Subaru_dude Posted November 19, 2006 Share Posted November 19, 2006 do fuel injected cars equipped with manual transmissions have a fuel pump cut off when you're moving above a certain speed above a certain rpm? i notice then if i'm rolling downhill in any gear above first and above 2000rpm, the engine has almost a hollow puur to it... but as soon as it hits 2000 and lower, it makes the same sound as iff i was not touching the gas at 4000rpm and then just barely touched it, it goes from hollow puur to "gurgle" exactly like when it goes from no gas above 2000 rpm to no gas below 2000rpm. it ALWAYS does this. So I know it's not a one-time freak thing. sorda "gurgles." the muffler isn't in that great a shape so the sound difference is alot more obvious. Quote Link to comment Share on other sites More sharing options...
GeneralDisorder Posted November 19, 2006 Share Posted November 19, 2006 You are just hearing the SPFI computer cutting the injector on and off - it does this under coasting conditions to save fuel. One of the reasons the SPFI is more economical for city driving than the carbs is the computer can just shut the injector down under closed throttle coasting. On a carb the engine vacuum will continue to pull fuel through unless you have some sort of (complicated) coasting relief system that can detect closed throttle, high RPM conditions - but even then they aren't 100% because the carb just isn't "smart" enough, and the systems are complex. GD Quote Link to comment Share on other sites More sharing options...
Subi81 Posted November 19, 2006 Share Posted November 19, 2006 GD is right I have verified the effect with my AF gauge. Coasting downhill = zero fuel injected. Quote Link to comment Share on other sites More sharing options...
bgd73 Posted November 19, 2006 Share Posted November 19, 2006 spfi also air bypasses quite nicely off the throttle. My sedans whistled. Quote Link to comment Share on other sites More sharing options...
DaveT Posted November 20, 2006 Share Posted November 20, 2006 You are just hearing the SPFI computer cutting the injector on and off - it does this under coasting conditions to save fuel.GD Good to know. I always thought the SPFI had more engine brakeing than my older carbed wagon, but wasn't sure why. If I could find a way to let some air in without turning the fuel on, would I get more engine braking? Quote Link to comment Share on other sites More sharing options...
nkx Posted November 20, 2006 Share Posted November 20, 2006 Good to know. I always thought the SPFI had more engine brakeing than my older carbed wagon, but wasn't sure why. If I could find a way to let some air in without turning the fuel on, would I get more engine braking? i know on my bike (93 cbr 600 f2) the engine braking is way higher than either my wrx or my wagon. it also has 12:1 comp ratio. i think the spfi CR is higher than the carb. the high-comp turbo guys seem to go for the spfi blocks in their builds. why do you want more engine braking anyway? even if youre doing a lot of downhill coasting, replacing pads should be easier than rigging up something to make the engine slow you down. Quote Link to comment Share on other sites More sharing options...
jeffast Posted November 20, 2006 Share Posted November 20, 2006 engine braking is leaving the car in gear while goinbg down a hill letting it keep your speed down, the answer to why, you must not have mountians where you live going down a pass brake fade sucks your brakes heat up and you don't have time to let them cool, and it can cause you to lose control if you ignore it long enough Quote Link to comment Share on other sites More sharing options...
mikeshoup Posted November 20, 2006 Share Posted November 20, 2006 Yeah, but these Subaru engines provide plenty of engine braking for Colorado. No need to do extensive modifications to increase engine braking, not that its really all that feasible to begin with. Quote Link to comment Share on other sites More sharing options...
jeffast Posted November 20, 2006 Share Posted November 20, 2006 i'm confused aside from an exhaust brake on deisels, how can you increase the engine braking power, nm forget it Quote Link to comment Share on other sites More sharing options...
mikeshoup Posted November 20, 2006 Share Posted November 20, 2006 i'm confused aside from an exhaust brake on deisels, how can you increase the engine braking power,nm forget it A person a couple posts up asked if they could increase engine braking by introducing more air into the engine... ... Quote Link to comment Share on other sites More sharing options...
jeffast Posted November 20, 2006 Share Posted November 20, 2006 oh thanks i skipped right over that one Quote Link to comment Share on other sites More sharing options...
Subaru_dude Posted November 20, 2006 Author Share Posted November 20, 2006 so, gearing down at stoplights and letting the engine slow the car down long hills saves me much gas, correct? Quote Link to comment Share on other sites More sharing options...
Carmen Posted November 20, 2006 Share Posted November 20, 2006 dad says that gearing down at high rpm can make the rings of the pistons to wear faster, very faster Quote Link to comment Share on other sites More sharing options...
Subaru_dude Posted November 20, 2006 Author Share Posted November 20, 2006 well then, i'll have to stop that, lol. i don't let it go too high when I'm gearing down, maybe only 3000rpm or so. never higher than that cuz it seems to me that going from a low rpm to a much higher rpm so quickly wouldn't allow time for the oil pump to get lubrication to the rings fast enough, resulting in some unecessary wear and tear. I just stay in a lower gear if I see the light changing ahead. Quote Link to comment Share on other sites More sharing options...
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