Too much carb?
07-24-2011, 08:19 PM
#1
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Too much carb? hey guys, im brand new to the carb world, and im trying to pick a carb for my LQ4. I started doing a little math on what the minimum CFM i should have, and i came up with 632 cfm using this formula:
364* 6000/3456 = 632
Now I want a quadrajet for my own reasons (from what ive read, theres a LOT of debate on carbs) but i was looking at some from jegs, and those are rated at 750, is that going to be too much?
If it helps, its a LQ4 with L92 heads, ASA cam, and dual plane GMPP intake.
364* 6000/3456 = 632
Now I want a quadrajet for my own reasons (from what ive read, theres a LOT of debate on carbs) but i was looking at some from jegs, and those are rated at 750, is that going to be too much?
If it helps, its a LQ4 with L92 heads, ASA cam, and dual plane GMPP intake.
07-24-2011, 08:27 PM
#2
the intake will help on that q- jet choice, but u will need some serious pump action on a throttle dump from idle or that carbs going to live up to the name my buddies and i gave it 25 yrs ago" quadra bog ". as for the cfm, no problem.how much cam are u running? idle circut might be a problem with too much duration .
07-24-2011, 08:34 PM
#4
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So when you say "serious pumping action" are you referring to vacuum secondaries or like a double pumper? the specs on the cam are 226/236 .525/.525 110lsa... excuse my noob-ness but im not sure on what idle circuit is. What carb would you recommend? I not totally sold on the quad, i might even take a quick fuel carb.
07-25-2011, 05:22 AM
#7
The majority of guys run a 750 or larger, even guys running near stock cam 5.7L motors. Carbs are rated on air flow, not displacement, and the bigger HP of the motor says you are moving big block type airflows. Your combo might actually need larger.
Ls motors with single plane intakes (which IMO would be a better match for your heads and cam by the way) often need a big accelerator pump shot to cure a bog when punching it off idle. Generally the dual plane intakes don't have that problem so you may or may not need to play with your accel pump.
Ls motors with single plane intakes (which IMO would be a better match for your heads and cam by the way) often need a big accelerator pump shot to cure a bog when punching it off idle. Generally the dual plane intakes don't have that problem so you may or may not need to play with your accel pump.
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07-27-2011, 07:54 PM
#8
TECH Resident
Unless you know of someone that has a large assortment of Quadrajet parts I would stick with an Edelbrock or Holley. There are a lot more easiy available parts for tuning and people with knowledge on how to "tweak" them.
The accelerator pump squirts fuel into your primaries when you acclerate...also, the secondaries if you have a double pumper.
I ran a Holley for several years and am switching to Edelbrock. Edelbrock is much closer to a Quadrajet and suppose to be an excellent street carb. You would be wise to get downleg or bottom feed boosters for street use. It would be more idle and low rpm friendly with its greater sensitivity to vacuum signal . The Edelbrock has bottom feed boosters.
A 750 carb should work well with a dual plane especially if you have a decent converter.
The accelerator pump squirts fuel into your primaries when you acclerate...also, the secondaries if you have a double pumper.
I ran a Holley for several years and am switching to Edelbrock. Edelbrock is much closer to a Quadrajet and suppose to be an excellent street carb. You would be wise to get downleg or bottom feed boosters for street use. It would be more idle and low rpm friendly with its greater sensitivity to vacuum signal . The Edelbrock has bottom feed boosters.
A 750 carb should work well with a dual plane especially if you have a decent converter.
07-27-2011, 08:31 PM
#9
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Your setup is very similar to mine. I started with an LY6 crate motor and ditched the efi and vvt, so it already had L92 heads. That said, I'm running a Holley "750 HP Street" carb and have been very happy. Still tweaking to get the best a/f ratio's (have twin analog AEM UEGO's) and haven't been to the track with it yet. I had to go up a lot on the squirter sizes from 27/30 it came with to 40/37, and changed the primary pink pump cam to black, to give it enough fuel on tip in.
If you're not familiar with carbs you'll either need to get familiar, pay someone else to tune it for you, or just get the simplest vacuum secondary carb you can for it.
If you're not familiar with carbs you'll either need to get familiar, pay someone else to tune it for you, or just get the simplest vacuum secondary carb you can for it.
07-27-2011, 08:44 PM
#10
The Q jet will work, but the 632 cfm thing is a general area that will work, i would never run less than 750 cfm on any LS motor asides from the 4.8, so you will be fine.
Run what you want, the simplicity, adjustability and dependability of a Holley win over most of us.
Run what you want, the simplicity, adjustability and dependability of a Holley win over most of us.
07-27-2011, 09:05 PM
#11
I run an 850 mighty demon on my 6.0, it likes it. I also had to go up on squirters, and tweek the jets a bit, but its worth it to get all the airflow possible into the engine.
The 850 Mighty Demon is closer in flow to a 1000hp holley
Its more about tuning the carb to your combo, than anything
The 850 Mighty Demon is closer in flow to a 1000hp holley
Its more about tuning the carb to your combo, than anything
12-18-2011, 05:59 PM
#13
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Your calculations are correct. Bigger is not better. No, not an 800 CFM on a 5.3!!! Throttle response is so much crisper and sharper with a smaller carb. MPG too. I have a 6.0 with a 670 Holley.
Super Chevy magazine tested 5 or 6 carbs on a 383. From a 700 Holley to a 750 holley they gained ONE horsepower - at like 7 grand!
Super Chevy magazine tested 5 or 6 carbs on a 383. From a 700 Holley to a 750 holley they gained ONE horsepower - at like 7 grand!
12-18-2011, 07:16 PM
#14
Your calculations are correct. Bigger is not better. No, not an 800 CFM on a 5.3!!! Throttle response is so much crisper and sharper with a smaller carb. MPG too. I have a 6.0 with a 670 Holley.
Super Chevy magazine tested 5 or 6 carbs on a 383. From a 700 Holley to a 750 holley they gained ONE horsepower - at like 7 grand!
Super Chevy magazine tested 5 or 6 carbs on a 383. From a 700 Holley to a 750 holley they gained ONE horsepower - at like 7 grand!
12-19-2011, 03:35 PM
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12-19-2011, 07:19 PM
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seems to me size has a bit to do with application too. My Street HP is for the spring thru fall time of year when I'm taking the truck to the track. I bought a smaller 650cfm Holley 4150 series with electric choke and vacuum secondaries for the off season. So far its still a bit lean... but it jumps off idle better (crisper) with the 650cfm then with the 750cfm carb.
