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Thread: Theory: Increased Engine load leads to better fuel economy

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Posts: 1-10 of 23
2013-09-30 20:33:00
#1
Theory: Increased Engine load leads to better fuel economy
Theory 1: Less load better fuel economy
General consensus says the more load the less fuel economy, the less load the better fuel economy is. This is how old school MPG gauges (vacuum gauges) used to work, the more you open your throttle, less vacuum, worse fuel economy. It makes sense on why, the more load (air), the more fuel is needed to keep the mixture from leaning out.

This is why increasing ignition timing can increase gas mileage too; by increasing timing you get more torque (until you reach the knock threshold), by increasing torque car has more power thus you use less throttle to keep the car going at the set speed.

Theory 2: More load better fuel economy
Metrompg.com did a warm air intake (WAI) test, one of the theories behind WAI is quoted below. Gasoline Engines are most efficient at wide open throttle and least efficient at part throttle.

Originally Posted by metrompg
Reducing the density of the intake air by heating it effectively reduces the power of the engine at a given throttle opening (relative to the same engine with a CAI). To do the same work as the CAI-equipped engine, the throttle of the WAI engine must be opened wider at a given RPM. This increases engine efficiency by reducing throttling or pumping losses:

"The air is less dense, so you get less horsepower at the same throttle opening, thus, you have to open the throttle wider to let in more air and get the horsepower that you need. That increases the efficiency because one of the primary causes of the well-known part load inefficiency of gasoline engines is the throttle loss." - (source)



Theory 3: Ignition timing
On my 9.5:1 Roller Rocker DE+T (with low-port intake manifold), I've noticed a rather weird trend too. The less timing I ran the better my MPG got. I started out at with 38-40* timing for highway speed cells. I got down to 30* before I ran out of road to test on (12 hour trip), I do plan on going lower then 30* on my next big trip. This is completely backwards of my 8.5:1 SR20DET (with Ported intake manifold), I kept on getting better MPG the higher I went, leaving it 40-42.

While both motors are SR20's, they are completely different animals internally, Roller Rocker is much more efficient internally then a DET. The more efficient the engine the less timing is required to get the same amount of torque out of each cycle. Having too high of timing would mean the mixture would finish burning while the piston was still pushing up, at WOT this would cause detonation, at partial throttle it will increase resistance, the same way it becomes harder to pump tires up the more pressure they have. What's odd is, on the dyno Roller Rocker's best torque was at 36-37* for that rpm range.

Hill climbing vs MPG
I occasionally get out of my small town hidden in a valley, ~1,400 ft above sea level, and drive to Fairfax area, ~300ft. If I fill up after a descending drive only, I will get around 30mpg. If I were to just turn around and drive back, ascending to 1,400ft, my MPG would be much higher, closer to 34mpg. I noticed the same trend when I went to Richmond (practically sea level). I always found it interesting that going 64 west over West VA mountains kept my gas mileage pretty high too, one would expect all of the hill climbing would hurt the gas mileage badly.

Now there are two possible explanations to this, either it's theory #2, or it's the fact that higher load columns will have less timing then lower load columns, thus decreasing the timing advance in the lower load would bring the MPG (less resistance from over advancing, ie pumping tires up example).
Last edited by Vadim on 2013-09-30 at 20-52-01.
2013-09-30 20:59:13
#2
My yd22 is more efficient @75mph than any other speed regardless of load towing etc.

The only way I can get better efficiency is to draught behind a HGV @ 56mph (hgv limited to 56 here)

Sent from my GT-I9300 using Tapatalk 4
2013-10-01 13:23:14
#3
Originally Posted by damienga15de
My yd22 is more efficient @75mph than any other speed regardless of load towing etc.

The only way I can get better efficiency is to draught behind a HGV @ 56mph (hgv limited to 56 here)

Sent from my GT-I9300 using Tapatalk 4


Do you by any chance have a timing map on hand? If not Nistune might have it, what RPM are you at at 75mph?
2013-10-02 00:17:19
#4
i roll up hills in 5th gear and cruise back roads in 5th gear, it rolls better then in 4th if im cruising albate acceleration. the throtal is open more but the lower rpm seems to reduce the fuel consumption, idk hat im talking about though.
2013-10-02 00:26:20
#5
~70mph avg
~2800rpm
Trip of 116 miles
Timing of ~26
6th gear
8.5:1 DET

37.8mpg
Last edited by unijabnx2000 on 2013-10-02 at 00-27-27.
2013-10-02 02:31:35
#6
Originally Posted by unijabnx2000
~70mph avg
~2800rpm
Trip of 116 miles
Timing of ~26
6th gear
8.5:1 DET

37.8mpg


What size tires? You have an 04 gearset right? Finally why so low timing with such low compression?
2013-10-02 03:10:26
#7
Originally Posted by Vadim
Originally Posted by unijabnx2000
~70mph avg
~2800rpm
Trip of 116 miles
Timing of ~26
6th gear
8.5:1 DET

37.8mpg


What size tires? You have an 04 gearset right? Finally why so low timing with such low compression?


22inch (after these are worn, im going with >23.5 inch)
no idea, my DET has always been weird like that.
Yes, 04-06 gearing.
Last edited by unijabnx2000 on 2013-10-02 at 03-11-33.
2013-10-02 03:13:06
#8
Originally Posted by unijabnx2000

22inch (after these are worn, im going with >23.5 inch)
no idea, my DET has always been weird like that.
Yes, 04-06 gearing.


Wow that's low RPM for such small tires. I'm running 24.7, and I'm at 2800rpm at 70mph, but I think I have an 03 spec trans...
2013-10-02 04:01:48
#9
Originally Posted by Vadim

Wow that's low RPM for such small tires. I'm running 24.7, and I'm at 2800rpm at 70mph, but I think I have an 03 spec trans...


based on the tires size, they should be 23.1 inch... but i measured them and they are 22
2013-10-07 04:15:00
#10
Alright got the first half of the trip done and do have some results. The trip is fairly simple, hit 81 south until 64 west, and stay on 64w until Saint Louis. We set out early in the morning ~50*F and by noon had to use AC. I tried to fill up at around the first 1/4 mark, which equals to 4 gallons. Cruise control was used for all runs, set to 70mph max, some roads had construction or lower posted speedlimits, which were followed with +5mph rule. I always fillup to two clicks, since a lot of pumps will punch out at different fullness.

Mods: SR20DE RR with lowport intake manifold,GT28r, avenir exhaust manifold, 6 Speed- 03 Gear set 2,750rpm @ 70mph, 3" DP and exhaust, FMIC, UR WP Pullley, NEMU with MAP sensor only (No IAT), Walbro 255, Nismo FPR @ 3 bar, Iridium BKR7IEX Sidegapped with .040" gap. EGR solenoid working (fuel vapor canister feed to intake manifold).
Suspension: 1" lower front, stock height rear. 1/32" toe in front, 1/32" toe in rear beam (4mm of beam shift to passenger side).

First fill up - Driving through Virginia: started out at 1,200 feet, got to about 2,200 feet, and back down to 1,200ft. Water temperature stayed steady at 185*F (180*F thermostat). No AC, 70mph, cruise control, max 30* timing at 2,800rpm. Out of 124 miles got 30mpg

Second fill up - Driving through West Virginia: started at 1,200ft to 3,400ft then down to 600ft. Water temperature stayed at 185*F, no AC, 65mph average, max 24* timing at 2,800rpm, 150 miles, 33mpg.

Thirst fill up - Driving through Kentucky: elevation stayed around 700ft. Water temperature was at 188-190*F due to sun rising (black car), 5% AC usage, 70mph mostly, max 24* timing at 2,800rpm, 150miles, 38mpg.

Fourth fill up - Driving through Indiana: elevation stayed around 600ft. I used a piece of cardboard to block the radiator, water temps were 206-212*F, with most of it being at 208*F, max 24* of timing at 2,800rpm, 70mph consistently, 100% AC usage, 180miles, 32.3mpg. Either warm water temps caused much worse MPG, or the previous pump punched out early

Fifth fill up - Driving through Illinois: elevation around 600ft. Removed the grill block, water temps were between 195-199*F. 100% AC usage, 70mph consistently, 150miles, 34.3mpg.


I will be testing some more on the way back, but so far it seems like having 24* of timing has been the main help with improving the gas mileage. This is weird, because the car made best torque at 36* on the dyno. I think water temperatures also play a role, it looks like you want to keep water temperatures around 190*F, but going too hot 205*F+ is detrimental. Using AC does seem to help increase water temperature, which in turn does seem to yield better mpg's .

I do think there is a connection between water temperatures, and best timing for best efficiency. I think colder water temps (=colder intake temp?), liked having lower timing which makes sense. I might try blocking the grill again and raising the timing too, but it's not a safe real world expectation to run that hot! Also since I am on a MAP sensor, it is more sensitive to heat changes. For the return trip I will install the MAF back and compare.
Last edited by Vadim on 2013-10-07 at 18-26-02.
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