Hypothetical build, flat top pistons @ zero deck, factory closed chamber iron heads 78cc, .039 head gasket, approximately 10:50-1 compression. Will I have enough quench to run pump premium or do I need aluminum heads?

You'd need to lose about 1.0 of compression off that combo for that to co-operate with pump gas and a mild street cam. Perhaps some D-dished pistons or better yet aluminum heads.

Agree!

Nope

I ran 10.0 comp in my old 440 with 906 heads and quench as I used the KB quench pad pistons. I ran the MP .557 cam because as we all know the cam has alot to do with it also. I ran 38 total timing using 92 pump and had no problem as I had no ping at all. In fact the eng is still running in a Duster as I ran it from 2006 until 2011 in my 63. I had no vacum advance on it as my dist is a race type and has no vacum advance on it. You could run 10.5 with iron heads if it is built right being the cam , cyl pressure and so on. Myself I would not push that much on todays gas if I was building an iron headed street car. I felt I was pushing it at 10.0. You would be better around 9.5 as many said here because if the build is not just right for pump you can still get away with it much easier at 9.5 comp. Ron

.039 is good quench but beware the thicknesses of head gaskets can be way off from the stated dimention (mike an old one of same type). You want the DCR reasonable enough for your octane so you can run an aggressive ign curve. On the aluminum there was a good discussion here (if I read it right) on that & that other than the weight savings off the front that there is no real magic as in more power from alum VS cast iron but you would get closed chambers and likely better ports (plus potential for more porting). Just me If the octane is set in stone (& proportioning in race gas in a DD gets old fast) I would lower the SCR/keep the quench (if you can with your piston/gasket selection) then max out the ign and ESPECIALLY get the CORRECT cam in there for your (driving) tastes

Your build is closed chamber zero deck pistons so you will have decent squish.

Your theoretical 78cc heads are more like 84cc. You may have up to 5cc in the valve pockets. Thus, actual compression of this theoretical build would probably be liveable if you kept the intake cool and the mixture right.

Calculator is 600 miles away or I'd do it now.

R.

I can tell you from experience. 9.7-1 214@.050" cam, 3.23, Coronet R/T convertible (heavy), .040" adjusted quench (all combustion chambers equalized and smoothed), stock intake and carb. Barely makes it on 93 octane, some ping when really hot.
Doug

Right now I'm running a 67 440 w/stock flat tops, .070 in the hole, #346 heads with .020 steel head gaskets. Calculates to 9:23-1. Eddy RPM intake, 440 Source distributor w/no vacuum advance, Summit 6401 cam and matching Crane spring. It runs fine on 89 octane w/no quench. It pulls pretty good now, I just want to build something with more oomph and I'm on a tight budget. I have a small block factory hi stall converter behind it and I want to keep my 3:23 gears to cruise. Any suggestions to improve on what I have would be appreciated.

With 9.23 compression I think you are right about where you need to be. Its pretty well dialed in. I'm guessing the heads must be milled to get that compression.

There may be torque in the tuneup, especially the spark advance curve. Not sure what you are starting with but bringing it in earlier may help (may also require premium fuel). Also, if not done already, I would suggest check and correct "true TDC" with the positive stop method. Timing marks on mine were almost 10 degrees off.

A more aggressive cam profile may help a little (eg comp extreme energy or Vodoo) but may not be budget friendly with possible new springs and maybe rockers.

Personally I'd tune it up and run it. If there is a tuning shop with chassis dyno nearby, that may be worth the money if they can dial in the advance curve and jetting.

I have a few 915 heads with std valve size and they are all 80 cc or less. I suppose if you do enough valve grinds w/o a valve replacement and no cut or clean-up on the head you could get to 84 cc.

adv the cam 4 deg. retune the dist subsystems. the exhaust is typically a restriction that needs reworking

Running Hedman headers, 1 3/4" with 2 1/2 exhaust pipes.

mandrel bends? loud (unrestricted) muffs?. uncorking the collectors & going out for a spin will tell you if a restriction from there rearward is holding you back excessively

Yes, mandrel bends and Thrush turbo mufflers that dump at the rear axle. Thought about calling Hughes and getting their recommendation on a cam for my set up.

Did you CC a combustion chamber in both heads? If not you should as those heads stock are close to 91.0+ on all the ones I've check. I'm thinking your true measured compression ratio is probaly around 8.5 to 1 or less
Have you jetted the carb. and maxed out the timing curve yet? If not do that also Little things add up quiclky to making a lot more power What size and brand carb are you using?

Well he did say factory closed chamber heads, 78cc. It's the 78 I'm questioning.

I've never measured a head myself so can't say, but there have been a lot of 84cc 915s or 516s mentioned over the years, IIRC.

R.

The factory BB "346" heads are one year only, 1971, and they are big in CC from the factory
As far as your post I didn't see him wanting to change to the factory close chamber heads on his original post, shame on me
Elmor, I wouldn't make the compression ratio much above 9.5 to 1 with any iron wedge head, quinch or no quinch on todays pump swill

Well, I need to up my reading comprehension.

But the OP needs to be clear on a number of things. First he said stock 78cc closed chamber heads. To me, stock meant not aftermarket. Then closed chamber heads meant 915s or 516s.

But later he said 346 heads which are definitely open chamber and no way are they 78 cc and could have been as big as 93cc as they rolled out the dealership door.

So, to the OP - what exactly do you have????????????? Have you even seen these heads' casting numbers or combustion chambers?

If you're dealing with the typical lazy Mopar open chamber, I'd go no more than 9.5:1 and that's only if I was using the KB "quench dome" pistons with the clearances right on.

R.

The engine I'm running right now has 346 open chambered heads they were cc'd at 88cc's, flat tops .070 in the bore, with .020 compressed head gaskets. According to Summit's calculator, I'm at 9:23 compression. I was asking about using the closed chamber heads because I have a set of 516's that have been mildly ported, with the exhaust valves opened up to 1.74. My original post was that I wanted to build another engine with zero deck flat tops using the 516's that I already have. The engine I'm currently running originally came with 915's, but they are long gone. If the 516's are really 84 cc's, perhaps I could use them on the engine I have to pick up some compression without going overboard. I am just trying to get some input from experienced engine builders. At some point in the not to distant future, I want to build another engine. Will zero deck flat top pistons work with aluminum heads? I am under the impression that aluminum transfers heat and you can run higher ratios without detonation. Am I wrong on this assumption?

Almost forgot, Cab, I'm running a 3310 750 Holley.

You're making general statements (blanket) and seeking definitive answers., no offense. The rule of thumb is if your safe on pump with iron you can increase 1point to compensate for thermal efficiency of aluminum. Now if your safe on 9.6cr iron can you get to 10.6 aluminum? I'd say you would be happier with 10.25 and again this all static without factoring cam events. My personal belief is 9.5 iron no quench open chamber. and 10.25 aluminum closed chamber with squish (quench .045) static CR. And keep dynamic CR (more important) at 7.5-8:1 iron and 8.25+ (the plus can go to even 9 with exact tune) with aluminum on pump. The lower numbers leave a little meat on the bone so to speak if the temp rises/humidity drops you get a bad tank ect ect. You will get all kinds of advice and some will tell you to run 10.5 ect. The further you push the edge the less room there is for error. In all fairness most parts come in large. I.E. Piston volume is 6 and not 5, head gasket are .40 not .39, chambers are a cc or so large as cast ect That margin keep a lot of "10.5" engines safe. You'll never have hard answers till you pay someone for their time, expertise, and equipment to sort everything out with exacts.