Finding Engine Compression Ratio Using a Manometer on SBC ?

[Tynan918]

I need to find my compression ratio and I don't want to disassemble my engine to figure that out.

I was reading an article and I see that there's another method to find your engines compression ratio without disassembling the engine.. it's used with a tool called a "Manometer"...

But can you use a "Manometer" on a SBC ?

Also, when I Googled a "Manometer", it doesn't show me specifically a "Manometer", it shows me an Oil Pressure Gauge from Harbor Freight, is this a "Manometer" ?

 

[Tynan918]

From what I understand a "Manometer" is a pressure gauge, and it measures pressure. Rather than cranking the engine to check the compression, you run the engine for about 5 seconds and record the pressure of that cylinder. You do all cylinders like that and record the pressure for all cylinders, and then run a compression check and there is a formula that you calculate so that you have your compression ratio.

I have these gauges, could I use one of these gauges as a pressure gauge to act as a "manometer" ?

 

[Supercharged111]

A manometer measures pressure differential specifically. I used one on a KC-135 to perform a cabin pressure test. It used 2 water columns.

 

[John Canon]

A manometer measures pressure differential specifically. I used one on a KC-135 to perform a cabin pressure test. It used 2 water columns.

That is true ... a manometer is a sensitive air pressure differential gauge with 2 hose connections. I used one with a pitot tube and 2 vinyl tubing hoses. The pitot tube feeds the static and the ram air pressures into the manometer. The device indicates the miniscule pressure difference to compute air speed.

Every story on the Nets is a copy/paste from one bogus description about using a "manometer" as a compression tester and applying some sketchy math. Unbelievable? Yes. There is even a compression tester on Amzon calling itself a manometer!

 

[carnutjw]

I need to find my compression ratio and I don't want to disassemble my engine to figure that out.

No, you don't.

 

[78Delta88]

Scratching my head on this one...

Like said, manometer measures pressure differential. For automotive work it is typical in the flow bench, and mainly used to measure flow through the head.

Car Craft magazine had the designs in one issue early 80's I built it, never did really work right for me but I was 17.

Below is DYI, built by another...

What your looking for is compression reading and even getting compression reading will not get you the Static CR. It might give you an idea but not the actual number.

Boost Compression Ratio Calculator

Below is just example of a 350 30 over with a 286 advertised duration intake. But note it has chamber CC as 85.58. So this is just example.

 

[RabbitHoleSS]

This engine your dealing with is a 305 right? Does it have flat top pistons? What does the front of the head look like? Casting number ends in 416? If yes, 9.5:1.....
If it has dished pistons, 8.5:1. Seriously, don't overthink it.

 

[Tynan918]

This engine your dealing with is a 305 right? Does it have flat top pistons? What does the front of the head look like? Casting number ends in 416? If yes, 9.5:1.....
If it has dished pistons, 8.5:1. Seriously, don't overthink it.

Not sure if it's flattop or if it's dish pistons...

Is 9.5:1 high compression ratio ?

 

[RabbitHoleSS]

Not sure if it's flattop or if it's dish pistons...

Is 9.5:1 high compression ratio ?

No. It's 87 pump piss friendly.Instead of that manometer,
Buy a cheap bore scope. They're handy as hell anyway , just put it in the spark plug hole. And look for valve reliefs.

 

[Tynan918]

No. It's 87 pump piss friendly.Instead of that manometer,
Buy a cheap bore scope. They're handy as hell anyway , just put it in the spark plug hole. And look for valve reliefs.

What's valve relief ?