How does Crank/Rod/Piston Weight affect mpg?

[patrickk1]

I just started a build on a 1963 GMC with a 305 V6. My original plan was a cummins 12v, but when I picked up a truck with a good running 305 v6, i decided I would build that motor. So far, I am modifying a gm 5 1/8 bellhousing to nv4500 trans adapter and also the sm420 bellhousing so I can use the nv4500 5spd trans with the 305. I though about re-building a 305 or 351M and lightening it drastically while doing an offset grind on the crankshaft to allow the use of different lighter rods and also pickup more stroke.

The more I thought about driving at relatively steady speeds, the more I started to think that maybe the heavy rotating assembly may help mpg as it has much more stored energy vs a lightweight setup. What do you think about this? Most of my driving is done on the highway.

Patrick

[bigblockv6]

I think it would not hurt to take away some of the rotating mass out of these engines specially in a light duty application, I would definitely take the 351M over the 305 engine. The engine will spin freer with less weight and that may actually improve in normal driving situations. The next time I build another GMC V6 I plan on using Chrysler 426 Hemi rods and have the crank ground and stroked for that but it will definitely be a 401 or 478 V6.

[patrickk1]

Originally Posted by bigblockv6

I think it would not hurt to take away some of the rotating mass out of these engines specially in a light duty application, I would definitely take the 351M over the 305 engine. The engine will spin freer with less weight and that may actually improve in normal driving situations. The next time I build another GMC V6 I plan on using Chrysler 426 Hemi rods and have the crank ground and stroked for that but it will definitely be a 401 or 478 V6.

I was thinking the same about the 426 Hemi rods. I saw somwhere that someone had used them at some time (they hadn't mentioned offset grind) and once I saw how much the crankshaft would have to get reground, figured an offset grind would be worth it for the extra stroke. Also with the offset grind, it would make up for some of the difference in connecting rod length. I haven't had a 305 apart yet to measure the rod length, but from what I have seen mentioned, they are long.

[lizziemeister'sV6]

Hi Patrick - Recently I had a 351E rebuilt - I had planned on using the 426 Hemi rods and the NOS 379 .040 over pistons I had - ( couldn't use the standard rods because the 379 piston pin height was closer to the top of the pistons) so figured this would be a good time to offset grind the crank for a little more stroke. Ended up the machine shop won't use the Hemi rods because of the amount you would need to grind off the crank journal and the difference in wrist pin diameter. So we went with longer custom billet rods to make up the difference in rod lentgh and left the crank alone - not a cheap overhaul but at least there's lighter and better rods. I would probably not do this again. BigblockV6 and I discussed this rebuild many times - he's a good source of knowledge. Cliff

[bigblockv6]

That setup was created by Babe Erson brother to Sig Erson of Erson Cams, done to a 478 that resulted into a 5500 screamer V6. It did require custom pistons since the Hemi rods piston pin diameter is considerably smaller. We did a member about 2 1/2 years ago building a bored and stroked 478 to a 523 based on the same idea but with custom built rods, I think it was Andrew Harmon but he seems to vanished, would have loved to here how is project turned out.