Sbc swap to bbc

[Sawlog]

Have any of you swapped your sbc to a big block? If so... how bad did it hit the mileage?
I'm getting 13 miles a gallon with my mild 350. Th 400 tranny with 3.42 gear. Was wondering what a big block would do. Thanks

[truckster]

Not in a truck, but in a full-size Caprice wagon back in the day. I went from 17-18 MPG to 10-11 MPG.

I've never had a big block truck that got better than 10 MPG, but none of the ones I owned had fuel injection or overdrive.

[AussieinNC]

Unless you need the additional torque of the big block for towing etc.....the mileage hit is usually around a loss of 30% ...as the 454 is approx 30% larger than a 350, so simple physics does the rest....then add in the inevitable additional use of that LOUD pedal....

[KQQL IT]

Originally Posted by truckster

Not in a truck, but in a full-size Caprice wagon back in the day. I went from 17-18 MPG to 10-11 MPG.

I've never had a big block truck that got better than 10 MPG, but none of the ones I owned had fuel injection or overdrive.

Our 99 has both and gets 10-12

[Rufas]

Ok not in a truck. I swapped a 468 BBC into my 67 Olds Cutlass. It has FiTech fuel injection, 700R4 trans, 3.36 rearend and Headmen headers. Typical mileage in town 10 mpg. Typical mileage on highway 11 mpg.

[weq92f]

Originally Posted by Rufas

Ok not in a truck. I swapped a 468 BBC into my 67 Olds Cutlass. It has FiTech fuel injection, 700R4 trans, 3.36 rearend and Headmen headers. Typical mileage in town 10 mpg. Typical mileage on highway 11 mpg.

That is real close to where I'm at.

-weq

[geezer#99]

I swapped a 396 into a 74 Chevy swb ss. Stock was a 350 2bbl 3ott with 3.40 gear.
The 396 had 10.25 to 1 compression, cam similar to a 375 horse 396, Torker intake, 750 Holley vs, headers. Optimized timing because I lived at 3500 feet.
Mileage never changed.
Got 17 mpg (about 14 US)

Traded that truck (and two grand)for a 70 chevelle ss454. LS6, m22 and 3.08 12 bolt. The guy I bought it from had replaced the solid lifter cam with a stock truck cam. Pulled down an incredible 22 mpg (18US). I swapped out the truck cam for a hydraulic with specs close to an L88. Mileage dropped drastically to 15 (12 US). It would fry the hides all the way into high gear though.
This was back in 1978 when gas was cheap and had more octane in it.

If you tune them right a bb should give the same mileage.

[jimijam00]

I get 10 mpg highway with a not-so-stock 427 in my C10. And that's with a 3.08 in the 12 bolt and a 28 inch tall tire.

[Speedracer39]

This is my 496 that was my upgrade last winter. I’m sure it gives me many more smiles per gallon than miles.

[Ziegelsteinfaust]

Most truck big blocks had a real measured 8-1 or less true compression ratio. So with the D chamber many had, and large chamber. This gave you a very slow flame propagation which in turn murdered mpg.

Bump the compression up to 9 or 10-1, and they can seriously knock down the mpg. For big engines.

Downsides are towing heavy is not in the cards unless you add a bottle of octane booster to the high test gas. Also your carb has to be spot on to get the AFR between 12.5 and 13-1 when under a big load such as a hill. Also hitting the hills with a trailer your cooling system will need to deal with the extra load.

My friend 496 with 9-1, oval port heads, and roughly a comp 265 cam. It hauled nearly 11,000 pounds up the cajon pass like nothing. Stock 7.3 power strokes couldn't keep with him. Granted it sucked down about 6 gallons if gas in 25 miles, but he kept the rpm at 4500. Right on peak torque, and it was happy doing it. If he drove it like he cared it could do almost 17 highway. Later he added a 4L80E, and it would not get better mpg. It just got to the high number alot more often, and at higher speeds.

If you watch your details, and spend money on the right parts. A modernized big block can do alot.

[Gigem]

If you care even a little bit about gas mileage, don't do it