When I started building them, they were intended for a .030"-over 292, with flat top pistons. Accepted theory then was, as Y-Block compression is so hard to achieve, an upper limit was never considered.

I now find myself with 318" displacement, and pop-up pistons. When originally chamber-matching the heads, I estimated final volume at 68cc's. With block zero decked, calculated CR is 9.5:1. However, due to abject laziness & wrongheaded assumptions (don't ask), I have ended up with 66.2 cc (smallest chamber). If CR variations due to volume are indeed linear (?), that comes out to 9.8:1. CR.

So, do I bite the bullet and disassemble/grind/clean & CC the heads again multiple times in trial & error attempts to achieve my original target volume (this IS a mild street motor), or will other variables with this particular application that might allow a higher CR (limited breathing, tight piston-head clearance, above average ignition curve/limit control, mixture effects of piston domes, etc.) give me enough leeway? (I would almost rather drag around a case of octane booster, that have to start over again on the heads). Of course, if more chamber grinding IS absolutely needed, better now than after the engine is installed.

6 VOLTS/POS. GRD. NW INDIANA

P.S. (rushed the last post so I wouldn't time-out and lose the text).



I should add: cam is Isky E4 (just slightly warmer than '57 OEM), degreed straight-up. Likely a negative factor re: CR limit.

There is also the complication of some pistons measuring .004-.005" out-of-the-hole. But (luckily), not on the smallest chambers.



I know that FoMoCo was encouraged to start posting the heads due to failures encountered after supercharger use boosted dynamic compression, but since no Y-Block had static CR as high (9.8:1), this adds the complication of gasket failure to likely pinging experienced on 93 octane.

6 VOLTS/POS. GRD. NW INDIANA

Daniel

   If it was my engine I would 1  Switch to a 12 volt system as it will not start hot on 6 volts

                                        2  Run it as is

                                        3 Get the thickest head gaskets and run it

                                        4 Put bolts trrough the top of the cylinder heads to support the combustion chambers from the top

                                        5 Get a cam with more overlap to lower the dynamic compression

  Of all these I like run it as is best. Pete

                                         

Pete,

Don't recall anyone complaining back in '55 that their poked & stroked hotrods with milled heads were hard to start? BTW, my previously mentioned hot/slow-cranking problem seems solved (fingers crossed) with fresh battery. Only time will tell, but HAVE noticed much improved cold starting. I guess carbon pile/BCI load test is only way to truly determine if it is overdue for replacement.

I will likely have to face up to reality, roll up my sleeves and get out the grinder. It occurred to me I might avoid disassembly/seal damage/cleaning hassle by fabbing a protective metal shield for the valves and wrapping the head in plastic. Two CCs of iron could easily be removed by just un-shrouding the intakes some more.


6 VOLTS/POS. GRD. NW INDIANA

The accepted rule of thumb for max static compression ratio for iron headed engines is 9.5:1 but this can vary somewhat depending upon the camshaft.  The later the closing of the intake valve, the more tolerant the engine will be for using a fuel with lower octane numbers.  Rather than use the static compression ratio calculation,  using the dynamic compression ratio can be a better indicator of what an engines tolerance for fuel octane will be.  I use 8.0:1 dynamic cr for 91 octane fuel while the use of 93 octane fuel can raise the max dynamic cr to 8.25:1.

 

Here are the numbers I used to calculate your dynamic compression ratio.  292 crankshaft?  You didn't say so that's what I used for the calculations.  The 60° ABDC closing for the intake valve is a rough estimate based on some Isky E4 numbers I had handy.  You didn’t mention what your piston dome cc’s so I simply based my value on what the typical Y domed pistons are.  I did come up with a higher static compression ratio than you did which also drives dynamic compression ratio up as a result.

 

 

Based on these values, your combination is on the ragged edge of detonation with 93 octane fuel when trying to maintain optimal ignition timing curves.

Lorena, Texas (South of Waco)

If you are dealing with a +040 over 312, then here are the new compression ratio (both static and dynamic) calculations based on the 318 cubic inch you mentioned.

Lorena, Texas (South of Waco)

i hate to bother you ted with the same question.i have a 314 using g heads.the pistons are flush and i used thick head gasgets so comp, will be more or less stock 57 312.cam is mild 222@050.nice idle with112.heads were not shaved.i dont think i will have a problem.its a 292 with 312 crank.

Ted,



Surprised rod length/bore Vs. stroke/etc. affected CR (?). Thought only displacement/volume was determining factor.



Actual specs:



6.200 Rod length

3.820 Bore

3.470 Stroke

.150 cut from Probe Piston domes

Best head gasket



Aside from providing "ragged edge of detonation" cushion, further grinding/chamber equalizing should also improve motor smoothness, justifying the effort.

6 VOLTS/POS. GRD. NW INDIANA

Daniel:

Re-read Ted's post.  He talks about two compression ratios, static and dynamic.  Your assessment of CR is of the static CR.  Dynamic is what the engine sees when it's running.

John - "The Hoosier Hurricane"

John,



Thought as much. Will assume that static CR IS fixed and entirely dependent on displacement/volume only (regardless of valve timing). Goes without saying dynamic CR affected by cam selection. Just wasn't aware bore/stroke/rod length also important factors. Don't want to belabor this point, as much relevant data readily available.

6 VOLTS/POS. GRD. NW INDIANA