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drag race carb jetting

7305 Views 10 Replies 6 Participants Last post by  Tech @ BG
For you drag racers out there...does anyone change the main jetting based on ambient temperature? The idea being that if the temp is lower the air will be more dense and therefore more fuel can me added. If so is there any general rule of thumb, like jet up 1 size for a 20deg temp decrease?

I'm asking because I went racing last weekend with an ambient temp of around 60deg...going again tomorrow and expect a temp of around 40-45deg and wondering if I can jet up and possibly gain a little power.

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there is a formula but i dont know it. i can tell you in stock car racing we start in may, at say 80's we'll be down to a 77 by july then back up to 80' s...82's in october
I am not sure of the rule, but keeping a log when you do race will help alot. If you get a cheap weather station and document humidity, pressure, and temp along with your times and jetting you will start to figure it out.

Hosking, Looks like you vary your jets by 5 sizes from 82 to 77 depending on the time of year and temp. Appears you are from canada so I'm going to guess at your temps

Spring/Fall night racing temp in the 40's
Summer night racing temp in the upper 80's

Please let me know if those are incorrect. As a rough guess to get started it looks like 1 jet size for every 10 deg.

Keep it coming.
yeah im trying to convert to farenheight .its about 50 to 85degrees
It's a little more complicated than that.

Temperature is only one variable, BP and density are just as important.
Since cold air is denser should he lean his mixture too ? just curious, im going to the track tonight too and its going to be in the high 40s
Did you read the article?

Here's another.

...and when your done there...check your stock configuration here.

From the sounds of it (without instrumentation and experience at the track) I would recommend you leave the carb alone if it's running well.

Only a few tenths to be had anyway, unless it's running really badly.:p
Changing Jets for Temp.

The colder the air is the denser it is. Hence you need more fuel!!!!

But wait its not that simple.

A carbureted engine works based upon the pressure differential of the atmospheric pressure pressing down on the fuel in the float bowl, and the low pressure or vacuum created by the downward movement of the piston. More O2 in the air means you’ll need more fuel.

The only way to know for certain what way to tune is to use a logbook, and a weather station where you can monitor the change in DA (Density Altitude) or ADR (Air Density Ratio). The Lower your adjusted altitude the more O2 is available in the air, the more fuel the engine will, the faster you will go. The Adjusted Altitude or DA/ADR comes from a combination of the Temperature, Humidity, and the Barometric pressure. You can be colder, but have an increase in humidity, and loss of barometric pressure and actually need to lean the carburetor out.

The easiest way to think about is to think about your body… When it’s real hot and sticky in the summer you just about break a sweat walking to your car never mind trying to run anywhere. There is less O2 in the air, and you need less fuel. Conversely in the fall when it’s cool outside you can run around, and still feel pretty good. Lot’s of O2 in the air, and you’ll need more fuel.

Bottom line, if you want to do this properly you’ll need to keep a good logbook, and have a weather station
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Hey tech, i tune my carb (edelbrock 750) with a vacuum guage, will this atleast put me in the ballpark when im at the track?

oh yeah and how much does a basic weather station cost? I keep a logbook but id like to get more in depth with it, and i think a weather station would really help out
Tuning with a vacuum gauge will help you set the idle, but not your WOT fuel flow.

You can generally pickup a Drag Race weather Station for a few hundred dollars. If you're bracket racing it's one of the best investments you can make that will pay for itself the first few times out.
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