Friday, February 15, 2013
BOOST, TIMING, & FUEL
On the dyno, we often jockey ignition timing and boost pressure levels, to enable us achieve maximum reliable power on a given octane. Do we go with more timing and lower boost, or less timing and more boost? Less timing advance and higher boost mean higher exhaust temperature (and hotter pipes) helping drive the turbo’s exhaust turbine harder and more quickly and higher intake charge temperature (less dense and more prone to deto) especially on two-strokes that have some of the intake charge spend time inside the hotter exhaust header pipe on the bottom half of each piston stroke, before the returning sound wave shoves it back into the exhaust port. More timing and less boost can have the opposite effect along with the increase in transfer of combustion heat into the heads while the expanding burning mixture spending more time there driving the pistons down instead of burning in the pipe(s), making them hotter! But what about fuel flow? We can make max HP at about 13/1 A/F, but we get the greatest combustion chamber cooling effect (and least power output) at 10/1. What shall we do? Jockeying all three about, and watching for/ listening to detonation is what we must do while we observe torque and horsepower on the dyno (or observe clutch RPM during field testing—was that 50 more RPM, or 50 less RPM?).
Some years ago DynoTech was contracted to create an optimal pump gas tune (ignition timing curve, fuel flow, and boost level) for a turbocharged Buell motorcycle that was being considered for production (details of the turbo Buell “Diablo” were made public in Cycle World, May 2010, page 44). We built five Buell X1 engines/ bikes for H-D to test with custom dished low compression pistons with tight squish band clearance to obtain the combustion chamber turbulence necessary for rapid flame speed (which results in less ignition timing required to achieve best torque). We also designed and fitted each bike with efficient side-mounted intercoolers with plastic pro-stock style air scoops (there’s a photo of one of the turbo Buell X1 test bikes on our motorcycle dyno on this website when you click on “About DynoTech”). H-D had loaned us engineering software to enable us to tune the front and rear (hotter running) cylinders individually. Their target was 125+ rear wheel HP (about 50% increase over naturally aspirated X1s). After trying 100’s of combinations of boost/ ignition timing/ A/F ratio we wound up with front/ rear ignition timing nearly identical to the stock factory curves with boost pressure and fuel flow that made over 130 RWHP! Part of the H-D endurance testing that these bikes ultimately passed included filling the bikes’ fuel tanks with pump gas and [underpaid] test riders would run in high gear, WOT (at a high speed test track in Alabama) non stop until the tanks were empty—with no electronic deto protection! According to the CW article, the turbocharged Buell X1 test bikes exhibited greater reliability than the normally aspirated high compression X1s.
The point is, people too often assume that we MUST retard ignition timing when we pressurecharge any engine. But if our goal is to achieve max safe HP on a given octane fuel, we should not be afraid to experiment with ignition timing