I've heard that increaseing compression will increase fuel effiency.
I am looking to go with the flat tops raising my compressio from 8 to like 9.2 and I was thinking of milling the heads how much better will my fuel economy be?

 

I don't think you would get much increase from milling but it's good to square things up. 9.2 is a good point to aim for. After that you have to get real involved.

 

to increase compression I know I've gotta shave some metal but If I get the heads milled a little will i have to have the intake modified as well or for that matter the block? and could I just take it all off the block instead of the heads??

 

will raising my compression this much cause me to fail the smog test?
I am getting new cats and a better ignition if that'll help

 

Increasing compression from 8 to 9:1 is only worth about 2% increase in power. Even if the mileage improved by 2% it would be hard to measure.

The cost difference between 87 and 89/91 octane fuel that you will likely need with higher compression will cancel any mileage improvements.

 

2%? I read somewhere that It was like 20% i dont know how accurate it was are you sure its only 2? and 4 for power,

 

You really should look to the pistons for taller height to get the compression up. Yes if you shave too much off you will have to mill either the manifold or the seating surface on the side of the head. 9:1 or above will probably require 91/92 octane depending on your operating altitude.

 

Anyone with a desktop dyno program should be able to verify the power increase for changing compression ratio. My information comes from David Vizard's "How to build Horsepower Vol 1. Everyone should buy this book! If it's wrong, someone tell me.

Pg. 36 has a chart showing the percentage power increase from raising compression. The percentage increase drops as compresion goes up.

8-9:1 = +2.0%
9-10:1 = +1.7%
10-11:1 = +1.5%
11-12:1 = +1.3%
etc.

I'll bet you can lower your axle gear ration by 10%, keep the 87 octane and spend less $$ per mile than a 9.5:1 engine with 91 octane fuel. Any other theories?

 

yes, i played with the dd2k and cr. those gains are close. i was surpirsed when i saw the change. i always went off the old runor of 35-50hp per cr. reality hit hard.

 

Now I'm confused...why is everyone always chasing flat top pistons and 9+ cr ratios if it doesnt do that much for you? Surely there is some reason for it to be so important! Somebody tell me quick cause I just spent and extra $100 for the 9:1 pistons

 

You guys really need to start thinking of an engine as a "system." All the components have to work together to produce the ultimate result of converting hydrocarbons into heat, noise, and kinetic energy. Components that don't work well together actually reduce the output potential.

In general, changing only one operating parameter of this somewhat complicated system does not have much affect on its efficiency.

The most significant change in power potential comes from improving volumetric efficiency (i.e., breathing). Volumetric efficiency determines the amount of fuel/air that the engine can "inhale," which correlates directly with the amount of power it can produce.

Increasing cylinder compression alone increases combustion efficiency only a little. However, increased compression allows you to substantially improve volumetric efficiency, if you use the right camshaft and induction systems. Volumetric efficiency is improved even more if you use the right exhaust system.

More cubes gives more power because more cubes inhales more fuel/air mixture for combustion. It's that simple. More volumetric efficiency gives more power from the same cubes.

 

I finally found a place that has the badger pistions that I wanted the 400-10f ones. The only problem I am getting conflicting information about what my compression will be, I've read on this forum that with 76cc heads the compression is like 9.1:1 but his book said it was like 10.3:1 or something, whos right?

 

Try this,
http://www.kb-silvolite.com/calc.phtml
Let us know if you need any help.

 

One other thing to consider if you are milling the block and/or heads for a comperession increase is valvetrain geometry with those non-adjustable rocker arms. Milling too much can mean having to buy a new set of pushrods.