compression ratio

[gasalley]

can anyone tell how to figure compression ratio without taking the heads off?
I have a sbford 302,stock pistons w/.030 overbore, `78 351W heads w/ 202 intake valves, milled(don't know how much). :?

[JimmySmith]

You would have to know what piston and what chamber CC you have to figure this. Later J.Smith 4621 racing

[desoto30]

the only way I'd imagine that this could be done would be to fill the cylinder @ tdc & bdc with fluid & calc from that,though sealing may present a problem

[Jordan1984]

You can buy a compression gauge from an auto parts store that you put in place of the spark plug and bump the engine over. It has a reset on it that locks it in place while you are bumping the engine from the driver seat, or you could have a buddy do it while you watch it. Put it in every cylinder and write it down on paper, then add them all together. Take that number and divide it by the number of cylinders, and you will get a round about compression ratio.

[MEMRACING62]

??? :shock: :shock:

[suicidebomb]

??? :shock: :shock:

Confused? me too :shock: :?

[MEMRACING62]

must some kinda new math!! :lol: :lol:

[wrightway]

You can buy a compression gauge from an auto parts store that you put in place of the spark plug and bump the engine over. It has a reset on it that locks it in place while you are bumping the engine from the driver seat, or you could have a buddy do it while you watch it. Put it in every cylinder and write it down on paper, then add them all together. Take that number and divide it by the number of cylinders, and you will get a round about compression ratio.


:shock: All that will get you is an average cranking psi :roll:

[Tod74]

Holy Cow My compression ratio is 245 to 1!!!!

[BEAST477]

Holy Cow My compression ratio is 245 to 1!!!!

WOW Tod, where do you get fuel for that? :shock:

[Jordan1984]

This is how they check for blown head gaskets, burnt/stuck valves, to make sure that it is blown or damaged and not something else. Its called a compression check. Take out the number 1 plug. Put the gauge into the number 1 plug. Crank the motor until the gauge reads at the highest point. Write it down. Do the same thing for cylinders 2 through 8. Add them up. Divide the number by the number of cylinders. This will give you a round about compression ratio. Example: number 1 cylinder - 9.5, 2 cyl - 9.6, 3 cyl 9.6, 4 cyl 9.5, 5 cyl 9.5, 6 cyl 9.4, 7 cyl 9.6, 8 cyl 9.5. This equals 76.2 total. Now divide 76.2 by the number of cylinders. 76.2 divided by 8 cylinders = 9.525. Your compression ratio is 9.5:1.

[Tod74]

This is how they check for blown head gaskets, burnt/stuck valves, to make sure that it is blown or damaged and not something else. Its called a compression check. Take out the number 1 plug. Put the gauge into the number 1 plug. Crank the motor until the gauge reads at the highest point. Write it down. Do the same thing for cylinders 2 through 8. Add them up. Divide the number by the number of cylinders. This will give you a round about compression ratio. Example: number 1 cylinder - 9.5, 2 cyl - 9.6, 3 cyl 9.6, 4 cyl 9.5, 5 cyl 9.5, 6 cyl 9.4, 7 cyl 9.6, 8 cyl 9.5. This equals 76.2 total. Now divide 76.2 by the number of cylinders. 76.2 divided by 8 cylinders = 9.525. Your compression ratio is 9.5:1.

Have you ever actually DONE a compression test? Or even held a compression gauge in your hand?

Just Curious.

[I972Nova]

I'd like to have one of those magic guages that tells you the compression instead of the cylinder pressure...lol... That must be the Master Mechanic brand...

[fla1976]

:idea: Oh yeah, the old divide by 8 rule.

With a compression average of 9.5, you should be able to run in the mid nines. :oops:
Here are a few sites:
http://www.not2fast.com/turbo/compression/compression.shtml
http://www.bgsoflex.com/cr.html
http://www.carcraft.com/techarticles/calculate_compression_ratio/index.html

[Jordan1984]

Yes I have done many compression checks and figured compression ratio on this method. You take 1 plug out and put a fitting into the plug hole. The fitting has a vacuum hose that runs to the compression gauge itself. This procedure is in every automotive manual that I have and have been told by many mechanics to do this. Haynes, ford thunderbird, mercury cougar, 1989 -1997 haynes repair manual, chapter 2 part C section 3, cylinder compression check is what I am refering to. Get any manual and look it up, or ask any full time mechanic, not a weekend backyard mechanic.

[MEMRACING62]

jordan, I think your confusing presure with ratio, look at the links fla left for you above ( the car craft one explains it very well) that should clear up the confusion.

[zipper06]

This is too funny :lol: , It is vertually impossible to check/get the compression ratio, without knowing about 6 factors, IE stroke,bore,piston dome or relief, valve relief, gasket thickness, and the cylinder head volume. Below is another formula on how to calculate the compression ratio ans it includes all the specs. needed.

Zip.

http://www.wallaceracing.com/cr_test2.php

[Jordan1984]

Try it sometime. Go to the calculator and type in all your specs. Then try the way I told you to. Bet you are within +/- .1. I know what I am reading in the automotive manuals. It says cylinder compression check. It doesnt say cylinder pressure check. If you dont have a manual, buy one and look at it.

[MEMRACING62]

compression check is only useful to determine condition of cylinder. not the ratio. fla & zip left some great links for ya to clear up the confusion. if a mechanic is telling you that you can tell compression ratio from the guage, he is dead wrong!!

[suicidebomb]

Well now I see what the problem is...........

[Tod74]

Yes I have done many compression checks .

Ok....

what is the compression ratio of an 8 cylinder engine that gets cranking compression readings of

234,237,232,234,233,235,231,236?

[suicidebomb]

Tod, I came up with 234, or about 16.5:1. How bout it? Am I within a tenth?

[Tod74]

....

[suicidebomb]

..........Tod ain't aswering, so I must be spot on..........Tod, give me a call, my number is BR549.

[I972Nova]

Im done trying to beat the dead horse. You my friend do not have a clue about getting a compression ratio and you should find a new mechanic if he is telling you that there is a guage that gives you compression and not just cranking pressure.

Get any manual and look it up, or ask any full time mechanic, not a weekend backyard mechanic.

I guess real world wrench turning experience and ASE certifications take a back seat to the brilliance you can attain by reading a Haynes manual :roll:

[TopspeedLowet]

Im done trying to beat the dead horse. You my friend do not have a clue about getting a compression ratio and you should find a new mechanic if he is telling you that there is a guage that gives you compression and not just cranking pressure.

Get any manual and look it up, or ask any full time mechanic, not a weekend backyard mechanic.

I guess real world wrench turning experience and ASE certifications take a back seat to the brilliance you can attain by reading a Haynes manual :roll:
Thanks for saying this DragsterJosh, I think a little miss reading or real general source material "grossly misunderstood by the reader" must have caused this nonsense. I would have thought that when every one was :lol:, at this nonsense, the author may have re-thought his answer prior to insulting other race junkers that are politely giving him a reality check . Cranking compression has no relation to static compression ratio, DUH... Cam timing will have the greatest role in manipulating the cranking numbers up and down. Advanced cam timing settings will give higher readings and retarded settings will give lower PSI numbers, no matter what the calculated COMPRESSION RATIO is, changing only the cam timing without any physical volume or ie, compression ratio changes.
PS, We don't have to be a full time mechanic's to understand printed material in a repair manual, but it appears someone obviously does.
Bruce
Retired master ASE tech

[radicalz]

Yes I have done many compression checks .

Ok....

what is the compression ratio of an 8 cylinder engine that gets cranking compression readings of

234,237,232,234,233,235,231,236?

Using his formula...I get the same answer as Tod...hey where...what the..

[THERATTLER]

not making fun or anything but you don't have a clue as to what you are talking about , if you think I don't know anything about motors , check out the picture by my name , I have built every motor ever been in this dragster , there is no compression guage that reads COMPRESSION RATIO , it reads pressure in pounds..SORRY :(

[Jordan1984]

SORRY FOR GETTING MY INFORMATION MIXED UP AND GOING BY WHAT 1 INDIVIDUAL HAS TOLD ME- HE HAS BEEN RIGHT EVERY OTHER TIME THAT I HAVE WENT TO HIM FOR INFORMATION. I DID GET IT CONFUSED. MY BAD. IM NEW TO RACING, GETTING MY FEET WET AND LEARNING AS I GO, LEARNING FROM MY MISTAKES AND I HAVE MADE ALOT OF MAJOR MISTAKES ON MY WAY BUT I HAVE LEARNED FROM THEM. I WILL NO LONGER GO TO THE MECHANIC WHERE I WAS GETTING MY INFO FROM, I WILL TURN TO HERE FOR MORE INFORMATION. What mistakes have I made? I ruined a 200 4r trans, broke lifters and pushrods, wiped out the lobes on 2 cams. Even as far as getting my arm stuck in driver side header, droping the 7/16 to the floor, and waiting cramped up for my dad to find me 4 hours later. It might be a street car, but my ad number is 1526368 if anyone wants to check it out.

[THERATTLER]

:lol: :lol: :lol: GOT YOUR ARM STUCK IN THE HEADER :?: :?: :?:

[cepx111]

SORRY FOR GETTING MY INFORMATION MIXED UP AND GOING BY WHAT 1 INDIVIDUAL HAS TOLD ME- HE HAS BEEN RIGHT EVERY OTHER TIME THAT I HAVE WENT TO HIM FOR INFORMATION. I DID GET IT CONFUSED. MY BAD. IM NEW TO RACING, GETTING MY FEET WET AND LEARNING AS I GO, LEARNING FROM MY MISTAKES AND I HAVE MADE ALOT OF MAJOR MISTAKES ON MY WAY BUT I HAVE LEARNED FROM THEM. I WILL NO LONGER GO TO THE MECHANIC WHERE I WAS GETTING MY INFO FROM, I WILL TURN TO HERE FOR MORE INFORMATION. What mistakes have I made? I ruined a 200 4r trans, broke lifters and pushrods, wiped out the lobes on 2 cams. Even as far as getting my arm stuck in driver side header, droping the 7/16 to the floor, and waiting cramped up for my dad to find me 4 hours later. It might be a street car, but my ad number is 1526368 if anyone wants to check it out.

Now we are getting somewhere, it takes a man to admit fault, very good.

Now, what you get by adding the compression readings up and dividing them by number of cylinders you checked will give you a "AVERAGE" compression pressure, not the compression ratio.

[Jordan1984]

Yes, arm stuck between the header and block. I was holding the flange against atop my forearm as I bolted it on, got 2 bolts in and went to remove my arm and my elbow pinned against the firewall, I couldnt bend my wrist or move my arm to get it out, and I dropped the wrench where I couldnt get it. That sucked.

[Tod74]

Yes, arm stuck between the header and block. I was holding the flange against atop my forearm as I bolted it on, got 2 bolts in and went to remove my arm and my elbow pinned against the firewall, I couldnt bend my wrist or move my arm to get it out, and I dropped the wrench where I couldnt get it. That sucked.

Do you Know Rudy? aka Hollowayshotrods?

ROFLMFAO!!! :lol:

[fatjodycat]

That does take a man to admit he is wrong Jordan......... but man i peeeeed my pants laughing from the responses of the other guys. Good luck to you Jordan.

[I972Nova]

Jordon I didnt mean to be rude earlier, but that seemed to be the only way to get through. It takes a big man to admit hes wrong... Just stick around here and you can learn alot. There is almost always someone to answer any question you may have, and just about everyone on here knows their stuff! Bruce and others have helped me with a some problems too. I brag about this forum and the quality of people on here.

IMO its so much better here than other places like yellow bullet.

[a350racer]

Jordan,
just in case it has not been made clear I want to chime in here and say it really does take a big guy to admit to being wrong. My hat off to you.
If you are in the business of learning you might want to check out a brand new site coming on line soon at motortecmagazine.com

a350racer

[Scooterz]

Hey Jordan,

Good job. To be honest, I think alot of people around here would be surprised at how much I do not know... but these guys have helped me alot too. Anyways, stick w/ the dudes around here who do the sport & maintain the equipment they race. Scooter

[fla1976]

Jordan, hope you have compression ratio straight now. Glad you owned up to your error. Keep on asking and someone will give you the correct info eventually.