So when the air is hotter or humidity is up how does it effect the motor. Do you have to decrease timing, add fuel, I'm kind of lost on this and today my car was running like a dog. It was about 95 and fairly humid, it reminded me of before I started modding it. What can I do to adjust these problems out.

 

i dont know if there s much you can do....outside air is a big factor in engine performance, especially with forced induction. there are more molecules of air in a cube of cool air compared to a cube of warmer air. more air you get per square inch, more power.

 

Hotter or humider (is that a word lol) = power loss since less dense air, or more correctly fewer oxygen molecules.

Check your AFR's under these conditions, if they're still around the high 12 to 13.x range then you cannot do much about it assuming timing is already optimal for that AFR (other than trying to somehow cool down the air). N/A cars will be affected by the air, much more so than FI. If on the other hand you see that it's richer then take out some fuel to account for the air temp (ie do you have an air temp correction map? I can't remember what all the Utec has for standalone mode off the top of my head now).

 

lol run your AC into the air intake

 

The UTEC has temp correction mode, however I haven't touched it yet. I'm not sure what it runs off of, I would assume the IAT but I'm putting a call in tomorrow.

 

Rather simple question: assuming for a minute that you cannot do anything to counteract the effests of temp and humidity (or for that matter, barometric pressure), does a MAF explicitly factor in these things ( i.e., temp, humidity and baro) in measuring the "mass" of the air flow? Is not the "mass" of the air flow what affects the engines potential (assuming proper fuel ratio and spark)?
I've got to (naively) assume that the manufacturers, in using a MAF, do so because it can take all of those factors into consideration in measuring the Mass Air Flow into the engine as the car is driven across a whole range of atmospheric conditions.

 

OK guys Im brainstorming here, so jump in and comment/ fix as you see fit...
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With hotter air, I read less slightly less AirFlow, but thats like from a day with 85*day/ 65*night. In that situation I could see a difference of ~2% in airflow just by the air temperature change alone.

- MAF detects LESS airflow (hot air), ECU will indirectly advance timing, but air is hot (balancing out?)
- MAF detects MORE airflow (cold air), ECU will indirectly retard timing, but air is cold (balancing out?)

Obviously, off the ~2% difference in airflow, that is a negligible number and it shouldnt do anything noticeable really, same with its minute effect on indirect ignition timing.

BUT.... the car still feels better when the air is cold, so there's SOMETHING going in the engine! Based on my notion, its got very little to do with measured airflow.

According to the 1995 FSM, ignition timing is determined/ affected by...
- CPS (POS)
- MAF
- ECTS
- TPS
- VSS
- Ignition Switch (wtf? I guess to know that the engine is running? lol)
- KS
- Neutral Position Switch
- Battery

So according to that, IAT sensor isnt taken into account for the purposes of Ignition Timing.

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OK thats my brainstorming, anyone here want to bust my chops, you're more than welcomed.

 

I wouldn't think so since the MAF is measuring air flow not air density. The correcting sensors would be the IAT sensor(maybe) and front O2s.
I've noticed my car running richer is this heat/humidity, and since my O2s are unplugged, I notice the AFR differences in the morning when it's cooler compared to afternoon when it's scorching on the same EMU map points.
I haven't logged base timing in this weather yet though.

 

one more thing (again just thinkin out loud)...
- Hotter air going into the CCs raises temps during combustion. Perhaps KS detects something and dials back timing a bit? and obviously colder air will push back that "threshold".

^^Thats a given that it happens. Real question is, is it something worth noticing or is it happening at such a small level that its negligible? Timing logs comparing same car with day and nite would be great for this

EDIT- according to 1995 FSM- IAT Sensor is "...not directly used to control the engine system. it is used only for the on-board diagnostics."

Yes on some car, GMs for example, IAT is indeed used to trim timing curves. There are sample maps laying around the internet of that sensor's effect on timing advance for the LSx motors. But its not the case for us it seems.

 

It measures MASS, so it takes volume and density into consideration all at once based on how much the "hot wire" is cooled.

 

Timing logs of same car comparing hot day / cool nite please . Lets get to the bottom of this like that, unless someone has some great input to add in this subject matter.

EDIT- oh yea, please overlay the line of IAT sensor's reading over the timing lines, 2 graphs preferably please for ease of reading.

 

LOL, that's what I get for thinking.
Thanks

 

There's a reason it's called a MASS airflow sensor guys...

 

So are you guys (DandyMax, Nismology - the deep thinkers) agreeing with post #6? In other words, cool air not only helps to cool the "hot wire," but (all other things being held constant), humid air ought to also promote more wire cooling. Conversely, with temp and humidity held constant, thin air at high elevations will reduce the wire-cooling effects. All of these will affect the signal that the MAF sends the ECU about the "mass" of the air being sucked/pushed into the engine.
Again, maybe we're all beating our gums about something we can't do anything about, but a better understanding is worthwhile to minimize the heresay and promote better understanding.

 

I agree with you except for the "humid air ought to promote more wire cooling" thing. Humid air is less dense. Less dense = less mass at equal volume so the cooling effect on the hot wire goes down. It's easy to mistakingly think that the water vapor itself would remove heat but it removes heat when it changes phases from liquid to gas. In order for it to do that, it would have to condense on the hot wire then re-evaporate. That could never happen because the temperature of the hot wire will always be higher than the temp of the vapor. But I agree with everything else.

 

I went over some logs from the 7th where I had some serious knock. My testing is always at night so it's cooler but I pulled over at a gas station to get a drink and left the car running for about 10 minutes. I jumped back on the highway and hit it and knock was all over the place. Looking back my IAT's where at 100 from idling while normally they are 65-75 for tuning. So would it be safe to say I should lower my timing when temperature is above X to avoid the knocking. Also the AFR seemed to register lower than normal.

 

How was the timing?

The parameters that, in my experience, have had the most affect on spark knock are ECT/IAT/AFR/Ambient temps.

 

Could you physically hear knocking, or was it just the knock indicator on the UTEC, was timing actually retarded by the ECU?

What kind of gas are you running? (ie octane)

I have run full WOT runs numerous times at the track at around 90-100F IAT temperature, and I'm able to run full timing advance on 94 octane, even though I will be a couple tenths slower from the less dense air.

 

I didn't hear anything out of the ordinary however the highways loud and the motor screams so who know's. How obvious is real knock. I run BP premium which is usually 91 octane out here. The UTEC is in full control of timing and knock so the ECU would have nothing to do with it but the UTEC will pull timing when knock is detected. Last night I lowered my knock parameters to see if that fixs it. AFR looks fine and I still haven't logged ECT but don't will start doing so. Last night I had zero knock but didn't let it sit idling for 10 minutes before I hit it.

The ambient was about 76 the whole night, the IAT varied from 78-105 depending on if I was moving or idling at a stop.

 

Here's snippet of my timing, remember it's hard timing.

2544 27
2701 27
2955 27
3274 25.5
3809 23
4357 23
4450 23
4480 23
4708 23.1
4610 23.4
4688 23.8
4775 24
4830 24.3
4901 24.6
4997 25
5073 25.3
5144 25.5
5224 25.8
5282 26.2
5367 26.5
5446 26.6
5500 27
5564 27.3
5659 27.6
5698 27.9
5777 28
5827 28.3
5878 28.6
5973 28.8
6038 29.1
6105 29.3
6301 29.6
6218 29.9
6265 30.1
6301 30.4
6397 30.6
6422 30.7
6459 30.8

 

That's something I don't like about hard timing. The ECU will adjust the timing based on MAF voltage and other consideration but hard timing throws that out the window. I think you're gonna start having to correct for ambient conditions/load and other variables like the stock ECU would.

You should really start by dyno'ing to see where peak torque is and making the timing curve the reverse of what your torque is (least timing @ peak torque and vice versa), then go from there. Seems like timing was just added wherever. A slow ramp up from low RPM doesn't seem optimal to me at all.

 

It's 27 from 2k up to 3k and then dips down to 23 for the shift point and then ramps back up to 31 at 6.5k. This somewhat mimicks the stock curve but much more aggressive. I would raise it as far as it would go without knock with road tuning. I drive for at least an hour while tuning. There closest dyno is about and hour and a half and I have no references for their ability nor would I know what to adjust for when I was there.

 

I have nothing to really add to this thread but I have always wondered how this effected performance. My car felt like a dog on the drive home today @ 90F typical humid summer FL day. This would most likely kill 1/4 times on the same car with the same launch on days with different weather. My car feels like a completely different animal when it cools down around here.

 

I talked with TurboXS today and the temp correction works off the IAT. He also said with a stock exhaust the timing is probably to aggressive which I kind of thought anyways. After doing some more research into the knock I have come to think it might be related to heat soak in the intake manifolds. This would make some sense as the IAT reading has nothing to do with the heat inside the manifold and once I'm up to speed for a few minutes the manifold would have cooled of a bit lowering combustion temps. Does that sound feasible.

 

Humidity plays a very small roll in engine performance, though it is a bit of a factor. Temperature and barometric pressure are what count the most. Aircraft takeoff performance calculations (which are a daily ritual for both the big guys and for small general aviation aircraft operating from runways at which you could have a runway length issue) ignore humidity entirely, temperature and barometric pressure are the variables they use. Humidity just isn't a significant enough factor.

For instance, assume we are at 1000' elevation and the temperature outside is 75F, and the barometric pressure is 29.92. A day with 0% humidity would see a density altitude of 2261'. Bumping the humidty to 100% and leaving the rest the same results in a DA of 2649'. You'd probably be hard pressed to notice a difference even on a timeslip unless every run you make is deadly consistent, I mean within a few hundredths of a second of the others. You change the temperature 10F in either direction and a consistent racer will definately notice a difference on the timeslips, same with barometric pressure.

Also keep in mind that relative humidity is a strange animal, it is dependent upon the temperature. a swing from 0% to 100% RH will have a larger effect on the density altitude the hotter the temperature is. At 45F - that 0-100% humidity change results in a 135' DA swing, at 90F, a 0-100% swing when it is 90F out results in a swing of like 637' DA. This is because relative humidity is based upon the air temp in that hotter air can hold more water vapor. RH is a measure of what % of water vapor is present in the air as compared to the maximum amount that the air at that temperature is capable of holding.

So that got pretty long and rambling and I'm sorry for that but here's the basic jist - if you're basing your decision to go to the track on the weather conditions - don't worry about high relative humidity, you probably won't even notice a difference between a 'humid' day and a 'dry' day. you will however notice a difference between a high temp day and a low temp day, or a high pressure day and a low pressure day.

 

Here is an excerpt from the POH of one of the aircraft I typically fly (Cirrus SR20 G2). I've circled the fuel flow and temperature. Piston aircraft have fuel flow (AFR) that is adjustable by just moving a lever. As you can see, colder days use more fuel at the same engine setting than hotter days do. That said, in your car you want to balance how much fuel you pull as the temp goes up with the fact that hotter temps can also mean more knock, so you have to be prudent so as to not pull too much fuel and introduce extra knock which is obviously a bad thing.

 

I would like to see someone do that lol
I wonder if the gains would outweigh the loss, if it's even possible.....

 

The engine sucks in a lot of air, I dont think a stock A/C could knock off enough temperature at that flow rate, but still it would be interesting.

 

You guys do realize that the AC compressor is clutched, right? Meaning that when you go WOT the compressor kicks off and you lose your AC... making this whole discussion quite moot. Even if it weren't, mikekantor is right in that the engine consumes far too much air for the AC system to provide any significant cooling with the piddly amount of cool air it kicks out.

 

Thats an electronic feature, and can be disabled, but yeah its a moot point.

 

Has anyone played with water injection to cool the intake charge and lower combustion temps?

 

That's a good point, never came across the topic on the forums. I know that it does help a bit with NA and more so Boost, would luv to get the VQ guys take on it....

 

I thought that its only for situations in which you push the engine so much that you get detonation. Seems like an extreme approach to me, especially with all the extra plumbing involved.

 

I agree, for a street NA it's of no use.

 

This website allows you to correct your drag runs to adjust for temp and elevation. When it is humid the air is more saturated with water and carries less oxygen. Same as with the higher the elevation the less oxygen.

http://www.greatlakesdragaway.com/co...onfactors.html