I've got about 10.5 compression on my 308....I run premium with 2 bottles of octane booster.....otherwise it knocks....I've tried it without it.....the engine definitely knows the difference!
The cam profile and cylinder pressure have a lot to do with detonation. I've seen high compression pistons used in conjuction with a large overlap cam that didn't have spark knock. Problem is when you get into it and get the rpm up then you are in trouble without the good fuel. Anyhow, I don't know where I'm going with all that...just facts that I've gathered over the years. Oh and the carb? I'd suggest a dominator...but it would never work! LOL A 600 would probably be fine if you decide to build the 250. Good luck!!!
You should definately invest in the Classic Inlines aluminum head. It is a much better design than the stock log head and is probably worth anywhere from 30 to 50 horsepower. Check their website for updates as to when they will be available...should be any day now. They will also have single 4-barrel intakes available and others but they are not ready yet...but soon. http://www.classicinlines.com/
At some point your cam, all cams, reach an RPM where they have peak cylinder compression. If your intake and exhaust are done right your actual compression will go over your static and theoretical compression. Some high performance ignition advance units have a bit of spark retard that can be tuned to prevent preignition at that RPM. Just because a cam reduces pressures at low RPM doesn't mean the pressures never come back up.
Sorry guys but I'm the 3rd one willing to know more details about this suggested upgrade for a I6 250's, anyone ? "A motorcraft 2 barrel for a 302 (a little under 300 cfm) will do you fine for now. After you get you`r cam and all that good stuff you`ll be wanting a 500 - 600 cfm carb ."
most of the 2bbl swaps for an I6 are either a holley/webber 5200 progressive 350cfm or a holley 2300 500cfm. they both go on the same way. im running the 5200 on mine with an adapter plate. everything under the plate is stock. it seems most people have gotten the best responce out of a forced induction set up. the crank is very well supported so the engine can be easily built to handle it
While it may be possible to run 11:1 or 12:1 on the street with 93 octane, I would not do a build based on it. For starters, fuel is changing, and more ethanol is sneaking its way into blends. It's a constantly changing issue and I wouldn't want a build to be dependent on the swaying influence of the eco crowd. Secondly, no matter how much overlap you run to lower cylinder pressure at the low end, you give up a lot of torque down low, and you have to run an advance curve so conservative that whatever horsepower you gained you are giving right back. You also kill vacuum for the operation of things like power brakes. Each point in compression yields about a 4% HP gain. Once you get into the 11:1 range, the curve you have to run will easily take that back. There really isn't any point in doing it.
Trick with the Motorcraft 2100 is watching the carb bore sizes. They're not all the same. These carbs came in cfm ratings from the mid 200's to 464 cfm. The bore size is cast into the driver's side of the bowl, behind the accellerator pump rod. You'll find em with .98 up to 1.23, the 464 cfm unit is a rare bird, it's bore size is 1.33. The Holley 2300 came in three sizes, two oif which are the most common----- 350 or 500 cfm. There's a rare larger 600+ cfm unit but these are hard to find, they rarely even come up on ebay.
