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Recommendations on Jet/Rod combination for a 383 using a Q-jet
Hoping someone has experience with a factory Q-Jet on a 383 stroker and can give me some advice.
I'm currently have a 73 Vette with AT, which I will (soon) be upgrading to a 383 stroker. The current (and nicely running) carb has 73/44 Jet/Rod combination, with DA secondary rods.
The engine build is pretty mild. The 383 is using a later-model block with a hydraulic roller cam. Intake is just an Edelbrock performer, heads are Edelbrock E-street 64 CC, with Headman Elite headers. I don't have the cam specs here, but expect it to be mild as I'm a big fan of idle quality.
Considering Q-Jets were run on factory high-compression Big Blocks, given my mild intake, and vortec-like flow numbers on the heads, I think I have enough carb.
The question is the right jet/rod combination. I'm considering going up to a 75/42 on the primary side. That would take me from a .266 metering area up to a .303. On the secondary side, I'm changing the DA medium-tip rods (.443) to CC medium-tip rods (.303). I'd like to get the AX (.40) short tip rods but they are not available.
Overall, that's a 14% increase in fuel flow on the primary vs. a 9% increase in displacement.
If I get the 75/42 combination, combined with my existing 73/42 Set, that should give me a good range (.266, .27, .289, .303) to work with.
Is this too much? Any thoughts or experiences? Am I thinking about this the right way?
Without knowing if you are starting with a lean or rich configuration under the conditions stated below, you are, in effect, chasing a rabbit wearing a blindfold. I would recommend that you first get a baseline map of your Air/Fuel ratios before you start changing rods and jets. The map should include idle, WOT to redline, and cruising in high gear at 1K RPM increments. There are plenty of wide band Lambda sensors available on the market, or you could have a local engine shop do the task for you.
Once you have the data available you should be able to precisely pinpoint which changes to make to your Q-Jet to obtain peak performance from your new engine.
DA is 454 sec. rod & is what I used when I set up a Q-J for a 383. Q-J 170 serial no. which may use different primary rods if you have an earlier Q-J. Used 77 jets & 41 rods which a Q-J tuner said was a little rich on the primarys, left it as that is stock L-82 jetting.
What is your sec. hanger- that can make a difference.
76 jets are common as they are used on std. 350s like L-48.
Use your carb just as you have it now. When you drive it on the new engine and learn how it performs, then address any differences. There are some very good articles and postings on how to tune a Q-jet, but don't try to do it before you baseline its current configuration.
I'm swapping out my base L48 q-jet into a 383 stroker. My assumption is that the jetting for that application would be too lean for a more aggressive 383. certainly I'll need to tune it in, but for breaking in the motor I'd like to get it closer and I'd rather break it in a little rich then a little lean. I was hoping some with a 383 and a q-jet could share their combo to see if my thinking was in the ballpark. One it's broken in I'll hve the car dyno tuned.
Are you running much cam? If so, you'll need a weaker power piston spring or she'll run rich. The PP spring is controlled by vacuum and the q-jet will think it's under a load when it's not, and enrichen the mix caused by the lower vacuum due to a longer duration cam.
A switch to the next larger needle and seat is a worthwhile performance upgrade. Stock is about .125". The next larger size is .135". The largest, .146", can overpower the float and cause problems, or at least that's my experience. AC Delco still sells these upgrades.
Rods and jets? 75/42 is pretty fat, at least it would be here in central TX (7-800 ft ASL). On the secondary side, DA's are a great baseline for initial tuning.
If you need a source for parts, here's a good place:
Did you change the primary rods or how did you come up w/ 48?
The Chevy dealer quoted 41 primary rods for 77 L-82 (& L48).
As far as I can tell, these were the numbers quoted by The Carb Shop who rebuilt my stock carb. I had this info written on a sheet of paper. I will try to find the original Carb Shop invoice to verify the numbers, but, with my filing system (everything goes into an empty
case) it may take a while.