Density Altitude Impact

**pajeff02** · 08-11-2013, 06:34 PM

Friday night, our car went pretty s-l-o-w at the track and I am curious on how it compares to your experiences. The car still sports the stock LS1 under the hood, but has been modded as shown in my sig. The track sits at just over 600 feet, and our D/A was around 1,700 feet on Friday night with the hot and humid conditions. Our car has never liked hot air, but I can usually muster a 12.7 or so out of it at around 104-105 mph even on a bad day. In good air she'll lay down a 12.4 and trap almost 108 mph.

This weekend was a completely different story though. I was launching fine with 60' times in the 1.7's (best ever is a 1.69), but the car wouldn't drop below a 12.9 and my trap speed was only 103 mph on all three of my runs. There was no head wind, and other than the hot and sticky air I am not sure what was going on. What do you guys see for a difference on "good air" versus "bad air" days with your cars?

**2002ssslp** · 08-12-2013, 05:52 AM

It's a combo of dirty, heavy air and the DA. Just put a blower and an intercooler on your can and you should be fine. lol

**pajeff02** · 08-12-2013, 02:46 PM

I think a cam is in order, Keith.

I forgot to mention - the class I entered requires that I run the car through the mufflers. I never do that on T&T days so no way to quantify the difference, if any.

**Firebirdjones** · 08-12-2013, 02:59 PM

It affects the car drastically.
With a carb (generally what I race) I have to jet up and down depending on the conditions if I want it to run at it's best.

With this fuel injection (and I assume you still run the MAF) That MAF is supposed to be able to compensate somewhat to airflow but as to how much of a swing it can take at the conditions is beyond me. While I ran an MAF I noticed drastic differences in performance at the track in different conditions so I don't believe the MAF makes big changes. However with bad air HP is down no matter what, and tuning only helps bring some of it back, not all of it, you can only fill the cylinders so much when you lose density.

When I ran alot back East in the summertime Cincinnati heat (only time tracks were open) it was pretty common to have DA figures in the 3,000 range if the ambient temps were in the 80's, which isn't great, the humidity back there just didn't help matters, and hotter temps it was obviously worse. I pretty much had the cars spot on in those conditions and ran pretty consistent. It was rare that we had DA numbers in the 1,000-1,500 range, and that was usually towards October when the weather cooled and right before the tracks closed, so tuning for that wasn't real world since I raced primarily through the summer months.

Out here tracks are open year round. In the dry desert air if you hit the tracks in winter/early spring/late fall I've seen DA numbers right at 1,000 which is fantastic compared to what I used to race in. I've run some of my best times here after a little tuning.

Best advice if you want to compare times, diagnose problems, and do any tuning at all,,,,,,is to keep track of your times and conditions and use the correction factor if you make any changes and want to compare timeslips apples to apples. Without the correction factor you are shooting in the dark and any changes you make to the car will throw you for a loop, because before you get back out of the pits,,,,I guarantee the DA has changed again and you won't know if you made a change in the right direction without the correction factor.

Living up here at 5,000 feet and having a track at 600 feet throws another curve ball in the mix. I compensate some with small easy changes, but I generally just run the cars around here a bit on the fat side and live with it. Better than risking a lean condition when I drive down the mountain.

**pajeff02** · 08-12-2013, 03:09 PM

I thought about charting the runs and based on what you posted, FBJ, that is probably what I will do. I'll have to go look up the correction factor thing and see what I get.

**Firebirdjones** · 08-12-2013, 03:16 PM

Originally Posted by pajeff02

I thought about charting the runs and based on what you posted, FBJ, that is probably what I will do. I'll have to go look up the correction factor thing and see what I get.

That's the best way if you want to tune or chart progress. I know some people scoff at the correction factor but in reality, it's a tuning tool more than anything else (for me anyway).

You can use an online correction factor that will go back in time. Using your timeslips with dates and time of day, you can go back on the chart. Weather conditions are recorded and kept. So dig out those old timeslips and see where you were at.

http://www.dragtimes.com/da-density-...calculator.php

**pajeff02** · 08-12-2013, 03:47 PM

I tried downloading their mobile app, but couldn't get it to go through. I used this site on my phone: Air Density Online

**Firebirdjones** · 08-12-2013, 03:51 PM

Originally Posted by pajeff02

I tried downloading their mobile app, but couldn't get it to go through. I used this site on my phone: Air Density Online

I don't even know what a mobile app is

I can bring it up on the desktop here though.

I'll look at the one you posted.

**transamtom** · 08-12-2013, 03:54 PM

I usually check the DA on Dragtimes when I get home and write it on top of the slips.

My car runs 12.2's and 3's in good air(sea level) at 1700 feet she is around 12.4,5,or 6 depending on how many times I hot lap her.

So anywhere from 2-4 tenths is normal losses due to air quality.

I generally don't bother to go anymore if the DA is plus 2000 or more,its not fun for me or the car

**transamtom** · 08-12-2013, 03:56 PM

Drag Racing 1/4 Mile times - DragTimes.com

Look under tools on the top header.

**Firebirdjones** · 08-12-2013, 04:21 PM

Originally Posted by transamtom

Drag Racing 1/4 Mile times - DragTimes.com

Look under tools on the top header.

Ooops, just realized that. My link was supposed to take you right to the DA calculator. So yeah, look under tools accross the top.

**pajeff02** · 08-12-2013, 05:52 PM

Is there any redneck math for tenths lost per so much gain in D/A... or do I just need to plug numbers into the calculator?

**35th-ANV-SS** · 08-12-2013, 06:42 PM

Not sure on that off-hand, but you could plot it out based on your runs so far and have Excel give you a best fit curve, then use the curve formula as a general guideline for any DA.

**Firebirdjones** · 08-12-2013, 06:52 PM

No real formula for that that I'm aware of, since every car is affected differently.

If you race the car enough you get a pretty good feel what to expect though.

Mine will swing about a 1/2 second in either direction but like I mentioned, I ran most of the time in similar conditions with DA in the 2500-3500 range, and I could make an educated guess on what the car would run before I got through tech and sure enough first pass I'd be within a few hundredths, maybe a tenth. A few times I'd make one time run just to see, then park the car, I knew what to dial in on the car for eliminations.

**2002ssslp** · 08-13-2013, 10:02 AM

Originally Posted by pajeff02

I think a cam is in order, Keith.

I forgot to mention - the class I entered requires that I run the car through the mufflers. I never do that on T&T days so no way to quantify the difference, if any.

I was thinking of a cam swap myself but I have to see if I am going to keep my SS or go for the Vette I always wanted. Let me know if you are doing the cam and please take pics as you go. I will help others who want to do the same thing. I would like to know how you keep the lifters in place while doing the swap.

**Firebirdjones** · 08-13-2013, 11:56 AM

Originally Posted by 2002ssslp

I was thinking of a cam swap myself but I have to see if I am going to keep my SS or go for the Vette I always wanted. Let me know if you are doing the cam and please take pics as you go. I will help others who want to do the same thing. I would like to know how you keep the lifters in place while doing the swap.

Bars/pins that slide through and hold them up after you rotate the camshaft. They sit in plastic trays but after some mileage they very rarely hold the lifters up on their own, so as a precaution, the pins are slid in.

**2002ssslp** · 08-14-2013, 07:35 AM

Originally Posted by Firebirdjones

Bars/pins that slide through and hold them up after you rotate the camshaft. They sit in plastic trays but after some mileage they very rarely hold the lifters up on their own, so as a precaution, the pins are slid in.

Thanks. After years of building Mopar engines I am use to a wet valley and pulling out the lifters.

**pajeff02** · 08-14-2013, 08:28 AM

I think wooden dowels are another viable option.

**Firebirdjones** · 08-15-2013, 03:27 PM

Did you compare corrected times with old slips yet Jeff? Anything wrong with the car or just weather conditions??

**pajeff02** · 08-15-2013, 05:26 PM

I checked the car out tonight while swapping back to the street tires and didn't see anything amiss. I also put together a chart at work today and will start keeping track of things. No correction yet, but the 0.5 second difference you mentioned is about exactly what I experienced.