Compression Ratios...How do i find them??? - Team Camaro Tech
Engine General Engine Discussion.

 
LinkBack Thread Tools Display Modes
post #1 of 15 (permalink) Old Feb 5th, 05, 04:33 AM Thread Starter
Tech Team
Craig
 
Join Date: Dec 2004
Location: State College, PA
Posts: 75
Send a message via AIM to cDuck28Z
Question

Alright, here's my problem. I've got this nice 1967 camaro 327 40over w/ an awesome roller rocker set in it. But its not cranking out the power i want, so i think i want to put in a new more aggressive cam. Im not sure what i sould put in, but ill get to that later. All the websites like ISKY cams, Comp Cams, and Crane Cams rate their cams by the compression. I dont know what the compression is because all the engine work was done before i got the car. Can anyone tell my how to find my compression???

1969 Pro Touring Camaro Convertible Project Car

Previously:
1967 Camaro SS Clone 327ci 4spd; 1968 Camaro RS Yenko Clone (1969 Stripes) 427BB; 1967 Camaro SS Clone 350ci 3spd

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
cDuck28Z is offline  
Sponsored Links
Advertisement
 
post #2 of 15 (permalink) Old Feb 5th, 05, 04:53 AM
 
Join Date: Apr 2000
Posts: 21,341
Smile

The easiest/hardest way to do this is to chase down the guy that built it and ask him what the motor has in it!! Hardest = probably can't find him...

Otherwise, post back the letters at the end of the VIN number string that is on the pad above the pass. side top water pump bolt and we will try to decode what it is IF it is still a "virgin"... The letters will be between "MA" and "MN" if it is a '67, 327 motor...

Also pull a valve cover and post back the long casting number between the valves so we can help with what heads are on it too.... That number should be like "3891462". It's the last three numbers we look at.. The "-462" here..

If it has been "molested", you will have to pull a head and see what pistons are in it

I have a feeling that if it idles nice and smooth you have a low compressioned, 210hp jobber and about the only way to perk it up is to install a set of 64 cc heads on it and then a good old CC 268HE OR Crane 272/272 Energizer hy-cam along with a set of cheap, 1.625" four tube, long headers and a 600 cfm Holley 1850 carb. on a Holley 300-36 hi-rise intake!!

Then she will run like a "scalded-dog" if geared and tired right.. At least my .040" over 327 did...

pdq67



pdq67 is offline  
post #3 of 15 (permalink) Old Feb 7th, 05, 07:52 PM
Senior Tech
Christopher
 
Join Date: Jan 2005
Location: Salt Lake City, Utah
Posts: 260
Send a message via AIM to ctrghtous
Post

is there a way to tell off of cylinder pressure with a compression tester?

Chris
68 camaro, sbc 385 (going together), holley 4150 750cfm, 4 spd, gm HEI, performer RPM intake
ctrghtous is offline  
 
post #4 of 15 (permalink) Old Feb 8th, 05, 03:48 AM
Gold Lifetime Member
Jim
 
Join Date: Dec 1999
Location: Yorkville, Illinois USA
Posts: 22,121
Post

Sorta, but not really. The closing point of the intake valve has a huge effect on this ##. If you knew what cam's in it, then yes, otherwise, not really. You could check it and see where it is... pretty much, with a near stock cam, under 150 = "lazy" 150-180 = "normal" over 180 = "hot"

The info asked for above will help some. If you can see a piston top thru a sparkplug hole with a flashlight well enuf to determine if it's flat, dished, or domed, that'll help too.

Could always work it backwards... fill it witl 87 octane, and keep advancing the initial timing till it starts pinging





To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.



To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
New website for my vert is coming together. Do ya like the slide show on the home page?
JimM is offline  
post #5 of 15 (permalink) Old Feb 8th, 05, 03:49 AM
Gold Lifetime Member
Jim
 
Join Date: Dec 1999
Location: Yorkville, Illinois USA
Posts: 22,121
Post

Sorta, but not really. The closing point of the intake valve has a huge effect on this ##. If you knew what cam's in it, then yes, otherwise, not really. You could check it and see where it is... pretty much, with a near stock cam, under 150 = "lazy" 150-180 = "normal" over 180 = "hot"

The info asked for above will help some. If you can see a piston top thru a sparkplug hole with a flashlight well enuf to determine if it's flat, dished, or domed, that'll help too.

Could always work it backwards... fill it witl 87 octane, and keep advancing the initial timing till it starts pinging





To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.



To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
New website for my vert is coming together. Do ya like the slide show on the home page?
JimM is offline  
post #6 of 15 (permalink) Old Feb 8th, 05, 05:00 AM
Senior Tech
Nick
 
Join Date: Nov 2001
Location: BARRIE ONT CANADA
Posts: 755
Post

Yes there is a formula.your compression lets say is 160:327x2=0.9785932 this is almost 10 to 1
if 180:327x2=1,1009174 this is 11 to 1
hope this helps Nick
: this means divide
MRCOOL_RSSS350 is offline  
post #7 of 15 (permalink) Old Feb 8th, 05, 12:06 PM
Senior Tech
 
Join Date: Feb 2003
Location: KC
Posts: 266
Post

Quote:
Yes there is a formula.your compression lets say is 160:327x2=0.9785932 this is almost 10 to 1
if 180:327x2=1,1009174 this is 11 to 1
Huh.... 160 and 180 are psi. 327 is cubic inches... I'm not following.

Like the others have said the only way to determine the static compression ratio is to look at the pistons and know the combustion chamber size, head gasket thickness, etc. etc. etc. If you take a compression reading it is more closely related to the dynamic compression ratio (DCR). If the compression reading is down (i.e. below 160) a larger cam (later intake closing event) will hurt the performance rather than help. You really need to match the cam with the static compression ratio and target an 8:1 DCR for iron heads or an 8.5:1 DCR for aluminum heads.
rpol78 is offline  
post #8 of 15 (permalink) Old Feb 8th, 05, 12:30 PM
Senior Tech
Nick
 
Join Date: Nov 2001
Location: BARRIE ONT CANADA
Posts: 755
Post

Yes, 160 and 180 is psi and 327 is not an engine, its a number. times 2 will give you your compression ratio. You can call it dcr or watever you want to call it, at least you will have a number that is close to the compression ratio. If the engine is worn out, then yes, the numbers will be out, but if the engine is fresh, I would say its very close. So take your compression, divide it by 327 times 2. That is the formula. I was given this by an old machinest. Nick

67 rs/ss 350 original eng,700r4, a/c, tilt, deluxe int,strato bench,Ca.emission,front disk,am/fm,floor clock,17" W/V Ralleys,Toyo T1S 255/40/17, 245/40/17
MRCOOL_RSSS350 is offline  
post #9 of 15 (permalink) Old Feb 8th, 05, 03:00 PM
Senior Tech
 
Join Date: Feb 2003
Location: KC
Posts: 266
Post

Well let's try the forumula. I have a 414 (402 +.060) that's running 9.7 static compression and 7.95 dynamic compression. On a compression test it pumps out about 175 psi (varies a little cylinder to cylinder). So the forumla is (175/327)*2 = 1.100917. So this is saying I have an 11:1 motor???
rpol78 is offline  
post #10 of 15 (permalink) Old Feb 8th, 05, 03:49 PM
Senior Tech
Al
 
Join Date: Nov 2002
Location: Vancouver, USA
Posts: 10,511
Post

The only way to know for sure is to remove a cyl head. Measure the volume of the chamber, measure the deck height, and measure any dish or valve notches in the pistons, and determine what kind of head gaskets it has. (Hopefully you don't have domed pistons.)

An educated guess method would be to determine chamber volume from a casting number on the head. You might be able to shine a light into a spark plug hole and see if you have flat top pistons, dished or whatever, and figure it based on some assumptions. Problem is, you can't really tell if the head has been milled or if the block has been decked, and those have a pretty big impact on CR.

If I couldn't track down the info from the builder, I'd pull a head and measure. Once you have it torn down far enough for a cam replacement, it's not that much more work to pull a head.

Compression ratio calculators are easy to find on the web - just do a google search. But they are worthless if you don't have accurate data.


To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
BPOS is offline  
post #11 of 15 (permalink) Old Feb 8th, 05, 07:01 PM
Gold Lifetime Member
Jim
 
Join Date: Dec 1999
Location: Yorkville, Illinois USA
Posts: 22,121
Post

rpol, while I'm not buying the formula, except maybe for stock cam grocery getters, I think the "327" should be replaced by the actual displacement of the engine in question. Doing it that way, I get 8.4:1 for your 414. off just as far, other direction...nope, still not buying the formula thing...





To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.



To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
New website for my vert is coming together. Do ya like the slide show on the home page?
JimM is offline  
post #12 of 15 (permalink) Old Feb 9th, 05, 04:55 AM
Senior Tech
Nick
 
Join Date: Nov 2001
Location: BARRIE ONT CANADA
Posts: 755
Post

Hey JimM At the time i had a 67 Camaro with a 327 so when I asked the guy he told me to use the 327 # so maybe the 327 is the displacement # I don't know but it is ease to find out by doing some # of engine displacement and engine compressions and see what comes up.Nick

67 rs/ss 350 original eng,700r4, a/c, tilt, deluxe int,strato bench,Ca.emission,front disk,am/fm,floor clock,17" W/V Ralleys,Toyo T1S 255/40/17, 245/40/17
MRCOOL_RSSS350 is offline  
post #13 of 15 (permalink) Old Feb 9th, 05, 08:24 AM
 
Join Date: Apr 2000
Posts: 21,341
Question

cDuck,

What VIN letters did you find??

pdq67



pdq67 is offline  
post #14 of 15 (permalink) Old Feb 13th, 05, 02:08 PM
 
Join Date: Oct 2004
Posts: 46
Post

Using the formula posted, it doesn't work out on the recent 355 I built. The cyls were c.c.'d @ 64. Going through the formula for mechanical c.r., I got 10:3. Here is my numbers in your formula; cyl. pres=205 divided by c.i.d. times 20 = 11.5. Now, if i use the same formula, and times it by 18 instead of 20, I get a more realistic answer, 10.39 or 10.4. If this formula worked for more than a few motors with a KNOWN c.r., then I mite buy into it, but I wouldn't put much faith in it without a reasonable amount of research. If anybody else has a known (key word) compression ratio 355 cu. in motor, plug the #'s in see what you get. Allways remember, "you can't go wrong, when you do it right".
19ynot52 is offline  
post #15 of 15 (permalink) Old Feb 14th, 05, 06:30 AM
Senior Tech
Nick
 
Join Date: Nov 2001
Location: BARRIE ONT CANADA
Posts: 755
Post

19ynot52 i agree with you about taking the head off to find out the correct cr, but he was looking for an easy way to do it without taking everything off. Weather this formula works or not, its up to everyone's finds. maybe if 18 works better than 20, then maybe that is the answer. We are just trying to help each other here, and im not trying to misslead anyone. To tell you the truth, those numbers were given to me in 1974 when i had my 67 327 275 hp car and believe me, i had to think very hard to try to remember what the numbers are, but i knew the 327 was one of the numbers because it matched my engine. Anyways, if it works, fine, if it doesnt work, take the heads off. Nick
MRCOOL_RSSS350 is offline  
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Team Camaro Tech forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address.
NOTE we receive a lot of registrations with bad email addresses. IF you do not receive your confirmation email you will not be able to post. contact support and we will try and help.
Be sure you enter a valid email address and check your spam folder as well.



Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome