How Speed Density works.

[TurboDave]

Lots of great info coming out here!!
This collaboration between Bob and Eric is the best thing to happen in our community in a long long time!!

Sent from my DROID RAZR MAXX HD using Tapatalk 2.

 

[Silver Ghost]

While this topic would make sense to post in the aftermarket ECM section, the latest developments in "SD" chips are going to bring up "howzit work" questions. So...

An ECM's job (among other things) is to fire the injectors such that the correct amount of fuel is injected based on the current operating conditions of the engine. How much to inject becomes the task. The way it is usually done is to calculate the amount of air that made it into the cylinder, factor that by the air/fuel ratio, convert to pulsewidth based on the injector size, and there you have it.

But wait, how do you know how much air "made it in" to the cylinder?

With a MAF (Mass Airflow) system, the MAF sensor measures the airflow rate, if you divide that by the engine RPM, and the number of cylinders that fire in one rev, you arrive at the amount of air that gets into the cylinder on the intake stroke. Easy math.

With a Speed Density system, we calculate the air based on the current operating conditions and the pumping characteristics of the engine (the VE table). If the engine were 100% efficient, then every cycle the cylinder would fill with enough air such that the pressure in the cylinder (or vacuum) would match what is in the manifold. Then the intake valve would close, this air would get compressed and used for combustion.

But no engine (that we will ever see) is 100% efficient, so we use a big table of percentages to help us calculate exactly how much air got into the cylinder. Since the efficiency changes with throttle/MAP and RPM, the table is 3d, with MAP and RPM as the axes.

So, with the engine displacement, MAP, and VE (there is a factor for density that is used also), we can calculate the amount of air in the cylinder.

We then use our regular math, factor in the air/fuel ratio and injector characteristics, and we have injector pulsewidth.

Its a lot of words, but does the above make sense?

Reply with questions, comments, corrections, clarifications.

Bob

 

[Silver Ghost]

If engine displacement is used to calulate the amount of air that enters the cylinder, how is the added displacement of a stroker engine, or deck height differences, factored into the math? Thanks.

 

[Royal-T-Ltd]

It all depends on how the program was written. I do have the SD2 chip set up to pull some timing if the airtemp starts getting high (150 F and higher).

Eric, for those of us injecting alky, where is the preferred mounting location for the IAT sensor?

aaron

 

[TurboBob]

If engine displacement is used to calculate the amount of air that enters the cylinder, how is the added displacement of a stroker engine, or deck height differences, factored into the math? Thanks.

Displacement is normally part of the base air- charge calculation, but for our purposes the easiest way to tune for it is to adjust the injector size.

Posted from the TurboBuick.Com mobile app

 

[Street Lethal]

Eric, for those of us injecting alky, where is the preferred mounting location for the IAT sensor?

aaron

More than likely in the plenum aaron...

 

[Royal-T-Ltd]

More than likely in the plenum aaron...

yeah kinda what i was thinking... but didnt know if they would rather have a reading pre or post alky.


aaron

 

[we4Mateo]

yeah kinda what i was thinking... but didnt know if they would rather have a reading pre or post alky.


aaron

Pre-alky is recommended but the tune can be compensated for a different scenario from what I've gathered so far.

 

[Eric Stage I]

I recommend the airtemp sensor before the alky nozzle, but you can tune it either way.

 

[TurboBob]

I recommend the airtemp sensor before the alky nozzle, but you can tune it either way.

agreed.

this is a common question, lots of opinions. But, like Eric said, you can tune it either way.

B

 

["Reaper"]

Bob I just bought a powerlogger at richard Clark's . Would it be better to have the powerlogger pro to run the new chip and would I have to buy a new powerlogger or is this one upgradable? Either way what would the price be?

Shaun

 

[TurboBob]

Bob I just bought a powerlogger at richard Clark's . Would it be better to have the powerlogger pro to run the new chip and would I have to buy a new powerlogger or is this one upgradable? Either way what would the price be?

Shaun

There is no new PowerLogger, the "Pro" just refers to the new+fancy software that works with the new SD2 chip.

This software upgrade is 100% free. No need to buy anything new. You will be able to download it as soon as the SD2 chips start shipping (very soon).

So to run the new SD2 chip, you just need to buy the new chip. Along with the PowerLogger, you need a MAP sensor, and probably a wideband (you most likely have these already).

Bob

 

[Pronto]

Will there be any additional 0-5v sensor logging abilities?

 

[TurboBob]

nothing new in that department, what do you want to log?

B

 

[Pronto]

It would nice to have more than 2. I'ld like to do fuel pressure, alky pressure and maybe trans temp as well...

 

[SCOOBY DOO]

Subscribed

 

[TurboBob]

It would nice to have more than 2. I'ld like to do fuel pressure, alky pressure and maybe trans temp as well...

Its on my list to expand the flexibility of channel 3. Beyond that gets a little bit more involved.

Bob

 

[Royal-T-Ltd]

Couple of questions Bob/Eric...
1)What are the advantages of Speed Density over the MAF systems?
2)Does one react faster?
3)Is there greater "tunability"?
4)Is idle quality different?
5)Does it operate under closed loop?

Sorry if any of these have already been gone over.

aaron

 

[TurboTdr3.8]

ETA on the chip , I hope you can make enough to not run out before all gets one .

 

[TurboBob]

Couple of questions Bob/Eric...
1)What are the advantages of Speed Density over the MAF systems?
2)Does one react faster?
3)Is there greater "tunability"?
4)Is idle quality different?
5)Does it operate under closed loop?

Sorry if any of these have already been gone over.

aaron

Some of this comes down to preference and/or opinion. But let me take a swing at it.

1. Speed Density has the advantage of "range", it is hard to build a MAF that reads well from 6 grams/sec (idle) to 800 grams/sec (fast car at WOT). MAF's experience flow reversals when tipping out of the throttle on a turbo car which affects driveability.

2. there is no practical difference in speed.

3. SD has more tunability only from the fact that it is a 3D table by definition. You can build a MAF system with the same level of tunability.

4. There should be no difference in idle quality when tuned properly. SD can have a "rolling idle" characteristic when the VE table is a little off at idle.

5. close loop is independent of the SD/MAF question, but in the case of the SD2 chip it is designed to be closed loop using the wideband. There is a mode built in to use the stock O2 sensor to tune part throttle with, but the wideband works better.

Bob