At what point does Q-jet need re-jetting?
03-20-2008, 12:10 PM
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At what point does Q-jet need re-jetting?
Stock Mk IV 454 MAG Q-jet. I've done a little searching but haven't seen a thread that combines engine mods with Q-jet rods/needles/hangars/etc. Will the current jetting supply a good mixture if I upgrade cam and intake? I've read the Q-jet craps out at 500hp but all I'm thinking about is 425-450 max at 5000-5200rpm. At this point I think my dream combo (good power, "cost efficient") would be large oval heads, perhaps Air Gap intake, and a compatible cam/valvetrain. I'm more interested in mid-range torque than peak HP.
03-20-2008, 10:04 PM
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I was thinking about this once before. If you have a carb lets say 800cfm, that is dialed in perfect on a 450HP engine. then you build that engine and it is now capable of 575HP. You put that carb back on. Why would you have to rejet? Doesn't the amount of gas that goes through the jets depend on the volume of air pulled through the carb? Isn't it linear, more air more gas? or do the jets reach a limit of how much gas they can flow, so you would need to jet up a little on the secondaries with the increase of flow with the new built engine?
03-21-2008, 12:43 PM
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Yes, the assumption is more air needs more fuel flow. At some point the jets would max out. Carbs are set up for a particular application. Change the application, you need to adjust the mixture. I imagine carbs are probably set up a little fat from the factory, so there's some excess capacity...to a point. I just don't know what that point is for a 454MAG Qjet.
03-21-2008, 12:49 PM
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I was thinking about this once before. If you have a carb lets say 800cfm, that is dialed in perfect on a 450HP engine. then you build that engine and it is now capable of 575HP. You put that carb back on. Why would you have to rejet? Doesn't the amount of gas that goes through the jets depend on the volume of air pulled through the carb? Isn't it linear, more air more gas? or do the jets reach a limit of how much gas they can flow, so you would need to jet up a little on the secondaries with the increase of flow with the new built engine?
I don't know that the same cfm carb would be "right" for both a 450 and a 575hp application. It may run on both but I doubt it would be the optimal size for both. In order to make a 125 hp difference you will need to move a significant amount more air through the motor. The carb may work on the larger hp motor but it also may be limiting it as well. Or, it may run great on the larger motor but be a bit oversized for the lesser hp motor.
This is where Richie Zul just amazes me. He gets massive amounts of horsepower from carburetors but they are also some of the most fuel efficient motors on the planet. The result of assembling a package of matched components.
03-21-2008, 01:10 PM
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Handfulz, looks like you and I were typing at the same time.
I actually did just what you are asking about with my old Formula 311. Took my 454's to 461ci, just a tick more compression (9.5), some work on the old stock heads, a custom grind from Cam Motion, EMI Thunder exhaust, Crane ignition and the good ole Q-Jets! I'm an old NHRA Super Stock guy so I love to play with Q-jets.
We actually came out with 509hp on one engine and 506 on the other. Took a bunch of playing with jets, rods and hangers. Trial and error. There are a couple things that limit the Q-jet.
1)The needle and seat have a very small orifice. They do make an oversized one though.
2) The fuel bowl is relatively small compared to other 750s hence in a high-flow application you run the risk of starving for fuel.
3) The primaries are so small, that in order for it to idle decent, I was forced to drill holes in the primary butterflies in order to get enough air at idle. Otherwise, you wind up having to crack the throttle open too much and the idle mixture goes south.
It was a bunch of work but I went with a more aggressive cam profile and higher hp goal than you. Your job should be easier at 420 - 450. It was cool getting the double-take looks at the dock when I raised the hatch, though!
I actually did just what you are asking about with my old Formula 311. Took my 454's to 461ci, just a tick more compression (9.5), some work on the old stock heads, a custom grind from Cam Motion, EMI Thunder exhaust, Crane ignition and the good ole Q-Jets! I'm an old NHRA Super Stock guy so I love to play with Q-jets.
We actually came out with 509hp on one engine and 506 on the other. Took a bunch of playing with jets, rods and hangers. Trial and error. There are a couple things that limit the Q-jet.
1)The needle and seat have a very small orifice. They do make an oversized one though.
2) The fuel bowl is relatively small compared to other 750s hence in a high-flow application you run the risk of starving for fuel.
3) The primaries are so small, that in order for it to idle decent, I was forced to drill holes in the primary butterflies in order to get enough air at idle. Otherwise, you wind up having to crack the throttle open too much and the idle mixture goes south.
It was a bunch of work but I went with a more aggressive cam profile and higher hp goal than you. Your job should be easier at 420 - 450. It was cool getting the double-take looks at the dock when I raised the hatch, though!
03-21-2008, 07:23 PM
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Yes, the jets are limited as to what they can flow. That's why there is a numbered system of jets for most carb applications. Any orifice is going to limit how much liquid can flow through it. Just as injectors are limited as to what they can flow. Even if you held the injector open, it is still limited as to what it can flow.
I don't know that the same cfm carb would be "right" for both a 450 and a 575hp application. It may run on both but I doubt it would be the optimal size for both. In order to make a 125 hp difference you will need to move a significant amount more air through the motor. The carb may work on the larger hp motor but it also may be limiting it as well. Or, it may run great on the larger motor but be a bit oversized for the lesser hp motor.
This is where Richie Zul just amazes me. He gets massive amounts of horsepower from carburetors but they are also some of the most fuel efficient motors on the planet. The result of assembling a package of matched components.
I don't know that the same cfm carb would be "right" for both a 450 and a 575hp application. It may run on both but I doubt it would be the optimal size for both. In order to make a 125 hp difference you will need to move a significant amount more air through the motor. The carb may work on the larger hp motor but it also may be limiting it as well. Or, it may run great on the larger motor but be a bit oversized for the lesser hp motor.
This is where Richie Zul just amazes me. He gets massive amounts of horsepower from carburetors but they are also some of the most fuel efficient motors on the planet. The result of assembling a package of matched components.
03-22-2008, 10:11 AM
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Read the spark plugs. They will tell you if you're running lean or rich. Adjusting the idle mixture is an easy thing and only "counts" at idle. Adjusting jet and metering rods is more involved, but your spark plugs are the best indicators if that modification is necessary and which way you must go.
