Factory '70-71 351C-4V compression ratio was almost 11:1 with closed chamber heads and flat top pistons. These engines were 98-octane Leaded-gas Premium-fuel only! With today's gasolines, 92 Octane premium unleaded is not enough to avoid engine-killing detonation unless you 'detune' the engine a bit or drive like you have a raw egg between your right foot and the gas pedal! With today's pump gas, 10:1 is about the limit. 9.5:1 is better.
351C-4V were available in both closed-chamber 63cc (70-71) and open chamber 78cc (72-74) chamber sizes. Your choice of factory pistons (dish, flat top or domed) makes the compression ratio anywhere from 8.7 to 12.3! I'll forget about the slightly different Boss 351 and 351HO heads since they're rare and you probably don't have those!
For aftermarket pistons, you'll need to know the piston pin height (from centerline of wrist pin to the top of the piston) so you can calculate the deck clearance (top of piston to the block deck surface). The piston catalogs should also give you the piston dish (and valve relief) and/or dome volumes.
All 351Cs had a deck height of 9.206 and a rod length of 5.780. To that you add the piston pin height. You'll also need to know your head gasket compressed thickness/bore diameter to determine the gasket volume.
Static compression ratio CR is calculated by the formula
(SV + PV + DV + GV + CV) / (PV + DV + GV + CV) = CR
SV= Swept Volume (pi x R^2 x H) or (3.14159 x (Bore/2 x Bore/2) x Stroke)
PV= Piston Volume (dish is +, dome is -)
DV= Deck clearance Volume (pi x R^2 x H) or 3.14159 x (bore/2 x Bore/2) x deck clearance (from top of piston to block deck surface)
GV= Gasket Volume (pi r^2 H again) 3.14159 x (gasket bore/2 x gasket bore/2) x compressed gasket thickness
CV= Chamber Volume - actual measurement is better, but book volume is approximate for stock heads. If heads have been milled for straightness, then volume will be slightly less... generally for every .006" milled off surface + 1cc
So, if I were to be running a ratio of 10.35:1 would I still be able to run on premium?
If my math is right, and what I have read is correct, with my current pistons (12cc relief cut) and deck clearance (zero), with factory heads (60.4cc) I should be at 10.075 with a .040 overbore. I have a feeling that this engine is going to be prone to pinging.
Still not sure clear what kind of engine we're talking about. There's no such thing as "4V closed chamber heads" for a W, and no C came with 60.4 cc chambers, to the best of my knowledge.
Anyway, there's really no way to answer your question since the amount of (static) compression you can get away with is going to depend on the type of head (you can run more with aluminum than cast iron), cam specs (increased overlap will effectively lower the compression ratio) and type of transmission, car weight, gear ratio, etc. will all play a role as well. Ultimately it comes down to the individual combination, but more info is needed to even begin to provide an educated guess.
Where to start. I have factory heads for for my 69 351W 4v. I have the comp cams 275deh flat tappet cam. If the calculator I am using is right, my dynamic compression is 10.075 and my static is 7.132. I have plans to put AFR 185 heads with the 60cc chamber on it this fall.
OK, now we're getting somewhere. Just to be clear, 351W 2V and 4V heads are exactly the same. The 2V/4V head and open vs. closed chamber distinction is a Cleveland thing.
Stock '69 351 4V compression, with the factory flat top pistons is 10.7:1.
I have no idea what your cam specs are, but if its a typical street performance grind, I can tell you from experience that combo will ping on modern pump gas. You'll need to add octane booster, some race fuel, or retard the timing in order to make it live. If you've added aftermarket pistons or opened up the chambers to reduce the compression to 10.075, you'll be right on the edge with iron heads and pump gas.
In my case, adding some aluminum twisted wedge heads (61cc chambers) was enough to stop the detonation when running premium pump gas. Although the compression may have dropped a fraction, its the superior heat dissapation of the aluminum heads that really makes the difference. That said, I'm running a manual transmission with fairly steep gears, and don't lug the engine. With an automatic (unless it has a very loose stall converter and 3.50:1 or higher numerically rear gears), I'd want to drop the compression down to 10:1 or below to be on the safe side, even with aluminum heads.
One other often overlooked factor is spark plugs. The factory 351W 4V plug was an Autolite (now Motorcraft) BF32. The period "performance" plug recommendation was a BF22. Standard 2V Windsors used a BF42. The larger number indicates a hotter plug, meaning it doesn't dissipate heat as quickly. Almost all the modern day spark plug reference charts list a single replacement plug for the '69 351W, which is the BF42 or equivalent. Those plugs are too hot for a high-compression engine, and will contribute to pre-ignition. If you're running the factory iron heads, its worth the effort to seek out the cooler running equivalents to the BF32 or 22. Newer replacement heads use smaller plugs, so the numbers will be different, but its still a good idea to make sure the plug heat range is appropriate to the compression ratio.
Thanks for the info! Now here is where it gets tricky. The block is a 72 D2 date code block, with 69 c9 code heads. I had the block decked to a zero piston to deck clearance. This, if memory serves, is how the 69 block was set up, with a shorter deck height. The stroke and everything else has stayed the same, so I am taking the numbers I am doing my calculations on from that. I do have aftermarket pistons, speedpro flat top, but they are fly cut for valve clearance and that gives them a 12cc volume, if i am recalling correctly. So, with factory stroke of 3.5", factory rod length of 5.955", bore of 4.040", head cc of 60.4, piston volume of 12, and cam that has an intake closing post of 63* after bdc. So basically, I need to run a little bit of race fuel in there to bump her up a bit.
Here are the cam specs: http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=888&sb=0
I wonder how hard it would be to rejet and run e85? Any prominent issues?