Well I found .040 in the difference between the stock pistons and the speed pro that I have. I guess at the end of the day I failed to research properly.
That's OK, its not Too Late Live and learn. The motor in my signature has Diamond pistons at 0 deck. It has aluminum heads and 10.54:1 SCR. Runs on 93 pump premium all day without any ping at all. I throw in some leaded race gas just for insurance when taking it down the track.
So did you get the Speed Pro Pistons from TA? Are you sure you have Speed Pros and not Sealed Power 392's? They come with the sadly usual short pin height of 1.955", rebuikder special. That would account for the extra depth in the bore. The Speed Pro hypers come in at a normal 1.985". The SP forged L2353's they list don't have the pin spec but I'm pretty sure it is the same as the hyper 1.985". I never had SP's that low in the deck but I haven't bought any lately. I'd pop one piston out and confirm what pin height you have before buying anything. Didn't your machinest take a cut off the block at all? I think you are saying that they took .030" off the heads, correct? If they didn't take off at least .030" take them back and get a total of .030" off. That would at least get you in the range of 69 cc there. Since you are dealing with a '70 block and heads you could always use the factory steel shim gaskets at .020" or spend the money on the .027" Cometic. Too bad Mike at AMP doesn't customise the .030" longer Pontiac Skat(?) "H" rod to fit the BBB anymore. That is what I have in my engine with the TA Spherical Dish pistons. Minimal cut off the deck and you have a "0". No stroking fuss. https://www.summitracing.com/parts/slp-392ap30 https://www.summitracing.com/parts/slp-h392ncp30/make/buick
Most replacement pistons are designed with the intention that the block is going to get decked during the rebuild,thus making them shorter. I would not panic. Have you decked or surfaced the block at all? You could run a .020” head gasket,or even a steel shim. One of my best engines has the pistons .045” in the hole,with 11:98:1 and an iron head. It was all planned that way when built.
I just bought a set of Speed Pro L2353's from TA, and can confirm the pin height is 1.985 and a 23 cc dish. I can say that after measuring everything, they wound up .018 further in the hole than I was expecting. Me and the machinist re-measured everything, to try to understand why, and never could figure out why it was (I never witnessed the deck measurement in person, but he swore it was 10.55 on the money) So in the end, we took another pass at the decks and now they sit .005 in the hole with stock rods and crank. head and intake alignment are still good, which makes me think the block was significantly taller than 10,57 to begin with. With .038 Orange Crush gaskets and 70 cc heads I come in at an exact 10.0:1.
My block has not been cut at all, heads had .008 cut and I haven't measured them to check cc volume yet. I'll measure them this weekend to check what they really are. The pistons are indeed the sealed power 392's, I guess I jumped the gun when I bought them. I'm beginning to see what people mean when they say you can't do a chebby rebuild on a Buick. I've done several chebby and a few furds in the past and I must say this one has been interesting to say the least.
If youve truly got the 392s , with a .072 depth now, your uncut deck (if everything else is to spec) is would have to be 10.58+ from the factory. No matter what you buy, youre still gonna have to adjust that deck height to get optimal quench. Your head CC will dictate the dish you need, but generally a 0 deck with a 30 cc dish is gonna get you in the high 9's for CR. I suppose you could go with custom pistons with a 2.02+/- pin height but I'd still cut the deck for a spec'd height and surface..
Same here ..too much emphasis is put on this,.. I had a 464,..with pistons .058 in the hole with a .080 gasket,....ran 11.60s at 115mph,....
X2 With the TA 212 cam you'll be fine with a lower compression engine. No worry about gas quality except on a hot day or racing(maybe). I'd CC a head and see what you have. It would not hurt to take another .020" off. They shouldn't have to take the valves out or anything. After they cut them they just need a good wash. I've had that done. Flat tops I'm sure and probably some race gas added or a big by large cam. Running .080" gaskets? Maybe flat tops? Unless you were intent on lowerering the compression for boost? It seems it would be excessively low compression if they were not flat tops.
They were Wiescos flat tops,..and to be 100% honest I'm not sure how the thick gaskets ended up on there but it didn't seem to care none to much,..ran well
Yes,Ross flat tops. It runs on 93 octane. Yes,an iron head and 11:98:1 compression. It’s only rattled once when I had my Sasquatch friend in the passenger seat during a pass.
Don’t over think it. If you don’t want to jack with new pistons or block decking (time, effort, expense), you could mill the heads to get less cc’s. Watch intake runner to head alignment (especially if you’ve already port matched), and use thicker intake gaskets. Or have the shop angle mill the heads. If so, make sure you run adjustable pushrods or rockers to keep hydraulic lifter preload in spec. As guys said, thinner head gaskets if you want more. Run good gas and sensible timing regardless of what you do.
A Chevy that's rebuilt improperly doesn't run better or worse than any other engine that's been similarly improperly rebuilt. There's no magic with the Chevy parts that makes them immune to screwups. A bum rebuild is a bum rebuild, no matter what engine it is. .058 + .080 = 0.138 The "danger zone" for detonation varies with engine family and with who you're talking to, but in general terms, it begins at .050--.080, and ends at .100--.120. Tighter quench than the minimum gives good turbulence which speeds flame travel. Looser quench than the maximum makes for an "open" chamber that burns right to the bore wall. Probably needs more timing advance due to lack of turbulence--a "lazy" combustion chamber. The problem is between .050 (some say looser than that) to .100 ( or a bit looser), where there's lots of super-heated end gasses, but the piston-to-head clearance is too tight for flame travel. Heated but un-burned end-gasses are "seed" for detonation. You built an "open" combustion chamber. I can believe you didn't have detonation problems. As said, you likely needed a few degrees more ignition advance than if the quench had been tighter, with the mixture closer to the plug and more mixture-motion (turbulence) in the chamber.
So, to simplify. IF the forged slugs in my 455 are indeed a bit low, as my 270 rwhp dyno and 13.78@99.6 would suggest. Would it be advisable to increase or decrease timing? Currently my dissy is regraphed to 18/34 It's a street car I run down the strip occasionally.
When you go to the track, play with the timing. If more timing produces a higher trap speed, the engine wanted more timing. If the trap speed goes down, the engine didn't want more timing. You have no idea how far in the hole your pistons are unless you have measured. Why worry about it? Does the engine have audible detonation on the street? If not, why worry?
IMO, the OP should correct this situation while he is in a position to do so. I would not rely on anecdotal evidence that others have run engines with similar measurements. There is plenty of evidence here to say that an engine with pistons that far in the hole will be rattlers on the street on pump gas. If that is the case for the OP, what is he going to do then?
We need to keep the whole picture in mind... he is most likely running a short duration cam, and will have a whole bunch of cylinder pressure. From the sounds of it those motors that ran "fine" had big cams in them. I too ran a 73 stock shortblock (pistons -.53 in my 4100 lbs GS with my iron head motor top half, with a 308S cam, an that thing ran 11.90's all day long.. But that's apples vs. oranges.. This makes a huge difference in cylinder pressure at the low/mid range rpm area where detonation occurs in this scenario. Pistons way down the hole and a small stockish type camshaft is the recipe for detonation. It's why the stock 70 motors ping so bad if you try and give them the timing they want to make power, on pump gas. I have had Buick guys take " Brand new rebuilt" motors out of the car, and send them halfway across the country to me, to fix this issue, usually it's 430's that have this problem. All owners report that with anything near the timing that is required to make it "fun to drive" they ping horribly. I would not assemble this engine with those specs.. have the decks cut at least .030 to get the pistons up to something reasonable. That is still not great, but more like how the factory put them together. Run a steel shim head gasket, spray it with copper coat to get it to seal, and you can tighten it up a touch more. Best option is a custom piston, but you have been subjected to that argument, I won't repeat it. you can lead a horse to water..... JW
TA212 is a short duration cam? The ping issue is why Buick ran such a slow timing curve from the factory.
