(800) 853-2651
SHOP NOW

Compression Ratio

M

Moondoggie

Member
What relation is there between 'compression ratio' and compression psi?' My Cub has a 6.5 to 1 compression ratio, according to the manual. With a 2.875" bore and 3" stroke, what does that ratio tell me about the compression PSI, if anything? If the piston is at the bottom of its stroke, there should be no compression. As the piston rises to the top of the stroke, does the volume of that cylinder decrease by a factor of 6.5? Just wondering if that 'ratio' is of any practical use.
 
There is nominally 14.7 PSI in the cylinder when it is at atmosphere, so 6.5 x 14.7 = 95 PSI, would have to look up the exact math.
 
Yes, the volume is reduced by 6.5. But lots of other factors, as David said, atmosphere pressure, the restriction of the intake system, lots of variables.

There's also the cam overlap to consider. The intake valve isn't fully closed before so many degrees past BDC. And rings are not a perfect seal, so some leakage there, which brings in a cranking speed variable.

Best bet is doing a comparison between cylinders, or a leak down test if you suspect a problem.
 
(quoted from post at 07:28:59 08/12/15) What relation is there between 'compression ratio' and compression psi?' My Cub has a 6.5 to 1 compression ratio, according to the manual. With a 2.875" bore and 3" stroke, what does that ratio tell me about the compression PSI, if anything? If the piston is at the bottom of its stroke, there should be no compression. As the piston rises to the top of the stroke, does the volume of that cylinder decrease by a factor of 6.5? Just wondering if that 'ratio' is of any practical use.
Click to expand...

Compression ratio (CR) is defined as; the volume at bottom dead center (BDC) divided by the volume at top dead center (TDC). CR is based on volumes and does not translate directly to pressure, since compression does not begin until the valves are closed. For this reason, refer to the manual for the engine cranking compression pressure in PSI.

Note: At BDC the total volume is the cylinder volume ((3.14 x bore x bore / 4) x stroke), or swept volume (Vs) + the combustion chamber volume or clearance volume (Vc).

Therefore we have: CR = (Vs + Vc) / Vc.

Clear as mud...eh
 
Compression ratio means the volume of the cylinder stroke is (in this case) 6.5 times the volume of the combustion chamber when the piston is at top dead center.
 
The math may not work out if your intake closes past BDC. The compression gauges have numbers for both psi and ratio on them.
 

Cam timing figures into it. Diesel engines and industrial gassers have sort duration came with very little overlap.
Make for excellent low end torque for both. Short duration is Essential to make a diesel start. However it's why diesels make rpm power but fall flat when revved.
The Fiat VM Motori diesel in the Dodge , Cruz and Canyon use variable timing twin overhead cams . To allow starting and high rpm , high HP per cubic inch operation for a diesel .
Compression pressure will also vary with cranking rpm. All other parameters identical. An engine cranked at 800 rpm will show more pressure than an engine cranked at 100rpm.
 
Thanks to all of you for the answers and info. Now I know why I avoided physics class in high school. By the way, the compression PSI results were: 78, 0, (yes, 0), 82 and 75, cylinders 1 through 4, respectively. No. 2 had an exhaust valve stuck open; when it was freed, compression in that cylinder was 72. Engine runs fine, very little grey smoke.
 
When you mention "Cub", I assume you mean Farmall Cub. From Tractor Data, the Cub engine is 2.625 bore and 2.75 stroke (and 6.5 to 1 compression ratio)which calculates out to 59.53 cubic inches. Commonly called the C60 engine ( for 60 cubic inches). The bore and stroke that you give calculates out to 77.9 cubic inches. Have you bored and stroked the engine? It is possible to calculate the compression pressure from the compression ratio or read it from a table.

<image src="http://forums.yesterdaystractors.com/photos/mvphoto25963.jpg"/>
 
You must log in or register to reply here.

Similar threads

F
1968 Ford 4000 Compression Pressures
Replies
9
Views
389
FishDuck
F
3
My Husqvarna 55 rancher chain saw quit
Replies
19
Views
504
DaveBN
D
B
JD 720 Gas Compression test
Replies
12
Views
464
BMAN in Detroit
B
J
1750 Gasoline, Compression Check
Replies
4
Views
377
Jon Wescott
J
waugh621
Cub compression
Replies
8
Views
444
Janicholson
Janicholson
Tractor Parts Tractor Manuals

We sell tractor parts! We have the parts you need to repair your tractor - the right parts. Our low prices and years of research make us your best choice when you need parts. Shop Online Today.

Copyright © 1997-2024 Yesterday's Tractor Co.

All Rights Reserved. Reproduction of any part of this website, including design and content, without written permission is strictly prohibited. Trade Marks and Trade Names contained and used in this Website are those of others, and are used in this Website in a descriptive sense to refer to the products of others. Use of this Web site constitutes acceptance of our User Agreement and Privacy Policy TRADEMARK DISCLAIMER: Tradenames and Trademarks referred to within Yesterday's Tractor Co. products and within the Yesterday's Tractor Co. websites are the property of their respective trademark holders. None of these trademark holders are affiliated with Yesterday's Tractor Co., our products, or our website nor are we sponsored by them. John Deere and its logos are the registered trademarks of the John Deere Corporation. Agco, Agco Allis, White, Massey Ferguson and their logos are the registered trademarks of AGCO Corporation. Case, Case-IH, Farmall, International Harvester, New Holland and their logos are registered trademarks of CNH Global N.V.

Yesterday's Tractors - Antique Tractor Headquarters
Website Accessibility Policy

Back
Top