I've heard the 6.4 Hemi is sensitive to humidity levels. What I think I'm seeing is high humidity having a noticeable impact on the 1320's 60s, even if the DA is good. Also, if the humidity is increasing even though the DA is dropping, the car isn't really picking up.

I'll post data later, but am interested if anyone else has experienced or suspected a similar pattern. Thx

So after pulling some data and digging into it a little deeper than usual, I think what I'm seeing is the difference in grains of water in the air for atmospheric conditions that come up with comparable density altitude values.

This is the identical car combination run at two different tracks this year, only one during spring and the latter during late summer. In both cases the track prep was very good and traction was not an issue. I ran 16.0 at MD; at Capitol I started at 19.0 and worked up to 21.5 cuz the car was hooking so well.

4.30.2022 at Mason-Dixon (elevation 541’)
-- 60 ----- 1/8 ----- MPH ----- 1/4 ----- MPH ----- Std to sea level ----- Atmospheric conditions = DA / Gr H2O
1.755 --- 7.623 --- 89.35 --- 11.930 --- 113.52 --- 11.94 @ 113.53 --- 59* / 24% / 29.42" = 650.6' / 18.0 gr
1.753 --- 7.674 --- 89.04 --- 12.007 --- 113.09 --- 11.99 @ 113.35 --- 61* / 22% / 29.40" = 802.7' / 17.7 gr
1.773 --- 7.685 --- 89.04 --- 12.012 --- 112.95 --- 11.97 @ 113.50 --- 64* / 21% / 29.34" = 1076.4' / 18.8 gr
1.769 --- 7.710 --- 89.00 --- 12.045 --- 113.75 --- 11.99 @ 114.41 --- 65* / 22% / 29.32" = 1176.9' / 20.4 gr

9.16.2022 at Capitol (elevation 118’)
-- 60 ----- 1/8 ----- MPH ----- 1/4 ----- MPH ----- Std to sea level ----- Atmospheric conditions = DA / Gr H2O
1.797 --- 7.799 --- 89.21 --- 12.164 --- 112.94 --- 12.09 @ 113.73 --- 71* / 61% / 30.00" = 1158' / 68.8 gr
1.796 --- 7.776 --- 89.92 --- 12.130 --- 113.52 --- 12.08 @ 114.17 --- 66* / 73% / 30.02" = 817.3 / 69.3 gr
1.798 --- 7.806 --- 88.96 --- 12.175 --- 112.83 --- 12.13 @ 113.39 --- Hot lapped
1.766 --- 7.732 --- 90.18 --- 12.099 --- 113.00 --- 12.07 @ 113.42 --- 65* / 81% / 30.03" = 779.8' / 74.3 gr

My hypothesis (?) is the higher water content in the air during the second session hurt the performance, although the higher tire pressures offset the corresponding MPH loss expected down track. Does this scenario fit w/ anyone else's experiences? Thx

Pics?

Humidity molecules displace O2 molecule's, they make the O2 spread out further away from each other resulting in less O2 in the combustion chamber resulting in less power.
Basic aviation training: grin: A.K.A., cold and dry air for best results in racing and flying Hot and damp air, no good

Yeah, I'm guild of having ASSumed comparable DAs meant comparable performance, simply cuz DA uses temp, elevation & humidity in the calc. But "today's lesson" is that just cuz the DAs are close doesn't mean the quality of the air (oxygen content) is going to be similar from an ICE's perspective.

Trying to compare, analyze and correlate data acquired at 2 different tracks, 5 months apart, on something like the effect humidity has on your combination, is not going to provide definitive conclusions.

Not without having a large number of runs at both places to determine how much difference it makes just by being in a different zip code. No two tracks are the same. Heck, even being in different lanes at some of these places can make a few hun difference.

Humidity and it's effect on ET (horsepower) is not linear. The higher the humidity, the bigger the difference is for a similar amount of change. 20 to 30%, not so much. 80 to 90%, a bunch. The combination has a lot to do with it. Cubes, heads, compression, fuel, and bunch of other things.

Actually the consistency between the two test sessions, given the differences between the two, is impressive.

Many laps and good records help to provide a clear picture of what your combo is doing.

As a long time user of Methanol I definitely agree....DA gives me a pretty good idea of what the car will run, but I have to account for the moisture in the air.

As a old time superstock racer once said, "you cant burn water....."

Where this really comes into play is running into the evening when the temps and the DA might drop but the humidity starts to climb.

I'd have to brush up on the math behind it, but the water displacing oxygen molecules makes more of a difference at lower engine speeds, I'm not surprised it mostly affected the 60'.

Also worth mentioning, the modern stuff adjusts timing based on air temp, so those losses are going to be in addition to what HP you lose just from the change in DA.

My primary reason for looking into the data was because of the change in 60 ft times. Here's another set of data from 4.9.2022 at Capitol w/ the same combination of tires / wheels / air filter as the other two listed above... and the 60-fts here are more in line w/ the other spring testing at Mason-Dixon than the last session at Capitol. FWIW, this was the first day w/ the new tires.

The DAs during this session started under 400' and dropped from there as the evening wore on, so these are the "best" atmospheric conditions the car has been run in w/ this combination.

-- 60 ----- 1/8 ----- MPH ----- 1/4 ----- MPH ----- Std to sea level --- Notes
1.728 --- 7.705 --- 90.32 --- 12.031 --- 114.44 --- 12.07 @ 114.19 --- 1:45 PM; car still hot from drive
1.714 --- ------ --- ------- --- 11.833 --- 115.80 --- 11.87 @ 115.61 --- 4:05 PM
1.805 --- 7.661 --- 91.91 --- 11.962 --- 115.26 --- 12.01 @ 114.91 --- 5:58 PM; launch felt "soft"
1.742 --- ------ --- ------- --- 11.918 --- 114.09 --- 11.96 @ 113.62 --- 7:13 PM
1.746 --- 7.594 --- 91.94 --- 11.893 --- 114.38 --- 11.94 @ 113.79 --- 8:11 PM

Yeah... I've been wondering how much is going on "behind the curtain" that I'll never see that impacts the performance. The car has "mysteriously" gained / lost about a tenth and 1 MPH between passes for which I'm unable to understand due to no other factors I can tell having changed. Haven't noticed that lately, but there are a number of runs on my spreadsheet where that stands out.

Humidity will certainly have an affect especially when you do not compensate for it with more timing to help burn off the water. We ad a TON of timing when the air gets really wet vs west coast conditions.

A lot of this calls back to the old high humidity and higher calculated corrected altitude affect horsepower. Lower horsepower on a car that hooks equals slower 60 foot times. That’s why we as racers love Fall temps and lower track temperatures.

Tonight's forecast

Conditions were really good last Friday night. Very low DA and the water grains were much less than the week before. Sure enough, my 60s were back.

Another case where the car "lost" about 1 MPH 1/8th & 1/4 for no known reason when conditions were actually improving (see 1st vs 2nd pass below).

The car got 3 good passes in, but unfortunately, the best pass they -- once again -- forgot to reset the clocks from King of The Hill 1/8th mile back to T&T 1/4 mile. The 7.509 is the car's best ever 1/8 ET and "should" have put the car solidly back into the 11.8s on that pass. Keefe Johnson watched that pass and said the car looked like it launched pretty hard... for 4300 lbs.

Not sure if the last pass spun cuz my line lock isn't working properly now, or the track was simply getting cold. Regardless, it was a waste of fuel & rubber.

9.23.2022 at Capitol (elevation 118’)
-- 60 ----- 1/8 ----- MPH ----- 1/4 ----- MPH ----- Std to sea level ----- Atmospheric conditions = DA / Gr H2O
1.740 --- 7.624 --- 91.46 --- 11.936 --- 114.85 --- 11.94 @ 114.94 --- 64* / 44% / 29.91” = 549' / 38 gr
1.720 --- 7.630 --- 90.54 --- 11.964 --- 113.75 --- 12.00 @ 113.52 --- 60* / 52% / 29.94” = 271' / 40 gr
1.686 --- 7.509 --- ??.?? --------------------------------------------------- 57* / 61% / 29.97” = 42' / 41 gr
1.906 --- 8.027 --- 88.82 --- 12.420 --- 112.44 ------------------------- 55* / 65% / 29.98” = -99' / 41 gr

Are you one of these guys that rush up to the lanes as soon as called. I have a few friends that jump as soon as called and they often spin or like you have the clocks or free set wrong. Let someone else jump and find the issues so it’s fixed for your pass. Sometimes it pays to sit back a little.

Capitol doesn't call people to lanes, except to make sure participants are ready for elimination rounds. For T&T and NT / Grudge runs, you can just pull into the lanes when you're ready... and willing to wait for a bit.

There's been a pattern recently where the next lane they call right after the KOTH round is done the track crew forgets to reset from 1/8th to 1/4...

is there a web page that gives da for any location/time ?

Way more to weather than D/A though. You could run 2 different tracks with similar D/A and run way different ET and MPH

Go here. Find your track. Click on historical and type in the date and time you want. https://airdensityonline.com/us-track-list/?filter=222

Doug

thanks

I’m coming around to the idea that the grains of moisture is more impactful on horsepower than the DA number. I have run good mph for the car at 3200 feet DA, but dry air and been disappointed at 400-500 DA with damp air. Cloudless sky is hard to beat. Thinking logically, the increase in air pressure from 100 feet to 3000 is relatively small. The increase in volume due to temperature is relatively small. Going from 10% humidity to 80% is huge.