-   General Efficiency Discussion (
-   -   Testing the IAT mod (

tasdrouille 05-09-2008 10:41 PM

Testing the IAT mod
So I decided to give a shot at the IAT mod on the TDI. In fact for this car, it's the FAT mod (Fuel Air Temperature) as we also have a fuel temp sensor to play with.

I used switches so I could toggle the mod on and off without too much hassle.

Here's the resistor attached to the fuel pump harness. Inline on pin 7 in my case. The other 2 wires are bridging pins 2 and 3 with a 1k pot, fooling the pump's piston movement sensor so I can dump in more fuel (hardly useful for FE...)

And finally the IAT sensor which is located post intercooler.

I did a couple of tests run.

Stock timing for the TDI is supposed to be between -3 to 4 at idle and between 8 and 12 at 3000 rpm. My car, as per my SG, was at -1 or 0 at idle and 8 at 3000 rpm, a bit on the retarded side.

With the mod on, my reported timing jumped to 2 at idle and 11 at 3000 rpm.

Here's a very boring video showing what happens when I switch the mod on and off at idle. Basically you'll see the timing switch from 0 to 2 and the temp from 85 F to -15 F when I switch the mod on.

I'll report on FE as I get some data.

SVOboy 05-09-2008 10:45 PM

Looking forward to seeing your results!

tasdrouille 08-21-2008 08:47 PM

1 Attachment(s)
I got my new toy in the mail earlier this week and decided to try to log some data to evaluate the impact of this mod on FE.

Here's a graph showing the injected quantity in mg per stroke during the course of my test (X axis in seconds). The black line is a moving average based on 60 data points (seconds).

I just did an A-B test as it took almost an hour of driving. The cruise control was set at 90 km/h the whole time and the car was never turned off. Run A1 started at a coolant temp of 85.5 Celcius and 90 Celcius was reached at the end of the run. All remainning runs were done at 90 Celcius.

During run 1, the ECU thought the intake air temp and fuel temp were -23.4 Celcius.
During run 2 the ECU saw actual intake air temp and fuel temp which were respectively averaging 45.22 and 50.4 Celcius.

The average injected quantity in mg per stroke was as following:

A1 - 11.9172
A2 - 11.2812
Var = 5.3368%

B1 - 8.4111
B2 - 7.9727
Var = 5.2121%

Let's average the variation at 5.5% to be on the safe side and account for unknown errors.

So it looks like the injected mass was actually higher with the mod. But wait, the ECU thought the temperature of the fuel was way cooler than what it really was and ajusted the volume accordingly.

Diesel volume varies approximately 0.125% per degree Celcius. The ECU thinking the fuel was 73.8 Celcius cooler than it really was, it should have adjusted the volume by approximately 9.225%, but let's use 9% just to be safe.

So, for any given mass the actual injected volume should be 9% smaller with the mod (at least during the summer), but since the injected mass was 5.5% higher the theorical net result is an increase in fuel economy just shy of 4%.

DonR 09-24-2009 11:45 AM

Do you have plans to use variable resistors to be able to find the "optimal" setting?


tasdrouille 09-24-2009 12:08 PM

In the end the fuel temperature had absolutely no impact on ignition timing, just the IAT had an impact.

The optimal setting is as much advance a you can get as long as you're still within spec. At idle max spec advance is 5 btdc. Whatever resistance you use will not give you more than 2 additional degrees.

Formula413 09-27-2009 01:05 PM

I tried playing with the IAT on my car. I actually found a potentiometer that could plug right into the IAT pigtail. I wasn't able to see any FE benefit or increase in timing, but the pot I found would only allow me to dial in an IAT as low as 40*F, so maybe I just needed to go lower. I can't remember if less resistance=colder IAT or warmer IAT, I would assume colder but I'm not sure.

tasdrouille 09-27-2009 04:09 PM

The higher the resistance, the colder the IAT.

All times are GMT -4. The time now is 05:33 AM.

Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2021, vBulletin Solutions Inc.
Content Relevant URLs by vBSEO 3.5.2
All content copyright