# 600 vs 800 cfm



## SDGoat619 (Mar 21, 2011)

According to the how to build pontiac V8's book by Jim hand the 455 motors should run a 800 cfm carb. I currently have a 600 that I am contemplating rebuilding. Should I scrap the rebuild and get an 800 or can I run a 600cfm on my 455. Any thought ? 

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## Eric Animal (Oct 28, 2007)

Personally, I wouldn't run a 650 on a 455....nothing less than a 750. If Jim hand says 800, then run an 800 ! Eric


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## SDGoat619 (Mar 21, 2011)

Thanks, I think that is what i am going to do


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## Mr. P-Body (Jan 20, 2011)

We use AED's 850 HO" carb for street builds. Reasonably priced and makes big power 
"right out of the box". 

FWIW

Jim


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## geeteeohguy (Feb 2, 2008)

A 600 would be right for a 326 or a 350. Even the 400's had 750's...a tripower is something like 1000cfm, and it came on little 389s.


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## 68greengoat (Sep 15, 2005)

Except for the super duties and HOs, even 455s had 750s until '75. Then 800s were on everything, including 350s and 301s..


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## BearGFR (Aug 25, 2008)

:agree with previous comments. 600 isn't going to 'git er done' even on a mild 455. It'd be like trying to run a marathon while breathing through a soda straw. 

Bear


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## SDGoat619 (Mar 21, 2011)

Thanks so much for all the great info. I have been having to add gas to my 600 carb in order to get the car to start when cold. Thinking that not enough gas was getting to the carb i was figuring that a rebuild was necessary. So this is good news, now I get to throw a bigger carb on it and get more power at the same time.


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## 66tempestGT (Nov 28, 2009)

lots of big engines also run 2 barrels and start fine if they are tuned right.


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## Thumpin455 (Feb 17, 2010)

SDGoat619 said:


> Thanks so much for all the great info. I have been having to add gas to my 600 carb in order to get the car to start when cold. Thinking that not enough gas was getting to the carb i was figuring that a rebuild was necessary. So this is good news, now I get to throw a bigger carb on it and get more power at the same time.


That symptom is something else entirely. The float bowls are leaking down, or the fuel is siphoning back to the tank. It could also b a problem with the choke if you only need to do that when its cold. You might have the same problem with a bigger carb and that would point to the fuel system. A bigger carb on a 455 wont hurt though. I still like Qjets, but Holleys work quite well, I ran them on 455s for years. I didnt see much difference between a 750 Holley and an 850 on my low compression 455s, but they will handle it. Any Qjet will work for a mild street engine with the right configuration.


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## goatcheese (Mar 25, 2011)

(CFM = CID x RPM x VE ÷ 3456) VE=Volumetric Efficiency (most street engines are at 90% or so.)


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## pontiac (Mar 6, 2011)

based on that formula, 600 is good for 5000 rpm on a 455.


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## SDGoat619 (Mar 21, 2011)

Thumpin455 said:


> That symptom is something else entirely. The float bowls are leaking down, or the fuel is siphoning back to the tank. It could also b a problem with the choke if you only need to do that when its cold. You might have the same problem with a bigger carb and that would point to the fuel system. A bigger carb on a 455 wont hurt though. I still like Qjets, but Holleys work quite well, I ran them on 455s for years. I didnt see much difference between a 750 Holley and an 850 on my low compression 455s, but they will handle it. Any Qjet will work for a mild street engine with the right configuration.


Thanks for the info, I hope a new carb (750 or 800) will solve my problem. I think that the choke does need some adjustment. I really hope that we fuel is not siphoning back into the tank.


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## jetstang (Nov 5, 2008)

My buddy had the same problem, his accelerator pump was clogged so he couldn't give it the squirt it needed to start. His was a Holley, I pulled the squirter out of the carb and anti flow back needle under the squirter was stuck, damn E85 gas. Reassembled carb and it fired right up everytime without issue. Look in the carb and see if it squirts..
As others have said, a 600 is fine, it may just leave some power on the table. You can jet any carb to run on any motor. Lots of race cars use 2 barrels and still put out 500 HP, and that's with a 500 CFM carb.
A vacuum secondary carb will only open enough based on the engines demand, so an 800 on a 301 will only open 600 CFM or so.


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## BearGFR (Aug 25, 2008)

goatcheese said:


> (CFM = CID x RPM x VE ÷ 3456) VE=Volumetric Efficiency (most street engines are at 90% or so.)


Here's the thing though - the big unknown in a carb that's "rated" at 600 CFM is that none of the carb manufacturers specify how much vacuum is required beneath the carb in order to get it to pass that much air. It's just like everything else in our "bigger is better" world. If it takes 10" of vacuum to get that carb to pass 600 CFM then it's going to suffocate the motor despite what the ratings and formulae say. Manifold vacuum is present only when the motor is trying to pull more air than it can get. That's why you'll need a carb that's "rated" at 800 CFM or more in order to get one that will supply an honest 600-650 CFM at very near 0" of vacuum, which is where that 455+ is going to want to be at full boogie. 

Oh, and on the dyno my 461 peaked at 97% VE, measured 

Bear


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## SDGoat619 (Mar 21, 2011)

jetstang said:


> My buddy had the same problem, his accelerator pump was clogged so he couldn't give it the squirt it needed to start. His was a Holley, I pulled the squirter out of the carb and anti flow back needle under the squirter was stuck, damn E85 gas. Reassembled carb and it fired right up everytime without issue. Look in the carb and see if it squirts..
> As others have said, a 600 is fine, it may just leave some power on the table. You can jet any carb to run on any motor. Lots of race cars use 2 barrels and still put out 500 HP, and that's with a 500 CFM carb.
> A vacuum secondary carb will only open enough based on the engines demand, so an 800 on a 301 will only open 600 CFM or so.


Yeah I think that is my problem for sure this should do the trick. Might have to upgrade non the less. I have noticed that the right side of the pump was not sending any fuel and now nothing out of either side. 

Edelbrock 1475 - Edelbrock Accelerator Pump Nozzle Kits - Overview - SummitRacing.com


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## 68GTO4004Spd (Jun 19, 2008)

If it were me, I would try to find a Q-jet 4bbl. But there is nothing wrong with Edlebrook it's a better choice than Holley IMO.


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## Thumpin455 (Feb 17, 2010)

Funny thing, I have fewer carb problems when I am running ethanol in my gas, even just 10%. That is all the 70 ran on when I lived in Nebraska, it was 89 octane and cheaper than 0% 87. It cleans things pretty well.

Anyway, if you want a carb SD I would be willing to do one for you.


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## jetstang (Nov 5, 2008)

Thumpin455 said:


> Funny thing, I have fewer carb problems when I am running ethanol in my gas, even just 10%. That is all the 70 ran on when I lived in Nebraska, it was 89 octane and cheaper than 0% 87. It cleans things pretty well.
> 
> Anyway, if you want a carb SD I would be willing to do one for you.


I think you are full of crap, serious??:willy:
I have had Holleys'/Edelbrocks for years and haven't had the issues I have now. Todays gas is such garbage, I have lost more needle/seats than ever before. Alcohol is corrosive and locks things up. Also, the junk gas that goes bad in 6 months is terrible, my garage wreaks of old gas right now, drained the Harley and took all the gas cans outside, and the smell remains..
I seriously need to use Stabil religiously instead of just letting it sit in the cabinet..
Just playin Thumpin..


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## SDGoat619 (Mar 21, 2011)

Got the 800 installed yesterday cant wait to take it for a drive. i need to adjust the timing first as the idle is really low now with the larger carb. I also installed a 1in 4brl - Qjet adaptor block. Really need to get my GTO Hood finished this motor needs the clearence of a hood scoop its getting tall.


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## Thumpin455 (Feb 17, 2010)

SD even with an RPM you should still have some room, I have like two inches clearance with the stock filter in the 70. If you had a 70-76 Formula then even half an inch is too much. How about some video of it running now?




jetstang said:


> I think you are full of crap, serious??:willy:
> I have had Holleys'/Edelbrocks for years and haven't had the issues I have now. Todays gas is such garbage, I have lost more needle/seats than ever before. Alcohol is corrosive and locks things up. Also, the junk gas that goes bad in 6 months is terrible, my garage wreaks of old gas right now, drained the Harley and took all the gas cans outside, and the smell remains..
> I seriously need to use Stabil religiously instead of just letting it sit in the cabinet..
> Just playin Thumpin..


yeah I am serious.  It isnt the ethanol causing the problems, if it were I would be having LOTS of them since I have a car running 100% ethanol now. Also I let it sit in the shop in red plastic cans, both gas and ethanol/E85. Now the cars that had E0 in them have a ton of varnish built up in the tanks, like my 67 Cougar. It will probably need a new tank because my fuel will clean it out and that will destroy an electric fuel pump, and clog lots of filters. 

You want to see the inside of the Qjet I have had E85 in since 07? Or the carbs I ran in Nebraska solely on E10? They are spotless, and the 850 Demon could be bolted back on just like it is. It has never had any fuel that didnt have ethanol in it. Ive had more problems with finding bad gas on my trips than anything with ethanol, its always the stuff that doesnt have any alky in it. Really chaps my ass that I would need to strain it as I fill. I am being 100% honest, I have fewer problems with the fuel system running ethanol, and the parts are all the same ones. 

Perhaps it is the additives you are putting in it, since I dont use any of them.


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## SDGoat619 (Mar 21, 2011)

sorry for the confusion but are you saying that i shouldn't be using a spacer/adaptor. The engine runs great once the throttle is engaged. But at idle struggles to get fuel. I am not sure how to post video. I will try and PM you video. 



Thumpin455 said:


> SD even with an RPM you should still have some room, I have like two inches clearance with the stock filter in the 70. If you had a 70-76 Formula then even half an inch is too much. How about some video of it running now?
> 
> 
> 
> ...


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## SDGoat619 (Mar 21, 2011)

problem fixed, all that i needed was to adjust the fast idle linkage. The 455 is now breathing like it should. Took for a quick run around the block, and my son 2yrs old couldnt stop sayin "vroom vroom daddy's car fast vroom vroom" these cars are so much fun to drive. lovin it


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