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what size carbs on my blown 671 sbc 350?

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darko

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Discussion starter · #1 ·
putting together a stock long block mid 70s chevy 350. Its probably going to have a comp cams 268 cam and start off at 15% underdriven. I got a bg220hr fuel pump and a msd digital 6 plus ignition. Just wondering what size carbs I should be running? Also would it be better to run vac or mech secondaries? I also was either going to run speed demon or holley's. I'll also be running an auto.
 
#2 ·
You need to buy carbs that are vacuum referenced. Meaning they are for a blower motor. A blower doesnt pull a vacuum, it makes pressure. So your power valves will not work unless you buy the correct carbs. Go to the blower sites like Weiand, Blower Shop, BDS and they have a CFM chart. It will ask various questions about your setup and will tell your cfm needed then divide it by 2. There is you carb size.
 
Discussion starter · #3 ·
prostreet69camaro said:
You need to buy carbs that are vacuum referenced. Meaning they are for a blower motor. A blower doesnt pull a vacuum, it makes pressure. So your power valves will not work unless you buy the correct carbs. Go to the blower sites like Weiand, Blower Shop, BDS and they have a CFM chart. It will ask various questions about your setup and will tell your cfm needed then divide it by 2. There is you carb size.
Click to expand...
i am already familiar with the modifications needed for the power valve. I was just curious what people were running/have runned and whether it would be better to go vac secondary or mech secondary.
 
#4 ·
I have a 502 with 8-71 blower. I run 2-750 mech secondary holleys. I run 8 lbs of boost.

I started out with 2-850 mech secondary holleys and it ran to rich at a idle and fowling plugs. These carbs were highly modified race carbs. The 750`s are stock blower carbs.
 
#5 ·
While a blower does make pressure that is under the case/rotors not above (where the carbs are). The base of the carb will see vacuum. A vacuum secondary carb could work but, I am not sure why you would want to do that.

Darko,
If you are going through the trouble and expense to put a blower on it why not make some power? It's not going to cost you any/much more. When you say "stock 350" what do you mean? Cast pistons? 2 or 4 bolt main? Cast crank? Stock rods? For a few more bucks you can build an engine to handle the power and you are going to want to turn up the boost, trust me.

Now if you are dead set on going with a "stock" long block, then a pair of 600-650 cfm carbs is all you will need. You are only going to be able to safely run about 4lbs of boost.
 
#8 ·
Also, take a look through comp cam's catalog for blower cams. Just doing a quick search, I found a 268 grind on their site for blowers. You should make more power with this type of cam. Invest in good head gaskets and head studs.:yes: A pair of 650's should be plenty of fuel.
12-400-4

Cam Style: Hydraulic flat tappet
Basic Operating RPM Range: 1,500-5,500
Intake Duration at 050 inch Lift: 218
Exhaust Duration at 050 inch Lift: 212
Duration at 050 inch Lift: 218 int./212 exh.
Advertised Intake Duration: 268
Advertised Exhaust Duration: 260
Advertised Duration: 268 int./260 exh.
Intake Valve Lift with Factory Rocker Arm Ratio: 0.454 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio: 0.444 in.
Valve Lift with Factory Rocker Arm Ratio: 0.454 int./0.444 exh. lift
Lobe Separation (degrees): 115
Intake Valve Lash: 0.000 in.
Exhaust Valve Lash: 0.000 in.
Computer Controlled Compatible: Yes
Grind Number: CS 260AH-14
Quantity: Sold individually.
 
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Discussion starter · #9 ·
67SS&99SS said:
Also, take a look through comp cam's catalog for blower cams. Just doing a quick search, I found a 268 grind on their site for blowers. You should make more power with this type of cam. Invest in good head gaskets and head studs.:yes: A pair of 650's should be plenty of fuel.
12-400-4

Cam Style: Hydraulic flat tappet
Basic Operating RPM Range: 1,500-5,500
Intake Duration at 050 inch Lift: 218
Exhaust Duration at 050 inch Lift: 212
Duration at 050 inch Lift: 218 int./212 exh.
Advertised Intake Duration: 268
Advertised Exhaust Duration: 260
Advertised Duration: 268 int./260 exh.
Intake Valve Lift with Factory Rocker Arm Ratio: 0.454 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio: 0.444 in.
Valve Lift with Factory Rocker Arm Ratio: 0.454 int./0.444 exh. lift
Lobe Separation (degrees): 115
Intake Valve Lash: 0.000 in.
Exhaust Valve Lash: 0.000 in.
Computer Controlled Compatible: Yes
Grind Number: CS 260AH-14
Quantity: Sold individually.
Click to expand...
i don't doubt that you will make more power with a 114ish lca cam, but i want to stick around 110 to keep the heat down in the cylinder from the cooling effect of the valve overlap. I'm probably going to run the xe274h cam that I got sitting here.
 
#10 ·
While I agree roots blowers do not like to be restricted, we need to look at what the blower is going on. We also need to think about how much boost will be made. I run 2 750 DP's on my 8-71 on a 383 small block but, I also make 12-14 lbs of boost. Since the blower is going to have to be underdriven so much the smaller carbs aren't going to hurt him (IMO). 1500cfms on a stock 350 that might see 4lbs of boost just doesn't add up.
 
#11 ·
Darko,
I don't know how much research you've done but, I see some issues with your combination. If you are going to run a 110 LSA that is fine for higher HP applications where blowing a little boost out the tailpipe won't hurt as bad. In your application 4lbs of boost is about all it will handle depending on what "stock" means to you. Now you add a cam with a 110lsa and blow 1.5 psi out the tail pipe, that is not going to be a very efficient engine. Please don't take this the wrong way, I am trying to help and not trying to knock your combination. If you are just after the "look" then go for it but, if you want to make power you'll have to make some changes.
 
#13 ·
I also agree with camaroman7d. I have seen alot of his posts about blower motors and he seems pretty knowledgable. I can tell you from experience if it is not right you will hate it. Been There Done That. I probably went thru 10 sets of plugs because my buddy who I bought the motor from said the 850 carbs would work. They actually flowed 930 cfm a piece which was 1860 cfm. Now for a all out race motor they were fine. Plus it ran cooler and once I put the 750`s on and started leaning it out it started running hotter. We were at Cruisin the Coast before Katrina and my buddy was behind me and he said I was killing him with the exhaust fumes. But once you get it dialed in you will never go back to naturally aspired motor.
 
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prostreet69camaro
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