VictoriaVette

So I was wondering if thee is a easy simple way to find compression ratio using just the head cc info? My stock 77 has 8.5 to 1 with 76cc chamber heads I do not know what thickness head gaskets were used from the factory. But if I use .028 head gaskets and go with a 57cc chamber heads can I find out compression ratio from just this? If so how? I used this link and input my stock info using .041 gasket and came up with 8.5 to 1 and then changed the the gasket to .028 and 57cc chambers and came up with 10,82 to 1 what do you think is this close?
Thanks

http://www.rbracing-rsr.com/compstaticcalc.html

gerry72

Ballpark? Kind of. Most will come in about a half a point lower nominally using "factory" production parts and specs. If you measured everything out, your factory 8.5:1 is probably closer to a bit below 8.0:1, and that's using the factory steel shim gasket, not a fiber gasket.

Machining differences and production tolerance stackup (deckheight, journal indexes, rod length, compression height, true chamber volume and such) will always make engine-to-engine variations the norm.

If you must know the true static compression, you have to make honest measurements.

dmaaero

For true c/r you have to cc everything, including the new heads !

63mako

You need accurate information for accurate results. There is also some great info in this thread.
http://forums.corvetteforum.com/c3-t...t-to-know.html

VictoriaVette

That is not possible I'm just trying to get a close figure I did cc the heads they were 56.7cc and the deck to piston was .026 but I could not cc the piston top because it was in the block. I was just trying to get a close compression ratio of the engine with the new heads.

63mako

Start a thread asking what the actual piston volume is on a stock 1977 L-48 and you will get a quick response. you have the stroke @ 3.48, piston to deck height @ .026, the bore size @4.000, the head cc @ 56.7, the gasket bore and the gasket compressed thickness (Refer to gasket), so the only number you need is the piston volume. I think it is a 12cc dish but not sure so ask in a thread.

MotorHead

Piston cc is all you need now for cr that will be close enough for most needs

yel76low

Assuming some things, but...

76 cc head
.041" gasket = 8.4cc
piston dish = 12.5cc (note, I just made this up to make it equal 8.5)

This combo gives CR=8.50

Now, using your new heads, gasket, and the same pistons...

56.7 cc head
.028 gasket = 5.8cc
piston dish = 12.5cc

CR = 10.70

cardo0

The only real way to know for sure is to get the piston to TDC and measure the cc volume just like you did the head chambers. U also need to seal the crevace above the ring with a little grease to prevent oil loss as u fill the vol. Problem is the plexiglass plate loses seal just when i get close to filling the empty cyl space above the piston with the (measured) oil. The engine is at a angle in the car and won't cooperate unless u tip the car on its side 45 degrees. This would be simple with the engine out of the car on a stand but i know u want to avoid that. Yep i burned at least a day trying to measure cyl/piston vol with eng in car.

My pistons would be similar but not exactly the same - its hard to nail the correct part number for each year Chevy sb. But mine had a large circumference ring surrounding the dish area and vlv relifes cast into the dish for even more dish vol. I suspect the dish vol was much more than 12cc. Also looking at stock chevy pistons it seems the compressed height was smaller too yielding even more space between piston crown and the block deck. Maybe u can locate your real piston part number.

Well thats all i got.
Hope it helps u more than confuses u double V.
cardo0

VictoriaVette

cardo0 that is what my pistons looked like also that is why I was thinking that it I took everything from the stock engine that gave me 8.5 to 1 with 76cc heads and then changed tho the 56.7cc & .028 gaskets would be the closest way to find out.

cardo0

Ok i understand as i recall attempting that also years ago. Problem is the Chevy published c.r. numbers tend to be a bit on the high side. Seems like Chevy would rather be conservative with c.r. for safety then generous with power. I spent hours surfing the net for pistons that could have matched mine to get dish size also when i did the top end. If i come across some good info for a '77 vette sb chevy piston i will try to contact u but that could be months from now.
So unless u have detonation problems on a hot day u shouldn't worry. U can also use a stethoscope to help listen for ping as u rev it up.

But really if what u got works don't worry as i think u got it right. Now u can help others here that want to do the top end up grade. I think u made some good choices without breaking your wallet and now have a fun car to drive. U may find even more power just by tuning the carb with an air/fuel meter. Develop your timing curve too.

Good job VV.
cardo0

VictoriaVette

Thanks I was going to try 19 instead of 17 for the timing and see how it does.

63mako

Using your numbers, 56.7 cc head, 4.000 bore, 12cc dish, .026 deck height, .028 gasket thickness, 4.000 gasket bore, 3.48 stroke your compression ratio is 9.98 to 1. Your piston is a 12cc dish according to NHRA.
http://nhra.com/competition/pistons.aspx

Little Mouse

Factory steel shim gaskets were listed at .018 compressed thickness

.041 thick gasket 4.166 bore to 4.100 bore you look them up there
both in the 9.0cc to 9.1cc compressed volume.

A 4.100 bore, 015 gasket is 3.2cc compressed volume.

Using your figures the highest compression I can come up with using a .015 gasket is 10.18 compression the lowest figure using .041 gasket 9.59.

If you want compression in between these two use the
victor reinz #5746, 4.100, .026 compressed.

www.competitionproducts.com. or I think napa stores have them.

to use the .015 gasket you have to for sure have a factory block that has not had any cutting on the decks. You need to keep
.035 piston to head clearance. .026 down in the hole add in .015 gasket you have .041 piston to head clearance thats plenty.