This is kinda of a off the wall question. This in no way means I think simulated computer dynos are 100% accurate. Here it goes....
I have three magazine articles that have vortec headed 355 buildups. All of them are claiming 9.5:1 or less compression ratio. I have a simulated computer dyno called Virtual Engine 2000. It goes into pretty good detail it what is of your engine. Deck height, rod length, head flow numbers, cams, ign timing, carbs, intakes, head gaskets, ect...When typing in the specs of the same vortec 355 buildups as the magazines I'm coming up with roughly 10.5:1 compression. This is with a flattop two valve relief piston. What I'm questioning is, when I dyno the motors with the 10.4:1 compression they come out to be what I think is a pretty accurate set of numbers. When I go to plug in the 9.5 compression the numbers are falling way below what I think would be accurate. I'm talking 50 to 60 horse less then what the magazine claims that motor makes. These compression ratios are all calculated with a zero deck height as well. Who is more accurate? I myself personally have never physically checked the compression ratio on anything I have ever built. I cant seem to fathom though that 355 CI, with a flattop single trench valve release (maybe -3cc's), 64CC head, zero deck height and a .028 head gasket would only yeild 9.5:1 comp. My only theory is, people want to read "pump gas motor". They know they cant make the kinda power they are making with these 355's with 9.5. So there caliming thats what it is anyways even though its over 10:1. What do you think? Weren't the old 327/300HP motors with 4 valve relief flattops and 64CC heads 10:1?? Thats 28CI less.

You've hit on a sore subject with me re: comp. ratios. "Advertised" ratios never check out to what they're supposed to be. In most cases they're
always less & no two engines are built identically anyway. Deck heights can vary along w/ combustion chamber volume differences. The tried & true method is to calculate it yourself w/ a burette & plug the values into the formula.
Then you know what you really have. BTW, Chevy used to advertise that 300 hp motor at 10.25 to 1 at ideal conditions with the thin steel head gaskets (.022).

I understand that no two engines are alike and there good be variations. But a whole point of comression?! I think differences in deck height on a small block and gasket thinkcnesses, might vary a few fractions of a point, but no ways a full point.

I have Virtual Engine too and when I recreated my engine the numbers were pretty accurate but I increased the compression up from the 9.52:1 it is to 10.5:1 it gained like 65 hp and 40 lb/ft too, weird huh, and almost all the builds I do on there that are'recreations" of real dyoed engines and the specs are almost exactly the same will make way less power than they should, I think it's the program.

A 350 with 64cc chambers, .040" quench, and flat top 6cc pistons always comes out to around ±10.2. A lot of mag build ups don't pay any attention to the quench distance and often use a .041" gasket with a .025-.030" deck. This lowers the CR but causes other problems. To get 9.5 or less with 64cc heads and proper quench requires dish pistons. Note also that a lot of guys have reported that Vortec head often have chamber smaller than 64cc. I've heard 60-62 is fairly common. Without measuring everything, it's pretty much a quessing game.

Let me start by saying that what you read you cannot always believe. i dont really trust magizines too much. i buy them like crazy and even have subscriptions, but what ever they say i subtract 25-40 hp before i subtract for power the power difference to the wheels. I sometimes get the feeling that their numbers are embellished. The way i figure it is if the engine makes as much power as they say it will then great but if not i wont be disappointed.

Let me start by saying that what you read you cannot always believe. i dont really trust magizines too much. i buy them like crazy and even have subscriptions, but what ever they say i subtract 25-40 hp before i subtract for power the power difference to the wheels. I sometimes get the feeling that their numbers are embellished. The way i figure it is if the engine makes as much power as they say it will then great but if not i wont be disappointed.

it's funny but when chevy hiperformance does the stimulated 1/4 mile runs with motors they dynoed they always have great numbers. I think with the roller vortec motor with gm hot cam they didn't make it into the 12's when they took it to the strip in a 3600 Lb car.

My calculations came out the same as Pat said. I just built a 355 with 4 valve relief flat tops, decked block, etc. with quench at .039. This is in a '99 Suburban with a LT4 cam (8* more duration on intake and exhaust that stock vortec cam). I just took it on a 2000 mile trip, ran the lowest grade available at the pump and got dead-on 19mpg with never a hint of a ping. Vortec heads on a computer worked like a dream for me...so far.

I understand that no two engines are alike and there could be variations.
But a whole point of compression?!
I think differences in deck height on a small block and gasket thickness'es, might vary a few fractions of a point, but no ways a full point.
According to "Car Craft magazine" who ran a test, Jan.2002, "decreasing the compression ratio"

[on a 440 C.I.D. Mopar engine,11.1 compression ratio, 230/238 @.050 .520/.534 lift on 110 LSA cam, single 4bbl carb, iron heads flowing 291 i/217 e, 850 speed demon on a street dominator...
by adding/stacking head gaskets...in order to drop the compression ratio...1 full point...each time...]

Compression ratio started out, at
[11.0:1]- 500hp @ 5400rpm, peak torque 531# @ 4500rpm, low rpm torque 455# @ 2500rpm
[10.0:1]- 483hp @ 5400rpm, peak torque 518# @ 4500rpm, low rpm torque 448# @ 2500rpm
[9.0:1] - 460hp @ 5400rpm, peak torque 505# @ 4400rpm, low rpm torque 436# @ 2500rpm
[8.0:1] - 440hp @ 5300rpm, peak torque 486# @ 4400rpm, low rpm torque 416# @ 2500rpm

According to their findings, basically, 1 point of compression = 20 horsepower, + some torque......
with the 0.030 engine setup they were using....60hp total loss going from [11.0:1] - [8.0:1]....Don.
..