Compression Test: These Numbers Sound Right?
Thread Starter
Melting Slicks
Joined: Dec 2001
Posts: 2,025
Likes: 1
From: Billings MT
Compression Test: These Numbers Sound Right?
I went ahead and checked the compression this evening while I was checking valve lash. Here are the numbers with corresponding cylinder:
1 - 185
3 - 185
5 - 184
7 - 184
2 - 186
4 - 185
6 - 190
8 - 185
They are all very consistent, but #6 is a little high. I think the lash may be a little loose on this cylinder, which would cause the intake valve to close sooner/open later, could this cause the higher number?
Also, do these cranking numbers sound right for my calculated 10.6:1 SCR with cam duration: 242/248 @ .050?
Thanks!
:cheers:
1 - 185
3 - 185
5 - 184
7 - 184
2 - 186
4 - 185
6 - 190
8 - 185
They are all very consistent, but #6 is a little high. I think the lash may be a little loose on this cylinder, which would cause the intake valve to close sooner/open later, could this cause the higher number?
Also, do these cranking numbers sound right for my calculated 10.6:1 SCR with cam duration: 242/248 @ .050?
Thanks!
:cheers:
Melting Slicks
Joined: Feb 2003
Posts: 2,529
Likes: 27
From: Auburndale Florida
2025 C2 of the Year ('64-'66) Finalist - Unmodified
St. Jude Donor '03-'05-'06-'07-'09
Re: Compression Test: These Numbers Sound Right? (LAvetteman)
Anytime the compression is within 5%, from ther highest reading to the lowest, the rings and valves are in great shape and doing their job!
:cheers:
:cheers:
Thread Starter
Melting Slicks
Joined: Dec 2001
Posts: 2,025
Likes: 1
From: Billings MT
Re: Compression Test: These Numbers Sound Right? (mbeeman350)
The largest % variance from the mean comes out to be 3.2%.
Excluding the erroneous high cylinder, the highest % variance from the mean is 1.08%.
Assuming there will be some variation between cylinder temperatures, ect., this sounds pretty darn close to me. I could have some gas sitting in the #6 cylinder as the couple times I opened the carb all the way dumped a LOT of gas, should #6 been open it could of got the lion's share of it. :lol:
:cheers:
Excluding the erroneous high cylinder, the highest % variance from the mean is 1.08%.
Assuming there will be some variation between cylinder temperatures, ect., this sounds pretty darn close to me. I could have some gas sitting in the #6 cylinder as the couple times I opened the carb all the way dumped a LOT of gas, should #6 been open it could of got the lion's share of it. :lol:
:cheers:
Racer
Joined: Dec 2003
Posts: 257
Likes: 0
From: Livermore CA
Re: Compression Test: These Numbers Sound Right? (VETDRMS)
Also, do these cranking numbers sound right for my calculated 10.6:1 SCR with cam duration: 242/248 @ .050?
Thread Starter
Melting Slicks
Joined: Dec 2001
Posts: 2,025
Likes: 1
From: Billings MT
Re: Compression Test: These Numbers Sound Right? (JJMURFS)
JJMURFS Yes, as expected my DCR is rather high: 8.5:1. This cam does have a lot of duration at .050, but seat-to-seat is kind of low: 280/286. With an intake closing at 66 BTDC.
ZD75blue I am running a 4.41:1 gear reduction starter good to 18:1 compression. It gets the job done and sounds :reddevil:.
Thanks for the input!
ZD75blue I am running a 4.41:1 gear reduction starter good to 18:1 compression. It gets the job done and sounds :reddevil:.
Thanks for the input!
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Team Owner
Joined: Jul 2001
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From: Lakeland, FL
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St. Jude Donor '03, '06, '17
Re: Compression Test: These Numbers Sound Right? (VETDRMS)
those numbers look good.. As others mentioned.. there can be a 5% variation...
Race Director
Joined: Apr 2000
Posts: 17,676
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Re: Compression Test: These Numbers Sound Right? (Oldguard 7)
My compression was 230-235psi across the board with Sportsman II iron heads and it runs great on pump gas and I run 40 degrees total advance. I did it twice once with the old CC270H cam and then again with CC282 solid and I got pretty much the same numbers. I have been told that up to about 240psi is good for the street then it is track only.
This is the trickiest part of cam/head/piston-compression ratio selection. The higher duration cams require a higher compression ratio to keep the dynamic compression up. That is the reason I am going with 11:1 pistons in my 406ci with the same cam you have, it will be interesting to see what numbers I get. :D
[Modified by MotorHead, 10:07 PM 3/23/2004]
This is the trickiest part of cam/head/piston-compression ratio selection. The higher duration cams require a higher compression ratio to keep the dynamic compression up. That is the reason I am going with 11:1 pistons in my 406ci with the same cam you have, it will be interesting to see what numbers I get. :D
[Modified by MotorHead, 10:07 PM 3/23/2004]
Thread Starter
Melting Slicks
Joined: Dec 2001
Posts: 2,025
Likes: 1
From: Billings MT
Re: Compression Test: These Numbers Sound Right? (MotorHead)
I'm surprised you didn't have any issues with detonation. What was your DCR?
With this cam and my calculated compression of 10.65:1 your 11:1 will most likely not be that much higher. I assume your elevation will have some effect on the compression limit with pump gas.
I am very happy with this cam, I think you'll be pleased. :)
Travis
With this cam and my calculated compression of 10.65:1 your 11:1 will most likely not be that much higher. I assume your elevation will have some effect on the compression limit with pump gas.
I am very happy with this cam, I think you'll be pleased. :)
Travis
Racer
Joined: Jul 2002
Posts: 275
Likes: 13
From: South Bend, Indiana
Re: Compression Test: These Numbers Sound Right? (MotorHead)
My compression was 230-235psi across the board with Sportsman II iron heads and it runs great on pump gas and I run 40 degrees total advance. I did it twice once with the old CC270H cam and then again with CC282 solid and I got pretty much the same numbers. I have been told that up to about 240psi is good for the street then it is track only.
This is the trickiest part of cam/head/piston-compression ratio selection. The higher duration cams require a higher compression ratio to keep the dynamic compression up. That is the reason I am going with 11:1 pistons in my 406ci with the same cam you have, it will be interesting to see what numbers I get. :D
[Modified by MotorHead, 10:07 PM 3/23/2004]
This is the trickiest part of cam/head/piston-compression ratio selection. The higher duration cams require a higher compression ratio to keep the dynamic compression up. That is the reason I am going with 11:1 pistons in my 406ci with the same cam you have, it will be interesting to see what numbers I get. :D
[Modified by MotorHead, 10:07 PM 3/23/2004]
Melting Slicks
Joined: Feb 2001
Posts: 3,330
Likes: 13
From: Europe , Luxembourg
Re: Compression Test: These Numbers Sound Right? (VETDRMS)
Hi
The compression test of my 427 CI .030 over solid lifter engine was from 161 to 177 PSI. Unfortunately, I used a push on type tester and they are known not to seal off so well .
The cam is a original L71 solid lifter cam with .520/.520 lift, 242/242 duration , 114 lobe ctr line angle.
Compression is 11.3 : 1 .
The engine had an overhaul about 10 K ago, .030 bore , new guts. The old heads only got a valve seat grind with creme only.
Gunther :seeya
The compression test of my 427 CI .030 over solid lifter engine was from 161 to 177 PSI. Unfortunately, I used a push on type tester and they are known not to seal off so well .
The cam is a original L71 solid lifter cam with .520/.520 lift, 242/242 duration , 114 lobe ctr line angle.
Compression is 11.3 : 1 .
The engine had an overhaul about 10 K ago, .030 bore , new guts. The old heads only got a valve seat grind with creme only.
Gunther :seeya
