Cylinder Compression for stock 1991 302?

[PSI4MX5]

What is the factory specs for compression across the board?

 

[Daggar]

The reading on your lowest cylinder should be within 75% of the reading on your highest cylinder.

 

[TheUser]

For a stock 5.0, compression should be somewhere around 150psi...I believe that's what mine was on my stock 5.0. More importantly, they should all be within 10% of each other - which I think is what daggar meant to say.

 

[hllon4whls]

Agreed somewhere around 150-160. As long as the cylinders are withing 10-15 percent of each other, you are good.

Assuming that 150 is the mean value, the rest of the cylinders should be somewhere between 135-165.

Whatever you determine to be the middle ground (mean value) multiply by 1.1 to get the high value, to get the low value multiply by .90. That assumes a 10% difference.

 

[Daggar]

For a stock 5.0, compression should be somewhere around 150psi...I believe that's what mine was on my stock 5.0. More importantly, they should all be within 10% of each other - which I think is what daggar meant to say.

Yep... pretty much the same thing. The Chilton Ford Engine Overhaul Manual calls for as much as a 25% difference between the highest and lowest cylinders for 87 to 93 H.O.s. However, if all of my cylinders were within 15% and one was at 25% then I'd be concerned about that cylinder. I'll agree that the 15% sounds allot better... especially when you're trying to make power. Using the 25% (or 75% depending on how you do the math), if your highest cylinder reads 134psi then your lowest can be no less than 101 (134 x .75 then round up). The 15% rule would dictate your lowest to be 114psi (134 x .85 and round up). If my cylinders were all within 15% and I had one that read higher than 15% but not higher than 25% then I'd leave it be. Hope I haven't muddied the water too much.

 

[hllon4whls]

Hope I haven't muddied the water too much.

Clear as mudd.