how much of an efect would changing to a bigger cam have on gas mileage? i have a .422/.444 lift 204/214 duration cam in it right now and want to go to a .462/.469 218/224 cam. about how many mpg in general.

also, how much of an efect would adding a 144 or 172ci blower have on it?

thx

 

The duration increase is the main factor in fuel efficiency at road speeds. Generally, (not always) more duration means more overlap and lower vacuum. That tends to lower the fuel charge velocity, cylinder charging, and BMEP at lower RPMs, which tends to reduce mileage. You just don't get quite aqqs much BANG out of each charge, and some is lost back into the intake and out the exhaust at lower RPMs.

Again, these are all theoretical tendencies, but seem to hold true most of the time. As for guessing exactly how much difference you might experience, I haven't got a clue.

 

anyone that tried to tell you how much MPG you will lose will be wrong. it totally depends on the entire drivetrain, as well as compression ratio and intake type/size etc... everything has an effect on fuel economy.

but i can try to help with some example combos I have had.

355, 2.77 gearing, 2400 stall. 234/244 @ .050, 112 LSA. B&M 144 Blower.

10 MPG city, 12 MPG highway. No vacuum advance. no overdrive. No lockup.

Going to 3.73 gearing in that same exact car did:
12 MPG city, 8 MPG highway. I gained fuel economy in town, lost it on the highway. bigtime.

Adding Holley TBI setup to blower and vacuum advance with CC timing:
16 MPG city, 11 MPG highway.

Adding overdrive/lockup:
17 MPG city, 21 MPG highway.

Adding bigger blower:
no change.

Changing to smaller cam (219/224 @ .050, 114 LCA)
19 MPG city, 21 MPG highway (no change highway, more fuel economy on the street. ) THAT was due to much better efficiency in the torque area. less duration around town meant more bang for your buck. I run 87 Octane most of the time, except when i crank the boost past 5 PSI. then i run 92 octane or race fuel if available.