Topic: Time to freshen up the F Model bracket bike

[scottly]

Even with closed-loop EFI which maintains the correct AF, an engine will not produce the same HP at altitude as it does at sea level. The changes in the weather conditions you guys are dealing with are too small to require re-jetting, and you don't have jets available in such small increments anyway. Yes, you might want to consider jetting changes if you're going to race at Denver, but otherwise adjust your dial-in, not your carbs.

[dragracer]

So here's the down and dirty for me, if I know the DA is high on any given day of racing, I have to acknowledge that my bikes will be slower because there is an adverse affect on my AFR. I can attempt to adjust AFR by tuning my carbs to a certain extent. This might mean re jetting. Also in the case of my nitrous bike that runs on an exact index, I may have to increase the nitrous or change the timing to get my power back to run the number.

Ultimately, the lower the DA, the quicker I'll go, the higher the number, the slower my bike will run. And that's all I really need to know to make adjustments to my ET or package.

[scottly]

So here's the down and dirty for me, if I know the DA is high on any given day of racing, I have to acknowledge that my bikes will be slower because there is an adverse affect on my AFR.

No, your bike will be slower due to less barometric pressure and/or higher temps.

[Bill/BentON Racing]

Nope, those are the factors that are going to change AFR! Lol, Bill

[dragracer]

That brick wall has my head hurting

[gschuld]

Billy, sounds like it's time to set up your data logging Wego III air/fuel system

George

[scottly]

You guy's either didn't read the article, or understand it.

"For many cars, major changes in density altitude do not necessarily demand tuning changes. If you’ve done research in this area, you may have run across information that shows that for every 2,000-foot change in altitude, you should lean a Holley jet size by one number. The problem with this idea is that carburetors are velocity-sensitive, not sensitive to air density."

This is an over-simplification, but as the density of the air flowing through a venturi goes down, so does the pressure differential developed in the venturi, and the flow of fuel through the main jet.

[Bill/BentON Racing]

Performance is sensitive to air density
We use DA as a tuning tool
Can go all year with just adjusting dial in, no jetting changes if you're bracket racing, heads up you change jetting etc.
The lower the DA the better the air
But you gotta remember Frank don't know sh*t or POP's and I know less! Lol 

[scottly]

Do your tuning on the dyno, and use DA to predict performance at the track.

[Bill/BentON Racing]

Bingo, you did stay at a Holiday Inn last night! Lol, Bill 

[scottly]

But you gotta remember Frank don't know sh*t or POP's and I know less! Lol 

Bill, Frank, and Pops, please don't misunderstand me. Everyone that contributes here knows a lot about something, but not a single one of us knows a lot about everything. Pops can tell you how to psych out your opponent at the line, which I don't get and never will, and how to adjust your dial-in based on DA, which I'm now learning.
Bill and Frank know more than I will ever know about piloting a bike down the quarter mile.
My strengths are electrical and tuning.
Together, we add up to one pretty smart feller!

[Medyo Bastos]

or a really fart smeller

[madmtnmotors]

This is an over-simplification, but as the density of the air flowing through a venturi goes down, so does the pressure differential developed in the venturi, and the flow of fuel through the main jet.

Therein lies the rub, and that's what Frank has been trying to explain. He runs more than one "index" class where the dial in is fixed, such as crazy 8's, where the class wide dial in ET is 8.88. If you tune at sea level, or at any level for that matter, with a DA down low (300' to 500' for instance) then show up at Memphis with a DA of 3500', how do you compenste for that lesser fuel charge through the carb venturi and still run your 8.88? It is the fixed index classes where these numbers have the greatest potential impact. Running bracket classes (staggered tree) you simply adjust your dial, but the fixed dial classes don't have that luxury. Sure, you could raise your dial to 8.99 or 9.2, but the rest of the crazy 8's class is still tuning for 8.88 and you dont get a handicapped tree either just because you can't hit the class ET... 

[scottly]

Wilbur, the DA is an extrapolated number, and shouldn't be used for tuning. Note that my dyno test at 4500' was extrapolated to 11,000'+. Should I have adjusted my jetting for 11,000' when the AFR was correct? I don't know the solution to Frank hitting exactly 8.88, but jetting isn't the fine tuning answer. Say you're running a 1.00mm main, just to keep the math simple, and you want to lower the AFR 1%. Got a .99? In reality, the 99 would have more than a 1% reduction in flow, given the same atmospheric conditions.   

[POPS 911]

FRANK,BILL,WILBUR : I JUST THROWED ALL MY AMERICAN MADE Weather stations, ET predictors, Wind Speed meter and my Laser heat temp gun in the trash. 1000's dollars of mis-information for years. Sorry I gave you bad information, Bought a $4.00 dart board [ made in CHINA ] at the DOLLAR STORE and put maybe DIAL IN on the board and will throw a dart to put the number on my dial in board on my TURTLE. Lets look at the past FRANK 10.32 SEL record at Valdosta my hand held read 3-400' and so cold we had on winter jackets and gloves = Memphis my junk hand held read 3500' and we were dying in 108 degrees heat and a track temp of 138 degrees and FRANK dial 10.72 w/ no wind. My fellow racer works at JEGS BIG warehouse here in Central OHIO and I will have him get me the number sold of those hand held weather analyzer w/ ET PREDICTION that cost $407.99 to $489.99 that they sale ........

[madmtnmotors]

I JUST THROWED ALL MY AMERICAN MADE Weather stations, ET predictors, Wind Speed meter and my Laser heat temp gun in the trash.

I would be happy to carry that trash can out to the curb for you...   

[dragracer]

Trying to figure out why a big company like Jegs would waste money on CA info and then listed all those factors that you input for DA in their log book. Someone should have told them how this DA craps is irrelevant.

[scottly]

WTF? Pops, I never said you gave bad information, or that your equipment was junk?? Frank, I never said DA was irrelevant??
Why is your Jegs log blank? It tells you how to use historical data to judge how DA will affect Your bike on the first page. With the example Pops gave, a 3900' change in DA was .4 seconds slower, so .01025 seconds per 100'.
Also, note the Jegs log includes a separate ratio for humidity. This is because the NWS formula for DA doesn't take humidity into account, only barometric pressure and temperature.
Peace guys.

[Medyo Bastos]

pops throw your american made junk this way, and throw in a recipe for me too! something like hobo stew


...Save a tree, eat a beaver...

[kmb69]

I read the article and got a lot out of it. I don't want to speak for Scott but I agree with what he is trying to convey.
Get your motor running as good as it gets on the dyno - jetting, timing, and such where you normally compete.
The DA, humidity, and barometric pressure data should provide the "guesstimate" for changes in ET/MPH based on power changes due to current atmospheric conditions.
The accuracy of such predictions is based on empirical data recorded over many sessions of correlating the atmospheric data to actual performance. Gotta log.
It requires significant changes in the atmospheric conditions to justify changing jet size unless you are on the cusp of needing a jet change under "normal" conditions.
And of course it assumes all track surfaces/conditions are all equal - NOT! and that you can repeat your reaction times to within .001 seconds. Right?
Just my $.02   

[Bill/BentON Racing]

Well, bracket racing...ok, index, you don't tune,adjust, change timing,carb adjustments, y'all ever owned Lectrons, they different,you going lose! We only use DA as an indicator what to expect, not I'll go up one size cause DA went up a measured amount. Yall reading too much into it, my .02.Bill

[POPS 911]

My old TAG SYSTEM weather station give me all the data from many pass runs,  [ TODAY at this time 80.6F - 50.69% RH - 29.08" HG - DA 2707' ] when you turn it on it gives you all the readings for the weather where you are standing and can change by the minute, while at Memphis I push the button for my numbers three weeks before at my home track, then I hit #3 button = PREDICTION and my old TURTLE with CV CARBS would be breathing labored on a pass ....... big problem for bike with CV carbs. Jegs also sells a Orange colored hand held for $3-400 dollars and yes they have it in their hand when they are waiting in line for JR.DRAGSTERS [ 35-50 MOMS and DADS ] ARE ALSO IN THAT LINE WITH HAND HELD racing cool running alcohol  motors . JEGS gives away run books away FREE at certain EVENTS at my home track = my book is a TAG SYSTEM hand held 9 buttons of information for 4 logbooks  for 4 different bikes. That small Orange hand held could be easily mounted on you bike and the data right in front of you, I did that for a year w/my ZIR and my TAG.

[dragracer]

One of those times when I truly wish I had a weather station to calculate DA and give me a predicted ET. Funny thing, I noticed over half the racers had trailer based weather stations and a third more had handheld's. Makes me wonder why all those serious bracket racers would waste their money on useless gadgets and not put that cash into those dragsters and door cars.

[Jim F]

it all sounds like best guess at the end of the day
the last race Fletcher and I were at was the double race at commerce.
the bike ran contestant 5.85 5.84 all day long. but the weather was crappy all day long.
when it started to cool off David dialed a 5.83 and I suggested a 5.82 from my seat of the pants weather station
he ran a 5.820. and we run lectrons and have never messed with the jets.
again its all best guess guys

2 cents worth Jim

[Bill/BentON Racing]

There you go data and guesswork. ...Jims right, damn Dave, you need to listen to Jim! Way to go Jim,  lol, see y'all on the 27th, Bill....I'll bring my handheld, we'll check /compare numbers. Frank, we gotta a Wally race the weekend after Rockingham. ........C'mon man!