[Kingtal0n]

Fuel economy

I spent alot of time increasing the fuel economy of whatever I drive.
I usually start AS SUGGESTED with a tune up. Mainly because your air filter and plug condition matter more than you realize.

Once you get those new plugs in there, you start looking at them religiously, because they are 1/2 the battle. A nice chunk of it anyways. You want them staying nice and white and clean, and with a hint of tan colour is ok too. Just a hint.

OK so, what economy should you have? 25+mpg from a random redtop with good compression, spirited driving half time, with 30-50% highway. Average stuff. I was able to achieve 32mpg~ from my redtop in a zenki, strict highway. with my father's 400rwhp redtop (built bottom end) we get 30mpg with the A/C on highway driving. So it is possible. And I have been up in the 420miles/tank region with a built sr20det as well, over 33mpg highway at one point.

Its part of the tune (why you are looking at the plugs comes back to that) but also how you drive;
how2drive:
#1 compression braking for long distances, pissing everybody off behind you is one major key. Why use gas rolling up that redlight? when you compression brake, not one drop of fuel goes into the engine. I set my PFC to not fuel the motor until 1000rpm, about 600 less than factory. It gives me an extra 6.2(X)~seconds of not using gas in 5th gear rolling to a stop. Every cent counts here.

#2 rolling resistance and weight is as low as possible, your car needs to be easy to push and weigh nothing. I take out the spare, the passenger seat, whatever I want. I grease the wheel bearing. Replace the center driveshaft bearing. put high quality synthetic oil in everything. Make sure your brakes are not dragging, take them off and clean them up, grease the backs of the pads. Push your car on a regular basis so you have a feel for how hard it is to push down the road. Get familiar with it's rolling resistance feel, so you can suddenly notice if there is any change.

#3 If you have a maf sensor, your off-throttle fueling will be wrong if you modified the factory intake air path. When you shift, air flows backwards through the compressor unless you have a bypass positioned right there nearby, and furthermore, it needs to be recirculated if you dont want the maf reading high voltage and splashing the cylinders with unwanted fuel. That stuff adds up, and it removes the protective coating of oil from the cylinder walls for an instant. With my Z32 maf positioned right near the compressor and the bypass operating late (because its on the cold side) I saw an additional 400-1500mV or more during shifts and during neutral position from rolling in gear reported by the maf, injecting an additional 3-10% of fuel duty cycle UN-necessarily every single time. To fix the first problem move the bypass near the compressor and set it to open position at idle, recirculated. To fix the second, move the maf a significant distance from the compressor and add a bend if possible.

#4 Briefly, Ill touch on the tune. You want 36-42* btdc of ignition timing during a light load 3000rpm+ highway cruise for most sr20det engines. I use 38* as a starting point for most cars, then log fuel injector duty over weeks at a time while slowly adding incremental timing. For the A/F I like to use 15.2 as a standard open loop target. I don't generally use closed loop, because the oxygen sensor will keep the A/F between 14.2 and 14.9 which is still richer than necessary for optimal fuel economy. If the car has a configurable wideband, I set the voltages such that 15.2 = .004v and 14.7 = 1.001v, that way the computer will hold the A/F around 15flat during closed loop. It doesn't make as much difference as you might think, the main thing here is the timing, everybody overlooks the timing. You need that full 36+ of timing or your mileage will seriously suffer.

Other things to think about:
Tire size, and rear gear. Get the S15 six speed transmission and the 3.6x rear gear if possible. Run a tall tire in the rear. Anything to drop your final drive ratio.
Wind resistance matters at highway speeds over 65 for most cars. Get your car lower to the ground if possible. If you modify the exterior, consider the implications. Air-dams, projections under the car that force air to move in a particular fashion may have an unanticipated effect on fuel economy.

The delay
something I have mentioned already that is worth re-exploring. the maf sensor reports a constant voltage to the ecu, whether the ecu cares or not. When you lift from the throttle, the ecu knows to cut fuel, because it watches the tps, and other sensors for an engine compression braking situation. The ecu is constantly calculating air fuel ratio based on maf voltage as well, and there is a delay between the air fuel being calculated by the ecu and the fuel cut caused by the tps. This delay is partially due to an algorithm within the programming, that is watching also the maf voltage on account of a situation when the tps stops working (signal lost / broken tps) the car can still be fueled based on the maf sensor voltage regardless, so you can drive it that way. Indeed disconnect your tps and watch your car drive pretty normal. During this delay, the suddenly shifting maf sensor voltage will partially be accepted and too much fuel will be injected, much more than desired, the air fuel may shoot to 10:1 or richer for a brief period. To partially compensate for this with tuning, you can tune the low value portions of the maf vE table to be extremely lean. The problem with doing that, however, is now the car runs super lean in those areas driving normally, because those areas correspond to actual driving conditions (while you are completely off the throttle trying to cut fuel that is the mass of air that was "supposedly" entering the engine, as reported by the maf).

The solution still holds, that if you set a high quality bypass to factory like conditions, it has to be open at idle, lightweight setting. The compressor is moving more air than the engine wants at most hot idle situations. This is should be given a place to go, back into the pre-compressor inlet, but far enough from the maf that it does not affect the maf sensor voltage. This will also put an above atmospheric condition at the turbocharger inlet helping spool. The literal reason is the chopping you hear from the compressor blade during a surge, that noise is the sound of friction with air molecules as they slide back past the compressor wheel to get away from the high pressure region on the other side of the blade. The left area of a compressor map, when boost would have been above what the pressure is on the inlet side of the compressor wheel but cannot be because the wheel can no longer hold and compress the outer most column of a column of air molecules, even just 1psi above atmospheric pressure. This is not a big problem when the exhaust housings are large and the compressor wheel is heavier. I am referring mostly to the t-28 crowd that this becomes a large problem when trying to use a large maf like the Z32 on a 2.0L engine. If you can let the pressure in front of the compressor wheel stay low, by producing a vacuum there, you will assist the compressor wheels motion instead of fight it. The faster you can get the compressor wheel to spin, the better off you are because it will be moving more air, and that air will only be going up in temperature primarily to the heat of compression, which you will not be doing when you are creating a vacuum (low pressure side, on the engine side of the compressor, due to the bypass you have open) situation so the end result will be that your compressor wheel is spinning faster at any given moment without heating it up as much as if we are compressing it. Ideally you would see that relief of this excess air to the atmosphere would be the most beneficial situiton, because any heat absorbed into it along the way to the compressor would be lost with it, so this bypass "problem" of recirculating only becomes a problem on a car equipped with a maf sensor. In either case you still want that bypass valve open whenever you are not demanding boost, to keep the wheel moving and out of surge.

...that your compressor wheel is spinning faster at any given moment ready for service when the bypass suddenly shuts. Imagine that situation now, the compressor has been feeding the bypass partially, and the engine partially, since the engine is still breathing (its spinning right? There is a vacuum source then regardless of throttle position) Now suddenly the bypass shuts and the situation is back to surge city if the throttle is closed, because the air has nowhere to go but we just said service, which is boost. What happens at wot: the throttle valve is wide open. Suddenly the bypass shuts and the compressor looks for a new place to put those air molecules. What does it find? An open throttle valve, which represents the path of least resistance for those air molecules. Can you imagine that a compressor wheel would rather push all those air molecules into an engine than slip backwards past the wheel? If you have been paying attention right now you should be thinking about the exhaust wheel. Remember that the opening of the throttle valve instantaneously does not generate any increase in exhaust gas production by itself, or temperature, because the situation is instantaneous we can consider the event before the next combustion reaction taking place. There is never a delay from the ecu in this situation because a sudden increase in maf voltage is always met with a large increase in fueling, but the next couple of combustion events that take place will still be at the same pressure as atmospheric at least, since opening the throttle valve we can consider the effect of atmospheric pressure compared to the enormous vacuum canister represented by the engine. Keeping in mind that the engine is literally representing a vacuum pump and it's dynamics due to modification would be mis-represented in the cases I am providing, since an engine does not need to produce much vacuum at idle to be a well performing vacuum pump at higher rpm. I am mostly considering stock engine t-28, engines that are effective vacuum pumps at idle, spinning their exhaust wheels easily enough to feed a cruising engine, when the throttle suddenly shuts they need a bypass open just as quickly to have a place to dump air molecules and so the wheel may keep moving forward freely without additional friction. Back to the exhaust wheel. If there were no wastegate, then the exhaust wheel would have a set speed for all air mass/unit time including temperature to get volume. And a given wheel speed also correlates to an exact mass/unit time of on the compressor side, also including temperature to get volume. Notice volume plays role with driving the turbine, and mass plays the important role of on the compressor side. If we reduce the volume of a given mass of air molecules by lowering temperature then the compressor will move more mass of air at the same wheel speed. But on the exhaust side we desire a large volume of air and therefore high temperature because it helps drive the turbine. Not that it needs to be said, the reason the compressor benefits(we benefit) from the low temperature is because engines horsepower is directly related to the mass of air ingested. The more mass we squeeze into the cylinder, the more fuel we can inject, and the more torque the engine will produce. So now once again back to the exhaust wheel, lets open the throttle fully and start filling the cylinders, and producing more heat and volume of air molecules to spin the turbine. The only times it does not, is when the wastegate is removing some volume to slow the exhaust wheel. This is a horrible situation because we are trashing perfectly good exhaust gas that could have spun the turbine faster. Nevertheless, boost pressure triggers the wastegate, and boost pressure says nothing about air mass because temperature is involved and the boost pressure tells us nothing about temperature. Now lets stop and consider that there are an infinite number of possible situations to consider with respect to the position of the exhaust and intake valves, and the pressures in every chamber and even the sound waves at any given rpm. When I say instantaneous I mean the infinitely thin slice of area under the curve, and for any of these ranging from idle all the way to WOT the ideal speed of the compressor wheel is it's maximum speed, as if durability were not an issue. If you had a maximum speed wheel at idle then there is no delay for full boost when you suddenly desire it if you can maintain that wheel speed once the demand is handled. Obviously we cannot spin it at max speed all the time, my point was so dramatic to illustrate our primary goal: wheel speed, and it should be obvious why now if it was not before.