With street (3.08) gears in a heavy car, how "big" a cam can you use before gas mileage starts to go away? I'm looking at the TA 212 or maybe something bigger (?) for my daily driver LeSabre, but don't want to kill what little fuel economy I have...o No:
James, You ask a very subjective question. But, I will tell you that I've gotten good gas mileage and HP from a TA212 in the past. -Bob Cunningham
Gas mileage and performance cams don't need to be mentioned in the same sentence!! Can't have one without loosing the other usually.
I've got 3.08 gears and run a TA413 cam / With a worked Holley 850 DP - with 28" tires I get over 15 mpg highway @ 65-70mph and can easily still punch up some 12.6's -12.8's on street tires .....this in a 4,100 Pound '69 GS (464) Convert.
When I had a 464 in my skylark, with Stg 2 street elim. heads, SP1, 1000 Race Demon, 254/268 dur., .550/.575 lift cam, I got 14.5 MPG highway with a 200-4r and 2.56 gears. The milege dropped about by half any other time :laugh:
After watching this board and cam discussions, boy do I know it's subjective! :Smarty: I'm just trying to see if there is a rough approximation, or "rule of thumb" to go by. I note that some cam descriptions say everything from "increases mileage" to "good mileage" (whatever that means) to "fuggetaboutit". o No: In my mind, engine combinations run along a line from "Bone Stock" (doggy performance, ok mileage) :boring: to "Performance" (where performance AND mileage can increase) to "Race" (big gains in performance, who cares about mileage?) :bglasses: . I'd love an engine that was way over in the "race" category, but reality dictates otherwise, so I'm trying to find out where the upper limit of "performance" overlaps with the lower limit of "race"...make sense? I want to have my cake, and eat it, too. :Brow:
small cam + good head flow + tight quench (.032-.035) = good power and gas milage. Quench is more inportant than anything else in my book. I have seen motors pick up 40 horse just with quench. Plus you can run better on pump gas. If you look at the Engine Master Challenge top five. They all had a tight quench. That is how they make all that power on 93 octane.
I was thinking about this during my last build, which I wanted to operate on 87 octane. It doesn't look like it would be possible to build a BBB with tight quench and keep the ability to run on low octane fuel. I'll use my latest 462 build for an example. I used the 9:1 federal mogul Hypereutectic pistons. Even if run at .005" deck height, and using a .027" cometic gasket to get the .032" desired, the shape of the piston does not provide very much quench area. Built this way, I would end up with 9.86:1 compression, and I doubt the amount of quench with these pistons would offset the .87 increase in compression. The 10:1 forged federal mogul/TRW pistons don't seem to be any better. It seems the best bet would be to have custom pistons made up to mirror the shape of the combustion chamber = $$$$$. For a typical low-budget street build, i would question the worth of this, since many BBB combinations are not driven lots of miles anyway, and it would take a long time to offset the extra cost of custom pistons and expensive gaskets with the improved gas milage. It would be nice to have off-the shelf, relatively inexpensive pistons available with good quench characteristics in mind, but I'm not holding my breath for that to happen. I've done some interesting reading on quench here - Combustion Space/Quench Info
So........quench is squeezing the mixture into as small a space as possible in order to get maximum efficiency from the combustion?
TA's new "conical dish" pistons would help in this area. They would help to focus the combustion event.
