I was going to go with a speed density tune for my boyfriend's car, but have been told it needs to be re-tuned when the weather changes...
I live in Texas, and the weather changes everyday.
Someone who has actually had this done, or knows what they are talking about please give me some advice on a s/d tune.

 

If your tuner gives you the means to change SA and fueling based on IAT output then your SD tune can keep up with Texas weather.

 

I am not an expert on sd and my car is kind of a seasonal car so it dosent see a whole lot of weather but cars didnt used to have maf sensors and they werent retuned every day so im pretty sure that the IAT will compensate for it.

 

SD tunes (either OEM or aftermarket) use the MAP sensor mainly to correct for weather/altitude changes.

 

SD will be using your MAP and IAT (as well as O2s for closed loop) ... so long as your tune is right it should be able to compensate... the biggest problem I've found is that if you tune in warm weather... then go to use the car in cold weather... you hit different cells that you may not have tuned previously ... so you have to hit those too... but once the VE table is dialed in there shouldn't be much variance

 

Geez...this is another rumor that really needs to be put to rest. I don't even know where this crap started, but it definitely isn't an issue. As stated, the MAP and IAT sensors will compensate for varying weather conditions. Just make sure you have your entire VE table dialed in, and you'll be set.

I run an OPEN LOOP SD tune on my daily driver, no issues.

 

It originated with the TPI systems. Supposedly they run exclusively on MAF and some do not incorporate a MAP sensor in the calculations. So deleting the MAF on MAF TPIs would require retuning (supposedly) or convert to a MAP based TPI system.

 

Thanks, this makes me feel a lot better...
Is there anything else I should know about the sd tune?

 

How about the adjustments for IAT though, how good are the adjustments on the stock table if running an SD tune ? Has anyone found that these adjustments need to tweaked ?

 

I've not messed with any of my IAT referenced tables, as I've not had any issues.

And thanks for the heads up on the origin of the rumors...makes sense.

 

The only thing that could cause tune problem is an exterme altitude change, but the fix would be to turn it off, and start it back up so it can grab the Barometer again.

A lot of commercial trucks have a seperate sensor for Barometer aside from the MAP sensor, as they are commonly driven over mountain ranges without being shut off.

MASS Air sensors can correct for changes in altitude while the car is running.

http://www.megasquirt.info/v22manual/mfuel.htm

this is a very good explination of the Speed Density fuel equation.

it is pure math.

PCM knows how much volume there is in a cylinder
PCM knows how big the injectors are

PCM can caculate how much fuel will be needed at 100% ve for a 14.7:1 mixture of air and fuel.

Air Fuel ratio is a ratio by mass (weight), NOT volume.
its 1 lb of fuel for every 14.7 lbs of AIR, air is not very heavy, think about it

but the mass of air does change a lot depending on temperature

The PCM caculates the air density from the intake air temp sensor, and the Barometer reading taken before then engine starts, when the key is ON.

Now in the fuel equation, the computer will look at the VE value on the table

It knows how much fuel for 100% ve, so if the VE table says, 50% at a certain cell, it will then take away 50% fuel imediatly. it then applies the enrichments or enleanments it needs based on other sensors.

Intake Air Temp + Barometer = Density (more dense, more fuel)

The pcm uses other sensors to more fine tune the engine, like warmup. PCM uses multipliers (or percent of change) at different coolant temperatures to add extra fuel during cold start warmup.

accelartion adds fuel, detected by increasing TPS voltage and (Power Enrichment) also detects speed of TPS by volts/second

O2 sensor feed back (used for fuel trim - fine tunes fuel to 14.7:1)

Target Air Fuel - a part of the VE table a certain A/F ratio is desired other then 14.7:1 this happens in OPEN LOOP

again, the computer can caculate a richer mixture because it knows exactly how much fuel it needs to inject for a 12:1 mixture

EVERYTHING goes back to an accurate VE table, that being the most importand thing in SD tuning.

Air temp sensor is also important. needs to know the temp of the air as accurate as it can as when it mixes with the fuel in the combustion chamber.

 

Never once had a problem with my SD tune during weather changes. Now, leaving the batter unplugged for more than five minutes and restarting the car.......thats a whole different story.