Speed density conversion tune

**nickp** · 02-12-2017, 04:46 PM

Hey guys, I have a new motor going together and I am curious how many of you guys switched to speed density. Pros and cons? How difficult was it to tune compared to mass air? Thanks guys

***SSlowBoat*** · 02-12-2017, 06:20 PM

Originally Posted by nickp

Hey guys, I have a new motor going together and I am curious how many of you guys switched to speed density. Pros and cons? How difficult was it to tune compared to mass air? Thanks guys

If you don't change elevation much and are boosted then sure. If you're n/a and want driveability then keep the maf. MAF is easy to tune man.

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**nickp** · 02-13-2017, 08:39 AM

I will be running nitrous but not much elevation change. Is there a performance difference between the 2 other than throttle response?

***SSlowBoat*** · 02-13-2017, 09:52 AM

Originally Posted by nickp

I will be running nitrous but not much elevation change. Is there a performance difference between the 2 other than throttle response?

maf is part throttle driving and driveability, when you go wot it uses both.

**nickp** · 02-13-2017, 10:19 AM

I will be running nitrous but not much elevation change. Is there a performance difference between the 2 other than throttle response?

**Injuneer** · 02-13-2017, 11:48 AM

The speed-density method is used to calculate the mass air flow. From that point on, the system works the same. That's why the PCM can keep the engine running even after the MAF sensor fails, and the system defaults to speed-density. The critical part of the speed-density calculation is the volumetric efficiency (VE). That's a table programmed in the PCM. If you change any part of the way the engine breathes... intake, exhaust, cam, etc..... you need to update VE tables. There is a utility available to help with that.

I'm not sure that elevation changes much. Elevation affects MAP, and MAP is part of the speed-density calculation, just like inlet air (absolute) temperature. That's how the density is calculated to convert volumetric flow (CFM) to mass flow (#/HR). Possibly an effect on VE???? Don't know.

As far as the nitrous, you tune the PCM to optimize for straight motor, then with wet nitrous you add the extra fuel and adjust the timing using external methods.... jetting the fuel to match the nitrous and retarding the timing with an external box. Since nitrous vaporization chills the incoming air, throwing off the speed-density calculation, you may (plate after IAT will not be as sensitive) have to carefully examine the plugs to insure your fuel/nitrous jetting is producing the correct results.

**MoeHorsePower** · 02-13-2017, 01:58 PM

I always hear that SD has a better throttle response than a MAF, I asked someone either they had a bad MAF tune and when re tuned in SD the so called lag cleared up, his response was he could tell that the SD was a millisecond faster in throttle response, I didn't even ask...

**nickp** · 02-13-2017, 03:42 PM

Awesome info, thanks guys.

**FastLT1** · 03-17-2017, 01:25 AM

Originally Posted by MoeHorsePower

I always hear that SD has a better throttle response than a MAF, I asked someone either they had a bad MAF tune and when re tuned in SD the so called lag cleared up, his response was he could tell that the SD was a millisecond faster in throttle response, I didn't even ask...

@MoeHorsePower, I know this thread is a little old, but do you do tunes in SD? Or only MAF?