Frost

iTrader: (45)

Alright.... so it seems I find myself responsible for keeping several cars with big-hits/stock-bottoms together and have a few tuning questions.

My current platform is HPT.

I was looking at the IAT/spark adder table and the bottom of the table is 14degF. I remember reading about someone grounding their IAT a while back. Grounding it's PCM input SHOULD force us into the last column (again, 14degF) on that table correct? It would be excellent if this were the case... just use one extra set of arming contacts to ground the sensor and we can tailor timing reduction VS.RPM.

I guess the real question is, what OTHER tables am I going to affect by forcing the IAT sensor low? I really didn't see any others along the way that would be significant.

jimmyblue

iTrader: (11)

IAT low will also affect the speed density air mass
computation. However "tricking" it to be more like
true (cold) air temp is better than "tricking" it to look
like hot (which people also do, to jack timing).

If I recall, the IAT thermistor has a higher reading
cold. So I don't think straight grounding it is what
you want. More like a series resistor inline with the
IAT and a normally-closed relay shunting that bit,
which 'noid power causes to open up and add the
resistance to IAT sensor value.

You have timing and PE adders that see IAT and
you can use the IAT adder to bump fuel if you
want.

Frost

iTrader: (45)

I'm gonna have to do some legwork then...

I was really thinking of more of a way to allow a max effort NA tune and then "switch over" timing (fuel isn't as critical; we can swap jets) without running a timing tuner box or full-time nitrous tables.

Wait.... thermistors' resistance falls with temperature... I remember making a time-delay circuit with one using a pot and a VDC source alone in series which takes advantage of a situation that in most cases you would wish to avoid. The initial voltage is high, but as the unit "self-heats" the flow of current increased until enough current was present to pull in a relay coil. The unit's resistance should be *VERY* high when it is is coldest. Assuming the PCM is looking at a voltage rather than current then the tie to ground should hold it to the bottom of the scale right?

jimmyblue

iTrader: (11)

There are NTC and PTC thermistors and the IAT/ECT
senders look to be negative tempco (Helm book pg
6-1377 has the table -100700 ohms at -40F, 9420 at
0F, 2796 at 77F, 177 at 212F etc.).

Frost

iTrader: (45)

I called mightymouse after remembering hearing him talk about doing some work with them back in the day... He linked me to the same info you have there. Turns out our ECT sensor follows the same resistance curve.

The bottom of the IAT table is 14degF and the value is about 16.2K ohm at that point. I'm worried about going much over target value since it's at the bottom of the table; any idea where one of these things goes to if it's declared in fault below accepted range? Hopefully it's not a failure, it just sits on the last column of the IAT spark adder VS. MAP.

I'll come back and update with results if anyone is intrested.

brad8266

iTrader: (23)

I think youre mistaken because people install resistors with the IAT to trick the PCM into giving them more power, which comes from additional timing.

brad8266

iTrader: (23)

Why not just get a nitrous timing tuner box and be done with it, no trickery involved?

Frost

iTrader: (45)

No.... not unless specifically tuned for it. As a matter of fact, if you get the IAT into effective high area it's gonna pull time. There is nowhere that I have seen in that table that EVER gives timing, only pulls it.


Those tuners ARE trickery.
It's a no brainer... They are expensive, they are an extra active [somewhat] complicated device, and they cause unexplainable issues on some peoples setups (misfires and other odd codes)... And if you can do it from the PCM rather than tricking the PCM.... well.... Not to mention that this will cost about 10 bucks with weatherpack connectors. Those timing tuners that intercept the CPS and delay it are REALLY expensive for what they are.

I am going to make a tiny boxed device with male and female weatherpack connectors so that it plugs right up by the IAC. When the nitrous is armed, a relay will switch out the IAT for a fixed resistor and give us a little LED on top on the contact side of the relay so that we know it's active and working.

When it's active, the IAT Spark Adder Vs. MAP's far left column (14degF) from 80-100kpa can be used to specify the degrees of retard for your nitrous shot.


JIMMYBLUE:
Looking at my buddy's 2000 Z files I can't find anything related to PE vs. IAT , only a PE modifier for ECT. Where should I be looking; the PE tab in HPT doesn't exactly have a lot on it, so I don't feel like I missed it...

brad8266

iTrader: (23)

Can you explain that to me a bit more, im interested in what you are talking about. I want to run advanced timing NA and then have it pull timing on the spray. If I can do that with tuning that would be nice since I have tuning software.

Frost

iTrader: (45)

I'm going to (when nitrous is armed) "replace" the IAT with a fixed resistor that forces the IAT into thinking it's as low as it can go. As far as I can see, outside of startup, the IAT at WOT only is examined in 1 table. That table exists solely to pull timing if very high IATs are reached; most of the table is 0. The table adds to your current spark map. Once the resisitor is brought in, we are locked on the far left of the table (IATs of 14degF, single coulmn) where you wouldn't normally end up (maybe if you live in Antarctica) and in these 3-4 cells (80ish kpa and up) you set the retard you want to have active while the nitrous is armed and it is subtracted from your ideal N/A timing table.

Keep your peak N/A tune and automatically pull timing just for nitrous without something that is intercepting the CPS and delaying it... and also costing a bunch of money.

brad8266

iTrader: (23)

I see, thats an awesome idea, i will have to look into trying that out and how to wire the arm switch so that the resistor will cut out the actual IAT when armed. That way i can knock like 5 degrees off just by arming the system. I cant believe I havent heard of this before. You didnt come up with it yourself did you?

I am getting a new H/C motor in like a week and plan to spray huge shots on it, so doing this would be very easy for me to tune to prevent knock.

I might go out and pull my IAT off and measure its resistance when hot and after it sits in the freezer for a while.

brad8266

iTrader: (23)

I was just going through my tune file and noticed that you can also adjust PE based on IAT as well, so that means that I would be able to pull a few degrees of timing as well as add some extra fuel when the nitrous switch is armed. Im liking this more and more already.

12secSS

iTrader: (30)

Although that doesn't work as fast as a switch. The nitrous slowly starts cooling off the IAT sensor, while the ECM slowly adjusts to supply more fuel based on the IATs. I have tuned some nitrous cars like this, but I also need to find a better, quicker way of adding fuel ... without multiple tunes.

NicD

iTrader: (7)

I built and ran an IAT tricker that activates off the arm switch about 3 years ago when I ran nitrous on my camaro. It's basically a double pole relay that when just sitting there it routes the signal through the IAT sensor but when active it goes to a resistor instead. I then use the IAT vs timing table to remove timing for nitrous. Obviously this renders the IAT sensor's normal job of pulling timing useless so I just make sure I pull enough to compensate for the different weather conditions. I now use this on my GTO and it still works. I had the guys at HPT track down another table for me in the 04 GTO cals that's a second multiplier based on ECT vs IAT so I could set it lower. The factory table that you can't see in the current production version of HPT only becomes active at 165 or something in that area. Below that the IAT timing retard just doesn't work. Just as a tip as well set it to pull like 14 deg in the idle range. This is a sanity check to make sure it's doing it's job before you spray as you will hear and feel the idle drop while sitting there when arming the system. PE vs IAT works instantaneously. Oh yea this works best on a MAF setup because the IAT changing drastically doesn't have as much of an effect on fuel trims, etc as it does in SD. Anyways I just rambled on here LOL!

dmoss69

iTrader: (6)

Frost

iTrader: (45)

I thought I came up with it myself until I did some searching

I was reading about dry nitrous tuning and realized that there must be a physical way to instantaneously force the IAT sensor low.

WHERE IN HPT IS THE IAT vs PE table????????? I am looking at an '00 file and I just don't see the table...

I think I'm gonna buy about dozen sets of weather pack connectors, small project cases and the parts to put them together. 1 small touch would be an LED on the OUTPUT CONTACT SIDE of the relay so you can be sure the box is ready to go.

I'm stoked.

brad8266

iTrader: (23)

i use ls1 edit and its righ there in the PE area.

NicD

iTrader: (7)

I don't believe that table exists in some of the GM cals.

brad8266

iTrader: (23)

I have a wet kit so cooling the IAT isnt a concern, and the IAT PE settings work juist like the timing one does, it should be instant.

12secSS

iTrader: (30)

Some 2000 and earlier PCM do not have it, but 2001 and newer do ... as well as Enrichment Rate.

Oh, don't get me wrong, it works good. But I am comparing it to say when you go into PE Mode and how quickly the fuel enrichment is changed. That is what I would like to achieve. The fueling does come perfectly in line by 4000 rpms if the run was started at 2400 rpms using solely the PE vs. IAT. I just hate the initial lean spike.