85 5.0 block vs. 94 5.0 block (gas mileage)
 

Hey guys whats up? I have a '94 GT and a year ago I broke the crank on it. When I did this I split the whole engine down the center. I found a '85 block real cheap that had about 70,000 on it. I set the block in (mild cam, stock heads) and put my factory headers back on, edelbrock upper and lower intake. I have notice a considerable drop in MPG. I have done some reading and found that the '85 block orignally only had 8.4:1 compression as opposed to the '94 9.0:1 compression. The '94 pistons are fly cut for valve clearance where as the '85 pistons are dished deeper. With this in mind, I thought about throwing on my '94 factory heads. These heads have 35,000 miles on them. Do you think this will help? I am wondering if this compression difference would be affecting my gas mileage. I am getting between 16 and 18 miles per gallon. On my old engine I had a e-303 cam, gt-40 heads, edelbrock upper and lower, with much more and was getting a good 22 miles per gallon. Anyone have any suggestions? I do not have any codes either. Thanks

Well, lemme take a couple stabs at the scenerio. But first, what 85 engine do you have? Do you know what cam is in there?

The 94GT computer doesn't really like cam swaps from the posts I've seen. A big *** cam will kill your fuel economy, that's for sure. The pistons are different in the 1985, and the heads are inferior to your heads. Why not make the swap? The 86 GT's flat top forged pistons make the valve clearance pretty tight for the fitment of E7's and with a cam there is no way. I don't know what pistons the 85 used, so I cannot advise you on that information. You may have piston to valve clearance problems if you do the swap. Another question I have is, how does the engine run? I was thinking the firing order changed in 1986, so if your 85 engine has a cam for an 85, the firing order would be different, that would really wack some stuff out.

Was the engine you got originally CFI, or was a it a 4bbl carb GT? Was it a Mustang engine at all? The addition of a roller cam should have helped fuel economy quite a bit, and I don't believe the CFI cars got the roller cam like the 4bbl cars in 1985.

Just throwing out some ideas. The more we know about the new engine, the better we will be able to assist you in figuring out what's going on.