calculating compression ratio with engine together.
#622933
02/24/10 04:04 PM
02/24/10 04:04 PM
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Joined: Aug 2003
Posts: 4,481
Mesa, AZ
Pat_Whalen
OP
super gas
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OP
super gas
Joined: Aug 2003
Posts: 4,481
Mesa, AZ
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Kinda curious. Is it possible to calculate compression ratio with the engine together. It would seem to me it wouldnt be too hard using a combustion chamber volume and p s i during compression stroke. Possible? Thanks. Cheers. pat 
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Re: calculating compression ratio with engine together.
[Re: Pat_Whalen]
#622934
02/24/10 06:31 PM
02/24/10 06:31 PM
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Joined: Dec 2007
Posts: 10,542
BROOK PARK, OH
WILD BILL
Senior Member of the Junior Dragster Club
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Senior Member of the Junior Dragster Club
Joined: Dec 2007
Posts: 10,542
BROOK PARK, OH
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If you know or have access to the #'s you can plug them in HERE 
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Re: calculating compression ratio with engine together.
[Re: WILD BILL]
#622935
02/24/10 07:02 PM
02/24/10 07:02 PM
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Joined: Aug 2003
Posts: 4,481
Mesa, AZ
Pat_Whalen
OP
super gas
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OP
super gas
Joined: Aug 2003
Posts: 4,481
Mesa, AZ
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Quote:
If you know or have access to the #'s you can plug them in HERE
Bill: The one number I'm missing, which I know is critical, is how deep in the hole the pistons are. That's why I was wondering if it can be done via some math and knowing the compression value of the cylinder itself at TDC.
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Re: calculating compression ratio with engine together.
[Re: DaytonaTurbo]
#622937
02/24/10 07:10 PM
02/24/10 07:10 PM
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Joined: Dec 2007
Posts: 10,542
BROOK PARK, OH
WILD BILL
Senior Member of the Junior Dragster Club
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Senior Member of the Junior Dragster Club
Joined: Dec 2007
Posts: 10,542
BROOK PARK, OH
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 Sorry but it sounds like if you wanna know your gonna need to pull a head 
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Re: calculating compression ratio with engine together.
[Re: AndyF]
#622939
02/24/10 10:25 PM
02/24/10 10:25 PM
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Joined: Aug 2003
Posts: 4,481
Mesa, AZ
Pat_Whalen
OP
super gas
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OP
super gas
Joined: Aug 2003
Posts: 4,481
Mesa, AZ
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Quote:
If the engine is on a stand you can measure the volume at BDC and TDC.
Engine is on a stand. How do I go about doing this? Same way to measure volume of a cylinder head? Thanks for renewing my hope, Andy.
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Re: calculating compression ratio with engine together.
[Re: Pat_Whalen]
#622941
02/25/10 10:55 AM
02/25/10 10:55 AM
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Joined: Nov 2003
Posts: 36,040
Lincoln Nebraska
RapidRobert
Circle Track
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Circle Track
Joined: Nov 2003
Posts: 36,040
Lincoln Nebraska
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On the stand I'd put a fair amt of (as suggested) ATF in there then turn it over several times to coat the walls etc then turn it upside down/drain it then turn it right side up & at BDC fill it & record the amt then upside down & drain it then right side up & turn to TDC then fill it & record the amt & do the math. Pre coating everything before you calculate will give you significantly more accurate reading I would think because if you dont, the amt left over that is coating the surfaces from the 1st quantity is going to skew the 2nd reading giving you a false reading of more or less CR depending on if you fill @BDC or TDC 1st. How much I do not know but I think w a bit more effort (to precoat) that you can end up w a pretty accurate reading on what you got.
live every 24 hour block of time like it's your last day on earth
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Re: calculating compression ratio with engine together.
[Re: RapidRobert]
#622942
02/25/10 03:47 PM
02/25/10 03:47 PM
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Joined: Oct 2005
Posts: 8,872
Ontario, Canada
Stanton
Don't question me!
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Don't question me!
Joined: Oct 2005
Posts: 8,872
Ontario, Canada
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NO, NO , NO , NO - do NOT precoat the cylinder - that will definately skew the numbers. Besides, all that filling and draining is unnecessary!
Do as Andy said.
Pull the rockers so the valves don't open. Flip the motor so the plug hole is up and level. Bring the piston to TDC, fill the cavity using a burrette (measuring in cc's). Record that volume. Bring the piston to BDC. Fill the rest of the bore using the burrette (this will take a LONG time 'cause burrette's hold surprisingly little). Record that volume and do the math. If you screw up, empty that cylinder and then do it again on a different cylinder so that any fluid residue doesn't skew the numbers.
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Re: calculating compression ratio with engine together.
[Re: Stanton]
#622943
02/25/10 04:15 PM
02/25/10 04:15 PM
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Joined: Dec 2007
Posts: 7,759
So Cal
HealthServices
Why would you even post that?
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Why would you even post that?
Joined: Dec 2007
Posts: 7,759
So Cal
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Quote:
NO, NO , NO , NO - do NOT precoat the cylinder - that will definately skew the numbers. Besides, all that filling and draining is unnecessary!
Do as Andy said.
Pull the rockers so the valves don't open. Flip the motor so the plug hole is up and level. Bring the piston to TDC, fill the cavity using a burrette (measuring in cc's). Record that volume. Bring the piston to BDC. Fill the rest of the bore using the burrette (this will take a LONG time 'cause burrette's hold surprisingly little). Record that volume and do the math. If you screw up, empty that cylinder and then do it again on a different cylinder so that any fluid residue doesn't skew the numbers.
Why do you need to fill it twice?
Can't you calculate the cylinder volume?
Bore and stroke you know. You can calculate this with the info you already have.
- The area the piston is down, including the area to the first compression ring
- valve relieves
- piston dome
- chamber volume
are areas that you are unsure of and measured with the piston at TDC. Make sure to measure to the bottom of the Sparkplug threads and turn the motor on the stand so the bottom of the sparkplug is at the top of the chamber.
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Re: calculating compression ratio with engine together.
[Re: HealthServices]
#622944
02/25/10 05:05 PM
02/25/10 05:05 PM
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Joined: Nov 2003
Posts: 6,293
Rock Springs
Bob_Coomer
master
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master
Joined: Nov 2003
Posts: 6,293
Rock Springs
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Quote:
Quote:
NO, NO , NO , NO - do NOT precoat the cylinder - that will definately skew the numbers. Besides, all that filling and draining is unnecessary!
Do as Andy said.
Pull the rockers so the valves don't open. Flip the motor so the plug hole is up and level. Bring the piston to TDC, fill the cavity using a burrette (measuring in cc's). Record that volume. Bring the piston to BDC. Fill the rest of the bore using the burrette (this will take a LONG time 'cause burrette's hold surprisingly little). Record that volume and do the math. If you screw up, empty that cylinder and then do it again on a different cylinder so that any fluid residue doesn't skew the numbers.
Why do you need to fill it twice?
Can't you calculate the cylinder volume?
Bore and stroke you know. You can calculate this with the info you already have.
- The area the piston is down, including the area to the first compression ring
- valve relieves
- piston dome
- chamber volume
are areas that you are unsure of and measured with the piston at TDC. Make sure to measure to the bottom of the Sparkplug threads and turn the motor on the stand so the bottom of the sparkplug is at the top of the chamber.
How would you figure swept volume then?
How would you know what volume is swept, from quench volume?
You need to know the volume at TDC, then any other volume or point of referance..but how will you know at what point the piston is down..Thats why it would be simple to just do TDC, then BDC...this will tell you swept, then quench..
Thats all you need.
Last edited by Bob_Coomer; 02/25/10 05:07 PM.
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Re: calculating compression ratio with engine together.
[Re: Bob_Coomer]
#622946
02/25/10 05:46 PM
02/25/10 05:46 PM
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Joined: Dec 2007
Posts: 7,759
So Cal
HealthServices
Why would you even post that?
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Why would you even post that?
Joined: Dec 2007
Posts: 7,759
So Cal
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Quote:
Quote:
Quote:
NO, NO , NO , NO - do NOT precoat the cylinder - that will definately skew the numbers. Besides, all that filling and draining is unnecessary!
Do as Andy said.
Pull the rockers so the valves don't open. Flip the motor so the plug hole is up and level. Bring the piston to TDC, fill the cavity using a burrette (measuring in cc's). Record that volume. Bring the piston to BDC. Fill the rest of the bore using the burrette (this will take a LONG time 'cause burrette's hold surprisingly little). Record that volume and do the math. If you screw up, empty that cylinder and then do it again on a different cylinder so that any fluid residue doesn't skew the numbers.
Why do you need to fill it twice?
Can't you calculate the cylinder volume?
Bore and stroke you know. You can calculate this with the info you already have.
- The area the piston is down, including the area to the first compression ring
- valve relieves
- piston dome
- chamber volume
are areas that you are unsure of and measured with the piston at TDC. Make sure to measure to the bottom of the Sparkplug threads and turn the motor on the stand so the bottom of the sparkplug is at the top of the chamber.
How would you figure swept volume then?
How would you know what volume is swept, from quench volume?
You need to know the volume at TDC, then any other volume or point of referance..but how will you know at what point the piston is down..Thats why it would be simple to just do TDC, then BDC...this will tell you swept, then quench..
Thats all you need.
Compression Ratio = Uncompressed volume ÷ Compressed volume
Uncompressed volume = Displacement + Compressed volume
Displacement = (Bore ÷ 2) X (Bore ÷ 2) X 3.14 x Stroke x 16.387
16.387 is the number to convert cubic inches to cc's.
Compressed volume = the volume you measured at TDC.
Quote:
Bring the piston to BDC. Fill the rest of the bore using the burrette ( this will take a LONG time 'cause burrette's hold surprisingly little).
Measuring the volume of the bore at BTC in addition to the volume at TDC just seems like a lot of extra work. With the bore and stroke you are just a couple clicks of the calculator from finding this value, just add the measurement from the TDC to it. Plus it's a big mess to clean up if you fill the bore at BTC afterwards.
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Re: calculating compression ratio with engine together.
[Re: HealthServices]
#622947
02/25/10 05:47 PM
02/25/10 05:47 PM
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Joined: Oct 2005
Posts: 8,872
Ontario, Canada
Stanton
Don't question me!
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Don't question me!
Joined: Oct 2005
Posts: 8,872
Ontario, Canada
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Quote:
Why do you need to fill it twice?
You're not ... the first check is the volume at TDC, then turn to BTC and fill the remainder for the second check. But it's only "filled" once.
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Re: calculating compression ratio with engine together.
[Re: 383man]
#622949
02/26/10 12:14 AM
02/26/10 12:14 AM
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Joined: Oct 2005
Posts: 8,872
Ontario, Canada
Stanton
Don't question me!
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Don't question me!
Joined: Oct 2005
Posts: 8,872
Ontario, Canada
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Quote:
Plus it's a big mess to clean up if you fill the bore at BTC afterwards.
How do you figure ??? You don't just flip the engine and dump the contents on the floor when you're done !!! You use your head and with some small hose or tubing you siphon the fluid out of the cylinder. Tilt the motor slightly and properly position the hose and it should get just about all the fluid out. Jeez !!!
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Re: calculating compression ratio with engine together.
[Re: HealthServices]
#622951
02/26/10 08:46 AM
02/26/10 08:46 AM
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Joined: Nov 2009
Posts: 3,410
Belpre,Ohio
CHAPPER
master
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master
Joined: Nov 2009
Posts: 3,410
Belpre,Ohio
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I don't care how you do it. I just offered some advice as how 'I' have done it. One thing I would like to know,,what is "BTC"?
If you like drag racing, support your local track.
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procedure.
