Possible to change the 6th gear
11-07-2012, 09:53 PM
#41
Registered User
This is an oversimplification, and could only be true if losses due to friction wend down at higher RPMs. It is true that you can achieve the same speed by running the engine at higher RPMs with reduced load, but the percentage of the total load that actually makes it to the wheels will go down. Basically, the load due to increased friction will go up, so the overall efficiency will go down.
11-10-2012, 01:25 AM
#42
Originally Posted by Weasel497' timestamp='1352241646' post='22133890
Actually, rpm doesn't play that much of a role in mpg. The reason being is that in a lower gear, you'll be in a higher rpm range, but the engine is also under less load. Think of it like a mountain bike. Depending on the load of the engine, the ecu will be sending more or less fuel through the injectors, no matter the rpm.
Bingo.
Half the guys talking about fuel increasing with rpm are correct, but for the wrong reasons, and I doubt any of them have ever even looked a properly tuned fuel map. For those of you who havent, Im about to blow your mind.
Heres a quick screenshot that I cropped the important parts to.
Y axis is load and X axis is rpm, the two blue parts are my 4200 and 5400rpm breakpoints at the same load. I have 4.77 gearing so thats where my 70mph and 85mph cruising points are at. As you can see, at the same load, the fuel values are actually the same (these are all tuned within 1% of 14.7 AFR) so initially, one would think well if your using the same fuel at 85mph wouldnt you be getting much better mileage than at 70mph because your going 15mph faster while using the same amount of gas? Yes this would be true, but when I speed up to 85mph, the load increases by about half a break point on average, so I do end up using more fuel, and get better mileage at 70mph, so Whiteflash is right, RPM technically doesn't matter, but its the increase in load thats necessary to get over the increase in friction (and wind resistance/rolling resistance if your going faster) that ends up biting you in the ass.
At least thats my non-engineering understanding of it.
11-11-2012, 05:39 PM
#43
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Its funny I had a 76 datsun with an LS1/6spd (t56) and 3.90 gears and I was maybe 1800rpm at 60mph in 6th and that was cam'd, heads etc (400rwhp) I was knocking down 30mpg in that car (granted it was light), in 5th or 4th the milage was not so much lol
Dropping some RPM would be nice to see some incrase for sure but payback would be LOOONG term for sure.
Dropping some RPM would be nice to see some incrase for sure but payback would be LOOONG term for sure.
11-13-2012, 09:04 PM
#45
11-14-2012, 04:24 AM
#46
11-14-2012, 04:48 AM
#47
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11-14-2012, 05:56 AM
#48
11-15-2012, 07:53 AM
#49
Registered User
actually S has pretty good mpg
I drove from SF bay area to LA, it was about 400 miles
I fueled full tank (12.5 gallons) before I reset the mileage meter, and my S didn't stop for fueling on the way.
so 400/12.5 = 32 MPG!!!
the bad thing is... our S is damn good and we are the enthusiasts lol
I drove from SF bay area to LA, it was about 400 miles
I fueled full tank (12.5 gallons) before I reset the mileage meter, and my S didn't stop for fueling on the way.
so 400/12.5 = 32 MPG!!!
the bad thing is... our S is damn good and we are the enthusiasts lol
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