I'm doing some research on the Honda CRF450. This is a single-cylinder engine. I'm interested in the fuel-injected models (not the older carbureted versions).

As far as I can tell, this engine doesn't have a camshaft position sensor, so there's no way to tell which stroke the engine is in. This means that the bike can only use the crankshaft position to decide when to inject fuel and when to spark, and it can't tell the difference between the compression stroke and the exhaust stroke.

Does this bike inject fuel AND spark during the exhaust stroke? I understand wasted spark systems, but I thought those only worked because there's no fuel during the exhaust stroke. Is there any way for this bike to avoid injecting fuel outside of the compression stroke, and if not, why is it okay for the bike to use "wasted combustion" with both fuel and spark firing at the wrong time?

3 Answers 3


In the dawn of fuel injection most cars used a simultaneous double fire system. During every crank shaft revolution the injectors spray half the fuel needed. This way half the fuel waits on top of the intake valve. When the intake valve opens the first half is dumped in and the injector sprays in the other half the fuel.

Similar thing happens with the spark. The spark fires both during the compression stroke and the exhaust stroke.

These systems work just fine without the necessity of a camshaft position sensor. The only real reason that cars grew camshaft position sensors is that only spraying the fuel in while the intake valve is open is more efficient and allows cars to run cleaner.

  • This is brilliant!
    – Greg d'Eon
    Nov 22, 2015 at 21:13

This article seems to suggest that the fuel injection system fitted to this engine doesn't draw it's power from the battery but instead relies on the revolution of the engine to provide power.

It wouldn't surprise me if this provides a timed "spike" in the electricity generated at a key moment for timing purposes as opposed to providing a smooth, constant supply.


The EFI software can watch the crank position speed and determine if it is in the compression stroke or exhaust (since the compression stroke will slow down while going up), and/or it can monitor the MAP sensor that reads the intake manifold pressure to determine intake vs power stroke, depending on the number of cylinders vs intake manifold configuration (MAP monitors one cylinder or multiple). Since yours is single cylinder, either technique would work. Don't forget that the EFI computer is sampling MAP and time for each location of the crank position sensor wheel, so it knows a lot of information over the course of one cycle.

  • This is incorrect. There is no manifold pressure sensor on the CRF. The CRF is absolutely unaware of whether the piston is TDC compression or TDC exhaust. It fires a fuel shot of every rotation as @vini_i stated. Dec 6, 2015 at 0:01

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