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Old 01-16-2004, 06:23 PM   #1
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Effects of temp on carb jetting

I'm wondering about the outside temp and how it affects jetting.

All other things being equal, given a 30 degree drop in temp, how much of an increase in jet size would be necessary.

I'm under the impression that the lower the temp (and humidity), the larger the jet size would be.


Thanks.
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Old 01-16-2004, 10:00 PM   #2
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You are correct in assuming that colder temps require more fuel. That is only if the A/F ratio was dead on to begin with. If it was, than I would say probably 2-3 steps (I'm no pro). But this is what I would do. Up the jets, run it and read the plugs. Did the MPH increase? If it did, and the plugs don't look like the tune is too rich, up the jets again. Continue this until the MPH doesn't increase or starts to drop and make sure the plugs look ok. If the MPH drops, back down a jet size.....James
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Old 01-17-2004, 01:04 AM   #3
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Thanks James.

I'm trying to ballpark the jetting to save some $$$ on the dyno.

My guess is I'm waisting my time though. I normally only drive in spring and summer. Trying to jet properly in mid-January on a Proform 750 (basically a Holley HP series) that will be run in 70-90 degree weather seems futile.

I'm within a couple changes now so I think I'll just wait for the dyno to fine tune.
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Old 01-17-2004, 03:31 AM   #4
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a dyno run gives you a basic tune up. but weather changes. my best sugestion is tune the car on the dyno and take a thermometer with you to get the temp reading in the shop. This is a base reading. Go to the track and run your car. turn the car off at the end of the run and coast intot he return lane and rad yoru plugs, see if your rich or lean or just right. Take the temp right before the run. Log your readings in some kind of binder statign the temp and the size jets used and if it was lean, rich or just right, also your MPH and e.t. do this in the return lane. My track hates it when I read my plug there but oh well. get back to the pits quickly and read the temp again if it is the same then go up a jet size to see if it makes it rich. get back in line and make another pass. Read the temp again right before the pass and log as before. seems liek a lot of work? it is, but it is a small price for being competetive. It all depends on how serious yuo want to get. I could tell you all the info on my jettign at what temps and what elevation, but it is different with each car. you should see what we log on our altered fuel car each pass. It takes a person just to do that on our 5 man pit crew.
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Old 01-17-2004, 07:13 AM   #5
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You can use a single wire O2 sensor to tune a carb. It's very easy and works great.
http://www.bob2000.com/carb.htm
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Old 01-17-2004, 08:23 AM   #6
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I've never had to change my jets in 4 years ..... whats the problem ?

It gets 110 in summer , and 15 degrees in winter here .
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Old 01-17-2004, 10:54 AM   #7
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Quote:
Originally posted by Musclestang
I've never had to change my jets in 4 years ..... whats the problem ?

It gets 110 in summer , and 15 degrees in winter here .
You are not making the most power possible all the time. Temperature, altitude, barometric pressure and humidity all effect the A/F ratio. If you are jetted dead on for 60 degree temps, with little humidity and run in the summer when it's 95 and humid, then your running rich in the summer. Running rich means you are not making as much power as you possibly could. The reverse is true if you are jetted dead on for the summer time and race in the cool fall weather (good air). Your car will be running lean without any jet changes. Cool air is more dense. More air into the engine with the proper amount of fuel equals more power! Most people drop thier et as the night gets cooler because they run slightly rich to keep a "safe" tune.

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Old 01-17-2004, 12:46 PM   #8
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I've always been told that temperature by itself is not the biggest factor in jetting. I believe the actually density of the air (amount of O2) has a much bigger bearing. Now I'm not an expert on air and I honestly haven't put forth much of an effort to learn. I asked this question on here a while back and a few people posted some real good info about air changes.
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Old 01-17-2004, 01:09 PM   #9
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I believe barometric pressure and relative humidity effects jetting more than anything. Cold air is good but if its soaked with lots of humidity and the barometric pressure is low it will do you no good. I always bring my barometer and hydrometer to the track and I tune with an exhaust gas temperature meter. I never have to vary my jets more than 3-4 jet sizes. When my tune is off a little it usually is a tenth or 2 slow at the drags. I would leave your jets as is since the dyno is probably indoors. Hopefully your dyno place has a barometer and RH% measuring device and you can do some tuning while your there. Good luck!
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Old 01-17-2004, 04:18 PM   #10
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Quote:
Originally posted by GreyLX306
I've always been told that temperature by itself is not the biggest factor in jetting. I believe the actually density of the air (amount of O2) has a much bigger bearing. Now I'm not an expert on air and I honestly haven't put forth much of an effort to learn. I asked this question on here a while back and a few people posted some real good info about air changes.
Temperature directly effects the density of the air. Think about how a hot air balloon works: The air inside the balloon is heated up. They don't humidify it, they just heat it. Hot air is less dense than cold air. That's why the balloon floats. Why do you think they make cold air induction kits? Some people use nitrous with turbos/blowers just to cool the intake charge from the heat caused by boost. John Lingenfelter used nitrous to cool the outside of the intake when he broke the speed record in his firebird. Temperature plays a major role in the density of the air; although altitude is the biggest factor.

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Old 01-17-2004, 05:23 PM   #11
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super_irish I had forgotten about that site and the O2 sensor. I was meaning to build on eof those and give it a shot. Thanx for the reminder.
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Old 01-18-2004, 12:33 AM   #12
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Dry Air Density (D)=1.325*(PB/TA)

PB is barometric pressure
TA is Absolute Temperature F. (T + 460)

Humidity has very little effect on air density, and lowers air
density.

Air density is measured in lb/ft^3.

Standard air density is .075 lb/ft^3.

Temp has the biggest influence on air density.
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Last edited by Mouse; 01-18-2004 at 10:56 AM.
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