Carb size question
#1
11-25-2011, 07:58 AM
Carb size question
396 BB currently with a Holley 650 double pumper. I just rebuilt the carb, but wanted you alls opinions on a 750? You hear ALL kinds of different opinions, but guess I wanted some more
. Anyway, would love to hear some of the big block guy sizes and positive/negatives I could see keeping the 650 now.
Thanks again
. Anyway, would love to hear some of the big block guy sizes and positive/negatives I could see keeping the 650 now.Thanks again
#3
11-25-2011, 05:16 PM
Some BB powerplants got a rochester 750cfm, while other higher horse plants got a Holley 780cfm. I am running a 750 quadrajet. Seems to get all the fuel it needs.
#4
11-25-2011, 07:39 PM
Hey Turbo 33. What heads are you running on your 396? There were a number of BB head combos. Without going into great depth, the basic differences were either oval port or rectangular ports, and closed or open chambers. You can find the stamping on the heads and it is worth the effort. Go to mortec.com and trace the number to find your heads. From there you will have a better idea for a carby choice, based on the performance and flow characteristics of your heads. You may be lucky and have a high performance engine. That would be great for you!
I did all that and found I had closed chamber oval port heads (3872702) known as the '702' heads. They create a high compression but the chamber shape is very shrouded. It is a 1966 engine, pretty much run of the mill for the day, producing about 325 hp when new. I dyno'd this in my 68 Camaro 5 years ago and it was making a measly 206 hp at the rear wheels. I put a new fuel pump on it, a holley mechanical replacement part, and it dyno'e at 262 hp, which is an amazing gain, and probably shows there was a problem with my engine. Since then I have rebuilt the car and a moderate engine rebuild. I have had new valves, springs, and furniture, to work with a new comp cam during a rebuild. I have been running a Holley 700 DP for six years, and I have decided to toss the heads and carby. I also put a Performer RPM airgap manifold on during the build, and overall, it has given a big HP boost and is fun to driver, but the limits with the heads are beyond the effort required to fix, without new pistons etc etc. So, I have taken a lot of advice from builders and hot rodders and gone for a vacuum secondary Holley 770 Avenger. I am putting on some small chamber Edelbrock heads, which will have a much better combustion chamber shape to allow the big valves to work. I will let you know how it all works out. I will be running a dyno on the current set up to track the hp changes (versus dollars spent) I use the car for street and somewhat of a daily driver, maybe more of a regular driver (haha) so the carby will make it all a bit easier to drive.
So, work out what you have and then decide what you want the engine to do for you. One of my more knowledgeable engine builders usually tries to promote smaller carbys over bigger, for street performance. He likes the higher velocities a smaller carby can deliver. It does only work if the rest of the set up can make the most of faster fuel/air mixtures, such as the intake runners, heads, valves, etc. For a basically stock engine, you might be better to use a basically stock carby size. The earlier replies mentioned a 780, and that is a good choice for top end increase on a stock motor. It might not be able to give improvement anywhere else, so weigh it up. If you have the time and access to a dyno, then run them both and the figures will tell the story.
I did all that and found I had closed chamber oval port heads (3872702) known as the '702' heads. They create a high compression but the chamber shape is very shrouded. It is a 1966 engine, pretty much run of the mill for the day, producing about 325 hp when new. I dyno'd this in my 68 Camaro 5 years ago and it was making a measly 206 hp at the rear wheels. I put a new fuel pump on it, a holley mechanical replacement part, and it dyno'e at 262 hp, which is an amazing gain, and probably shows there was a problem with my engine. Since then I have rebuilt the car and a moderate engine rebuild. I have had new valves, springs, and furniture, to work with a new comp cam during a rebuild. I have been running a Holley 700 DP for six years, and I have decided to toss the heads and carby. I also put a Performer RPM airgap manifold on during the build, and overall, it has given a big HP boost and is fun to driver, but the limits with the heads are beyond the effort required to fix, without new pistons etc etc. So, I have taken a lot of advice from builders and hot rodders and gone for a vacuum secondary Holley 770 Avenger. I am putting on some small chamber Edelbrock heads, which will have a much better combustion chamber shape to allow the big valves to work. I will let you know how it all works out. I will be running a dyno on the current set up to track the hp changes (versus dollars spent) I use the car for street and somewhat of a daily driver, maybe more of a regular driver (haha) so the carby will make it all a bit easier to drive.
So, work out what you have and then decide what you want the engine to do for you. One of my more knowledgeable engine builders usually tries to promote smaller carbys over bigger, for street performance. He likes the higher velocities a smaller carby can deliver. It does only work if the rest of the set up can make the most of faster fuel/air mixtures, such as the intake runners, heads, valves, etc. For a basically stock engine, you might be better to use a basically stock carby size. The earlier replies mentioned a 780, and that is a good choice for top end increase on a stock motor. It might not be able to give improvement anywhere else, so weigh it up. If you have the time and access to a dyno, then run them both and the figures will tell the story.
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