NOTE: I did a search on this and after going through a few pages of results, I didn't find what I was looking for.


One of the problems I run into at my local track is a substantial change between weather conditions during time trials and eliminations. I've gotten my car to run very consistantly, but when conditions change, my ET changes, and since I like to dial very tight, that can be problematic.

The varying conditions that I could measure are:

• Temperature
• Dewpoint
• Wet Bulb
• Relative Humidity
• Barometric pressure
• Air Density

Obviously, the last one; Air Density, is calculated using Temp, Humidity & Pressure, plus Altitude, which is constant at 1,273'.

Is there any rule of thumb, using any combination of those factors, that would let me make reasonably accurate corrections in my dial-in, as conditions change?

ANY rule of thumb would be better than the absolute guesswork I'm doing right now.


Also, on a semi-related question; is there any advantage or disadvantage to bypassing and disabling the MAF, in order to force the PCM into Speed Density mode? This would be strictly for the track of course.

 

No rule of thumb. I spent a few years finding data on web sites dealing with density altitude and HP adjustment. Also looked for sites dealing with corrections to standard, dyno conversions plus lots of other conversion formulas and finally came up with lots of formulas to use in my laptop using Excel spreadsheets. It's not guarenteed accurate but as the weather changes it gives me a close enough idea what I'll run.

Just spend the money and buy a Perform Aire or Tag weather station with ET pridiction.

 

I looked around a bit at what's available, and I think that's a pretty good suggestion.

Think I'll do that.

 

The basics of what you need to convert. Best mph/ET pass with no extra variables such as a tail wind. Simple calculation to determine how much HP was used to get the best run as uncorrected hp. A dyno correction formula is needed to convert the HP numbers to a standard day (correction to standard) as a base setting. As weather conditions change, the base setting for a standard day gets converted for weather (correction from standard). This hp result is then recalculated to see what ET the car should run. Every time you get a better correction to standard run with no modifier such as tail wind, this becomes a new base setting. All runs should be slower or equal to this. Tuning can lower the base setting but you shouldn't be doing tuning during time trials and eliminations.

Lets say a 3500 pound car ran 12.0 at 114 mph. (close enough that the same HP is converted from both numbers). This means you're using 400 hp to run those numbers but depending on the density altitude, the engine will probably have a higher number if on a dyno because the dyno results are converted to sea level on a standard day. At my local track that run with a DA of 5000 feet on a normal race day means the engine is actually making 453 - 457 hp if put on a dyno. Now as the day got hotter and the DA increased to 5700 feet, a predicted ET would only be 12.07 because the car would only be making 392 hp at the higher altitude instead of 400 hp. There's no guarentee the car would run 12.07 since the increase in DA also means the temperature increased and everything else got hotter. If it was hot and sunny, the track could be slippery and you'd spin and slow down. It's a close enough calculation that if I decided to dial in the 12.07, there's little chance I would break out. Not impossible but I would have a better chance of running close to the dial than breaking out unless some variable made me go faster that I had no control over. Chances are the car would also run a bit slower than predicted unless you have a car that can run the same numbers every time.

 

Well in Orlando at Orlando Speed World, which is about 30 feet above sea level I use this formular.

Baro: .3 = -.15 on et
so if the baro is usually 30.00 and goes to 30.30 then that is a solid .15 in the quarter. (this would be very rare) This is used mostly from one week at the track to the next week.

Temp: 30 degrees = -.1 on et
Same here 30 degrees is good for .1 in the quarter.
In the winter when we get a 35 to 45 degree night the track is swamped with people. 50 to 60 degrees will bring them out also.

This is crude but seems to work down here. I have compiled lots of data in a spreadsheet with Temp, humidity, baro, amount of fuel in the tank, tire pressures, track conditions, etc.

 

Jim -

That's the kind of thing I was looking for.

I'd suspect that it'd work anywhere, because it's based on changes, not absolutes. Sort of like 100# dead weight = 1/10th or 100# rotating weight = 3/10th.

I'll try it out at my local track and see how it works.

 

Glad I could help.
Now you have taught me something; 100 # of roatating weight is good for 3/10's. I never new that.

So weld wheels and skinnies, Aluminum DS, are the key factors here.

Gotta keep the custom steel .084" thick DS, I am going from a 3000 rpm launch to a 4000 and see if it will stick. To do a 5000 rpm lanuch I am going to need taller tires (more sidewall flex and footprint) and send the converter back to JW for a 4500 to 5000 or more fix.

Thanks,
Jim

 

Yeah, I learned that about 20 years ago. I forgot which magazine ran the test, but they essentially ran a car with steelies and street tires, then bolted on some race wheels with 3.5's and skinnies up front, and dropped 3/10ths. I was pretty shocked at the time as well, but you can't argue with results.

 

do welds and skinnies really equal 100 lbs. of difference?

 

do welds and skinnies really equal 100 lbs. of difference?
-------------------------------------------------------

Tire and Rim Inertia HP/TQ Loss is greatest in 1st 60 Feet
where Tire Angular acceleration is greatest

and can be well over 100 Lbs.+ of addition to the Effective Weight of the RaceCar when accelerating

 

In this article it did. I think. It's possible that they actually had less of a weight savings. I would be lying if I said I remember the details well, but I THINK it was because they had wide steelies at both ends and went to ultralight racing rims, including a 3 or 3.5 inch wheel up front. Something like that.
I believe some of it was made up in the tires too. Please try to understand...
Back in those days I used to smoke so much dope that the Zig-Zag man had a tattoo of ME on HIS arm, so it's a miracle that I remember ANY of the article.


For reference, I have a set of modern, stamped steel, 8" van wheels that are 27lbs each. I have 28x9 M/T ET drags on them and another set with a CAST aluminum wheel and Hoosier Quick Times on them. Fully assembled, there is a 10lb weight difference, and I believe the Hoosiers are actually a heavier tire.


Anyhow, I've used the 3/10ths formula for computing weight & ET savings, for years, and it's a pretty accurate predictor.