Changed out my secondary Jets in the AFB on my 65 Gran Sport yesterday - went up one size to .086 ( If my memory serves) - did this on top of new metering rods I picked up from Carmen, "tri-Stepped" with .675/.55/.48 (again, from memory) Took the car out for my Sunday 'Test and Tune" and did 7 runs - all in the 14s, with the worst being 14.73 @ 98.5, up to my new personal best of 14.58 @ 99.3. Damn! All these runs were measured on a G-Tech meter. I read up on them this week, it seems most agree the ET's are about right if it's calibrated, but the MPHs run about 5 mph high (as the G-tech measures at actual speed at the end of the qtr, not the average of the last 60 feet like at the track). So that puts me at about 93-94 mph, which seesm alot more realistic. Now my question - I seemed to have the carb dialed in rich for the cold weather (32 degrees this am), but have no idea what to do in the hot weather - go leaner than factory specs? on the rods or jets or both? Any ideas? Thanks again -
I assume you mean .0675/.055/.048? Th metering area is proportionate to the difference between the squares of the diameters. .086" with .048 varies as (.086)^2 - (.048)^2, .086" with .0675 varies as (.086)^2 - (.0675)^2, etc. If you change the jet, you change all functions, but larger effect on the power step. For warm weather I'd just drop 2 sizes primary, 1 secondary, leave the rods alone, and try it out.
every g-tect must be different, i took mine to the track with me and its only 1 mph faster and 1-2 tenths faster. made about 5 runs. why did you go richer?
Yeah - there must be differences in every G-Tech. When I took mine to the track, mine read about .2 tenths slower than actual, but about 5mph faster. I wont really know how accurate it is until I go back, but it's a great tool for incremental gain... And yeah, missed a decimal place on the rods.... I went richer by gut, to compensate for the any additional air the doc mod provided, and to see if it had any effect in the cold air.. Seems like it did. Next order from Carmen I'll pick up an assortment of jets and mess with it come spring/summer. Maybe I'm just wasting my time, but it sure has been fun anyway. thanks for all your feedback...
when i did docs mod i went up one size then i saw they did docs mod on a olds on a engine dyno they went up too but then went back to stock made 12 hp more at 5000 rpms. i went back to stock my car was 2 mph faster. try it you might like it.
yeah - averaged about 15.0 before the rods, about 14.8 after, and this AM after the jets about 14.6. But all this is sooooo dependant on launch, was launching real well this AM, never broke loose once and was able to leave with some RPM. Funny, cause I though the tires would be too cold to bite. Kept the rears at 30 lbs, and no problems, where the same tires at the same pressure at 50 degrees can break loose like they're greased. I'm sure it has somehtign to do with the roadway too, it rained alot since my last out, maybe washed the area off....
The g-tech has to be used on a level road to be accurate....if you're going uphill, it will read it as additional acceleration (and mph) and skew the results. That could account for the differences between a stretch of road, and a track. when testing, try to use the same stretch of road. To figure the actual fuel metering change you have to compute the AREA of the jet, and subtract the AREA of the metering rod... Area = pi * radius (squared) then compare that figure to the result with the new jet/rod combo. Once upon a time, I found a great chart online with jet/metering rod combos for the Q-jet carb.
my test area is slightly uphill, not alot - debatable whether a car would roll on the grade. It could cause some noise on the G-Tech. And while I agree MPH is a good indicator of top end charge, I always run fastest after my best 0-60 times.
Here is the cart with area of various jet/metering rod combos. It only goes up to a 0.080" jet, but you can do the math to figure out your AFB. http://www.corvette-restoration.com/resources/technical_papers/Q-Jet_Carb_Tuning.pdf
Since all relevant areas are either circles or the remainder of circles, Pi, etc. can be completely omitted from comparisons, and the math made easier by simply using the squares of the jets minus the squares of the metering rods.
I've written an Excel spread-sheet that compares new vs. old settings for any combination of jets and metering rods (4G, QJ, AFB, AVS, TQ) as % mixture changes - e-mail me for a copy.
you said on sept-18th you ran a 14.71 at 98.7 with stock jets and rods, that day must of been in the 50s degree weather. that degrees in temp between 32 and 52 is good for 1.5 tenths alone.
with docs mod you will have a little less bottom end right off the line,same with a richer carb less bottom.
I'd have to look in my notes on the September runs, but that temp seems about right, I always run at about dawn, when it's coldest. Kitabel, am very interested in your spreadsheet, I'll shoot out an e-mail. It would seem that a little killing on the bottom end helped me out some - It was always a fine line between a good launch and just going up in smoke. It's an open diff. I've been jacking the rear with 50 lbs in the RT airbag and running heavy - full tank and toolbox. I'll be glad when I can shed some of that weight.
Now that I think of it, neither the Doc mods or carb changes seemed to kill bottom end much at launch, my 0-60 times are better than ever. I was averaging about 6.5, now I'm between 5.9-6.1. Maybe I'm just better at launching the car....
it might of hurt it right off the line just a little maybe first 10-20 ft but that a good thing, spinning kills your 60ft times you know that. with docs mod you will make up for that and more in mid and top end. what air cleaner are you running
