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427 Carb Jet Sizing
Doing a little bit of research for future carburetor jetting. Need some input from everyone that has a 427 (or similar) size engine.
First of all, what size is your engine? What altitude did you set up the carb for? What size carb do you have on your engine? What size are the main jets? What size are the secondary jets? What size are the squirters? And finally, how does it run? |
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1000 feet 770 Street avenger main 74 secondary 78 Squirters 35 After fixing intake leak it purrs at idle and runs strong at WOT |
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Good idea.
Roush 427 SRTW. Altitude is at sea level. Carb is a ProSystems 850 DP. Mains are 76/86 Squirters are both .045 Runs slightly rich, but is freakin wild when you let it loose. Mileage is 10 around town and 12 on the open road with a .80 fifth gear/ 3.46rear end. My goal is to lean it down a bit but not sure where to start. Squirters seem quite large to me but there is no hesitation on throttle tip in or steady cruise at light throttle. Still, after reading the setups on quite a few others, (with similar engine sizes and double pumpers) I'm thinking of taking the mains to 72/80 and the squirters to .37. and, yes, I would make one change at a time but how in the hell do you gauge the secondary mixture without a wideband? It's not like I can put my foot into it AND try to watch a vacuum gauge at the same time. I'm too busy trying to go straight! And I already had it on the dyno where they tuned it for max power. That's where the current jet sizes came from. Not sure that's best for every day driving. It seems like I'm pouring a lot of fuel into those cylinders during normal driving. Thoughts. |
Power valve size?
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650-750CFM double pump will alleviate some of your issues, spending a few dollars more to get it tuned and tweaked on a chassis dyno (not engine dyno as you cannot take in to account the effects your drivetrain has on your engines tune) and you'll really wake up your engine. Having spent days with my cars on chassis dyno's, after first listening to my engine builder tell me what I needed based on his engine dyno, I can tell you what I stated above is FACT and not conjecture...... Bill S. |
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The dyno tuning was done on a chassis dyno. I had the same thought about over carbing, but the engine came with a 770 Street Avenger and when I ordered the new carb from ProSystems, I asked the same question and he recommended the 850 DP. |
How much vacuum are you making?
Bill, I would agree that a lot of guys typically put carbs on that are too big. In this case, I think Jim is on the fence. Usually, when the carb is too much for the engine, it shows lean as the engine isn't efficient enough and can't suck hard enough on the carb to produce a good signal. In this case, he's possibly running a little rich, which could be just a jet change, air bleed change, or if he has lower vacuum, it could be opening the power valve prematurely. |
I seem to remember I'm running about 12 on the idle vacuum.
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Bill S. |
Buy an LM-1 by Innovative Motorsports and tune the carb yourself. Most carbs I have seen used on Cobras are running WAY too rich. That being said, an 850 cfm should work. Re-jetting will help immensely.
David A. here is a interactive selector for you..... Holley Interactive Carburetor Selector |
I have the same Roush 427 SRTW #460 with a BIGS Performance Stage Five 750 CFM double pumper. The primaries are 75 and secondaries are 82, primary power valve is 4.5, primary and secondary shooters are both 0.035, the idle feed is 0.033 and the bleeds are 0.073 Idle and 0.036 Main. Not exactly sure what all of this means but that's what is says on the card. There is no choke on this carb. It doesn't backfire when slowing down or hesitate when you step on the gas. I pretty sure this is unrelated but when I go hard around a corner it sort of flutters. Other than that it runs great. I can get 16 MPG on the freeway at 65-70 with 0.82 OD. I've averaged 13.5 MPG since purchased. I did change the distributor springs last summer to pull in full advance sooner in the mechanical advance. I think the weaker springs helped the gas mileage.
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Thanks Ward, just what I was looking for!
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It's not that easy just to compare what others are using. You didn't give Intake, Heads, exhaust and air cleaner. What color are the plugs? What type of plug? The gas mileage does not look great at cruise, but it's good for around town. Check your power valve for a possible problem. I'm running a Pro System 780 on a 359w with 76/85 8.5 PV and 11.5" vacuum and my plugs look good and gas mi on cruise is about 18. I tried leaning it out one afternoon and when I reached 72 primary jets the car surged.
I have very good heads, 1 3/4 primaries and a 3"id muffler. My air cleaner lets alot of air through. I could tell you a story about the difference the exhaust alone made on my carb tune. It was significant. |
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