RPM vs Injector Pulsewidth
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RPM vs Injector Pulsewidth vs MPG Bare with me and I'll explain why I'm posting this here.
I have 2.73's in the rear with my A4. I scan my vehicle with HPT on a somewhat regular basis. For kicks tonight, (and thinking about what a gear change would do to this car for the sake of fuel economy), I scanned and added visual gauges showing my IPW.
I set my cruise at various speeds, mainly 60, 65, 70mph, and switched between 4th gear and 3rd gear. Driving in 3rd with 2.73's is mathematically equivalent to 3.90's in 4th. What I discovered was the opposite of what I expected. IPW actually DROPPED in 3rd vs 4th gear. For example, on a flat road at 70mph in 4th gear (1750 rpm) my IPW would range between 5.0-5.5, dropping to 3rd (2500 rpm) would level down to 4.3-4.7 or so. I repeated this test many times with the same result. Going from 3rd gear to 4th gear I'd see the jump up as well.
Just tell me if it's possible that the 2.73's are lugging the motor too low at highway speeds and causing highway driving to be less efficient. If not, what am I missing? Should I be looking at something besides IPW to give me an idea how much fuel is being consumed? (such as the flowrate PID) Also could the FPR be playing a role? Thanks guys for any words of advice.
I have 2.73's in the rear with my A4. I scan my vehicle with HPT on a somewhat regular basis. For kicks tonight, (and thinking about what a gear change would do to this car for the sake of fuel economy), I scanned and added visual gauges showing my IPW.
I set my cruise at various speeds, mainly 60, 65, 70mph, and switched between 4th gear and 3rd gear. Driving in 3rd with 2.73's is mathematically equivalent to 3.90's in 4th. What I discovered was the opposite of what I expected. IPW actually DROPPED in 3rd vs 4th gear. For example, on a flat road at 70mph in 4th gear (1750 rpm) my IPW would range between 5.0-5.5, dropping to 3rd (2500 rpm) would level down to 4.3-4.7 or so. I repeated this test many times with the same result. Going from 3rd gear to 4th gear I'd see the jump up as well.
Just tell me if it's possible that the 2.73's are lugging the motor too low at highway speeds and causing highway driving to be less efficient. If not, what am I missing? Should I be looking at something besides IPW to give me an idea how much fuel is being consumed? (such as the flowrate PID) Also could the FPR be playing a role? Thanks guys for any words of advice.
Last edited by Rhedalert; Jan 9, 2008 at 05:03 PM.
Thread Starter
TECH Regular
iTrader: (21)
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From: Long Island, NY
here's one of my shorter ones I saved when I was testing this out.
Look from frame 375 then you'll see the transition at around 540 when I shifted from 3rd to 4th. Ans again between 1220 and 1270. Thanks!
PS if you need my scanner config file, I'll try to host it somewhere (wouldn't upload for me here)
Look from frame 375 then you'll see the transition at around 540 when I shifted from 3rd to 4th. Ans again between 1220 and 1270. Thanks!
PS if you need my scanner config file, I'll try to host it somewhere (wouldn't upload for me here)
assuming that the AFR is stoich during all these transitions is constant (it should be) the amount of fuel injected should be directly proportional to cylinder airmass (dyncylair=AFR*IPW*IFR) this makes things easier because you can look at the cylinder airmass and really see the fuel consumption. dyncylair is that cylinder airmass. thus:
dyncyl air at lower rpm is about 0.25g/cyl, while after the shift you get about 0.17g/cyl. looks like it's eating more gas in 3rd right? maybe...
i say maybe because this is an amount of air PER CYCLE not PER SECOND. in order to convert it to per second, you gotta consider rpms. airflow=airmass*rpm/120 luckily, this conversion is done for you, it's your airflow, MAF or DynamicAirflow.
if you look at the MAF figures between the shifts, the 3rd gear gets 30-31g/sec, 4th gear gets around 25-28g/sec. thus, 4th consumes less fuel, even though the IPW is higher.
RPMs play a huge role. pulsewidth is only half the story, how many times you fire the injector per second is the other half, the same, if not more important. also remember that usually VE goes up with RPM, so by revving higher you're naturally digesting more air, and thus demanding more gas. you want the good millage, keep the revs low. watching airflow figures is the easiest way to determine which gear/rpm to use.
dyncyl air at lower rpm is about 0.25g/cyl, while after the shift you get about 0.17g/cyl. looks like it's eating more gas in 3rd right? maybe...
i say maybe because this is an amount of air PER CYCLE not PER SECOND. in order to convert it to per second, you gotta consider rpms. airflow=airmass*rpm/120 luckily, this conversion is done for you, it's your airflow, MAF or DynamicAirflow.
if you look at the MAF figures between the shifts, the 3rd gear gets 30-31g/sec, 4th gear gets around 25-28g/sec. thus, 4th consumes less fuel, even though the IPW is higher.
RPMs play a huge role. pulsewidth is only half the story, how many times you fire the injector per second is the other half, the same, if not more important. also remember that usually VE goes up with RPM, so by revving higher you're naturally digesting more air, and thus demanding more gas. you want the good millage, keep the revs low. watching airflow figures is the easiest way to determine which gear/rpm to use.
