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Unread 10-26-2008, 12:57 AM   #31
Pacfanweb
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^Dollars to doughnuts says that in real life, a larger carb will out-perform that 456cfm one your calculations came up with.

Yes, the 456cfm will be more responsive or "snappy" off idle, but shortly after that, the larger carb will be spanking it like a frat pledge.

I agree about using a spread bore carb, such as the Holley spread bore, or a Quadrajet. Best of both worlds there, you get the responsiveness of the smaller venturi at part throttle, and the power of the larger carb when you get into it.

But overall, most anyone will be more pleased with the larger (600cfm in this case) carb.
Vacuum secondaries I also agree with....you can even put a stronger spring in the diaphragm to prevent it opening until whatever RPM you deem it necessary....again, gets you best of both worlds.
Basically, if you have a square bore (e.g., a standard Holley) that's 600cfm, you'd be running a 300cfm 2bbl until your secondaries open....that gets you that responsiveness everyone seeks, plus the power over 3000rpm that most folks also want.

Again, this setup will outperform that smaller carb pretty much every time, if both are set up the best they can be.


I should say that I'm speaking from the viewpoint of a street-driven vehicle...not a primarily off-road Jeep. If all you want to do is rock crawl, then by all means get the small carb, because you'll never see the other side of 3000.
But I'm thinking this guy wants a Jeep that will run really well on the street, and be good off-road....and in that case, the 600 Holley is his best bet outside of a well-tuned Quadrajet.

I'm also assuming the setup he wants to run....304, with mild cam and aftermarket intake/headers. A 500cfm carb will choke that engine. The same engine with the 600 will run off and leave the smaller-carbed one.
And yeah, it's correct that the majority of driving is idle-3500, but that's not what a guy builds a V-8 with a cam/intake/headers for. If one were to run a dyno test on his engine with a 450-ish cfm carb, you'd see it falling on its face over 3000rpm. Definitely after 3500. But the 600cfm carb with vacuum secondaries would probably only be behind the smaller carb under 2000-2500, and after that it'd be killing it.

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Unread 10-26-2008, 05:19 AM   #32
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A 304 can handle a 6 pack just fine, it's all in the cam and heads. However, this will move your power band way up into the rpm range. Great for mud bogs and sand drags, not so good for trail riding. High hp is good for the street, but lowend torque is much more valueble on the trails. This is how the senerio was explained to me. Take a 5 speed manual tranny, torque is 1st, 2nd, and 3rd gear, HP is 4th, and 5th gear. How much do you use 4th and 5th gear out on the trails?
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Unread 10-26-2008, 09:30 AM   #33
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Pacfanweb, 79cj7stone stated in his original post a mild cam. These traditionally sign off below 5000RPM's. There is also no mention of either head or bottom end modifications. Couple this with a manual transmission (which will not allow for high RPM stall converter launches), the platform he is working with, the DD factor, and I think we are looking at an engine that will need to perform at it's best off idle through the midrange.
While you and I disagree on carb size (though this isn't a huge deal due to both of us agreeing on the use of a vacuum secondary unit), I simply can't find a way to agree with wendell.

Quote:
A 304 can handle a 6 pack just fine, it's all in the cam and heads.
Any way I shake it in my mind, a manually transmissioned, virtually stock 304 simply will not be able to handle the amount of CFM presented at WOT. Both Holley and Rochester 2BBL's were used for these applications and I can't come up with one in my mind that doesn't flow below 298CFM. Given that, a six pac will net you the equivalent of a 900CFM mechanical secondary carb set up at WOT. While possibly acceptable for a circle track engine or use with a high end stall converter carrying and holding high RPM's, I just don't see how a virtually stock 304 with a manual transmission will be able to recover enough manifold vacuum from a standing launch to do more than bog. One could attempt to limit throttle at launch, then transition to WOT at a specific RPM, but this kills any performance advantage and for the amount to trouble would be easier to simply go with a 4BBL. I believe there was a vacuum operated secondary carb offered in the 60's(model 2300). This would be the only way to get this amount of carburetion to work.
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Unread 10-26-2008, 07:12 PM   #34
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The engine will RUN okay with the 6-pack which doesn't exist anyway, but it darn sure won't run that great.
Just depends on the 6 pack....if he used the GM style, with 3 Rochesters, I think that's vacuum operated. So it'd run on the center carb, then kick the others in later, so it'd run, but not as good as a single carb.

As I mentioned earlier, if he's only after the "cool points", then it'd be okay. If he wants optimum performance, a 6-pack or Tri Power isn't the way to go with a 304.

And as far as the smaller carb or larger one, I've had a bit of experience with that on 304's.....had a stock 79 that had the stock 2bbl...put a Holley 500cfm 2bbl on it....it ran better, then put a Perfomer intake and a 600, ran even better.
The friend I mentioned earlier, had a 304 built almost exactly like OP wants, the 600cfm lit that thing up once we tuned it right.
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Unread 10-29-2008, 05:43 PM   #35
79cj7stone
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alright so i goin with the 4 barell 600cfm carb on the intake highrise or low rise
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Unread 10-29-2008, 06:58 PM   #36
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Edelbrock Performer.
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Unread 10-29-2008, 07:54 PM   #37
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alright and on the cam wat do u suggest
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Unread 10-29-2008, 10:23 PM   #38
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Quote:
Originally Posted by 79cj7stone View Post
alright and on the cam wat do u suggest
Use the Performer cam, too. Can't go wrong there.

I've also had some experience with 304's with Comp Cam's 268H and 260H...both are sort of in the same range as the Edelbrock Performer cam.
Both the 304's I helped with that had those Comp cams ran pretty good, I thought. Both had Performer intakes and 600 Holleys.
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Unread 10-30-2008, 06:37 PM   #39
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ok i found a cj5 today with a strong AMC 360 strong meaning it has a cam, 4 barell, highrise and is just tight nothing is leaking just wat i wanted in my 304 do u think i should get the jeep and use the engin and other thiung i need from it or do you think i should just rebuild the 304 this jeep is sellin for a pretty good price wat do u think?
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Unread 10-30-2008, 06:56 PM   #40
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Quote:
Originally Posted by 79cj7stone View Post
ok i found a cj5 today with a strong AMC 360 strong meaning it has a cam, 4 barell, highrise and is just tight nothing is leaking just wat i wanted in my 304 do u think i should get the jeep and use the engin and other thiung i need from it or do you think i should just rebuild the 304 this jeep is sellin for a pretty good price wat do u think?
If you have the $$$, and the 360 is in good shape, jump on it. Once you go to the bigger engine, your only regret will be "not having done it sooner."
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Unread 11-02-2008, 02:04 PM   #41
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ok thanks that is what i am hoping to do just gotta make some $$$ now
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Unread 10-18-2009, 03:59 PM   #42
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I know this is an old thread, but I found this today on the Speedway web site....

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Unread 10-18-2009, 10:31 PM   #43
godfather901
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Weld your own intake... There are always 6 pack mannys at swap meets but finding one for that engine is not gonna be easy - the import tuners weld their own intakes all the time i'm sure you could find someone. I commend your effort as 6 packs though amazingly wicked to tune would be freaking amazing - especially with some stacks coming out the hood. Webers are the carbs your looking for btw.
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Unread 10-19-2009, 11:17 AM   #44
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Off Topic: The funny thing about carbs, we switched from a 2 bbl to a four and get better mileage now since the primaries are smaller........just saying
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