Does anyone know what difference air temperature makes on optimum jet size? I noticed that after a cool/damp spell here in Michigan my plugs looked dark.

I thought that cooler air was more dense and would require more fuel (larger jet size) to keep the ratio right on, but it seems that the opposite is happening.

I also noticed the same thing this spring when temps were cool - #67 Primary / #72 secondary were right on. Then as temps got warmer I had to increase jet size to keep plug color correct. I was running #72 Pri and #76 sec when the temp was in the 90's, but had performed other mods to the motor along the way.

It would be nice to have a calculator or chart that could correct for temperature and humidity changes. Then I could keep it jetted perfect without wasting my plugs.

Thanks in advance for any insight.

BTW - my combo: Holley O-3310, 10.4:1 383, Air Gap, CC 282S cam, Trick Flow 23* heads, 1-3/4 headers, 3000 stall, 3.55 gear.

My combo: Holley O-3310, 9.8:1, 355, RPM, CC 282S cam, Sportsman II heads, 1-5/8 headers, 2400 stall, 3.55 gear.

Hot weather: 72P/78S

Cooler weather: 72P/80S

Hot weather definately makes the 72/80 combo a little on the rich side, but 72/78 seems about right so far.

Just curious, what's your ET?

The idea is to vary the jet size by changing the jet AREA by a calculated percentage.
It's like an altitude correction.
Ideally you would take the air density and moisture into account and come up with a percent change from standard.
Jegs and Summit sell barometric guages and calculators to do it.
I think I saw how to do it in a magazine a while back.
It was either Circle Track or chevy High performance magazine.
You might check with Holley and see if their web page has any info.
David

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67 RS 327
69 Camaro Vintage Racer
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Thanks - Maybe my changes affected jetting more than temp, because if I understand you guys, optimum jet size SHOULD increase as temp drops. Maybe the very high humidity (driving in blinding rain) had something to do with it. I'll check out the Holley site.

Ballistic - I'm running pretty consistently in the 12.4's at 108 mph on BFG drag radials, but have been down into the 12.2's when I can get it to hook. I just picked up some Hoosier Quick Time Pro's, but need to play with the accelerator pump before I'll be able to get it to launch right and take advantage of the extra grip.

Looks like we have similar combos - what's your ET?

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68 Camaro, 383 small block with TH350 trans. 12.4's and never trailered.

[This message has been edited by Eric68 (edited 08-23-2001).]

So far I've managed a 12.6 @ 107 with full exhaust and McCreary Road-Star tires.

One thing I'm considering changing is the 2400 stall converter for a 3000. Since my cruise RPM is ~3000 @ 60 mph with the 3.55's, I thought maybe transmission heat would be a problem due to converter slippage while cruising. How well does it work in your case? Or do you not drive 60? http://www.camaros.net/forum/wink.gif

Sure, I drive it everywhere with the 3000 converter - drove across Michigan for the dream cruise last weekend. Cruised about 62 mph at 3100 RPM the whole way (except when it rained hard and I felt the back end floating) http://www.camaros.net/forum/biggrin.gif I do have an external tranny cooler. Mileage sucks though - 11 mpg freeway, but then I didn't build it for mileage.

I'm still a little baffled on the jet thing though. I found an interactive calculator (hopefully the link below works) but I am unsure whether I should select "correct to standard" or "correct from standard". "Correct to standard" has jet size increase with temperature, while "correct from standard" has jet size decrease as temperature increases.
http://click.hotbot.com/director.asp?id=1&target=http%3A%2F%2Fwww%2Ebgsoflex%2Ecom%2Fholley% 2Ehtml&query=Holley+temperature+humidity+correction&rsource=LCOSWF

Hmmmm, maybe I'm missing something, but as I understand the concept of that page, you enter a jet size that works well, along with the weather conditions under which it was tested and correct to standard.

Then, after computing a standard jet size, you should be able to calculate what jet size you need under any weather conditions by entering that standard jet size and computing from standard.

What doesn't add up is that if you run the program with the default weather conditions and jet size, it finds a standard of 74 for the 72 entered, but then if you enter 74 along with the same weather conditions and correct from standard it computes a 75 when I think it should return the original 72.

The other odd thing is that it seems humidity does not affect jetting - at least not by itself according to that calculator. If I change humidity from 1% to 99% the recommended jet remains the same. Oh well, maybe I'll not worry about it - I'm pretty sure (based on other sources) that a 20* increase in temp means you should drop 1 jet size.

If I find something genuinely useful I'll post it for the rest of you guys.