXi V-AFC to attain acceptable air/fuel ratios. Plus we had fuel for future mods.
Along with the injectors, we installed an AEM high-flow fuel rail and adjustable fuel pressure regulator to enhance our control of the fuel system. The rail and pressure regulator went in flawlessly and also added a touch of color under the hood. With the regulator set at 45 psi (stock pressure), we performed another run.
The engine went from dead lean to rich, the air/fuel was now 12.6:1 at idle and 11.5:1 to 11.8:1 at wide-open throttle. On the brighter side, power figures were up substantially with increases of 10 to 15 hp across the board. First, we adjusted the AEM regulator, trimming the fuel pressure from 45 to 41 psi. On the second run, air/fuel at idle was still rich at 13.4:1 and at wide-open throttle, 12.1:1 to 12.4:1. We kept the fuel pressure at 41 psi, wanting to maintain a good spray pattern from the injectors (injector spray pattern can be affected by low fuel pressure).
From here, we went to work on the APEXi V-AFC. First, we fixed the air/fuel ratio at idle, leaning the injectors by reducing duty cycle at idle. The MoTec meter showed a lean spot when the VTEC lobes were activated, so the V-AFC was programmed to richen this point. At the top end, the injectors were again leaned out to get closer to the original 12.6:1 to 12.8:1 from the B16A.
After a couple of runs, we duplicated the air/fuel ratios from the B16A engine. The result was nothing short of amazing. From simple fuel tuning, we were able to run at a safer air/fuel ratio and generate considerably more power than before. Peak power increased from 135.9 hp and 104.3 lb-ft of torque to 159.7 hp and 123.8 lb-ft of torque--an increase of 23.8 hp and 19.5 lb-ft of torque. Large increases were realized in the mid-range where we were able to see gains of 15 to 18 hp at some points. And that's a tuning job well done.