Sxynerd

Does lowering the compression and a having a lighter rotating mass equate to better MPG in a Forced induction motor while it's not seeing boost?

...or more simply, an NA motor with less compression?

(Pump gas)




Ideally, lighter rotating masses do usually equate to better efficiency in a numbers world but shouldn't cam profile, weight and gear play a bigger role?


I'm asking these questions to design a 5.4 2v or 4v build setup for turbos and I'm trying to find the "sweet spot" in efficiency like many of the new fords have. I'm not trying to make big numbers, 350rw ish.

Derf00

In a nutshell,

The most efficient compression ratio for FI (for face value) is somewhere around 8.5:1 or 9:1. Anything more or less you get less per lb of boost also known as dimishing returns.

For more info.
http://www.xcceleration.com/cr-boost%20101.htm

Sxynerd

So does less compression in the lower ranges translate to better fuel economy off boost or does it lessen the burn efficiency of the fuel too much to matter?

In my mustang even at the profile & Hp range I was at I still managed 22-24mpg on the hwy. I own the fact that an F150 with old tech won't ever be as good as new tech but there has to be a happy medium that's in my favor somewhere.

(I get the irony of a "happy medium" and "my favor" being contradictory.) lol

Derf00

Burn efficieny is not directly related to compression ratio. Matching the Air/Fuel ratio for your application will have a greater impact on efficieny than anything else. If you run rich, you're wasting fuel. If you run lean you run hot which lowers efficieny.

What AFR (Air/Fuel Ratio) you run for your application is what will dictate burn efficieny. Running too lean or too rich means a slower burn (less efficient).

I'm curious you don't know this stuff, with a 600rw stang previously in your stall? Or, are you going for the 'cheap laugh'?

Sxynerd

I know what you're saying and I get it completely, I'm looking to see if CR has an effect not if it's the only factor. I know air/fuels role.

school boy

I would think cam profile and air velocity at cruising rpm's would play the biggest role. Creating a lot of torque at those rpm's should help carry the car easier. But with this type of ports and cam would be tiny and have no top end hp. Like def00 said, air/fuel ratio is going to be your best friend. If youre shooting 350 rwhp, install a set of h beams and forged pistons and gasket match everything for smoother air flow. Use pi cam with advanced timing to bring down torque curve into a lower rpm's. Or just custom grind a set of cams for what you are looking for. I know higher compression feel peppier but I really need to read up more on the effects on lower rpms and with milage. Good luck, I enjoy threads like this, gets me brainstorming