JSM Posted February 21, 2009 Share Posted February 21, 2009 I was reading this thread and I understand why the need to not run 100% but why not 95%? Why is 80% the magic #? http://forums.hybridz.org/showthread.php?t=118512 If fuel pressure is regulated properly, is 100% all that bad? Or is it to difficult to get even pressure to all X amount of injects at 100%? Why don't they sell injectors based on the 80% rule instead of 100% since no one runs or no one should run at 100%? http://www.injector.com/faq.php#faq2 What do the terms “static†and “duty cycle†mean? An injector in an engine turns on and off very quickly to control the amount of fuel delivered. The amount of time an injector is turned on and delivering fuel is known as the duty cycle. This is measured as a percent, so 50% duty cycle indicates that the injector is held open and held closed for an equal amount of time. When the engine needs more fuel, the time that the injector stays on (its duty cycle) increases so that more fuel can flow into the engine. If an injector stays on all the time, it is said to be static (wide open, or 100% duty cycle). INJECTORS SHOULD NOT GO STATIC IN A RUNNING ENGINE! If an injector is static in a running engine (open 100% of the time), that injector is no longer able to control fuel delivery. It is just “along for the rideâ€. This could be an indication that the injector is too small for the needs of the engine. Injector duty cycle should usually not exceed 80% in a running engine at any time. Quote Link to comment Share on other sites More sharing options...
X64v Posted February 21, 2009 Share Posted February 21, 2009 I believe over 80% duty cycle they have a tendancy to not open/close properly, they end up floating like valves. From http://www.hondata.com/techduty.html: ...it is not recommended to exceed 80% duty cycle with replacement injectors. Otherwise you will run the risk of the injector not being able to open and close quickly enough, which causes the injector to float half open. This will lean the mixture out, which is not going to [be] healthy for an engine at high revs and under full load. Quote Link to comment Share on other sites More sharing options...
garvice Posted February 21, 2009 Share Posted February 21, 2009 The reason companies don't sell injectors based on 80% duty cycle is for a few reasons. One is marketing (Our injector is bigger then yours), the other is to set a standard. People buy larger injectors because they need to flow more fuel. Companies see this and want to show you the maximum capacity of their injector. When was the last time you bought a hard drive? Companies use to sell hard drives based on a metric GB I.E. If it had 1000 MB, then they would call that 1 GB and sell it as such. In reality a GB is 1024 MB. Then there was also lost space due to indexing tables etc. This seems to be getting worse, the last hard drive I bought was a 500GB hard drive and it only had 470GB of useable space. (I don't know what sort of math they are using to get those sort of numbers). I like that manufacturers use the 100% duty cycle measure. It is not how much fuel you will get through the injector (As you will never use such a high duty cycle), however it makes comparisons between different companies easier. It also means that injectors can be easily bench tested to determine their condition (You don't need a PWM signal to bench test, just hold them open). What is worse is when injector companies sell them based off of hp rating. That really grates me. Quote Link to comment Share on other sites More sharing options...
WizardBlack Posted March 28, 2009 Share Posted March 28, 2009 Crappy injectors will flutter much beyond 80% duty cycle. Good injectors can push a bit past 90% duty cycle (EVO and some RC Engineering Injectors I played with). Some injectors will max out and stay stable at ~100% duty cycle (Subaru STi Injectors). Quote Link to comment Share on other sites More sharing options...
WizardBlack Posted March 28, 2009 Share Posted March 28, 2009 I like that manufacturers use the 100% duty cycle measure. (As you will never use such a high duty cycle), Some cars do. STi's float pretty darn high from the factory (like, low 80's) and a few bolt-ons can push them to 100%. Quote Link to comment Share on other sites More sharing options...
garvice Posted March 28, 2009 Share Posted March 28, 2009 Some cars do. Wow, didn't realise that. So at full fuel usage they are always open? Anyway, still like that the manufacturers use 100% duty cycle as a measuring stick. Quote Link to comment Share on other sites More sharing options...
WizardBlack Posted March 28, 2009 Share Posted March 28, 2009 Wow, didn't realise that. So at full fuel usage they are always open?Anyway, still like that the manufacturers use 100% duty cycle as a measuring stick. Yeah, they refer to the injector going into DC mode essentially. No more square wave; just solid 12V. Quote Link to comment Share on other sites More sharing options...
Matt Cramer Posted March 31, 2009 Share Posted March 31, 2009 It's a rule of thumb. We sell injectors based on their maximum flow rate as that is the industry standard and easiest one for someone with a flow bench to measure, but our guidelines for sizing injectors and the horsepower charts on the injector catalog page assume an 85% duty cycle, which is the rule of thumb we typically use. Quote Link to comment Share on other sites More sharing options...
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