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Old 05-05-2020, 05:26 PM   #31 (permalink)
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The lowest RPM in the highest gear with the engine in lean-burn gets me the best mileage.


If I'm on a long stretch of road I'd rather be going 50+ mph
1000rpm is 30mph and it gets over 100mpg but you lose lean-burn if it drops below 1000rpm and there isn't much reserve torque for anything but a completely flat road.

On a warm or hot day with windows closed and air conditioning compressor off
50mph, 1700rpm, 90mpg
58mph, 2000rpm, 85mpg
64mph, 2200rpm, 80mpg
70mph, 2400rpm, 75mpg
70-78mph can be acheived under lean-burn which has a 65-70mpg(lower at lower altitudes and if you are near 10,000 feet you might see 80mpg as the limit but with less air drag I've cruised in the 75-78mph range at 80mpg on a high density altitude(hot and high altitude, no AC) day) can be achieved on perfectly flat roads and after 78mph the car typically will drop out of lean burn when VTEC kicks in when you are above 2700rpm.

The brake specific fuel consumption is a smaller factor than air drag in a car and for the engine, you want the throttle plate as far open as possible without triggering fuel enrichment, which usually means the lowest engine speed that can accomplish the task. The Honda Insight does this with lean-burn to reduce the pumping losses and keeping the engine at roughly 80-90% load at a low RPM relative to its smaller engine displacement and power output capability. Cars with CVTs can target, although they might not necessarily do this or do it well depending on the CVT, for a specific engine load or throttle position to maximize throttle opening while keeping the engine at the lowest speed possible. If you can achieve a high enough engine load to where the engine couldn't produce more power through throttle opening and there isn't much excess power, you are now in a position where it's better to hold that load than to do pulse and glide(engine-off).

Most engines are overpowered or their transmissions won't bring the engines to a low enough RPM for this to be the result and we need to use engine-off coasting to drive with enough load to add efficiency.

In my Insight, if I use a speed higher than 40mph, I might as well use lean-burn, anything below that and pulse and glide makes sense, but I don't care anymore because gas is not above $3/gallon and my driving below those speeds without traffic behind me is far more uncommon with the driving I do today(pre-COVID) than it used to be. ..now I'm just barely driving at this point

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Old 05-06-2020, 12:08 PM   #32 (permalink)
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Grand Cherokee Diesel Jeep 2016 mileage

Quote:
Originally Posted by hat_man View Post
Is there a general rule of thumb for better/best FE in certain RPM range?

So where do YOU get YOUR best mileage?
This Jeep, got the best mileage at 57MPH in combined 80 city, 20 Hwy (28MPG)

At 100% Hwy, I achieved 40MPG!
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Old 05-06-2020, 04:13 PM   #33 (permalink)
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Hi all, I have an 07 Legacy wagon automatic w/lockup torque converter and live in a hilly part of VA and for me with that car 55-60 mph seems to be about optimum, any slower than that and i come out of tc lockup and then mileage really suffers. At 60 on level ground I am at about 2350 rpm. Best mileage under real world mixed driving conditions is 26-27 mpg in mixed mostly highway driving. Iím running 5-30 synthetic oil and factory transmission fluid (synthetic). Any suggestions on how to get better mileage with this boat?
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Old 05-06-2020, 08:00 PM   #34 (permalink)
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Quote:
Originally Posted by cerec View Post
This Jeep, got the best mileage at 57MPH in combined 80 city, 20 Hwy (28MPG)

At 100% Hwy, I achieved 40MPG!
Which Jeep? Those are rather extraordinarily claims.
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Old 05-06-2020, 08:08 PM   #35 (permalink)
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Quote:
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Which Jeep? Those are rather extraordinarily claims.
I also wonder what city has 57mph speed limits...lol
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Old 05-06-2020, 08:46 PM   #36 (permalink)
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I have not done a specific test but my 05 Civic's engine doesn't seem to run efficiently at lower RPMs. Whenever I try to get more MPG by keeping RPM down such as by locking the torque converter and cruising at 1500 RPM in 4th gear going 35 (the soonest I can without the engine lugging) I have always gotten lower MPG than when I leave it in D3 around town up to a sustained speed of 40+ MPH.

I also seem to get my best MPG at higher speeds. When I was on my way to North Carolina to visit a friend it was raining most of the way there so I was going 60-65 most of the way and I averaged about 33 MPG. On my way back the weather was good so I was going 70-80 MPH at 3000-3400 RPM and I averaged 38 MPG! Sure there are other variables, but these results seem to reflect my experiences around town. I don't have any significant aero mods either, about my only aero mod is I lowered my car by 1.5 inches.

I have a Crower stage 1 cam and other engine mods though, so my results may be different with the stock cam, I started manually controlling torque converter lockup after I installed this Crower cam. I'm thinking that my Crower cam might just not be efficient at lower RPMs, but I'm not sure.
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Old 05-06-2020, 09:38 PM   #37 (permalink)
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Quote:
Originally Posted by EcoCivic View Post
I have not done a specific test but my 05 Civic's engine doesn't seem to run efficiently at lower RPMs. Whenever I try to get more MPG by keeping RPM down such as by locking the torque converter and cruising at 1500 RPM in 4th gear going 35 (the soonest I can without the engine lugging) I have always gotten lower MPG than when I leave it in D3 around town up to a sustained speed of 40+ MPH.

I also seem to get my best MPG at higher speeds. When I was on my way to North Carolina to visit a friend it was raining most of the way there so I was going 60-65 most of the way and I averaged about 33 MPG. On my way back the weather was good so I was going 70-80 MPH at 3000-3400 RPM and I averaged 38 MPG! Sure there are other variables, but these results seem to reflect my experiences around town. I don't have any significant aero mods either, about my only aero mod is I lowered my car by 1.5 inches.

I have a Crower stage 1 cam and other engine mods though, so my results may be different with the stock cam, I started manually controlling torque converter lockup after I installed this Crower cam. I'm thinking that my Crower cam might just not be efficient at lower RPMs, but I'm not sure.
What engine management are you using with your cam and mods?

If you haven't remapped your fuel tables (and to a lesser extent your ignition timing values, as they're fuel dependent), they're probably a mess.
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Old 05-06-2020, 09:50 PM   #38 (permalink)
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Quote:
Originally Posted by Ecky View Post
What engine management are you using with your cam and mods?

If you haven't remapped your fuel tables (and to a lesser extent your ignition timing values, as they're fuel dependent), they're probably a mess.
I'm unfortunately using the ECU that came with my JDM Honda Stream VTEC engine and it's not programmable. It's just a stock Honda Stream ECU. There are 2 aftermarket options available for these engines: Kpro and AEM EMS. Kpro isn't an option because I have an auto and AEM EMS isn't an option since I can't afford $1300. Other than that it would be a nice option though, I could use it with my D17 now and use it with a K24 in the future when I swap.

I don't think the mods I did messed anything up too badly though because my MPG is no worse than stock and performance has greatly improved, so it is still an improvement even though I'm probably not getting the most performance or efficiency possible out of my mods.
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Old 05-06-2020, 10:18 PM   #39 (permalink)
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Makes sense. I don't want to get the thread too off topic but the issues you can expect to see from changing how the engine breathes are thus:

Big cams (broadly speaking) move the torque curve up. You lose torque at low RPM, and gain it up high. There's no such thing as a cam profile that does both. I haven't looked into the exact functioning of the D17's VTEC system but my guess is that it's either 1) a low and a high lift lobe, and each lobe drives both valves depending on VTEC, or 2) a low and a high lobe, where the low lobe actuates one valve at low RPM and the high lobe the other, and a locking pin allows the high lobe to operate both valves after VTEC engagement. This arrangement is more likely.

More lift and duration degrades torque and combustion quality at lower RPM. This can be seen in extreme cases such as in classic large domestic V8s which have loping, rough idles from their high lift and duration cams. VTEC can minimize this, depending on how it operates, but you'd be trading mid-range for high end torque in a best case.

At low load the engine runs in closed loop. The O2 sensor can compensate for differences in airflow, but the ECU has to relearn the fuel trims on every startup, and you may find it runs rich for a very brief time when changing throttle, as it expects more air. There will likely be some changes to cylinder scavenging if the low cam has changed, which would have a minor effect on what ignition timing is needed.

At high load, most likely the ECU goes open-loop and relies on its fuel tables without O2 feedback. If you're seeing a 15% difference in torque, for example, you'll be running 15% more rich in the mid-range and 15% leaner up high, since the tables won't be corrected and that air would have been moved from one spot to another. If the base target AFR was 13.5:1, that could mean at (for example) 4000rpm it could be as rich as 11.7:1 AFR, and up around 7000 it might be as lean as 15.5:1. I doubt from a cam alone you'd get 15% more high-end, but the principle applies.

My 2 cents, and my word of caution about changing anything past the butterfly valve without adjustment of ECU maps.
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Old 05-06-2020, 10:42 PM   #40 (permalink)
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Quote:
Originally Posted by Ecky View Post
Makes sense. I don't want to get the thread too off topic but the issues you can expect to see from changing how the engine breathes are thus:

Big cams (broadly speaking) move the torque curve up. You lose torque at low RPM, and gain it up high. There's no such thing as a cam profile that does both. I haven't looked into the exact functioning of the D17's VTEC system but my guess is that it's either 1) a low and a high lift lobe, and each lobe drives both valves depending on VTEC, or 2) a low and a high lobe, where the low lobe actuates one valve at low RPM and the high lobe the other, and a locking pin allows the high lobe to operate both valves after VTEC engagement. This arrangement is more likely.

More lift and duration degrades torque and combustion quality at lower RPM. This can be seen in extreme cases such as in classic large domestic V8s which have loping, rough idles from their high lift and duration cams. VTEC can minimize this, depending on how it operates, but you'd be trading mid-range for high end torque in a best case.

At low load the engine runs in closed loop. The O2 sensor can compensate for differences in airflow, but the ECU has to relearn the fuel trims on every startup, and you may find it runs rich for a very brief time when changing throttle, as it expects more air. There will likely be some changes to cylinder scavenging if the low cam has changed, which would have a minor effect on what ignition timing is needed.

At high load, most likely the ECU goes open-loop and relies on its fuel tables without O2 feedback. If you're seeing a 15% difference in torque, for example, you'll be running 15% more rich in the mid-range and 15% leaner up high, since the tables won't be corrected and that air would have been moved from one spot to another. If the base target AFR was 13.5:1, that could mean at (for example) 4000rpm it could be as rich as 11.7:1 AFR, and up around 7000 it might be as lean as 15.5:1. I doubt from a cam alone you'd get 15% more high-end, but the principle applies.

My 2 cents, and my word of caution about changing anything past the butterfly valve without adjustment of ECU maps.
You are totally correct, great information! This is a great cam, the idle quality is just as good as stock and I don't feel a loss in low end torque, if anything it feels a bit better. The mid range and top end improved noticeably though. The main difference I noticed is that the car now pulls strong all the way to redline, the power doesn't drop off as it approaches redline anymore. It now feels like it would keep pulling strong past redline if it wasn't for the rev limiter.

IIRC what Crower did is they kept the non VTEC cam lobes the same and they increased the lift and duration of the VTEC lobes so low end torque and idle quality are retained, but the engine breathes better at higher RPMs when VTEC comes on. Or something like that, it's been a while.

Great point about air fuel ratios. My Scanguage shows AFRs, but I don't know how it calculates it or if its reading can be trusted. For what its worth it shows low 12s at full throttle though, so that should be good. I told myself I had enough gauges when I added a dual temp gauge for my oil and trans temp and an oil pressure gauge, but you are making me want to get an AFR gauge too lol.


Last edited by EcoCivic; 05-06-2020 at 10:44 PM.. Reason: Spelling and clarification
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