Confused about compression ratio

[DW]

I'm building a 69 383 hp engine for my 64 Belvedere.  The goal is to build a nice street engine that is more powerful than stock, but has good vacuum for power brakes and good idle.  I'm thinking about using one of the Engle cams, maybe the K-56 or K-58.  I was thinking I should try to get a 10:1 compression ratio, but I have 2 questions.  First, should I really go for that much.  Seems like reading other posts it'll still work on pump gas.  But I noticed on Engle's site that the comments for both of these cams mention they are for 9:1 engines.  Is that carved in stone or will the cams work even better at 10:1, and they are saying 9:1 is a minimum?

My second question is how to get 10:1.  I have 906 heads.  The books say they have 83cc chambers.  I was going to use KB Hyper pistions.  They have 2 383 pistons.  The KB162 is a flat top with reliefs for a 5cc head volume.  As you can see in the attachments, thats 9:1 with a zero deck and .04 gasket.  The KB400 would get me to 10:1 with it's -6cc volume.  Should I go with the KB400's and get the extra compression?  Is the HP gain I would get out of that worth it to risk getting closer to a detonation point?  You can see the pistons here:

http://kb-silvolite.com/performance.php?action=search&mfg=CHRYSLER&EngSize=383&RodLen=6.358

[694spdRT]

Most of the 906, 346, 452 heads I have seen are around 88-90cc unless they were milled.

You should physically measure the chambers to get an accurate number.

I would not go above 9.5 to 1 with open chamber iron heads on a street engine.

Also, steel shim head gaskets are 0.020" which will gain a little compression if need be.

[SeattleCharger]

   Anyone run 10:1 with super unleaded?   I was considering if this would be ok.

[DW]

Thanks 69, so I guess most of the guys running 10:1 are using aluminum heads?  Any suggestions on how to measure the chambers.  I remember reading somewhere that you pour water into them and measure how much it takes to fill them up.  Is that right?  Is there some kind of tester or measuring device specifically for this?

[694spdRT]

I measured my chambers using a sturdy piece of clear plexiglass with a tiny hole drilled in it and a 10cc syringe. Just secure the plexiglass and carefully fill the cylinder with water while keeping track of the amount of water put in until the air inside the chamber is gone. I used a thin layer of grease to help seal the valves from the water and it worked for me.

[DW]

Thanks, I'll give it a try.

[mally69]

i have a 440  with 906 heads    i run domed 10.34 compression plus the heads are decked 7 thosanths  so i guess around  10.4:1 compression ratio   runs just fine with 93 octane

[Chryco Psycho]

I would never used KB pistons , spend a few extra $$ a use the Speed Pro L2315F with a steel shim gasket , deck the heads 10-20 thou & be in the mid 9:1 compression range
906 heads are 88 cc approx , , if you can use a set of 516 or 915 heads with the larger 1.81 exhaust valves installed you will have a quench engine withj a composition gasket & just under 10:1 with out milling the heads at all

[DW]

Chryco, do you not like KB in general or is it that they are hyper vs. the forged Speed Pro's?  This is intended to be a budget rebuild for a mildly modified street engine.  There's a guy on ebay selling KB's for $259 a set.  Summitt has the Speed Pro's for $367.  I always read that hyper's were better for a mild street build.  On my Coronet R/T there won't be a question I'll spend the extra bucks for the good pieces, but frankly I've decided to use the Belvedere as sort of a practice run before I start building my Coronet.  Both cars need some sheet metal work and an engine rebuild.  I haven't done either before, and I want the Coronet to come out really nice.

So, considering that, would you still go for the Speed Pro's? 

Thanks to all for the input.

[Runner]

       i ran my 452 at 10.0-1 compresion with a 87cc 906 head and a .039 thiink felpro gasket and had no issue before bolting on the e heads and getting the cr up to 10.4.    the 383 in my 68 satillite is 9.4-1 compresion with 78cc 516 heads and it is very sensitive to octane.    cam choice will play a huge roll in what cr you can get away with on pump gas.

   speed pro pistons are ok  but the are friggen heavy.  i put them in the 440 in my daughters car.the bob weight was just over 2700 grams.i have kbs in my 452 low deck motor and  id buy them again if the situation was right .  i needed a .040 over 400 piston for a short rod 3.75 stroke crank and they were the only one that made  piston that fit the bill for me.   ross was the only other company that made a short rod piston but i believe it was in a 4.35 bore only and was double the price.   it came down to going with kbs or putting a 383 bacxk in the car. easy choice.   


       could also buy the 440 source 431 stroker kit, that would make it easier to get the cr up, give you a much lighter rotating assembly, give you new rods, and a really nice quality piston ( diamond) and butt loads more torque.

[DW]

I've been to 440 Source's site many times and drooled over those stroker kits.  I'd love to do that with this car but I just don't know if it's worth the expense.  I don't know how long I'm going to keep it.  I just ended up with it cause a guy I knew was selling it and I got it for cheap.  Thought I'd fix it up some and sell it, getting some resto experience before doing the Coronet.  Oh well, plans change.  Maybe I'll break down on the stroker kit if the budget allows.  My 14 year old son is begging me to keep it so he can have it when he gets his license.  'Spose I can fool myself into thinking that's a good reason to keep it.

When you say the cam will play a big role in what I can get away with compression wise, what aspect of it should I be concerned with?  Duration, lift, LSA?  Can you give me a general formula, like more duration = more tolerance for bad fuel, or something like that?

[Runner]

im probably not the right person to answer your cr and cam question.  it has to do with valve timing events more than lift.  the more valve overlap you have, the more cylender presure you bleed off. that calculator your using at kb will figure it out for you if you input the nubers at the bottom for dynamic cr.

[Chryco Psycho]

longer duration cams will bleed off more pressure at lower rpm allowing a bit more compression , at high rpm the increased overlap helps draw in more air so you can actually mildly supercharge & build higher compression & at high RPM detonation will not be an issue generally