[512chevelle502]

I am trying to figure out the compression ration on my 502 BBC. I have found a lot of great information here and some resources for doing the calculations online.

One thing I can't seem to get a number on, that I need for an accurate calculation is my piston volume.

The ZZ502 I have comes with these pistons
On Summit racing: Chevrolet Performance 12533507 Chevrolet Performance Forged Pistons | Summit Racing

Info from Chevy: Engine Parts: Big Block Pistons and Piston Rings | Performance

The summit link mentions that it is flat with a single valve relief cut. I would assume that that relief cut would be adding volume to my combustion chamber. I can't seem to find any place that lists that volume.

I have tried to reverse engineer what that volume must be by using GMs stated compression ration of the engine with there heads and these pistons and I keep running into the fact I need to REMOVE volume of (as it treat the pistons as domed) to get to GMs stated CR.

The site I am using for the calculations: https://www.gofastmath.com/Compress...atic_And_Dynamic_Compression_Ratio_Calculator


My Data:
Cylinder Bore: 4.47"
Stroke: 4.00"
Cylinder Head Chamber Volume: 108cc (Edlebrok 6055 RPM Performers, apparently these were tested at this volume by some previous owner/shop, the receipt also says "Milled Cylinder Heads .040", So I think this makes sense since the heads are listed as 118cc from Edlebrok )
Head Gasket Thickness: .039" (compressed)
Head Gasket Bore: 4.540"
Deck Clearance: 0.00". (I calculated this one on another site, but seems suspect?)
Connecting Rod Length: 6.135"
IVC @ .050: 41.5 (Lunati Voodoo)

What is throwing me off a bit is that GM claims the engine is 9.6:1 with 110cc heads. I have 108cc heads and depending on what number is put in for Piston volume I end up
0cc: 9.69:1
6cc: 9.27:1 (I have read 6cc is maybe normal for a relief cut for valves?)

To hit GMs claims of 9.6:1 I have to use a -2cc in the calculator (as in dome piston removing volume) to come up with their CR of 9.6:1 and 110cc heads.
If I throw that number in for my pistons I get a CR:
-2cc: 9.84:1

Note: I am also aware that there are other things that would be increasing my chamber volume such as the gap between my rings and the deck, my number would be end up with a lower CR. Just ball parking it for now.

This exercise is all just for fun and to learn more about my engine and how things work. I am not super worried what the number is, I just want to know what it is, or get reasonably close.

Short summary:
1. Does any one know the volume of GMs pistons they put on the zz502 crate engines (Chevrolet Performance 12533507 - Chevrolet Performance Forged Pistons)
2. Does a deck height of zero make any sense? I remember when I was rebuilding my engine back in feb that things did seem pretty flush, but I wouldn't swear it was "0" and I wish I would have measured it at the time!

 

[Malibu ss 64]

They have a small dome, 2-4 cc.

 

[512chevelle502]

So, in the calculations that would be removing from chamber volume, which would make sense giving numbers I have been seeing.

The would give me Static Compression of 10:1 and a Dynamic Compression of 8.5:1, Assuming my calculation of deck clearance is correct.

 

[68Chevele]

$267 for 1 BBC piston from GM? that's like over $2000 for a set of 8, holy crap!

 

[512chevelle502]

$267 for 1 BBC piston from GM? that's like over $2000 for a set of 8, holy crap!

I guess so, they just also happen to be the pistons they put in the engine when you buy short/long blocks.

Seems like if you were to build one there might be better options pice/performance wise.

 

[Aml383]

$267 for 1 BBC piston from GM? that's like over $2000 for a set of 8, holy crap!

I'm in the wrong business lol

 

[Aml383]

Speakin of not knowin up from down lol
Which end of this head goes to the front?
Theres different hole patterns on each end of the head

 

[David Bates]

Speakin of not knowin up from down lol
Which end of this head goes to the front?
Theres different hole patterns on each end of the head

Well, it'll be the front on one side of the engine and the rear on the other. Just a WAG!

 

[Aml383]

Well, it'll be the front on one side of the engine and the rear on the other. Just a WAG!

Haha ...hey easy big guy !
Never been in these parts before lol

 

[GoFast Beaumont]

I am trying to figure out the compression ration on my 502 BBC. I have found a lot of great information here and some resources for doing the calculations online.

One thing I can't seem to get a number on, that I need for an accurate calculation is my piston volume.

The ZZ502 I have comes with these pistons
On Summit racing: Chevrolet Performance 12533507 Chevrolet Performance Forged Pistons | Summit Racing

Info from Chevy: Engine Parts: Big Block Pistons and Piston Rings | Performance

The summit link mentions that it is flat with a single valve relief cut. I would assume that that relief cut would be adding volume to my combustion chamber. I can't seem to find any place that lists that volume.

I have tried to reverse engineer what that volume must be by using GMs stated compression ration of the engine with there heads and these pistons and I keep running into the fact I need to REMOVE volume of (as it treat the pistons as domed) to get to GMs stated CR.

The site I am using for the calculations: https://www.gofastmath.com/Compress...atic_And_Dynamic_Compression_Ratio_Calculator


My Data:
Cylinder Bore: 4.47"
Stroke: 4.00"
Cylinder Head Chamber Volume: 108cc (Edlebrok 6055 RPM Performers, apparently these were tested at this volume by some previous owner/shop, the receipt also says "Milled Cylinder Heads .040", So I think this makes sense since the heads are listed as 118cc from Edlebrok )
Head Gasket Thickness: .039" (compressed)
Head Gasket Bore: 4.540"
Deck Clearance: 0.00". (I calculated this one on another site, but seems suspect?)
Connecting Rod Length: 6.135"
IVC @ .050: 41.5 (Lunati Voodoo)

What is throwing me off a bit is that GM claims the engine is 9.6:1 with 110cc heads. I have 108cc heads and depending on what number is put in for Piston volume I end up
0cc: 9.69:1
6cc: 9.27:1 (I have read 6cc is maybe normal for a relief cut for valves?)

To hit GMs claims of 9.6:1 I have to use a -2cc in the calculator (as in dome piston removing volume) to come up with their CR of 9.6:1 and 110cc heads.
If I throw that number in for my pistons I get a CR:
-2cc: 9.84:1

Note: I am also aware that there are other things that would be increasing my chamber volume such as the gap between my rings and the deck, my number would be end up with a lower CR. Just ball parking it for now.

This exercise is all just for fun and to learn more about my engine and how things work. I am not super worried what the number is, I just want to know what it is, or get reasonably close.

Short summary:
1. Does any one know the volume of GMs pistons they put on the zz502 crate engines (Chevrolet Performance 12533507 - Chevrolet Performance Forged Pistons)
2. Does a deck height of zero make any sense? I remember when I was rebuilding my engine back in feb that things did seem pretty flush, but I wouldn't swear it was "0" and I wish I would have measured it at the time!

the dome volume is variable. Have a look at the picture. It is higher in the Center than on the outsides. There is a good valve relief. Mine was .018 down the hole. Don’t quote me but I thought it was 2 or 5cc dome. Couple companies make direct replacement pistons for the 502, I’m sure they list the dome height.

 

[Blown1932]

Assuming my calculation of deck clearance is correct.

Did you measure piston to deck height or calculate it based on machining that was performed? I didn't see anything in your original post about machine work (maybe I missed it), is this a stock ZZ502 shortblock or has it been decked? If its stock then I'd guess the pistons are down in the hole a bit (likely no where near zero deck). I'd measure it as like all the variables it can make a significant difference. Example for reference: IF its .030 down it is over a half a point of compression in a 502 and would leave the squish at ~.069 with your gasket (i'd prefer it to be much tighter)

 

[512chevelle502]

Did you measure piston to deck height or calculate it based on machining that was performed? I didn't see anything in your original post about machine work (maybe I missed it), is this a stock ZZ502 shortblock or has it been decked? If its stock then I'd guess the pistons are down in the hole a bit (likely no where near zero deck). I'd measure it as like all the variables it can make a significant difference. Example for reference: IF its .030 down it is over a half a point of compression in a 502 and would leave the squish at ~.069 with your gasket (i'd prefer it to be much tighter)

The deck high was taken from the Chevy website / PDF. Unfortunately I did not measure when I had it apart. I calculated the deck clearance with another calculator that used stroke, rod length, piston compression height and of course deck clearance.

As far as I know the short block as not been decked and should be factory dimensions.

On another note the receipts I have the heads were milled .040", Which is I assume why they tested them at 108cc rather than the Edlebrock stated 118cc.

 

[512chevelle502]

Hard to tell, but it does look like #1 and #6 are at TDC in this picture and maybe I am seeing that the piston is down in the deck just a bit and not flush, especially if I zoom in on #6, like a millimeter or so, which is pretty close to @Blown1932's "IF" example of .03". That puts me into a 9.43:1 compression, rather than 10.1, all else being equal.

I also double checked and my Fel-Pro 17048 is .039" compressed.

 

[68Chevele]

Years ago I warned my former service manager about putting nitrous on his ZZ 502. I told him GM's tolerances for that kind of power as they are way too tight in the pistons and ring clearances, he sprayed it anyway and blew it up. Bought another 502 and is doing the same thing. Some people never learn.

 

[Wolfplace]

To answer a question or two,,,
No zero deck is highly unlikely,,
The nominal number for the piston depth in the hole is .020 down in a standard undecked 9.800 block assuming it is actually 9.800
Compression height of almost all 4" stroke BBC pistons is 1.645 with a stock length 6.135 rod.
There are exceptions but they are not relevant here
This is about as close as you can get if you don't measure

Other numbers to plug into a good calculator
Piston head will be approx .020 smaller than the bore
Rings in a BB will be approx .300 down

 

[512chevelle502]

To answer a question or two,,,
No zero deck is highly unlikely,,
The nominal number for the piston depth in the hole is .020 down in a standard undecked 9.800 block assuming it is actually 9.800
Compression height of almost all 4" stroke BBC pistons is 1.645 with a stock length 6.135 rod.
There are exceptions but they are not relevant here
This is about as close as you can get if you don't measure

Other numbers to plug into a good calculator
Piston head will be approx .020 smaller than the bore
Rings in a BB will be approx .300 down

Thanks for the details. From my pictures I posted .020" certainly seems possible or even likely.
I remember seeing a calculator once that took into account the rings and their depth in doing the compression calc. I may try and find that and see what come out with this additional info

Based on my new found knowledge, albeit not measured directly, I think my Static CR is 9.6:1, with dynamic coming in at 8.18:1

I am wondering if I might play with thinner head gaskets? I ARP studded my heads (and block and rods) when I put it back together, so it would not be all the difficult or costly to switch out if I am really not getting what I can out of the engine.

 

[Wolfplace]

Thanks for the details. From my pictures I posted .020" certainly seems possible or even likely.
I remember seeing a calculator once that took into account the rings and their depth in doing the compression calc. I may try and find that and see what come out with this additional info

Based on my new found knowledge, albeit not measured directly, I think my Static CR is 9.6:1, with dynamic coming in at 8.18:1

I am wondering if I might play with thinner head gaskets? I ARP studded my heads (and block and rods) when I put it back together, so it would not be all the difficult or costly to switch out if I am really not getting what I can out of the engine.

You could go to the Cometic MLS which can be had at .027 assuming your surfaces are smooth & flat enough,,,, but for the price you are not going to see of feel anything except for a lighter wallet

 

[ekitch]

So you studded the main caps...did you check the main bearing clearances? Last time I tried to use studs on an engine (small block) it opened the clearances up too much, went back to bolts. There is a difference in the clamping force.
Are you sure that block hasn't been decked? If it has not and your using stock pistons with a head that is only 2cc less, you should be in the ballpark of 9.6:1. I just checked and the stock 502 head gaskets are .041 compressed thickness, so your .039 would slightly increase the compression.
What cam?
Once you get that motor together and running and broken in, you can take a compression test, that will really tell the story of where your at.
On a side note, when I mocked up my new ZZ454 block, crank, rod, and piston, the piston was down approximately .013" in the hole. So I will get the block decked .010" to get .003" down.

 

[512chevelle502]

Thanks to everyone for the great information!

So you studded the main caps...did you check the main bearing clearances? Last time I tried to use studs on an engine (small block) it opened the clearances up too much, went back to bolts. There is a difference in the clamping force.
Are you sure that block hasn't been decked? If it has not and your using stock pistons with a head that is only 2cc less, you should be in the ballpark of 9.6:1. I just checked and the stock 502 head gaskets are .041 compressed thickness, so your .039 would slightly increase the compression.
What cam?
Once you get that motor together and running and broken in, you can take a compression test, that will really tell the story of where your at.
On a side note, when I mocked up my new ZZ454 block, crank, rod, and piston, the piston was down approximately .013" in the hole. So I will get the block decked .010" to get .003" down.

Yes, had the crank checked and ended up selecting a proper new set of bearing, I forget the specs right now.

As far as I know the block has not been decked. It was a new in 2015 zz502 short block when the previous owner had it installed. The reason I rebuilt it was the distributor gear gave out and took out my bearings. The engine had just a few thousand miles on it and think it was not installed properly by the last builder, or just bad luck.

For the record, the CAM I just put in was a Lunati Voodoo Hyd. Roller.
Advertised Duration (Int/Exh): 282/290;
Duration @ .050 (Int/Exh): 231/239;
Gross Valve Lift (Int/Exh): .600/.600;
LSA/ICL: 110/106;
Valve Lash (Int/Exh): Hyd/Hyd

I took the time and measured the Cam after install and it checked out perfectly.

I have about 3500 miles on my rebuild, it runs amazing. I drive it daily. After my initial 500 mile break-in I switch the carb our for a Holley Sniper EFI system, including all the associated fuel upgrades.

I am really just interested in learning everything I can and it dawned on my I didn't really know my compression ratio.

This was my first engine rebuild and after everyones input I am wishing I taken some additional measurements when I was in the guts of it all!

 

[ekitch]

Sounds good, glad you were successful! How do you like the cam? Does it pull hard?
What heads are you running, the original 502 heads? Are you happy with the Holley sniper?

BTW there is an excellent video on youtube by PowerNation where they went thru a long process to calculate the exact compression ratio of a ZZ427 engine. They used some grease around the bore and set the piston down about 1-inch and found that volume first. After that a lot of calculations. They came up with 10.37:1; Advertised is 10:1