Sean, The reason my inquiries started was due to the person working on a set of heads of mine decided that to fix some straightness issue he needed to mill 50 thousandths off them. This took the compression ratio from 10.5 to 12.5 That upper number also happens to be the upper limit allowed in the PSMCDR series. So, I don't know if I got the full story, I'm hearing everything secondhand. Running under that series was the goal, not having a 12.5 compression ratio which requires high octane racing gas to operate. Yes, I still have the same gaskets (0.039 spec) but I also now have valves that may be close to hitting the pistons. So, I was essentially wondering if a thicker head gasket would provide similar quench performance and move the valves back further from the piston face as well as some relief on compression ratio. Otherwise I may need to scrap some very expensive heads. Brian PS There may be other solutions, that is the first one that came to mind.
There are a few different options that you can do to correct the situation. Different pistons Machine pistons to lower compression Use thick enough gaskets to reduce compression ratio to 9:1 making pistons so far in the hole that quench is taken out of the equation but lose all the benefits that quench brings to the table. If in you already have pistons you have your heart set on using then you can have them milled to reduce compression, depending on what style dish you have will determine the milling strategy. If you have dished pistons with a quench pad then to compensate for the extra .050" that was removed from the head, the top of the piston should be taken down .050" everywhere EXCEPT the quench pad to maintain quench, maybe even an extra .010" to make up for the quench area?(have to do the math to see what you need to get you where you need to be) So, what kind of pistons do you have?
I woukd start by measuring the cc volume of the heads, measure the piston to deck height, and calculate the compression ratio. It sounds like you may just need some custom valve reliefs cut into your pistons. I dint think it is possible to go up 2 full points of compression with 50 thou removed from the head. Maybe half a point.
This was supposed to be a budget build, I wasn't planning to be competitive speed wise, just hoping to be consistent. So, a set of Speed Pro L2262F40 pistons were used. Sean you are correct, my math was off. The heads have been CCed and the compression ratio is up 1.25 rather than two full points. This does get pretty close to matching the chart here: http://www.wallaceracing.com/headmill.htm Piston to deck height is zero as that is what is generally noted as the best scenario for achieving optimal quench . And why I posted in this thread. Over time I've gotten more picky about who does what work(and trying to do more of it myself, so there is no one else to wonder about just my own ineptitude). This is the final lesson for me. With that said the person who worked on the heads has been doing head work for 30+ years and in particular Pontiacs, so I am a bit surprised by this situation. At this point, I almost want to forget about this engine as I eagerly await results of a Buick 350 build.
You should never have to scrap some very expensive heads. I'm sorry they were cut so severely before you fully understood what else was involved. Near .050" warpage? Wonder what the story is there and what collateral damage went along with that? Double check that math and actual parts as suggested, doubtful it's 12.5. As far as the limits of compression and pump gas, temperature ignites the fuel way before pressure in this case. [This comment was hanging on the computer long before the preceding one, ignore anything that seems out of place]
.005 milling gets you a 1cc reduction in combustion chamber volume, so .050 should lose 10cc. Are these standard heads or Stage1? Maybe you should post your actual numbers, and we will check your math.
C) Square off the block so both sides are perpendicular to the mains and then order custom longer connecting rods to raise the pistons in the bore, then use an appropriate head gasket to set quench. Choose pistons for the compression you want. D. Square off the block so both sides are perpendicular to the mains and then custom offset-grind the crank for some extra stroke. Pushes the pistons higher in the bore at TDC. Use an appropriate head gasket to set quench if the pistons are at/near zero deck. If the crank can't be easily offset-ground enough to get the pistons high enough, you'll need longer rods, taller pistons, or more material taken off the block. Or you weld the crank to make a major change in stroke. Stroke is 2x the distance the journal is offset ground--.020 offset grind lifts the piston .020, but = .040 increase in stroke. Be sure the pistons clear the counterweights of the crank. Choose pistons for the compression you want, keeping in mind the extra stroke will make for higher compression than the pistons are "advertised" at. FIFY.
Larry - these are actually Pontiac heads. Standard ‘68 d-port Pontiac heads are listed as 72 ccs but are known to often be 75 ccs. These are Pontiac’s 1968 round port Ram Air II racing heads. I know how much was milled and I know current volume of 56 CCs. Using the Pontiac head milling chart I linked above, I get about 9 CCs of change. Which actually puts the heads at their NHRA class racing spec of 65. The change in compression in this scenario is 1.2 points. Using the d-port spec of 72 gets to the change of 2 points. Brian Y.
If you find your static compression is higher than you want then look into a custom cam as the designer can set it up to bleed off some compression. Also be sure to check the valve to piston clearance once the cam is degreed in.
Pontiac heads have different attributes than B455 heads. My understanding is that quench affects things in the same theoretical way but mixture motion and other related effects having completely different size, shape and relationships of proportions offer more variation to the package overall. *Bleed off compression taken to mean "works with the engine to address both air flow and cylinder pressure curves in a manner that sacrifices nothing yet keeps the engine fairly safe." One way of dealing with excess cylinder pressure is to lower the load to the engine, such as what happens when a lighter vehicle with more gear and a higher converter rpm spends way less time accelerating past the lower part of the rpm band where load induced detonation is more likely to rear it's head. If your cam guy is worth his salt, there won't be much bleeding off as much as moving the powerband to a useful range.
Or mill your block to leave .013 piston to deck clearance and use .027" thick commetic head gaskets By far the best way is minimally decking the block and getting pistons with the right compression height but at 750$ a set minimum its too expensive for most imho Fwiw the bigger the bore the more p-h clearance you will need as well as the more piston to wall clearance you have. There is many things to look at but its almost always safe at .040 with steel rods until the bore gets bigger, i run .038 with my 464 and have no issues and i run 10.92:1 on 92 octane
Pontiacs can not handle as much compression as buicks unless you use an aftermarket efficient chambered head like KRE's fwiw, totally different animal, i build both and there isnt much in common in how i build a buick 455 vs a Pontiac 455
