MicroSquirt.com read only forum (Jan.15,2018)

Spenser309,

An overly rich condition (or a mis-fire) can definitely fry the cats, as the excess fuel is consumed in the cats themselves. However, a lean condition generally does not harm the cats (but does result in higher emissions while lean).

BTW, you don't need a wide band sensor to achive to a lean mixture (as long as you don't want both lean and EGO feedback). You can tune it to stoich, then adjust the VE and EGO limits as described in the manual to get lean mixtures.

Lance.

Spenser309 wrote:and fry the cats

The only way lean mixtures will fry cats is if it's lean enough to misfire. This limit is highly engine-specific. Otherwise, the lean mixture will cool off the cats and make them ineffective.

It can. I'd say the Bug is now tuned 95% correctly and milage has gone up by about 2 mpg. If I were tuning for maximum fuel economy, it would probably have gone up more.

When tuning my MR2 for fuel economy, I saw the economy jump from 28 to 34.

Well I solved my tuning problem. That wasn't the problem.

I did some mpg tests and my car gets 22.5mpg on the street and 43.1mpg on the freeway. I usually get 28mpg mixed.

So the only thing I can think of is the weight of the car is the problem. The car weighs 3000lbs and it has a 150hp engine. So all the go and stopping is killing the mileage. What do you guys think?

Look at the spots in the map you hit while accelerating and decelerating that you don't while cruising on the freeway. Specifically:

-Is Deceleration Fuel Cutoff turned on? It should be.
-Consider dipping into a richer AFR at a slightly higher manifold pressure.
-Your medium-to-high load timing might be too retarded.
-If you have staged injection, that might not be handled well.
-Your acceleration enrichment settings are WAY too rich.

Something else to consider is whether or not one of those two measurements is a fluke.

Say my accelaration is too rich 1000-2000rpm. If I were to lean it out wouldn't that put more strain onto my engine???

My A/F right now is 14.65

Those two measurements are not a fluke and I'll explain why.

If I were to drive only on the streets and never touch the freeway my mpg would be 20-22.5mpg. Now my mixed mileage is 28mpg, that is 50% street and 50% highway. I can do 120+ miles on the freeway in 3 gallons, you do the math.

If you don't believe me since this is the internet and a lot of stuff gets made up you can come to Southern California and I will empty out my tank and put a 1 gallon of gas and you can follow me on the freeway till I run out of gas and see my odometer.

I even drive 10mph over my sweet spot. If I were to drive at my sweet spot I can easily pull off 45+mpg.

My main concern is the street MPG, I am over the EPA number but I want more.

So would having say a 15-15.5 A/F at 1000-2000rpm put more strain on my engine?

Whoa! I'm not distrusting you, I'm just suggesting possible reasons for your observations. It doesn't benefit me in any way imaginable to ask you to prove yourself. If you know the reading isn't a fluke, then that's good enough for me!

Back to the other reasons. You say your acceleration is too rich in that RPM range. 14.65 doesn't sound too rich assuming you are talking nearly-ambient manifold pressures. What do you think about the other items on my list?

Well my car can coast for a good amount of time. The problem is it eats up too much fuel on accelarations. I was told that the SAFC II won't work because the throttle needs to be pushed at least 40% for it to work.

My timing right now is 20 degrees. My compression is 9.2 and I use mainly 89 octane by shell or Chevron. I'll be switching to 87 octane soon since it's a 4 cylender.

I don't think there is fuel cut off on decelaration, just the rpms drop to around 1100-900rpm and the car can coast 1/8 - 1/4mile.

Here you go, this will clarify a lot.
http://www.nissannews.com/site_library/ ... pecs.shtml

Okay, now I'm lost. Why would you mess with the SAFC when your megasquirt gives you full control over everything?

Decel cutoff happens every time you lift the throttle, like on upshifts, which happen all the time when accelerating using a manual transmission. Every OEM uses decel fuel cutoff because it's free mileage that you don't have to do anything to achieve.

Timing is at 20 degrees where? If it's at 20 degrees at high manifold pressure, my guess is that's way too retarded and will cause poor mileage. If it's at 20 degrees under boost, that might be too much.

Octane has nothing to do with the cylinder count of the engine. It has to do with burn rates and detonation supression. You use the octane you intend to use when you tune the car. Lower octane can sometimes produce better mileage given you aren't detonating, but what REALLY improves mileage is careful EMS tuning on a dyno.