Pistons...

[86IROC]

For months now, I have been trying to figure my exact compression ratio. BT used DTD & some rough estimates I had to give me a range of where my CR was sitting at. One day I was chatting with RobF and a light bulb went off in my head. I typed in www.wiseco.com (http://www.wiseco.com) - an POOF! Their website popped up. graemlins/hump.gif I sent a brief message explaining my problem and asking for the exact cc of the valve reliefs in my pistons, as well as how they derrived the advertised CR.

When I bought them from Summit a few years ago, I was told that they would yeild 9.6:1 CR w/ 64cc heads, a .041" gasket and a zero deck. TPIS techs had told me that I needed to stay around 9.5:1 CR. So, I felt that these were the pistons for me. After months of aggrivation from TPIS... I called AFR, Comp Cams, and a few other manufacturers for some straight answers and some good ole bench racing. We came up with the following: ZZ9 cam too small for what I wanted, 9.6:1 CR was too low for my application, and a 2400 stall and 3.42's weren't going to cut it. Anyhow, back to my orignal intention - I got a prompt reply from Wiseco that answered all my questions. Two thumbs up to their customer service dept. By plugging in the info he gave me , I got roughly 11.32:1 CR. graemlins/devil.gif Finally I have an answer after months of searching.

Here is the email that I recieved back from Wiseco:

"5.4- CC valve relief's in the 2000 catalog they were advertised as 10.3:1 w/
.020 deck clearance and .038 gasket 67. cc head
in the 2002 catalog they are advertised as 10.7:1 w/ .038 gasket 0.0 deck
and 67.0 cc head
I' m not sure where the info was advertised but that's what my catalogs show
for the last 2 publications and we just started advertising at 0 deck this
year prior to that it was always .020 deck on the Pro-Tru parts .
Thanks hope this helps
Dwayne"

Anyhow, I know this is a long post. But, after reading that email... doesn't it make you wonder what CR your pistons really are? Summit tells me 9.6:1 w/ 64cc heads, their new catalog says "9.6:1 CR w/ 67cc heads." Yet, the company that makes the pistons tells me that they are really 10:1+ CR! Oh well, it all worked out for the best & there is a happy ending. smile.gif

[ June 28, 2002, 20:26: Message edited by: 86IROC ]

[WS6 VERT]

Amazing...

Thats a whole lot of compression :eek:

[james85z28]

all I can think of about the compression descepancy is this. In most catalogs for pistons they list the compression with a particular piston type at a standard bore, a deck height, and a chamber size. Then they have you add a certain amount of compression for bore oversizes. Since Summit only lists +.030 bores size pistons maybe they just published what was in the catalog without realizing the table was for standard bores.

[ragtop88]

To expound on what James is saying, most pistons you see in "generic" catalogs typically assume a a stock SBC piston to deck height of 0.025" unless a different compression height on the piston is used....i.e. some of the FM stuff uses a shorter height and have a .035" deck height....these are obviously uncut heights. SO what exactly is your setup? chamber size, gasket thickness and deck height? SOme quick number crunching on my CR checker says with those pistons and a 030 over bore you'd have to be running something on the order of 58cc chambers AND a thin gasket in order to hit that CR.....whcih I would say may be risky with a zero deck height as it may be belwo the minimum piston to head clearance. Just curious. Thanks

[86IROC]

TOM - .004" deck clearance, 64cc chambers, .029" compressed gaskets, (2) -5.4cc valve reliefs. .030" overbore, 3.75" stroke

FYI - this is the formula I used to get that figure: S.V. = 3.1416 X Bore X Bore X Stroke / 4.

C.V. = (chamber volume - dome volume + deck clearance volume + gasket volume) X .061, then...

Compression ratio = S.V. + C.V. / C.V.

This all comes from the back cover of my AFR catalog.

[ June 29, 2002, 13:00: Message edited by: 86IROC ]

[WS6 VERT]

What head gaskets are you using, specifically, what is the gasket bore? I'm using GM P/N 10105117 which is a .028 compressed thickness and a bore of 4.100"

[86IROC]

I believe they are ROL or Detroit Gasket Bernard. The guy I used to work for got them for me... $20!

[ragtop88]

Doh....forgot about the 383.....i was figuring for a 355....yeah i get something very similar as your number 11.2:1. Right now your sitting about .033 on the piston to head clearance.....typically a minimum of .035 to .045 is used on steel rods.....you may want to double check that though..hate to see the pistons come in contact with an imovable object....something to think about.

[Chvywolf]

Direct quote from High Performance Chevy Magazine:

Mechanical contact between the piston and the cylinder head is not advisable

[WS6 VERT]

OK, assuming you have a 4.100 gasket bore, he is what I come up w/ (warning, I'm a NOVICE)

10.67:1

Here is how I came up with that:

Gasket: 4.100" x .029" = 6.27cc
Heads: 64cc
Piston down from TDC: .004"
Assuming you were to have flat top pistons, your deck volume @ TDC would be .84cc, now add your 5.4 x 2 (10.8cc) valve reliefs to the .84cc = 11.64 deck volume. Add your heads and gasket and you get 81.07cc of Total Combustion Volume.

P.S. I did NONE of the math, just repeating what DD2000 says... tongue.gif

[ragtop88]

i believe combined the dish volume is 5.4 cc....atleast th eflat tops I'm using are a combined 6cc volume....10.8cc would be a lot of dish from VR's....unless they are really hogged out to avoid some high lift cam. I've got a spreadsheet that I've checked before versus some other calcualtion and to some known CR's...it works pretty good.

[WS6 VERT]

Hmmmm, all I saw was this:
(2) -5.4cc valve reliefs So assuming its a total of 5.4cc, then you are looking at 11.36 : 1