Interesting Fuel Economy Observation
- Thread starter Big Blue24
- Start date
displacedtexan
$500 PREMIUM MEMBER
addictedtodezel
New member
I'd like to figure out why I can't get much better than 15 mpg no matter how I drive with my setup. Cruising at about anything above 55 I have 1000* egts and about 11psi of air, so I'm definitely getting to much fuel. To lose of a afc spring or what? It's 4x4 with 3.54's and 35" tires.
Tate
What?
I did the Calgary to Regina run last summer, and got the best mileage in my 12v that I've ever had, which is about 21 mpg. That was running 120km/h (75 mph) the whole way. That was with my 64mm and 15 degrees timing. With twins and 18 degrees, my best has been about 19.5 mpg, with 17 being the average.
Big Blue24
Comp Diesel Sponsor
Update:
Just filled up after running 95% highway driving 30 miles each way to work at an average speed of 65-70 MPH. 17.4 MPG I also checked boost when cruising at 65 MPH and it hovers around 3-5 psi depending on grade, wind, etc.
It's time to retard the timing a little and try again, the consensus seems to be that I should try 18* instead of my current 26.4*, so I'm going to shoot for 18* and see where it ends up when I get the pump shaft nut torqued down tight.
Just filled up after running 95% highway driving 30 miles each way to work at an average speed of 65-70 MPH. 17.4 MPG I also checked boost when cruising at 65 MPH and it hovers around 3-5 psi depending on grade, wind, etc.
It's time to retard the timing a little and try again, the consensus seems to be that I should try 18* instead of my current 26.4*, so I'm going to shoot for 18* and see where it ends up when I get the pump shaft nut torqued down tight.
Big Blue24
Comp Diesel Sponsor
Another Update:
I moved the timing back from 26.4* to 18*. My truck idles a little bit quieter and it seems to have a little smoother off-boost power delivery.
On the highway, the motor is nice at quiet at 60 MPH, before, it didn't quiet down till about 65 MPH or 5 psi boost.
Cruising boost is up 1.5 to 2 psi. At 60 MPH it now cruises with 4-5 psi and 70 MPH is 6-8 psi...... Interesting! If someone told me cruising boost changed with different timing settings, I wouldn't believe it until I seen it. I suspect the later fuel deliver equates to more burn in the exhaust stroke and therefore higher cruising boost.
I made a nearly identical trip to California yesterday, it was about 60 miles shorter than my last one with 26.4* timing but the same freeways, just exited a little sooner.
I set the cruise at 73 MPH and the hand calculated fuel economy was 19.93 MPG. Better, but not significantly different enough for me to come to a conclusion. I plan to follow up with my usual 90% freeway in-town testing at 65-70 MPH to see the difference at that RPM range.
As far as power, It does feel like I detuned the top-end power, I'd guess 10-15 HP peak loss. The midrange feels slightly less powerful but the spool-up and low-end horsepower have increased.
I moved the timing back from 26.4* to 18*. My truck idles a little bit quieter and it seems to have a little smoother off-boost power delivery.
On the highway, the motor is nice at quiet at 60 MPH, before, it didn't quiet down till about 65 MPH or 5 psi boost.
Cruising boost is up 1.5 to 2 psi. At 60 MPH it now cruises with 4-5 psi and 70 MPH is 6-8 psi...... Interesting! If someone told me cruising boost changed with different timing settings, I wouldn't believe it until I seen it. I suspect the later fuel deliver equates to more burn in the exhaust stroke and therefore higher cruising boost.
I made a nearly identical trip to California yesterday, it was about 60 miles shorter than my last one with 26.4* timing but the same freeways, just exited a little sooner.
I set the cruise at 73 MPH and the hand calculated fuel economy was 19.93 MPG. Better, but not significantly different enough for me to come to a conclusion. I plan to follow up with my usual 90% freeway in-town testing at 65-70 MPH to see the difference at that RPM range.
As far as power, It does feel like I detuned the top-end power, I'd guess 10-15 HP peak loss. The midrange feels slightly less powerful but the spool-up and low-end horsepower have increased.
kas83
New member
With your speed, are you running around 2100 rpm? I ran the numbers in a calculator, came out 2,080. Might try bumping your timing up a few degrees if that's your cruise speed alot, keep it in the sweet spot as was mentioned earlier.
Interesting findings Big Blue. I'm looking forward to playing around with my truck after I pick one up.
Last edited:
Big Blue24
Comp Diesel Sponsor
Cruising EGT's did go up about 50-75, this setup cruises between 700 to 850. Before it would cruise 650 to 800.
WOT is too difficult to compare from memory. It's hard to keep it WOT the same amount of time, on the same hill, with the same outside temp, back to back etc.
If your real question is will increased timing lower EGT readings on the gauge, the answer is yes. Where the heat is going, is another discussion.
Gasoline Sucks
slow 12v
wow interesting. Im around 17 now and with the 35s 3.55s rpm is at 2k at 80 and very low boost due to this big hair dryer. Are you running a stock hx35 on your truck blue?
Big Blue24
Comp Diesel Sponsor
Stock HX35 with a boost elbow, stock wastgate with stock non-drilled divider.
I have been acquiring parts for a compound turbo setup for the daily driver. I'm not sure if i'm going to run this HX35 on top or go with an He351.
Gasoline Sucks
slow 12v
JQ- use the nitrous 
My egt is around 7-800 and my afc spring is pretty tight. I think this turbo is hurting me a bit on the highway due to lack of boost, im going on a trip today but with all this WMO in the truck I dont think I will get an accurate average.
My egt is around 7-800 and my afc spring is pretty tight. I think this turbo is hurting me a bit on the highway due to lack of boost, im going on a trip today but with all this WMO in the truck I dont think I will get an accurate average.