I have been datalogging pretty religiously and noticed that although my coolant temp is pretty contant due to fans, the intake air temperature seems to fluctuate from run to run. I noticed crappier times when IAT is over 100* I assume there is some adjustment being made by the PCM when it reads high IATs which adversely effect drag strip performance. I don't understand how IAT works completely. It is just based on the temperature reading of the IAT sensor in the intake tract right? Seems like on some runs it is inexplicably high. The temp also tends to climb fast on some runs and not on others (don't know if ambient temps are responsible). I assume cool air sucked into the motor cools the IAT sensor although during burnouts it seems that IAT goes up pretty quick. Once the car is in motion it goes down pretty quick. I have a RDP intake by the way which is powdercoated black and has a black silicone elbow where the IAT sensor is located. I wonder if having the hood up for cooling has the unwanted effect of causing the intake to absorb heat from the sun causing the IAT to go up. I have iced the intake on occasion but actually believe now that the thing to do is to ice the area containing the IAT as this will probably have a bigger effect. Any opinions here? I'm starting to see why folks are interested in relocating the IAT sensor to an area with ambient air and no sunlight.
Yes. IAT is taken from the sensor at the elbow. The PCM expects the sensor to be approximately 4" from the throttle body. Yes. Speculation is that the PCM starts pulling timing as the IAT goes over 90*. I've never been able to verify that. There's no doubt that hotter air is less dense, so less O2, so less power. You IAT is going to be determined mainly by the temperature of the air passing over it. The reason you get very high temps standing still or at slower speeds is that you're sucking hot air from the engine bay as well as ambient temperature air from outside the car. At higher speeds there's enough air coming through the front of the car to pretty much keep the heated engine bay air flushed. There's always going to be some difference from ambient air since the radiator will heat the air somewhat and your intake sucks some of it's air from the engine bay. Personally, I'm pretty skeptical that icing your manifold does much of anything for intake air. I do believe that it helps cool down your engine. The manifold is probably the largest heat radiating surface area on the motor when it's sitting still and off. Common belief is that the PCM also pulls timing/performance when the coolant temperature goes over 190*. Cool the whole engine down and you have lower air temps in the engine bay and cooler air getting sucked in the intake. About icing the manifold (and maybe even moving the IAT sensor) thinking that a cold manifold cools the air passing through it. Your engine displacement is 6.1L. The engine is moving something close to that volume of air every 2 revolutions. 4000 rpm: (2000 x 6L)/60s = ~200L per second? 6000 rpm: (3000 x 6L)/60s = ~300L per second? It turns out that your manifold has about the same volume as the engine. That means that any single bit if air takes about 0.018 to 0.03 seconds to pass through the manifold? I don't know, but I'm wondering just how much heat that volume of air can absorb in that amount of time... I think the volume and time spent in the air intake might be about the same? (Cam was going to take readings with a probe to characterize all this but wussed out when it came time to drill his manifold). I don't know if the IAT sensor readings are damped or how fast it reacts to temp changes.
Thanks Dave. I'm going to keep a close eye on IATs next time at the track and play around with Ice and wet towels to try to keep the intake tract cooler. Maybe I'll try just dumping an ice chest in the engine bay and when I start the car it will act as a big blender
I think the first thing is to try to get a handle on the effects and what the PCM may be doing. 2 things. IAT and ECT. It's possible the PCM also reacts to oil temp, trans temp, etc. but if it does, I think that would be secondary. You're concerned with the standing 1/4 mile. Log IAT, ECT, Spark Cyl 1, and AFR along with your other stuff. ECT: Our coolant systems are massive but it's dependent on the volume of air passing through the radiator. The SRT engineers said they tested full throttle runs through the desert in over 100* heat for a full tank of gas (35 - 40 minutes?) without the coolant boiling over. They would have been going well over 100 mph with plenty of air moving through the radiator. If you log ECT in a 1/4 mile run you can watch it climb steadily through the run and go well over 200*. I'm not sure what the PCM does in response to this? If it's timing, you should be able to see your timing change during the run. I'm guessing it wouldn't change fuel for ECT but I don't know. IAT: Cooler, denser air has more O2 and can burn more fuel. Without changing anything else, you would expect to see your AFR change with IAT. Same amount of fuel but denser or thinner air. If the WOT added fuel is a fixed amount, then you would expect to see a richer AFR at higher IATs, leaner AFR at lower IATs. The PCM adjusts fuel while in closed loop to get 14.7:1. It does not monitor AFR in open loop (WOT). In WOT it adds a fixed amount of fuel from the "PE" table (this is what you adjust with the Predator or in a tune). Assuming no other changes are made you would expect to see your AFR change during a 1/4 mile. Let's say it's cold out. The engine runs in closed loop (only takes a few seconds) and adjusts fuel for 14.7 with 60* air. Now you go WOT and add X amount of fuel from the PE table. Let's say that adding X (fixed amount) equals adding 10% fuel and in WOT you get 12.0 AFR. Now let's say it hot out. The engine runs in closed loop and adjusts fuel for 14.7 with 90* air. It's going to take less fuel since the air is less dense. Now when you go WOT and add X from the PE table (same amount as before). Since your "base" fuel is less now, X might be 15% more fuel and you'd expect your AFR to be richer. I'm thinking if you don't see that then the PCM must be adjusting fuel in response to IAT (a fixed amount relative to IAT). Remember it does not monitor AFR in open loop. Then again, I don't know how the PE table works. Is it a fixed amount of fuel that's added to the base or is it a percentage increase? Ok... maybe the PCM reacts to ECT. I think the major factor is going to be IAT. From the previous post I think the overwhelming factor is going to be the temperature of the air ENTERING the CAI. Until we find out different, I don't think the air temp changes much between the air filter and the intake port in the head. I could very well be wrong, but I don't think there is enough inside surface area in the CAI and manifold to conduct enough heat fast enough to raise the temp of that volume of air at the rate it passes through there. (Your radiator is much more efficient due to the huge amount of surface area of the fins). I don't think icing the intake and manifold (or insulating the CAI) has much effect on the air passing through them. I don't think it's going to cool the air (or heat it) as it passes through there. Getting the entire engine cooler WILL drop the engine bay air temperatures so the air will be cooler entering the CAI. I think, far and away, the largest gain would be achieved by drawing air only from outside the engine bay. CAI inlet down on the ground in the front wheel well or better yet, sticking out completely in front of the car well above the hot asphault (in the sunlight the asphault is radiating a lot of heat and it's much hotter down close to the track). Straight up through your hood would be ideal. Relocating the IAT sensor. People claim to see lower IAT readings by moving the sensor towards the filter and away from the TB/engine. If this is true then the implication is that the temperature of the CAI it's attached to effects it. Let's just take the extreme and say you attach your IAT sensor to the outside of the grill. Now the IAT reads ambient air temp. It does NOT read the temp of the air entering the engine. IF the pcm adjusts WOT fuel in response to the IAT we would expect to see a richer AFR. PCM thinks the air is denser than it is. This might have some small effect but you still have less dense air. Less O2 so less fuel burned and less power. Again I believe the actual air temp would be the most significant factor and fooling the PCM might only give marginal results at best. What would hurt you is if the IAT is reading the temperature of the CAI material and the air passing through it is much cooler. This is a possibility. Is that little blue temperator sensor that sticks out into the CAI insulated from the temperator sensor housing? That I don't know. I think if this was the case you would expect to see leaner AFRs at WOT. I expect all this was factored in by the engineers. Bottom line: You need to get cold air ENTERING the CAI. I think fooling the PCM, cooling or insulating the CAI housing, cooling or insulating the manifold, etc., is marginal at best and probably pointless. Maybe you want to make sure the IAT sensor is reading the air temp and not the housing temp once you get cold air going through it.
Believe it or not, I actually understood most of that:hypno: My take home lesson from my datalogging is that low IATs are important to a good 1/4 mile run. There is some argument as to weather the PCM adjustments happen at 65* or 90*. Either way those first few seconds of the run are critical and if the IAT is high the run will suffer. The IATs will not drop from a burnout (they'll go up). The car needs speed to clear the hot air from the system.
