2 650 carbs on a blown 392???

[billlsbird]

....Hello Everyone, I talked to my engine builder today about what carburation he would recommend for the blown 392 hemi that he's building for me. He said not to use any more than 2 650's. But I've always thought that this size blown motor would need 2 750's. Plus that's what the Wiend web site recommeded {2 750's}. He said that 2 650's would be more than enough carburation for my application. What does everyone think about this??? ... Thanks, Bill

oh, 392 hemi in a '32 ford. 8 to 1 compression. 6:71 supercharger, turbo 400 w/ trans brake, 3000 true stall. 4:11 true track rear end. Holley blue fuel pump w/ -8 an line.....

[kitz]

This is from BDS ..............

The size of carb(s) or CFM required for a given application can be calculated by the following formula A: {(CID x RPM) ÷ 3456} x {Boost ÷ 14.7) + 1} = CFM required. The amount of CFM required will determine carburetor size and quantity. If you try to use a carb with less CFM than required, performance and economy may be greatly reduced. Bigger is not always better when selecting carbs that are 30% over what is required, you may encounter problems in fuel distribution.

Kitz

[billlsbird]

....Thanks Kitz! so two 650's probably won't be enough carburation. It looks like somewhere between two 700's & two 750's.... Thanks again! Bill

[valvebounce]

Be sure to externally reference your powervalves if you're running holleys.

[billlsbird]

...Thank You Valvebounce, I am going to run the supercharger carbs. These would already be set up like this, wouldn't they??? Bill

[valvebounce]

I suppose? I'm not really familiar with fancy purpose built stuff. I had to modify the powervalve set up on the holley for my blown engine.

[techinspector1]

OK Bill, let's do the drill.

Kitz has provided you with the formula, so we'll plug in some numbers. You have 8.0:1 static compression ratio. BDS recommends a max boost of 8 psi at sea level for use with pump gas at that scr. Let's assume you will limit the revs to 6,000 with a street motor. (dependent on cam operating range).

So, CID x RPM divided by 3456 = 680 CFM (naturally aspirated @100% efficiency at sea level).

Boost (8) divided by 14.7 plus 1 = 1.54

680 times 1.54 = 1,047 CFM.

Now, again, this is using 100% efficiency at sea level. The elevation in downtown L.V. is 2028, so the motor will inhale 6% less air than it would at sea level. If you really wanted to be real about it, you could therefore multiply 1,047 times 0.94 and determine that the motor will pass 984 CFM. In this example, two 500's would get you there with some to spare. If you wanted to cover all the bases so that the motor would operate at sea level also, you could mount a 500 as the primary and a 600 as the secondary.

[billlsbird]

.....Thanks Tech, whoops I did the formula wrong! It's bored .030 over so I went;

1) 398 {cid} x 6000 {RPM} divided by 3456 = 691.

2) 8 {boost} divided by 14.7 = .5442

3) 691 x .5442 = 376

4) 376 + 1 = 1376

5) 1376 divided by 2 = 688 cfm for each carb.

BUT what I should have done was;

1) 398 {cid} x 6000 {RPM} divided by 3456 = 691

2) 8 {boost} divided by 14.7 + 1 = 1.54

3) 691 x 1.54 = 1064

4) 1064 divided by 2 = 532 cfm per carb....

oh & I forgot to change my home town but I'm back in Calif at sea level again.... my bad....

I also thought I'd may be run a little lower compression ratio {7 to 1} so I could run a little more boost {10 pounds} but even then I'd still only need a total of 1160 cfm divided by 2 = 580 cfm per carb.... which would be two 600's..... THANKS again Tech!!! Bill

[mopar34]

I have a friend with a Mopar 440 running two 500's and that is more then sufficient. He doesn't run a blower. 500's should be all you would need, perhaps 600's max.