Carb Size
Thread Starter
Burning Brakes
Joined: Aug 2005
Posts: 751
Likes: 0
From: Lufkin Texas
Carb Size
Can someone tell me the best size carb for street application? 650,750, vacumn secondaries or mechanical?
Engine specs
350 L82 Block and crank new pistons flat top making 9.5 to 1 comp with the 64cc heads engine builder says 465hp 450 torque
The cam has a .465 / .488 lift and 300 / 300 sae duration.
THE RPM BAND IS 2500 TO 6000 RPM RATING Roller Rockers
Intake Edelbrock performer
Pro Comp Aluminum heads 2.05 and 1.6 valves screw in studs and stud girdles
So looking for carb help thanks
Calvin
Engine specs
350 L82 Block and crank new pistons flat top making 9.5 to 1 comp with the 64cc heads engine builder says 465hp 450 torque
The cam has a .465 / .488 lift and 300 / 300 sae duration.
THE RPM BAND IS 2500 TO 6000 RPM RATING Roller Rockers
Intake Edelbrock performer
Pro Comp Aluminum heads 2.05 and 1.6 valves screw in studs and stud girdles
So looking for carb help thanks
Calvin
Race Director
Joined: Dec 1999
Posts: 19,610
Likes: 778
From: Forked River NJ
I hate to say it but his HP estimates are extremly optimistic. I was running a way bigger cam with better heads with a 350 and was around 420 HP. As far as a carb, I like mech. secondaries. I was running a 750 Holley on mine. My 750 is for sale also.
Thread Starter
Burning Brakes
Joined: Aug 2005
Posts: 751
Likes: 0
From: Lufkin Texas
Originally Posted by Gordonm
I hate to say it but his HP estimates are extremly optimistic. I was running a way bigger cam with better heads with a 350 and was around 420 HP. As far as a carb, I like mech. secondaries. I was running a 750 Holley on mine. My 750 is for sale also.
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
i think you should be in the real 650 -750 range, i'm running a speed demon 750 but I think a 650 speed demon would have worked well also. you need to decide if you want the float of vacuum secondaries or the directness of mechanical secondaries
Drifting
Joined: Jul 2005
Posts: 1,562
Likes: 2
From: Spring Texas
I am not a real gear head, but it occurs to me that if the max that the heads will flow is for example 280 cfm, times 2 560 cfm, that at some point say 125, 130, 135% of the head flow rate is going to be the maximum effective carb. I don't know what formula to use but someone like Grandsportc3 or tt or Norval should be able to shed some light on this. It is only common sense that you can't stuff more air/fuel in the heads than they can handle. On your L48 I suspect that without better heads, cam, etc. that a Lars tune on your q-jet will get you about as much as you can expect.
Race Director
Joined: Dec 1999
Posts: 19,610
Likes: 778
From: Forked River NJ
Originally Posted by 71rdster
I am not a real gear head, but it occurs to me that if the max that the heads will flow is for example 280 cfm, times 2 560 cfm, that at some point say 125, 130, 135% of the head flow rate is going to be the maximum effective carb. I don't know what formula to use but someone like Grandsportc3 or tt or Norval should be able to shed some light on this. It is only common sense that you can't stuff more air/fuel in the heads than they can handle. On your L48 I suspect that without better heads, cam, etc. that a Lars tune on your q-jet will get you about as much as you can expect.
The 280 cfm is not rated for the whole head. It is port size. That is not the correct way to determine the carb size. There is a formula for this. It is based on engine size and rpm and the efficiency of the motor also. I don't have the formula handy but it is out here. Do a search or google it and you should find it.
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Drifting
Joined: Nov 2002
Posts: 1,951
Likes: 1
From: Shore NJ
I had a similar combo to yours with better flowing heads and the specs for the HP and TQ were way off, they sounded very good but the car just didn't move for the 420 HP and 435 TQ it was to have.
I think a 650 or 750 cfm carb would work fine for your application...
I think a 650 or 750 cfm carb would work fine for your application...
Drifting
Joined: Sep 2004
Posts: 1,532
Likes: 4
From: Fairview Heights Illinois, near Saint Louis MO, STL C3 Shark
The forumula can be found by searching for "engine volumetric effeciency". Pretty much an engine will never have 100% volumetric effeciency, if it did, a 350 would only really "need" somewhere around 400 cfm or something like that.
Safety Car
Joined: Dec 2004
Posts: 4,876
Likes: 1
From: Jersey Shore Exit 98
These motors came from the general w/650's & 750's you can't go wrong w/either. Smaller cfm will not give you the top end & larger cfm's will get you pass everything on the road except the gas station.
my .02
my .02
Pro
Joined: Jan 2002
Posts: 624
Likes: 0
From: Marietta ga
Originally Posted by toupstrio
Can someone tell me the best size carb for street application? 650,750, vacumn secondaries or mechanical?
Engine specs
350 L82 Block and crank new pistons flat top making 9.5 to 1 comp with the 64cc heads engine builder says 465hp 450 torque
The cam has a .465 / .488 lift and 300 / 300 sae duration.
THE RPM BAND IS 2500 TO 6000 RPM RATING Roller Rockers
Intake Edelbrock performer
Pro Comp Aluminum heads 2.05 and 1.6 valves screw in studs and stud girdles
So looking for carb help thanks
Calvin
Engine specs
350 L82 Block and crank new pistons flat top making 9.5 to 1 comp with the 64cc heads engine builder says 465hp 450 torque
The cam has a .465 / .488 lift and 300 / 300 sae duration.
THE RPM BAND IS 2500 TO 6000 RPM RATING Roller Rockers
Intake Edelbrock performer
Pro Comp Aluminum heads 2.05 and 1.6 valves screw in studs and stud girdles
So looking for carb help thanks
Calvin
Drifting
Joined: Sep 2004
Posts: 1,532
Likes: 4
From: Fairview Heights Illinois, near Saint Louis MO, STL C3 Shark
Originally Posted by Jims79
These motors came from the general w/650's & 750's you can't go wrong w/either. Smaller cfm will not give you the top end & larger cfm's will get you pass everything on the road except the gas station.
my .02
my .02
It is quite simply that the engine creates vacuum on intake stroke of piston, unless you are using a supercharger or turbo, that volume of air on a 350 will not exceed around 400 cfm total (my numbers could be off slightly) Nascar uses 350 cfm 4 barrels if my memory serves me. I'd kill for that kind of performance.
Le Mans Master
Joined: Sep 2001
Posts: 7,739
Likes: 630
From: Unreconstructed, South Carolina
I too feel those hp & tq figures are too rosey ... suggest your combo will run out around 350-375hp, 380-405tq @ crank. If you still have your Qjet, suggest it be gone through by a pro ... it's about 750 cfm and will do a great job on a street car. Cam sounds like Speed-Pro P/N CS1171R .465"/.488", 298*/308*@.006", 224*/234*@.050", LCL 112*, overlap 71*, pwr range 2.5K-6K. (latest SP spec is shown as 300*/300*@.006" ... but ???). Elgin E1015P virtually same cam.
Originally Posted by toupstrio
Can someone tell me the best size carb for street application? 650,750, vacumn secondaries or mechanical?
Engine specs
350 L82 Block and crank new pistons flat top making 9.5 to 1 comp with the 64cc heads engine builder says 465hp 450 torque
The cam has a .465 / .488 lift and 300 / 300 sae duration.
THE RPM BAND IS 2500 TO 6000 RPM RATING Roller Rockers
Intake Edelbrock performer
Pro Comp Aluminum heads 2.05 and 1.6 valves screw in studs and stud girdles
So looking for carb help thanks
Calvin
Engine specs
350 L82 Block and crank new pistons flat top making 9.5 to 1 comp with the 64cc heads engine builder says 465hp 450 torque
The cam has a .465 / .488 lift and 300 / 300 sae duration.
THE RPM BAND IS 2500 TO 6000 RPM RATING Roller Rockers
Intake Edelbrock performer
Pro Comp Aluminum heads 2.05 and 1.6 valves screw in studs and stud girdles
So looking for carb help thanks
Calvin
Race Director
Joined: Dec 1999
Posts: 19,610
Likes: 778
From: Forked River NJ
A 350 will need more than 400 cfm. Maybe a low po motor only turning 3500 rpm but for a performance motor even turning 5000 rpm you will need at least a 650.
Nascar using a 350. No way. Maybe the restricter plates only allow a certain cfm but they are using highly modified in the 750 cfm range.
Nascar using a 350. No way. Maybe the restricter plates only allow a certain cfm but they are using highly modified in the 750 cfm range.
Safety Car
Joined: Dec 2004
Posts: 4,876
Likes: 1
From: Jersey Shore Exit 98
Originally Posted by ruby76
Your right about coming with 650 and 750, however, the wonderment of the q-jet is that it actually ONLY flows what is needed by the engine, generally around 400 cfm, but is capable of larger flow numbers if/when warranted.
It is quite simply that the engine creates vacuum on intake stroke of piston, unless you are using a supercharger or turbo, that volume of air on a 350 will not exceed around 400 cfm total (my numbers could be off slightly) Nascar uses 350 cfm 4 barrels if my memory serves me. I'd kill for that kind of performance.
It is quite simply that the engine creates vacuum on intake stroke of piston, unless you are using a supercharger or turbo, that volume of air on a 350 will not exceed around 400 cfm total (my numbers could be off slightly) Nascar uses 350 cfm 4 barrels if my memory serves me. I'd kill for that kind of performance.
Race Director
Joined: Aug 2004
Posts: 10,628
Likes: 132
From: Duvall, WA
Originally Posted by Gordonm
A 350 will need more than 400 cfm. Maybe a low po motor only turning 3500 rpm but for a performance motor even turning 5000 rpm you will need at least a 650.
Nascar using a 350. No way. Maybe the restricter plates only allow a certain cfm but they are using highly modified in the 750 cfm range.
Nascar using a 350. No way. Maybe the restricter plates only allow a certain cfm but they are using highly modified in the 750 cfm range.
Thread Starter
Burning Brakes
Joined: Aug 2005
Posts: 751
Likes: 0
From: Lufkin Texas
Originally Posted by Cory1970
Are you sure about those cam numbers.. What is the durraton at .050''???