| T O P I C R E V I E W |
| Markley02 |
Posted - Nov 11 2011 : 12:05:08 PM
I actually didn't know if I should post this in electronics or under rx7 stuff. I am trying to install a hot-wire MAF into a 2nd gen rx7 TII. The car is running a RTEK 2.0 and I have a hot-wire MAF laying around. My issue is that the stock flapper reads 5 volts with no air flow and when maxed out with airflow reads 0 volts. The hot-wire MAF reads 0 volts with no air flow and 5 volts when maxed out. I started ask questions about this probably 4 years ago on another forum, but I never got around to it until now. Plus, this forum is probably a little better suited having both electronics and a rx7 sub-forum. I only have very basic electronic knowledge BTW.
It is my understanding that I should be using a LM358 to do this. I should wire it like a Inverting Amplifier correct? I think I am going to have to use variable resistors to try and get the voltages of the flapper and hot-wire in sync with each other. What range resistors do I get?
Yes, there are products out there that can do this already in the car world. The downside is they charge around $800 for it. |
| 4 L A T E S T R E P L I E S (Newest First) |
| Aaron Cake |
Posted - Nov 19 2011 : 10:20:17 AM
quote:
Originally posted by Markley02
There is a couple reasons why. First is accuracy. A hot-wire is way more accurate than a flapper AFM. It is even more accurate than a speed density setup. Plus, with a speed density setup you have to worry about your VE being thrown off all the time. Second, the ricer whistle demands a blow-thru setup so it wont run rich at shifts. Third is the limits of the RTEK. The RTEK is limited by the map sensor. Which only goes up to 1 bar. A hot-wire MAF in a blow-thru setup will compinsate for the pressure. This is where the hole thing might go down the tubes. I dont know what algorithms the ECU uses to determine load at this point. If I give the ECU a standard voltage coming from the MAP sensor, will it still be able to hit the higher load cells? Will that have an effect on it going into closed loop for idle and cruising conditions? I dont know at this point. Which is why I want to try and do this cheaply right now.
That's an interesting idea, but ultimately I think you're sort of looking in the wrong direction. The stock ECU is pretty crude. With an RTek, at least it's an adjustable crude ECU, but crude nonetheless with poor resolution. While a hot wire may be more accurate, the stock ECU has no way to care. It's going to treat the signal from a hot wire AFM the same as the flapper door AFM. You're feeding a mono crystal ear phone with a $2,000 amplifier. The ECU has no higher load cells than would would be considered 100% airflow on the AFM. The MAP sensor is basically just used for fuel cut and to stage in the secondaries. It has little effect on the fuel curve.
It would help if I knew what you were attempting to do as far as mods. Since you mentioned the 14 PSI limit of the stock MAP sensor being an issue, I can assume that you're looking at a turbo upgrade (since the stock turbo is basically just blowing hot air at that point). In that case, you need to stop, and move in a different direction. While an RTek is an option, it's really only good for stock turbos and hybrids, to about 300HP. The RTek only allows you to control 720CC injectors. Anything larger and it can't turn them down because it is still bound by the limits of the stock ECU. The problem with a stock ECU is not necessarily not being able to get it to belch out enough fuel (just drop in larger injectors) but the lack of timing control. The stock ECU timing curve is very aggressive. Big turbo cars need far less timing. Plus, tuning the transition of the injector stage will be a big deal. You will blow engines around 4000 RPM several times before you get it "right".
Standalone is the way to go. Speed density is fine. Or pick a standalone that supports an AFM directly (most do, and will run a hot wire unit). |
| Markley02 |
Posted - Nov 13 2011 : 3:53:32 PM
quote:
Originally posted by Aaron Cake
Does the hot wire MAF actually produce a 0-5V signal directly from the unit?
An inverting amp would be the easiest way. Here's a general overview:
http://www.play-hookey.com/analog/experiments/basic_op_amp_inverter.html
I'm sure the datasheet of the LM358 has an example of how to connect it up as well.
Just wondering, why? With AFM equipped 2nd gens making 400HP, it seems like a lot of work to go through.
Oh, an S-AFC is also capable of inverting an AFM signal.
For the most part the hot-wire is 0-5v. They sometimes top out a little over 5v, but that would probably be around 600hp. So in this application it would be fine.
There is a couple reasons why. First is accuracy. A hot-wire is way more accurate than a flapper AFM. It is even more accurate than a speed density setup. Plus, with a speed density setup you have to worry about your VE being thrown off all the time. Second, the ricer whistle demands a blow-thru setup so it wont run rich at shifts. Third is the limits of the RTEK. The RTEK is limited by the map sensor. Which only goes up to 1 bar. A hot-wire MAF in a blow-thru setup will compinsate for the pressure. This is where the hole thing might go down the tubes. I dont know what algorithms the ECU uses to determine load at this point. If I give the ECU a standard voltage coming from the MAP sensor, will it still be able to hit the higher load cells? Will that have an effect on it going into closed loop for idle and cruising conditions? I dont know at this point. Which is why I want to try and do this cheaply right now.
I tried reading the datasheet. I am not that good at this stuff yet. I might just have to research more.
quote:
Originally posted by audioguru
If you bias the (+) input of the LM358 at +2.5V and the voltage at its inverting input resistor is +5V then the output of the opamp will go near 0V if the output load current is low.
When the voltage at its inverting input resistor is 0V then the opamp output will go to +5V if the output load current is low.
The input resistor might load down the source voltage to it. We don't know how much resistance will load it down.
Maybe you could use the second opamp in the LM358 as a buffer amp with a very high input resistance (330k ohms) so that the source is not loaded down. Then the resistors for the inverting opamp are 10k ohms each.
Setting the bias at 2.5v did make sense to me, but I have no idea how to do that. lol. I have no idea what you are talking about when it comes to doing the buffer amp thing. I understand there is a second opamp in the LM358. Just something else I am going to do some research on.
Thanks guys. I should have stated in the first post that I am really good at cars, but I feel like a nun in a whore house with this circuit stuff. |
| audioguru |
Posted - Nov 12 2011 : 10:28:39 AM
If you bias the (+) input of the LM358 at +2.5V and the voltage at its inverting input resistor is +5V then the output of the opamp will go near 0V if the output load current is low.
When the voltage at its inverting input resistor is 0V then the opamp output will go to +5V if the output load current is low.
The input resistor might load down the source voltage to it. We don't know how much resistance will load it down.
Maybe you could use the second opamp in the LM358 as a buffer amp with a very high input resistance (330k ohms) so that the source is not loaded down. Then the resistors for the inverting opamp are 10k ohms each. |
| Aaron Cake |
Posted - Nov 12 2011 : 10:22:43 AM
Does the hot wire MAF actually produce a 0-5V signal directly from the unit?
An inverting amp would be the easiest way. Here's a general overview:
http://www.play-hookey.com/analog/experiments/basic_op_amp_inverter.html
I'm sure the datasheet of the LM358 has an example of how to connect it up as well.
Just wondering, why? With AFM equipped 2nd gens making 400HP, it seems like a lot of work to go through.
Oh, an S-AFC is also capable of inverting an AFM signal. |
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