Join Date: Mar 2008
Location: Kapuskasing Ontario Canada
Mentioned: 7 Post(s)
Throttle bodies' vacuum bypass adjustment screw (one per TB) does not allow any air flow between TB's. Vacuum bypass screw simply adjusts volume of extra air drawn over individual TB's throttle valve at idle position to balance both TB's vacuum levels at 1300 rpm idle.
I left the right's TB's screw shut, and adjusted the left TB's screw to balance vacuum at 1300 rpm idle. Right TB's vacuum draw increases significantly over the left TB's as you start opening throttles, then tapers back down close to the left TB's vacuum draw at over 4000 rpm and up. Idle quality and smoothness, throttle response and starting ease all benefit from TB's vacuum balance adjustment, with or without the already added coupler hose. With the TB's vacuum equally balanced at 1300 rpm, the added coupler hose channels an equal rate of vacuum pulse between the two, via the vacuum access fittings which bore is smaller than that of the left TB's IAP sensor fitting. Some Honda 4-stroke parallel twins already have a TB vacuum coupler hose.
A 180-degree crank offers the most uneven firing interval: 180 and 540 degrees. It also gives the lowest pumping losses, as displacement in the crankcase remains roughly constant throughout the cycle. A 360-degree crank gives even firing, but both pistons moving together offers the same mechanical balance as a single.
The 6" long 1/8" ID vacuum hose connecting both TB's also affects IAP sensor signal... Adjusting the TPS to raise voltage output into spec range at WOT which raises TPS output voltage just above idle position spec range, adds fuel at low to mid throttle opening to help cure the lean surge, notably near 2800 rpm. It also affects ignition timing advance.
ECU controlled fuel metering: D-J Method and α-N Method
When the engine load is light like at idling or low speed, the ECU determines the injection quantity by calculating from the throttle vacuum (inlet air pressure sensor output voltage) and engine speed (crankshaft sensor output voltage). This method is called D-J method (low-speed mode). As the engine speed increases, and the engine load turns middle to heavy, the ECU determines the injection quantity by calculating from the throttle opening (throttle sensor output voltage) and the engine speed. This method is called α-N method (high-speed mode).
Last edited by invader; 05-31-2012 at 10:38 PM.