Measuring compression ratio, and rear end gearing... questions

SadbutTrue

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May 1, 2002

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Granada Hills, California

Jun 14, 2004

#1

• Jun 14, 2004

• #1

Another thread of mine with more than one topic at once that are completely unrelated...

First, will you feel a difference, either in gas mileage or seat of pants, between 3.25 and 3.50 (ish on both) gears? Trying to decide...

Second, to measure compression ratio, do you simply take the ratio of volumes of the displacement of one cylinder (plus the compression ratio, i guess) to the volume of the combustion chamber? Seems like that would make sense, but only if the piston comes all the way up to combustion chamber,which i'm not sure is the case.

Thanks for words on both.

 

6Stang7

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Jun 1, 2003

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Jun 14, 2004

#2

• Jun 14, 2004

• #2

If you do feel a difference between 3.25 to 3.50 then it won't be a big one. The raito change there isn't that much. I think you had the right idea going on the CR messurement(didn't really follow what you said too well). You messure CR by taking the total cylinder volume at BDC (inclunding the combustion chamber) to volume of your combustion chamber.

 

kslushy

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#3

• Jun 14, 2004

• #3

SadbutTrue: You're on the right track. Normally, the piston will not come all the way up to the deck of the block, but will be "submerged" a bit, usually a fraction of an inch. You need this volume, as well as the volume that the head gasket adds in addition to the "cylinder" volume that you can easily imagine, to find the compression ratio. A dished/valve relief and domed pison will therefore lessen or raise the CR depending on which your pisons have, if you're not using flat tops. Depending on how accurate you want to get, you might also want to figure in the top piston ring land height into the equation.

 

SadbutTrue

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Jun 15, 2004

#4

• Jun 15, 2004

• #4

hrmm...i wouldn't imagine it would be easy to find out what pistons i have in my car... so... whats a stock 69 351w's compression?

 

kslushy

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#5

• Jun 15, 2004

• #5

did you try google?

otherwise, this site has a lot of great info. direct link to ford compression ratios:

http://mustangii.com/headswapcompression.asp

 

SadbutTrue

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Jun 15, 2004

#6

• Jun 15, 2004

• #6

nice link, you wouldn't happen to have another site up your sleeve that says how to tell a 69-72 head from a 73+ one would ya? I assume theres a way with codes on the head, but I have no clue what most of those mean. The engine I bought was from a engine rebuild shop taht supposedly builds engines back to stock specs... but the engine I bought wasa "69-74" type.

 

kslushy

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#7

• Jun 15, 2004

• #7

It should be the standard "C=1960's, D=1970's, and the following number= the year in the decade" type decoding.


Does that answer your question? If the engine is still in the car, then pulling the valve cover sometimes shows the year of the engine, ie 71 or something between two of the middleish pushrod holes. Otherwise, the stamping is usually on the underside of the head.

What kind of rockers does it have?

 

LMan

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Aug 10, 2002

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#8

• Jun 15, 2004

• #8

look here for additional info:

http://www.network54.com/Forum/119417

 

Max Power

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Jul 31, 2003

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St Paul

Jun 15, 2004

#9

• Jun 15, 2004

• #9

If I am not mistaken, the 69 351w was 9.5:1 for the 2v models and 11:1 for the 4v models.