Too Much Carb?
Posting Guru
Joined: Nov 2004
Posts: 1,111
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From: Turlock CA
Originally Posted by Scouder
Factory 4bbl manifolds work OK, but you just about gotta have somebody help set it on the engine.
-Scouder
-Scouder
So my reccomendations would be get a friend or a engine hoist to change a stock manifold.
jd
Post Fiend
Joined: Mar 2004
Posts: 6,056
Likes: 8
From: Union, Washington
Originally Posted by Scouder
I'm not buying the "too much carb" thing. I say run it. The world is full of 350 Chevs running 750's every day. I certainly wouldn't buy another carb if you already have the 750.
-Scouder
-Scouder
Cargo Master
Joined: Jul 2003
Posts: 2,328
Likes: 14
From: Colorado
I run a 1968 cast iron Cobra Jet manifold. It flows good, I guess... I don't have anything else to go off of except the stock 2bbl. I do know that the CJ is taller than all the others I've seen.
Senior User
Joined: Nov 2005
Posts: 176
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There is one potential problem with the Edelbrock Performer RPM which is the lack of an exhuast crossover. Correct me if I'm wrong, but this may affect winter driveablity, if indeed you live in a place that gets cold weather during the winter months and you drive your rig during that time. A substitute would be a discontinued edelbrock F427 dual plane manifold. It is just an aluminum version of the cast iron Cobra Jet intake manifold. And of course, it contains the exhuast crossover. I have one on my 390 in my F-250 and there is definate positive difference between the F427 and the the S code manifold that I was using.
Elder User
Joined: Feb 2005
Posts: 669
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I find myself conflicted on the carb size issue. To me, the carb cfm # is the capability of the carb. So a 750 holley is capable of up to 750 cfm. That doesn't mean that it will force 750 cfm into the engine. The engine is only going to pull in what it can. It seems to me that, as long as the jets are sized properly, you'd be better off with a little higher cfm than too little. I've heard many times in this forum that a 600 cfm is large enough for the 390. I really don't know for sure but my engine seemed choked down with the 600 on it. For me, I've changed to a 750 Holley, vacuum secondary. I've got the engine back in so I'll know soon if I was wrong to switch it out or not. I'm a hard head and I have learn the hard way...LOL.
Tracy
Tracy
Post Fiend
Joined: Jan 2003
Posts: 8,600
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From: Iowa
The other thing which has been mentioned before, is that it is a vacuum secondary carb, and that the throttle bore size is the same as a 500cfm Holley 2 barrel.
What this means is, at part throttle cruise, or even light acceleration, what you have is the equivalent of a 500cfm Holley 2 barrel. It's only when the airflow through the primaries gets to a point where the engine could benefit from more air that the secondaries are engaged. Even then,they only open enough to give the engine as much as it needs.
Now if it was a 750 double pumper with 1:1 linkage, like most of those Chebbies are running, THAT would be bad!
What this means is, at part throttle cruise, or even light acceleration, what you have is the equivalent of a 500cfm Holley 2 barrel. It's only when the airflow through the primaries gets to a point where the engine could benefit from more air that the secondaries are engaged. Even then,they only open enough to give the engine as much as it needs.
Now if it was a 750 double pumper with 1:1 linkage, like most of those Chebbies are running, THAT would be bad!
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Post Fiend
Joined: Mar 2004
Posts: 6,056
Likes: 8
From: Union, Washington
Originally Posted by FalconStng
I find myself conflicted on the carb size issue. To me, the carb cfm # is the capability of the carb. So a 750 holley is capable of up to 750 cfm. That doesn't mean that it will force 750 cfm into the engine. The engine is only going to pull in what it can. It seems to me that, as long as the jets are sized properly, you'd be better off with a little higher cfm than too little. I've heard many times in this forum that a 600 cfm is large enough for the 390. I really don't know for sure but my engine seemed choked down with the 600 on it. For me, I've changed to a 750 Holley, vacuum secondary. I've got the engine back in so I'll know soon if I was wrong to switch it out or not. I'm a hard head and I have learn the hard way...LOL.
Tracy
Tracy
Post Fiend
Joined: Mar 2004
Posts: 6,056
Likes: 8
From: Union, Washington
Originally Posted by rusty70f100
The other thing which has been mentioned before, is that it is a vacuum secondary carb, and that the throttle bore size is the same as a 500cfm Holley 2 barrel.
What this means is, at part throttle cruise, or even light acceleration, what you have is the equivalent of a 500cfm Holley 2 barrel. It's only when the airflow through the primaries gets to a point where the engine could benefit from more air that the secondaries are engaged. Even then,they only open enough to give the engine as much as it needs.
Now if it was a 750 double pumper with 1:1 linkage, like most of those Chebbies are running, THAT would be bad!
What this means is, at part throttle cruise, or even light acceleration, what you have is the equivalent of a 500cfm Holley 2 barrel. It's only when the airflow through the primaries gets to a point where the engine could benefit from more air that the secondaries are engaged. Even then,they only open enough to give the engine as much as it needs.
Now if it was a 750 double pumper with 1:1 linkage, like most of those Chebbies are running, THAT would be bad!
Senior User
Joined: Mar 2004
Posts: 375
Likes: 0
From: East TN.
ive got a 750 holley on my windsor, but it revs to 7200 nicely. but it aint something i would want to drive everyday either. i think its too much for a 352, ive got a 600 and an s code intake on my 390. but since youve got it already at least try it and see what it does. it isnt gonna cost anything (other than some gas).
Cargo Master
Joined: Feb 2004
Posts: 2,055
Likes: 167
From: Central Texas
Bear pretty much nailed it here. I have a 400M in a Bronco that is fairly well built(cam headers, etc) I use a 600 for around town and when I offroad it. I then switch to a 750 when I use it for highway towing(my boat). The 750 works fine in on the highway and pulls strong, but looses it off idle. The 600 really works well in town. The differance is the velocity through the venturies. The 600 will have higher relative velocity and provides better mixing of fuel/air, thus the lowend is snappy. It does run out on the top end though. Incidentally there is very little differance between them as far as MPG goes. If I had to choose one to stick with, it would be the 600 just for the lowend.
Fleet Mechanic
Joined: Nov 2004
Posts: 1,857
Likes: 5
From: Northern CA
Bear, I think that I already know the answer to this but, will different CFM carbs (4BBL only) affect manifold vacuum?
gw
gw
Originally Posted by Bear 45/70
Actually 2 barrels are rated at 3 inches of vacuum and 4 barrels are rated at 1 1/2 inches of vacuum so they are not rated the same and you need to take that into account because a 500 CFM 2 barrel in only 355 CFM rated at 1 1/2 inches like a 4 barrel is.
Fleet Mechanic
Joined: Nov 2004
Posts: 1,857
Likes: 5
From: Northern CA
Well, I re-read some of the previous posts and seetwisted, indicated that carb size affects vacuum as I suspected, but if you're pulling 20in vacuum with a 750CFM, wouldn't that be plenty signal for the carb to work efficiently?
b62
b62
Post Fiend
Joined: Jan 2003
Posts: 8,600
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From: Iowa
Originally Posted by Bear 45/70
Actually 2 barrels are rated at 3 inches of vacuum and 4 barrels are rated at 1 1/2 inches of vacuum so they are not rated the same and you need to take that into account because a 500 CFM 2 barrel in only 355 CFM rated at 1 1/2 inches like a 4 barrel is.
Originally Posted by Rusty70f100
Something interesting to note here:
Holley 500cfm 2 barrel throttle bore diameter - 1 11/16"
Holley 750cfm 4 barrel throttle bore diameter - 1 11/16"
I dont think we would be complaining too much if he was putting a 500cfm 2 barrel on it. This 750 also has vacuum secondaries, so if the engine doesn't need the extra air, it doesn't get it. It's not like he's got a double-pumper. So what's the big deal here?
Holley 500cfm 2 barrel throttle bore diameter - 1 11/16"
Holley 750cfm 4 barrel throttle bore diameter - 1 11/16"
I dont think we would be complaining too much if he was putting a 500cfm 2 barrel on it. This 750 also has vacuum secondaries, so if the engine doesn't need the extra air, it doesn't get it. It's not like he's got a double-pumper. So what's the big deal here?