More NA POWER part 5: 100 bhp per liter in 1.7 liter B6ZE

My engine tuning project has finally achieved its goal - to make 100 crank hp per liter in naturally aspirating form. Current maximum figures are 173 hp of power and 181 Nm of torque (crank). And max figures are not all, torque peaks up at 4000 rpm and there is usable power up to 8000 rpm. Let’s first take a brief look at the previous modifications and then inspect the current setup a bit closer.

In 2008 basic bolt ons with oem cams produced 132 crank hp and 151 Nm (105 rwHp and 124 rwNm). That was all I could get out of a 1.6 liter engine with bone stock internals and a larger RX-7 AFM.

In 2009 the engine was bored to 1720 cc with high compression (11:1) Toda Racing pistons. The head was mildly ported with oem valves which were left untouched. Also Toda Racing 272/9 cams (in&ex) and solid lifters with stiffer springs, measured to be good for 8300 rpm with safety margin, were installed. Conrods were still the old ones with about 140 000 km behind. There were no adjustable cam gears and the catalytic converter was bone stock, also possibly clogged. Van Kronenburg Management System’s fully adjustable ECU was wired to have a command on the injection and ignition. This setup produced 156 hp / 175 Nm (121 rwHp / 147 rwNm). After the build I had almost two years of happy high revving when the original conrods finally gave up. It luckily happened with no drama as the engine suffered a spun rod bearing.

Forged Toda Racing pistons (3 mm oversize) and CP Carrillo A-beam rods, 110 grams less than stock.

In 2011 as the engine was torn apart for the bottom end rebuild the cylinder head also went under maintenance. The block received a factory new long nose crank, Carrillo A-beam rods, ACL Race Line bearings, Toda Racing pistons and Toda Racing light weight flywheel - all dynamically balanced.  New valve guides were installed and stock valves received a three angle grind to improve the head flow. Old cam gears and worn cat were replaced with Toda Racing adjustable gears and Catco’s metallic 400 cell unit. Instead of the standard 1.0 mm thick head gasket a 0.8 mm thick optional part from Toda Racing was installed to increase the compression ratio to about 12:1. I also wanted to lower a bit high intake air temperature and fabricated a sealed airbox that was supposed to produce a ramming effect in higher speeds. With revised cam timing this setup produced an estimated 160 crank hp. Crank power is estimated because this time a hub dyno was used and the measured figures were 132 rwHp and 142 rwNm. I was getting closer to my 100 crank hp per liter goal!

During the 2011 dyno session it was noticed that the original throttle body started to restrict the flow (a vacuum was building in the intake manifold over tb). Another visit to the hub dyno was made in 2012 with a new 60 mm diameter throttle body installed and ported to the intake manifold. The result was a bit disappointing with only minimal gains (135 rwHp and 145 rwNm). At this point the airbox started to be the main suspect for creating a bottleneck.

AT Power 60 mm 'Shaftless' throttle body required some porting to the intake manifold.

Finally this spring I made the latest to the dyno (inertia type once again). A wanted to make a back to back comparison with the old airbox and the new “cold side” cone filter intake. I prepared for the dyno session by checking the valve lash and noticed that they all were on the loose side. Meticulous adjustment to the specs (0.20 mm intake and 0.25 mm exhaust) was done. I also checked that the intake manifold was port matched to the head. First pulls on the dyno with the old airbox indicated restricted flow above 5500 rpm. Peak torque was 159 rwNm at 5000 rpm and after that the curve sank to about 150 rwNm. Max power was 138 rwHp.

After that the new intake was installed and it was time for the final pulls. And there it was! The torque stayed up till 6750 rpm, and maximum output of 173 crank hp (144 rwHp) was achieved.

What is the most noteworthy, in my opinion, is that this motor runs on a stock intake manifold, although port matched. The head porting is also mild, I would call it a “fast street porting”, and stock sized valves are used. The potential of producing current figures has always been there but the output has been hampered with various restrictions. In the very beginning it was the displacement and cams. Then it was the flow of the cat, improper cam timing, restrictive throttle body and intake plumbing.

So, what is the current bottleneck? Evidently there is something restricting the flow above 7000 rpm. Intake runners, stock valves, current cam profiles, exhaust manifold, 2” catback exhaust pipe or something else. Stretching the power band 800 – 1000 rpm higher would be good for another 15 to 20 hp. We will see…