is bigger better?
Thread Starter
Burning Brakes
Joined: Feb 2006
Posts: 1,223
Likes: 4
From: Regina Saskatchewan
is bigger better?
I purchased a 650 cfm Edelbrock Thunder Series a few years ago for my automatic ,cammed / vortec head - L48. Always in seach of 'a little more power' , I put in a 383. Same heads , and carb, bigger cam. Now I'm OK with the 383 , but I'm thinking it should be putting out more , for what it is. Both my buddy and and the local speed shop guy have built many drag cars , and both tell me I need to go to at least an 800 cfm carb. Both say I don't need a DP.
I'm looking for an increase in torque , NOT hp. I can count the # of times I've zipped the engine to 5 G on one hand.
So can someone explain how a bigger vaccuum secondary carb is going to increase the bottom end? I can't see it. At 1000 rpms the engine is going to flow 100 cfm whether I have a 650 carb or an 800. At 2000 rpms both carbs would pull 200 cfm. It wouldn't be until I hit 6000 rpm or so that I'd run out of cfms with the smaller carb. So I'd loose HP , but not torque.
Both these guys can't explain it , they just say thats just the way it is.
Also - I've been on one of the carb sites (can't remember - Edelbrock / Holley?) and they say a 650 is fine.
So if I get a bigger carb - will I roast my tires?
I'm looking for an increase in torque , NOT hp. I can count the # of times I've zipped the engine to 5 G on one hand.
So can someone explain how a bigger vaccuum secondary carb is going to increase the bottom end? I can't see it. At 1000 rpms the engine is going to flow 100 cfm whether I have a 650 carb or an 800. At 2000 rpms both carbs would pull 200 cfm. It wouldn't be until I hit 6000 rpm or so that I'd run out of cfms with the smaller carb. So I'd loose HP , but not torque.
Both these guys can't explain it , they just say thats just the way it is.
Also - I've been on one of the carb sites (can't remember - Edelbrock / Holley?) and they say a 650 is fine.
So if I get a bigger carb - will I roast my tires?
CF Community Team
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The smaller carb will give you better low and mid range torque than the larger carb. The larger carb will give you more horsepower at higher RPM's. I am running a 650 with mechanical secondaries on my 383 in my 61 Vette. The car pulls like a big block at low RPM's and pulls nice right up to 5,500 RPM's. I guess it depends on how you drive. If you are always up around 6 grand, go for the bigger carb, like a 750 CFM. If you want great driveability and low and mid range torque the 650 with mechanical secondaries is great.
Race Director
Joined: May 2006
Posts: 18,706
Likes: 2,573
Remember what Chevrolet used on the 70 to 72 LT-1, a 780 CFM vacuum secondary Holley. Today the closest approximation is one of the 750 CFM vacuum secondary carbs. The strength of the secondary spring combined with the vacuum signal on the primary side will determine how much the secondary opens. The 600 CFM wil generate a higher vacuum signal than the larger carb, at that point the strength of the springs determines which one opens more or less. I hope I explained this clearly.
Last edited by MelWff; Aug 4, 2011 at 03:51 PM.
Race Director
Joined: Apr 1999
Posts: 11,520
Likes: 13
From: CORVETTE 77 385 C.I. TEXAS
A Holley 750VS should be better than the Edelbrock on the 383.
Maybe you would like a Holley 650VS better.
I did a carb topic to help the guys out. Note TQ.
http://www.corvettefaq.com/c3/carbcomparo.htm
Maybe you would like a Holley 650VS better.
I did a carb topic to help the guys out. Note TQ.
http://www.corvettefaq.com/c3/carbcomparo.htm
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
I purchased a 650 cfm Edelbrock Thunder Series a few years ago for my automatic ,cammed / vortec head - L48. Always in seach of 'a little more power' , I put in a 383. Same heads , and carb, bigger cam. Now I'm OK with the 383 , but I'm thinking it should be putting out more , for what it is. Both my buddy and and the local speed shop guy have built many drag cars , and both tell me I need to go to at least an 800 cfm carb. Both say I don't need a DP.
I'm looking for an increase in torque , NOT hp. I can count the # of times I've zipped the engine to 5 G on one hand.
So can someone explain how a bigger vaccuum secondary carb is going to increase the bottom end? I can't see it. At 1000 rpms the engine is going to flow 100 cfm whether I have a 650 carb or an 800. At 2000 rpms both carbs would pull 200 cfm. It wouldn't be until I hit 6000 rpm or so that I'd run out of cfms with the smaller carb. So I'd loose HP , but not torque.
Both these guys can't explain it , they just say thats just the way it is.
Also - I've been on one of the carb sites (can't remember - Edelbrock / Holley?) and they say a 650 is fine.
So if I get a bigger carb - will I roast my tires?
I'm looking for an increase in torque , NOT hp. I can count the # of times I've zipped the engine to 5 G on one hand.
So can someone explain how a bigger vaccuum secondary carb is going to increase the bottom end? I can't see it. At 1000 rpms the engine is going to flow 100 cfm whether I have a 650 carb or an 800. At 2000 rpms both carbs would pull 200 cfm. It wouldn't be until I hit 6000 rpm or so that I'd run out of cfms with the smaller carb. So I'd loose HP , but not torque.
Both these guys can't explain it , they just say thats just the way it is.
Also - I've been on one of the carb sites (can't remember - Edelbrock / Holley?) and they say a 650 is fine.
So if I get a bigger carb - will I roast my tires?
Scott
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
Another 750VS vote here. 
...and definitely get a secondary diaphragm spring kit (#20-13) while you're at it so you can get dialed in properly.
Not exactly apples-to-apples, but I wasn't satisfied with only 650 on my re-cammed (solid) L-82/350.
...and definitely get a secondary diaphragm spring kit (#20-13) while you're at it so you can get dialed in properly.
Not exactly apples-to-apples, but I wasn't satisfied with only 650 on my re-cammed (solid) L-82/350.
Pro
Joined: Jul 2011
Posts: 511
Likes: 0
My last car was a Firebird with a 400 small block. I had a Holley 750 double bump. I was a little over-carbed. Not bad, but a little - I had to feather it off the line.
You can absolutely over-carb an engine. A 750 is the biggest carb I would EVER put on a small block - and on a 350 I'll stick with a 650.
Really mine was a 383 too - 400 block with a 350 crank. 750 was fine - but 751 would have been too much.
You can absolutely over-carb an engine. A 750 is the biggest carb I would EVER put on a small block - and on a 350 I'll stick with a 650.
Really mine was a 383 too - 400 block with a 350 crank. 750 was fine - but 751 would have been too much.
