Carb Advice...Is 770cfm Too Much? - Team Camaro Tech
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post #1 of 17 (permalink) Old Sep 26th, 05, 07:05 PM Thread Starter
Mark
 
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Carb Advice...Is 770cfm Too Much?

OK, I'm finally building the motor.

The lower end is stock LT1 (not LT-1), bored .030 over. I'm using an LT4 hot cam (.525" lift, 218/228 duration). Aluminum LT1 heads, GM roller rockers (1.6:1). The cool part is the GM LT1 aluminum carb intake, with a hole for an HEI.

700R4 with a lock-up converter, and posi 3.42 rearend.

I want to use a Holley Street Avenger carb, either the 670cfm or the 770 cfm. Is the 770 too big? Holley recommends it for over 400hp, I think I might get close to 400, but probably not over. Any suggestions?
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post #2 of 17 (permalink) Old Sep 26th, 05, 09:21 PM
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Tom
 
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Re: Carb Advice...Is 770cfm Too Much?

I believe you are talking about a 350ci motor right ? I would go with the 670. the engine could be happy with a 600-650 cfm but, I would try the 670. IMHO you only want a 750 or 770 on strip only 350's. I think a 750 or 770 cfm on a 383 plus cubic inches.

69 Camaro -originally a LM1 car. 327, Edelbrock E-streets, hooker, DUI performance distributors, ultradyne cam, stewart, TRW etc. Vintage Air, AGR steering, Corbeau GTSII seats, 700R4 transmission, 12-bolt w/Eaton 4.11:1
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post #3 of 17 (permalink) Old Sep 26th, 05, 10:12 PM
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Al
 
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Re: Carb Advice...Is 770cfm Too Much?

I'd choose the 770. Vac secondaries are pretty flexible, and you might want more later. It should run well with either.


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post #4 of 17 (permalink) Old Sep 27th, 05, 07:33 AM Thread Starter
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Re: Carb Advice...Is 770cfm Too Much?

I had to chuckle when I read these two posts! I'm just as confused now as when I started!! LOL

Anyway, yes the motor is a 350, I guess after the over bore it is technically a 355.

Anyone else with an opinion?
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post #5 of 17 (permalink) Old Sep 27th, 05, 06:49 PM
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Re: Carb Advice...Is 770cfm Too Much?

Check out my responses in https://www.camaros.net/forums/showth...ric+Efficiency. Too much info to retype here. In summary use the 670 unless the 770 is a spread bore carb. Yes I know, others have used 750 square bore carbs on their 350SB without issue, but the gain is minimal compared to the loss of streetability and headaches setting it up right.

Dave
========================
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI
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post #6 of 17 (permalink) Old Sep 27th, 05, 07:16 PM
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Al
 
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Re: Carb Advice...Is 770cfm Too Much?

Not to start a war or anything....your 400 HP 350 will run just fine with either carb. Consider the the fact that that GM used a 780 Holley on the 302 in z/28's back then. While rated at 290 HP, 350 or so HP being more realistic, they still weren't over carbed. The key is that they used a vacuum secondary carb. (square bore at that).

The Street Avengers are vac secondary, and they basically only use as much CFM as the engine can pull through it. The 770 will work just fine on your 350, and should you decide to jazz it up down the road, you won't need to worry about being under carbed. Scads and scads of folks run 750 VS (3310) Holleys on warmed over 350's without issue, as Dave said.

As far as headaches in setting it up - I doubt it will be difficult. Start with stock jetting, and make sure the floats are adjusted correctly, adjust the throttle linkage and the idle mixture, and have fun.


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post #7 of 17 (permalink) Old Sep 29th, 05, 07:55 PM
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Re: Carb Advice...Is 770cfm Too Much?

Did a google search for carburetor sizing on google and found this article on chevy high performance. They took a 383 stroker, set it up on a dyno with the 02 sensors in place to monitor unburnt fuel and exhaust gases and such and then put holleys of about 5 different sizes and dyno'd the motor and posted all the dyno results on the web. it will open your eyes about bigger is better when it comes to carbs. There is a basic formula for figuring this that goes something like cubic inches X 2 and then 85% of that. So for a 350 its 85% of 700 which work outs to be 595 cfm. im putting a 600 or a 650 double pumper on mine. There is really very little to be gained by over carbing a car and with gas prices the way they are now why give these creeps any more of your money than you have to. ALthough their testing agrees the car will run good with a 770 or even a 1050 for that matter but what you get for the bigger barrels is minimal. It was a very good read and a real eye opener for me.

Heres the link for the article.

http://www.chevyhiperformance.com/te...ose/index.html

Steve
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post #8 of 17 (permalink) Old Sep 30th, 05, 08:36 AM
 
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Exclamation Re: Carb Advice...Is 770cfm Too Much?

What gear in the rear? what tire height? roller camshaft? Usually a 670 cfm is going to be sanppy at busting the tire loose idle-5500, but a 770 cfm is going to pull hard from 2000-7500
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post #9 of 17 (permalink) Old Sep 30th, 05, 08:38 AM
 
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Re: Carb Advice...Is 770cfm Too Much?

Carb jetting on a dyno does not relate to real-world driving. I recomend 2 jet sizes bigger than the dyno, because the dyno is not making uneven load on the engine. We find this all the time when running a dyno for engines used at the speedway and the dragstrip.
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post #10 of 17 (permalink) Old Oct 1st, 05, 11:20 AM
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Steve
 
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Re: Carb Advice...Is 770cfm Too Much?

I'm in the same boat. Everytime I think I have my mind made up on a size I change my mind. Is a 600 or a 650 a good choice for a 350? From what I have read (understood that a dyno is not a real life application) and the formula's I have seen a 570 should be plenty enough for a 350/350 horse build. Thats what my application will realistically end up being. I know alot of guys brag up their 400-425 horse 350's but they have never been on a dyno and the ones that do are sometimes disappointed. away a little sober when the car is only making 350 or 375 and never reaches the 400 mark. I know there are a lot of 400 hp 350's out there but in my town horsepower is always conversationally inflated.
I am seriously considering a 570 street avenger, vacuum secondaries and manual choke on mine. Will I be disappointed?

Engine Specs:

350 punched 60 over
9:5 to 1 compression
Strange 373 posi
Muncie M-21 close ratio
64 C/C updated heads that are tweaked a little and will be ported and polished
a 69 z/28 winters foundry intake
a comp cams extreme energy split pattern cam. I dont know the specs off hand but it is listed as a slightly noticable idle cam nothing crazy here no roller cam.
15" wheels and have not chosen a tire height but would welcome recommendations.
Just a nice driveable streetable camaro.
Realistic expected rpm range 1500-5500. It will probably not see a race track or 7000 rpm's in its lifetime 6000 maybe but certainly not everyday all day with a 4 speed. for this application I am wondering if a 570 would be starving it out. The book formula says a 530 cfm carb is enough buit i'm having a little trouble with that. Seems small.


Steve
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post #11 of 17 (permalink) Old Oct 1st, 05, 01:28 PM
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Re: Carb Advice...Is 770cfm Too Much?

No, you won't be disappointed at all. A "street driver" will spent 90% of it's time below 4000 rpm, running on the primaries. Small primaries and high velocity's build low rpm torque, and on the street, low rpm torque is what buys the smiles. It will pull hard to 6000 or better, when you want it to, without losing it's "snap" at street rpm's.

Too many people spend lotsa money building a motor with a 4500-6500 rpm power band when they rarely if ever run the engine in that range.

Years ago, I ran a mild 406 with a 650 DP carb. It was awesome on the street, and it ran 108 in the quarter (with zero traction). Gearing was 3:73, shifts were at 5800 or so. It idled like a stocker at 500 rpm. Pure smiles, all the way.





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post #12 of 17 (permalink) Old Oct 1st, 05, 04:35 PM
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Re: Carb Advice...Is 770cfm Too Much?

Well stated Jim. Too many times I see people build thier motors to be faster than the neighborhood mustangs, but complain about all the problems they compromised for. You just gotta decide which sand-box you want to play in. You can go as fast as you like if you've got the $$. But be realistic. If you plan to drive your car on the street, there is no need to design it for 7000 RPMs.

Dave
========================
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI
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post #13 of 17 (permalink) Old Oct 1st, 05, 04:42 PM
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Re: Carb Advice...Is 770cfm Too Much?

Quote:
Originally Posted by 69x77
Did a google search for carburetor sizing on google and found this article on chevy high performance. They took a 383 stroker...Steve
Good article Steve, thanks for posting.

Dave
========================
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI
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post #14 of 17 (permalink) Old Oct 1st, 05, 06:48 PM
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Steve
 
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Re: Carb Advice...Is 770cfm Too Much?

Quote:
Originally Posted by JimM
A "street driver" will spent 90% of it's time below 4000 rpm, running on the primaries. Small primaries and high velocity's build low rpm torque, and on the street, low rpm torque is what buys the smiles.

Of course... Smaller primaries = Greater air signal at streetable rpm's so it will still be snappy when cruising but when you really wanna feel the pull, grab the secondary's. With the M-21 I should be able to manage the rpms depending on my driving mood. Sounds like the ticket to me.

ok putting a check mark besides carb. hmm now lets see about that ignition. GM HEI (KISS) or ignition box, coil and Mallory unilite. I loke the GM HEI but I hate that gig giant monster cap sticking out there like a castle tower. and what about advance???? Mechanical or vacuum. I'm assuming Vacuum with the vacuum secondaries on carb or does it matter???

Doesn't seem like things have changed much in 30yrs. The one I had when I was 17 was primer grey and so is the one I have now.
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post #15 of 17 (permalink) Old Oct 2nd, 05, 10:12 AM
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Re: Carb Advice...Is 770cfm Too Much?

Quote:
Originally Posted by 69x77
...hmm now lets see about that ignition. GM HEI (KISS) or ignition box, coil and Mallory unilite. I loke the GM HEI but I hate that gig giant monster cap sticking out there like a castle tower. and what about advance???? Mechanical or vacuum. I'm assuming Vacuum with the vacuum secondaries on carb or does it matter???
Unless your motor is very radical and destined for the 1/4 mile you should always have some sort of idle advance that is sensitive to engine load (a.k.a. vacuum advance). In addition you need and advance mechanism that works with engine speed (a.k.a. mechanical advance). So it's not a matter of one or the other, and it has nothing to do with vacuum secondaries.

If you don't like the GM big-body HEI, but like the KISS of GM HEI, check out http://www.davessmallbodyheis.com. He will convert a breaker point distributor into an HEI system for you that will look stock, fit well and will be 100% GM HEI. Pertronix and others make similar conversion kits, but the nice thing about GM HEI is the ability to find parts just about anywhere, and they are proven to run for 10s or 100s of thousands of miles.

Dave
========================
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI
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