specific head mod questions - please advise

[falcon fanatic]

I've already started porting an M (flat log) head and it will go to the michinist soon. Before it goes, I need to finalize my plan so here are some questions for the experienced among you.

I'll be raising the compression to above 9:1 by milling (best target cr??). But on an otherwise stock 200, is it worth any gain to:

1. Mill the log to accept direct mounting of my weber 38? I'm concerned that such a big hole will have too low port velocity to run well at low revs.

2. install bigger exhaust valves but leave stock intake valves? I've got a header and big exhaust. I'll have the pocket ported intake and the bigger carb, but with a stock cam big intake valves seem like they'll cost me too much on the low end which is were I'll generally be.

3. Hardened seats: Most beneficial on exhaust, intake, or equally important to both?

 

[XFlow_Fairlane]

I would mill for direct mounting. if you only bore a hole big as the bores then it will be ok. since the weber is a 2bbl non progressive carb maybe a steel port divider to make a dual plane intake and epoxy it in? that would help keep up the velocity and make it more tuneable (richen and lean the front and rear halves on their own) with a dual outlet header this could be a nice combo.

I would bore the two holes for mounting the carb then open up the hole into a large oval shape and then epoxy in a .100" or so plate with some big fillets on the bottom and sides then have it all milled at once so the carb can be bolted and sealed well. put a O2 sensor bung on each outlet of a dual out header (use a spark plug anti fouler dealy form the help section of the parts store. cut in half so you get a weld bung and a plug from each one. only like $7 a pair) any cheap o2 sensor off anything and you are set to go for serious tuning.

nick

 

[CZLN6]

Howdy Falcon Fanatic:

In general, all of the mods you've listed tend to have a greater effect at higher rpm ranges, say 3,000 and up. That is very dependent on cam timing.

Our guideline for CR is to match the octane of the gas you intend to run, with a stock or near stock cam. For example if you want to run 85 octane gas shot for a CR of 8.5:1. If you want to run 91 octane go for a 9.1:1 CR.

Several factors effect CR and knock or pre-ignition. Headers and exhaust- add .1. Cam timing- More cam overlap allows more CR. Elevation- for every 1,000 feet above sea level add .1 to your goal CR. Deck clearance of .030" to .040" add .2. For ever .010" over .050" start subtracting .1 from total CR. Ambient temp and elevation also effects how much cr you can run. Higher elevation and lower temp= more CR, lower elevation and higher temp= less CR. Smoothed and polished chambers also help to resist knock. Lower gears-more, higher gears- less.

If you're building your head now and the block later, plan for your final outcome and accomodate the interium. It will save you money and you'll be happier in the long run.

In general, all else being equal, 1 point of CR is worth and honest 4% increase/decrease in output.

Milling the log and mounting the carb directly will be an advantage at max flow. It will also add hood clearance. It is kinda confusing to add a larger volume log head and then choke it off with a funnel adapter. Adding an "M" head and a direct mounted carb will be most noticeable at higher rpms.

Larger exhaust valves are a good idea, but again their value will be most noticeable at higher rpms. If your stock valves are reusable I'd touch them up and use them. If you have to replace your exhaust valves then step up to larger ones. What larger exhaust valves are you planning to use?

It is worthwhile and cost effective to back cut the intake valves along with a three angle performance valve job.

Hardened seats or inserts are most beneficial on the exhaust side, but of value on both intake and exhaust. Typically head machinists set a wider margin on the exhaust to help with heat transfer and longevity. The exhaust valves live in a much more hostile environment then the intakes.

Hope that helps.

Adios, David

 

[falcon fanatic]

David,
Thanks. I'll go back and double check your book before I go to the machine shop. The thing is, this engine (small log, autolite 1101) is pretty wheezy at high revs even with the dual header and big exhaust.

Now, most of my experience is with ricer engines. Take a 1.5 liter honda civic, add header, a high flow cat, a high flow intake and advance the timing a hair and you pick up power across the band, but especially at high revs. Even with a stock cam you gain top end without much or any low end sacrifice.BUT, do too much porting and you end up with a dog.

I'm figuring the same will apply here. bigger carb, more open intake manifold, bigger exhaust valves, dual outlet header coupled with an increase of cr (i don't know what mine is, but I'm pretty sure this engine is stock.) will also give me more top end, even with a stock cam, but with little or no sacrifice on the bottom end. Bigger exhust valves should increase flow without hurting the bottom end, too.

How much bottom end I will lose depends on the engine's ability to maintain port velocity. The question then becomes: If I open up the carb flange do I lose too much velocity? Obvously, the aussie head does fine, but then it has long runners. The runners on this head are so darn short It's almost like they designed it to run at high revs, but then misteriously choked it down with a little carb. What were they thinking? I'm very confused about what to do.