I was curious about how a cam with a higher powerband will affect gas mileage. Right now my car has a cc306 and on the highway i can feel the powerband starts around 1800 rpm at about 75 mph. If i cruise below that powerband will it waste more gas, or will it not change?

Ive never gotten a straight answer as to why cams affect gas mileage other than the typical answers about lope, duration, overlap, tune.... yadda yadda yadda

I noticed in my old formula with the cc305 cam it would get stock like gas mileage (31 mpg) and it cruised alot better at 1500 rpm at 60 mph. I always thought it had to do with the powerband starting lower.

-Stu

 

The "power band" is usually the wide open throttle (WOT) readings we see from engine or chassis dyno runs. Most folks change cams primarily to increase WOT power for more aceleration. Cruising at part throttle is a very different engine operating condition. The cam timing can have a large effect on part throttle engine efficiency and therefore fuel economy.

Cruising at 60 mph or so at 1500 or maybe less rpm requires very little throttle plate opening so very little air is getting into the engine. That reduces the "effective" compression ratio to maybe 5 or 6 to 1, or even less, just because there is so little air getting in. If there is considerably more overlap than stock and/or the intake valve closes later than the stock cam (both usually occur with an atermarket performance cam), the dynamic compression ratio is lower than stock, and the already low "effective" part throttle CR is further reduced, so fuel efficiency takes a hit. Also, more overlap allows more dilution of the intake charge with exhaust gas and/or allows intake charge to pass thru into the exhaust. Again efficiency takes a hit.

Obviously this will change with the duration and overlap of the cam. There's really no "free lunch" with valve timing. If you want more torque/power at WOT, especially in the rpm range above normal curise (2000 ++), you have to expect lopey idle, and reduced low rpm drivability and often fuel economy.

There are exeptions, however. By using fairly aggressive lobe profiles, you can get more "area under the lift curve" with fairly short durations and low overlap. You pay for it with more load on the valvetrain parts which means more costly parts and a somewhat shorter life. Lunch is still not free.

Of course, variable valve timing (VVT) can really change this. You need to be able to vary the phasing of at least one of the cams, or both to get both the performance and also the drivability and fuel economy desired. With a single cam, SBC's aren't there yet.

So I guess yadda, yadda, yadda... applies here.