What's my CR?
Thread Starter
Drifting
Joined: Apr 2002
Posts: 1,743
Likes: 37
From: Stockton Ca
What's my CR?
I saw something on UT that makes me question my CR.
I have .60 over SB w/TFS 62cc heads.
218-224 w/1.6 RRs.
Flat top pistons w/2 valve reliefs.
I always thought this would result in 10-1.
The video says low 9-1.
I runs best on 32* and has never pinged!
If I ever freshen the engine some porting is planned.
Shaving the heads would not be a big problem! lol
R
I have .60 over SB w/TFS 62cc heads.
218-224 w/1.6 RRs.
Flat top pistons w/2 valve reliefs.
I always thought this would result in 10-1.
The video says low 9-1.
I runs best on 32* and has never pinged!
If I ever freshen the engine some porting is planned.
Shaving the heads would not be a big problem! lol
R
Old Pro Solo Guy
Joined: Dec 2017
Posts: 8,094
Likes: 4,449
From: Marlton NJ
You would need to know the piston dome or valve reliefs in cc's, and head gasket width, and deck height.
Lots of variables.
http://www.wallaceracing.com/cr_test2.php
But with typical 7cc valve reliefs, uncut .025" deck, thick .041" head gasket, I get 10:1 (9.9)
Likely higher if block decked.
Video may have been talking about '71-82 dished L48 pistons, not '68-70 flat-tops.
Lots of variables.
http://www.wallaceracing.com/cr_test2.php
But with typical 7cc valve reliefs, uncut .025" deck, thick .041" head gasket, I get 10:1 (9.9)
Likely higher if block decked.
Video may have been talking about '71-82 dished L48 pistons, not '68-70 flat-tops.
Drifting
Joined: Jun 2025
Posts: 1,722
Likes: 598
I saw something on UT that makes me question my CR.
I have .60 over SB w/TFS 62cc heads.
218-224 w/1.6 RRs.
Flat top pistons w/2 valve reliefs.
I always thought this would result in 10-1.
The video says low 9-1.
I runs best on 32* and has never pinged!
If I ever freshen the engine some porting is planned.
Shaving the heads would not be a big problem! lol
R
I have .60 over SB w/TFS 62cc heads.
218-224 w/1.6 RRs.
Flat top pistons w/2 valve reliefs.
I always thought this would result in 10-1.
The video says low 9-1.
I runs best on 32* and has never pinged!
If I ever freshen the engine some porting is planned.
Shaving the heads would not be a big problem! lol
R
How MUCH was your block decked? How much did the machinist mill the decks ?
When at true TDC, how far from deck is crown of your piston down in the hole ? 0.025", 0 nothing (aka zero deck) ?, 0.010? How far ?
which Piston part number YOUR piston?
which Gasket part number YOUR head gasket?
Thread Starter
Drifting
Joined: Apr 2002
Posts: 1,743
Likes: 37
From: Stockton Ca
I had my engine professionally machined and assembled.
I picked the parts, they did the rest to a turn key vehicle that passed smog at that time!
Not sure of specs.
has run long and strong!
I picked the parts, they did the rest to a turn key vehicle that passed smog at that time!
Not sure of specs.
has run long and strong!
Drifting
Joined: Jun 2025
Posts: 1,722
Likes: 598
Not sure of specs equivalent to not enough specs to make a remote determination of SCR.
If I had the motor in my shop I could disassemble and measure; we're not doing that.
My best GUESS based of what you state and what is also likely: scr approx 10:1 - 10.25:1
Relative height of first compression ring groove helps dictate ability to withstand detonation/pre-ignition/spark-rattle; the closer groove is to crown, the more prone to damage the piston is.
More silicon in aluminum piston alloy makes alloy more brittle aka less ductile. More silicon in alloy also is less expansive.
forged alloy pistons can sustain more ignition abuse (More ductile)
than do cast alloy,
than do cast hypereutectic alloy (Least ductile)
*edit-add-
seems you indicate you picked the parts; with that, it seems you might provide P/Ns for piston & gasket to help others in helping to determine your CR?
If I had the motor in my shop I could disassemble and measure; we're not doing that.
My best GUESS based of what you state and what is also likely: scr approx 10:1 - 10.25:1
Relative height of first compression ring groove helps dictate ability to withstand detonation/pre-ignition/spark-rattle; the closer groove is to crown, the more prone to damage the piston is.
More silicon in aluminum piston alloy makes alloy more brittle aka less ductile. More silicon in alloy also is less expansive.
forged alloy pistons can sustain more ignition abuse (More ductile)
than do cast alloy,
than do cast hypereutectic alloy (Least ductile)
*edit-add-
seems you indicate you picked the parts; with that, it seems you might provide P/Ns for piston & gasket to help others in helping to determine your CR?
Last edited by Rebelyell; Aug 29, 2025 at 05:28 PM.
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Drifting
Joined: Jun 2025
Posts: 1,722
Likes: 598
