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Old 04-16-2010, 07:42 AM
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Fuel System Design - SBC

I currently have a 350 SBC with flat tops, 76cc 487 casting heads, Crane cam 100042 (Dur 266/266, Lift .440/.440), Performer intake with 1 " spacer topped with a Edelbrock 1406 (600 cfm). The fuel pump is a Holley 110 gpm unit plumbed with a 3/8 input line. Plugs are Accel shorties and Distributor is a Mallory HEI.

The trans is a TH350C with the lockup wired up. Rear gears are 3.42 in a 10 bolt posi corporate axle. Tires are 235-15 T/As.

The truck is used for daily commuting and cruising. Current mileage is 14-15 mpg with the AC off.

My question is whether I can get significant mileage increases if I reconfigure the fuel delivery by changing to an 80 gpm pump, and/or a regulator with a return line. I know that changing to a EFI system or a quadjet might be preferable but those options are not currently on the table.

From what I can determine the 110 gpm pump is a little overkill for the motor and my style of driving. I estimate the engine to have about 300hp. Using the fuel pump sizing calculation of HP x .23 gives me a fuel requirement of 69 gpm.

Would going to a lower volume pump improve the mileage? On the surface it seems it would.

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Last edited by Houston54; 04-16-2010 at 07:49 AM. Reason: Add info
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Old 04-16-2010, 07:48 AM
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No. carb has a float that cuts off the fuel when the bowls are full.

If you want better mpg then look at the carb jetting and the ignition timing curve.

have you read a plug or installed an 02 sensor?

what is the ignition timing set at (initial, total mechanical, vacuum advance)???
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Old 04-16-2010, 07:57 AM
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Currently have initial timing set to 4 degress with the vacuum advance plugged and 32 degrees at all in. Plugs have not been checked lately but were running a bit dark before. Need to check them again.

No O2 sensor in place. The truck is a 54 Chevy we built to drive. Have 21K miles on it in the first two years. This motor was put in at 9K due to a oil pressure issue.

I have not changed any of the jetting in the carb. Have read where they are jetted rich from the get go. I beleive they sell a tuning kit for around $35. Is there a specific size to go to or is it a trial and error process?

I tried a search on this subject and found some information but it did not address all my questions.
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Old 04-16-2010, 08:11 AM
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I think you need more timing (especially with those smogger heads). read this

http://www.corvette-restoration.com/.../Timing101.pdf

use the vacuum advance on the manifold source and get an adjustable vacuum advance canister.

carb adjustment is trail and error. I recommend you install a cheap one wire 02 sensor and use a volt meter to measure the a/f ratio at cruise.
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Old 04-16-2010, 08:15 AM
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here is a nice article how to tune a holley but a lot of the information is universal to any carb. and there is a nice 02 sensor write up at the end.

http://www.bob2000.com/carb.htm
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Old 04-16-2010, 08:51 AM
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If the o2 sensor is to monitor exhaust qxy. content he will need a broad band to see the full spectrum. As for fuel economy making the burn more efficient is the best way, the next best is probably to make the gear a little taller. Personally I would probably save my nickels and lean towards a FI setup. It is just so MUCH more efficient, can't improve on technology. If you have a few hot rod buddies then go together and buy a broad band setup and then you can all take turns tuning your fuel supply, just a weld in bung and a plug when your not using it.
Bill
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Old 04-16-2010, 10:14 AM
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sure a wide band 02 would be best, but not a requirement.

A simple and cheap 1-wire with a voltmeter will give enough information to tune the carb for better highway mpg.
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