Fuel economy Q

[guitaraholic]

Well, an engine management system will definatly help you don't "need" on to get good fuel economy. It's a little bit of a misnomer that you can either tune for power or fuel economy. True, things like HUGE injectors and sometimes a High Flow fuel pump can bring down fuel economy, but I still believe you can get good fuel economy on the stock ECU,, and here's how

The stock ECU (well with a daughtercard of some sort for bigger injectors, MAF, ect), has two modes, open and closed. During open loop (I hope I don't mix these up). The computer is checking signals from the MAF and O2 sensor, and probably a host of other sensors. It is trying to achieve the optimal A/F mixture at all times. This is usually as steady state driving conditions such as cruise. The computer doesn't care if you have a turbo that can put out 600whp (at least not at cruise). It just cares if you are running rich or lean.

On closed loop, (Full throttle, start up, and a few other conditions), the computer goes to the maps and doesn't care about A/F ratio. It does whatever is in the maps. It's at this time when you "tune" for power, and yes, you mileage will be horrible if you romp on it all the time. The more power you make the worse your mileage.

I have a 98 240sx w/ a 98 Sr20det engine with for now the stock T28. I have the usual bolt one (550cc injectors, Z32 MAf, chipped ECU, Walbro, FMIC, Full 3" exhaust) and I get 29-30mpg on the highway. In fact, I get over 30mpg if I am doing over 75 mph constantatly. I can't figure that one out except maybe I am driving where the engine works more effeciently. I can run up to 15psi of boost with no probs. So it makes plenty o' HP and still get civic-like mileage..

Hope that helps

[RHD95S14]

Originally Posted by guitaraholic

Well, an engine management system will definatly help you don't "need" on to get good fuel economy. It's a little bit of a misnomer that you can either tune for power or fuel economy. True, things like HUGE injectors and sometimes a High Flow fuel pump can bring down fuel economy, but I still believe you can get good fuel economy on the stock ECU,, and here's how

The stock ECU (well with a daughtercard of some sort for bigger injectors, MAF, ect), has two modes, open and closed. During open loop (I hope I don't mix these up). The computer is checking signals from the MAF and O2 sensor, and probably a host of other sensors. It is trying to achieve the optimal A/F mixture at all times. This is usually as steady state driving conditions such as cruise. The computer doesn't care if you have a turbo that can put out 600whp (at least not at cruise). It just cares if you are running rich or lean.

On closed loop, (Full throttle, start up, and a few other conditions), the computer goes to the maps and doesn't care about A/F ratio. It does whatever is in the maps. It's at this time when you "tune" for power, and yes, you mileage will be horrible if you romp on it all the time. The more power you make the worse your mileage.

I have a 98 240sx w/ a 98 Sr20det engine with for now the stock T28. I have the usual bolt one (550cc injectors, Z32 MAf, chipped ECU, Walbro, FMIC, Full 3" exhaust) and I get 29-30mpg on the highway. In fact, I get over 30mpg if I am doing over 75 mph constantatly. I can't figure that one out except maybe I am driving where the engine works more effeciently. I can run up to 15psi of boost with no probs. So it makes plenty o' HP and still get civic-like mileage..

Hope that helps

Good information but you did switch the names around.

Closed loop would be the first paragraph, and open loop would be the second.

[joe3180]

Originally Posted by guitaraholic

I have a 98 240sx w/ a 98 Sr20det engine with for now the stock T28. I have the usual bolt one (550cc injectors, Z32 MAf, chipped ECU, Walbro, FMIC, Full 3" exhaust) and I get 29-30mpg on the highway. In fact, I get over 30mpg if I am doing over 75 mph constantatly. I can't figure that one out except maybe I am driving where the engine works more effeciently. I can run up to 15psi of boost with no probs. So it makes plenty o' HP and still get civic-like mileage..

Hope that helps

i have the same setup but on an 89 240sx. and i get the same mpg has he does...28-30 highway. its great. before i installed the maf, inj, front mount, t28 turbo, clutch and tuned the ecu, i had all stock except the exh and fuel pump and was getting crappy mpg like 17-20. i dont know what exactly i did to change it but im happy with the result. im also boosting 15 time to time. i was thinking it was the tune that did it but there is no way for me to really figure it out very easy so im not going to and just enjoy. hope it helps in someway.

[steve shadows]

Honestly the best way to really really GET THE MOST out of your fuel economy is to take full control of your fuel and ignition maps

I suggest investing in a Power FC L Jetro unit.

Then reducing the fuel injector time on the low load cell portions of the map by 15% in every cell up to 4K rpms and eveywhere below load cells half way up the map. This should yield cruising AFRs of around 14.8-15.0 and also more low end torque.

Also I have found that the factory OEM Timing maps are WAY too advanced on the low load cell portions of the map, unless you are running about 100+ octane.

This may be due to the quality and octane suggested in Japan.

What you really need is about - 10 degrees less timing in every cell of the map to get the most efficient driveability and fuel economy as well as torque out of the SR 20det

Not 41 degrees of timing but 21 degrees of timing...dead serious