Im doing this swap to my own truck in the next couple of months (with ported 862's, ferrea valves and a custom comp cam, ARH headers ect, projected just shy of 500hp @ the crank), but my neighbor who has a identical truck to mine (except his is a 2500 GMC Sierra with the same 4x4 and 6.0) wanted to know if there was any gas mileage gains to the swap?

If he did his swap, he would JUST get some stock 862 heads, replace the tiny valves with the larger factory valves and get a valve job and mill .010 off the head to clean it up- thats it. With a stock "LQ4" and those heads, the compression ratio would be just shy of 11:1- he won't change the stock tiny camshaft or anything else and wants to know if it would significantly increase gas mileage. He works construction and cares only about towing and gas mileage. Personally, its a 6,400lb truck... gas mileage is that legendary thing you don't get but talk alot about but anyway...

He currently gets 11mpg average (I get slightly better) with an all stock engine, stock exhaust ect. I told him just a head swap to jack the compression would "maybe" get him to the 14mpg average range- has anyone done JUST a head swap or compression increase and recorded the gas mileage difference?

 

Nobody has ever just swapped on a set of 862/706 heads on a 6.0 for gas mileage?

 

It might get him to the 12 or 12.5 range, but its not going to do any miracles.

 

I may be completely wrong but isn't the bore on the 6L a 4" bore whereas the 4.8/5.3s are smaller? So would running the 862s be possible? Someone please correct me if I'm wrong

 

It's possible to bolt 862 heads on a 6L but the compression is a lil high for 87 octane.

 

You can bolt small-bore heads on the big-bore engines just fine, you just have to take compression into account. It's when you try to bolt heads designed for a 4+" bore on a small-bore block (eg L92 heads on a 5.x) that you start to run into issues with valves contacting the cylinder wall.