bassn_07

I'm trying to learn a few thing since I plan on hitting the track here and there. I've done a search on DA and found a few threads where they mention DA not being a big factor on SC cars, why is that? I'm sure it's a simple logical answer but I can't think of it.

chiromikey

da is the calculation of pressure/density differences for different altitudes, temperatures, and humidities...all of which affect the amount of oxygen in the air. f/i cars have the ability to help make up for a decreased oxygen content by compressing more air (oxygen) into the cylinder so da has less of an effect on f/i cars than it does on n/a cars.

n2o does the same thing (to an even greater extent depending on jets) by supplying significant amounts of oxygen as well as cooling the charge air.

bottom line is that the more oxygen, the more power you're going to make!

bassn_07

Thanks. I found this on a different site. I guess my next question is how little does it effect our SC engines? Is there a calculation that determines a percentage per second in your ET's?

"Basically a lower DA is better. Lower DA means more dense air, more dense air means more air molecules entering the engine, and the amount of air an engine can injest is what determines how much horsepower the engine can make. So more oxygen is good, and since air gets less dense as you go up in altitude, lower Density Altitude is good. Any engine will make more HP at a lower DA than at a higher DA, all else being equal. Turbo and supercharged engines are not effected AS MUCH by changes in density altitude because they create their own artificial atmospheric pressure in the intake manifold, but they are affected somewhat and still adhere to the 'lower DA is better' rule, they just don't suffer AS MUCH when the DA goes up."

chiromikey

the percentage is up for debate. nhra states to only use half correction factor for f/i cars. however, nhra only corrects for altitude and altitude is only one part of the equation that makes up da. so they're not calculating for pressure, humidity, or tempurature which all have an effect on hp.

E55Pilot

Well not quite. If the turbo or supercharger is running into efficiency problems because it's too small and you are asking for too much boost a high density altitude is a bad thing and can push the Turbo/supercharge outside of it's optimum P/R (Pressure ratio).

One thing that many people forget about is the pressure loss due to what's ahead of the compressor face (Airfilters, airbox and intake tubing) add the loss due to High DA and you could have a decent amount of loss that now you're asking the compressor to make up.

The bottom line is that you want the highest atmospheric pressure at the face of the turbo/supercharger compressor face possible.

bassn_07

What brought this to my attention is dragtimes.com DA calculator. At the very bottom they have 3 different categories listed, how accurate would this be? I'm trying to keep a personal log to see my progress over time and DT's chart is something I would like to use. It's seems obvious that #3 is the one we want to use (E55's) but would a moderately modified E55 be in that class " Extensively Modified Supercharged and Turbocharged Engines".


1. Stock and Mildly Modified Naturally Aspirated Engines
2. Extensively Modified Naturally Aspirated Engines
3. Extensively Modified Supercharged and Turbocharged Engines

chiromikey

evan (cruzinquick) is the guy to ask regarding accuracy of da corrections on a s/c motor...

bassn_07

Maybe he'll chime in.

Point2Point

There's a difference between turbocharged and supercharged cars. Many turbo cars can compensate for the lower atmospheric pressure through higher boost. The wastegate won't actuate until the maximum absolute pressure is attained.

Atmospheric Pressure + Boost Pressure = Absolute Pressure

For example, if atmospheric pressure is 2psi lower at altitude, the turbocharger boosts 2psi higher to achieve the same absolute pressure. Of course, there are limits to boost pressure increases due to the design of the turbocharger. There's a point at which the turbocharger will be making more heat than boost.

Superchargers have no similar way to compensate for the lower atmospheric pressure. There's no way supercharger boost can be increased on the fly. As such, the supercharged motor suffers from lower atmospheric pressure in a similar way to a normally aspirated motor. You just can't swap to higher boost pulleys on the fly to compensate for changing atmospheric conditions.

An E55 will be significantly slower in Denver, Colorado than in Los Angeles, California. A 996 Twin Turbo won't suffer much, although having to spin the turbo to higher speeds will result in slower full boost response.

juicee63

If just for personal use , there are two ways you can go.

1. Just use the dragtimes calc, whatever one you chose always use the same one so your numbers are factored the same on all your runs.

2. Use the calculator and then take a .50 to the da number. So if the calc says 2200 Da use 1100 and calculate your times based on the new 1100 ft level.


You can use your calculations to better organize your runs and mods, setup, weight, etc. Note as much detail on all your runs as possible.

I have over 1000 slips in my car all with calculations, a correction is only as good as the inputs , so check and crosscheck all your weather data. Take a reading or ask one of the folks with a weather station preferrably on the top of a trailer and away from the staging lanes. If you buy a handheld they can vary wildly so take several readings and see what can be repeated.


I once asked a professional NHRA funny car driver how weather effected his car. I phrased it like this " I guess weather has little effect on your car since you have a few thousand hp"

answer: Actually weather is a tremendous factor , the track surface temp, the pressure, every 1 degree must be accounted for"

"I said 1 degree", he goes on,

"with every degree change the cars electronics are dialed up or down changing the fuel mixture and % of throttle".

To him weather was the #1 consideration.

For you just note the da on your slip and be consistant with how you "correct to sea level" as long as you keep the consistant record, you will be able to get very accurate with predicting times from -1000-+5000 ft. My car ran within 1/100th and 1mph of what I predicted it would run. I usually can dial within 1/10th and 1mph but the pros take the decimal out to 1/100th or even 1000th.



Fun stuff, I can tell your going to be addicted

bassn_07

No doubt brother! I could almost guarantee you that I'll attend most of these meets you boys put on. I never even been to one yet but I could tell it's going to be a memorable event. I'm just glad I signed up early because that shiz filled up quick.