Compression Check (Compression Ratio?)

[73Swinger18]

I just finished putting my 318 together. It was bored .020 over with flat top pistons and larger valves in the stock heads. I did a compression check on it today and each cylinder was 135 PSI. Is this close to stock? I am wondering if there is any way to estimate the compression ratio with the numbers I got. Thanks!

 

[Down Shift]

I just finished putting my 318 together. It was bored .020 over with flat top pistons and larger valves in the stock heads. I did a compression check on it today and each cylinder was 135 PSI. Is this close to stock? I am wondering if there is any way to estimate the compression ratio with the numbers I got. Thanks!

135/14.7 = ~9.18. 9.18:1 is your DYNAMIC compression ratio. Dynamic compression is static compression minus lost pressure due to the intake valve closing after BDC (into the compression stroke). Your static compression ratio really depends on how radical your camshaft is. Probably somewhere between 10 and 11 to 1?

 

[iw378]

I'm curious as well. My cranking compression is 158 to 160.

 

[73Swinger18]

135/14.7 = ~9.18. 9.18:1 is your DYNAMIC compression ratio. Dynamic compression is static compression minus lost pressure due to the intake valve closing after BDC (into the compression stroke). Your static compression ratio really depends on how radical your camshaft is. Probably somewhere between 10 and 11 to 1?

Thanks for the reply. I guess it's better than I expected. I was worried it was gonna be a turd at low speeds with the Comp Cam (20-223-3) and 2.76 gears, but she moves along real well! Have my initial timing at 18 BTDC, but not sure if I'm going to keep it there. Still need to re-curve my distributor.

 

[MopaR&D]

Thanks for the reply. I guess it's better than I expected. I was worried it was gonna be a turd at low speeds with the Comp Cam (20-223-3) and 2.76 gears, but she moves along real well! Have my initial timing at 18 BTDC, but not sure if I'm going to keep it there. Still need to re-curve my distributor.

That's good to hear I think I was one saying it would be a turd lol... Did you end up using a stock converter?

 

[RustyRatRod]

No way in hell to determine static or dynamic compression from cylinder pressure. No way, no how. Not yesterday, not today, not ever. The only way to determine true static compression ratio is by measuring.

Once static is determined, you can plug in your intake valve closing event into one of the many formulas and figure dynamic from there.

135 PSI is fairly low and in the stock range. If I had to hazard a guess, I'd say somewhere around 8-8.5:1 static, but again, there's no way to tell but measure.

 

[73Swinger18]

That's good to hear I think I was one saying it would be a turd lol... Did you end up using a stock converter?

Haha, "turd". Yes, I used the stock converter. I was prepared for the worst, but I am quite pleased.

 

[273]

If you didn't do anything to increase your CR how can it be anything other than stock and is probably what RustyRatRod said. If you used flat valves any milling of block and heads and or taller pistons would increase it.

 

[RustyRatRod]

Haha, "turd". Yes, I used the stock converter. I was prepared for the worst, but I am quite pleased.

Low compression does not necessarily mean turd at all. It is all in the combination. Too many people get caught up in thinking they need a 10:1 engine to make power. Just aint so.

 

[73Swinger18]

Low compression does not necessarily mean turd at all. It is all in the combination. Too many people get caught up in thinking they need a 10:1 engine to make power. Just aint so.

It was my cam, stock torque converter, and rear end setup that people warned me about. All in all, it worked out well for it's intended purpose.

 

[273]

If you look at DCR of different performance engine combos are a lot closer than static cause of cr and cam combo so one doesn't necessarily have more actual cr than the other. Just If you have a low compression engine it just limits how large of a cam you can go and same with the reverse if you got too much static cr then it dictates a rather large cam.

There's nothing wrong with going with a rather mild combo decent heads with 9:1 CR and 260- 270 ish cam for a mainly street car. Especially most on here seem allergic to converter and gears. Can make a lot power that way and if that's not enough your probably building too small of an engine or its something more of a strip car and or weekend play toy.

 

[clinteg]

No way in hell to determine static or dynamic compression from cylinder pressure. No way, no how. Not yesterday, not today, not ever. The only way to determine true static compression ratio is by measuring.

Once static is determined, you can plug in your intake valve closing event into one of the many formulas and figure dynamic from there.

135 PSI is fairly low and in the stock range. If I had to hazard a guess, I'd say somewhere around 8-8.5:1 static, but again, there's no way to tell but measure.

Exactly! I only get 150 psi cranking on my engine according to my gauge (motor is actually 11:1 static, 8:1 Dynamic). That compression reading isn't going to tell anyone anything about static or dynamic compression. 150 psi is about what a stock motor would crank I'd guess, but this motor is far from stock. The cam bleeds off a lot of cylinder pressure and you won't know that from a compression test. Static CR is calculated from volume of the cylinder at TDC and BDC of the piston and dynamic is calculated from static CR and camshaft intake valve closing event. Without that information, you don't know squat. Whoever is saying to take the psi and divide by 14.7 to get dynamic compression makes no sense whatsoever. It's no wonder people get so misleaded on the internet.

 

[69_340_GTS]

What's "14.7" anyways? I'd like to see the "formula" that was used to get that number!

 

[73Swinger18]

What's "14.7" anyways? I'd like to see the "formula" that was used to get that number!

14.7 is the avg atmospheric pressure at sea level.

 

[clinteg]

14.7 is the avg atmospheric pressure at sea level.

Which is relevant how?

 

[73Swinger18]

Which is relevant how?

Lower compression at higher altitudes.
Technical Articles at Greg's Engine & Machine

 

[clinteg]

Lower compression at higher altitudes.
Technical Articles at Greg's Engine & Machine

All that website is giving you is an equivalent cranking compression for different elevation, nothing more. Still irrelevant to the topic. You can't figure out CR based on compression testing. Period.

 

[73Swinger18]

All that website is giving you is an equivalent cranking compression for different elevation, nothing more. Still irrelevant to the topic. You can't figure out CR based on compression testing. Period.

Yet, relevant to your question. 14.7 is a lot better estimate than 4.3 or another number. So we all agree, it's not precise. Maybe not even close. But, it does make some sense. I wonder who out there knows their actual CR and would like to see what they get when they divide their cranking pressure by 14.7 or whatever variable for your altitude. Anyone??

 

[273]

The VE of a running engine is gonna have a huge effect on the real cr.

 

[clinteg]

Yet, relevant to your question. 14.7 is a lot better estimate than 4.3 or another number. So we all agree, it's not precise. Maybe not even close. But, it does make some sense. I wonder who out there knows their actual CR and would like to see what they get when they divide their cranking pressure by 14.7 or whatever variable for your altitude. Anyone??

OK I'll play. 150/14.7 = 10.20. Actual Static CR = 11.0:1, Dynamic 8.0:1. Someone said earlier this is supposed to be Dynamic CR but obviously that isn't even close. Before I had the motor rebuilt, it had 225 psi cranking. 225/14.7 = 15.3. Actual Static CR = 13.9:1. Dynamic unknown. Numbers obviously aren't close to anything and I wouldn't use this method. Mostly because it's just plain wrong. You have to understand that a compression test says nothing about what the differential in cylinder volume at TDC to BDC is. Without that, you won't know how to get dynamic CR either.

Let's take an example. Say you take a motor cranking 175 psi. It's 10:1 Static and 8:1 Dynamic (known and calculated values). Now let's change the camshaft with a later closing event on the intake valve, all other aspects of the camshaft the same. The engine now cranks 150 psi cylinder pressure because it's bleeding off some pressure before the valve closes. Is the Static CR changed now? No. The cylinder volume hasn't changed. Has the Dynamic CR changed? Absolutely. Now the engine is 10:1 Static and the Dynamic could now be, say, around 7.5:1 and more pump gas friendly. People tend to want to gravitate to the Static number to know whether something runs on pump gas or not. Static does not matter in the grand scheme of things. It's all about the Dynamic CR.

My point is, one cannot get Dynamic CR from a compression test. Dynamic is calculated from Static CR and camshaft intake valve closing event. A compression test won't give you the Static CR either to calculate Dynamic. So in all, you get no accurate information for CR from compression testing. There are several things compression tests are good for, but getting accurate CR's is NOT one of them. If you want to get a ball park figure by using the "14.7" method, be my guest. It may be ball park, but ball park in another county!

 

[yellow rose]

Low compression does not necessarily mean turd at all. It is all in the combination. Too many people get caught up in thinking they need a 10:1 engine to make power. Just aint so.

Exactly right.

But it's nice to run compression if your willing to tune for it.

A good, well thought out 9:1 engine will make power. Lots of circle jerk engines are limited on CR.


I did build my latest engine at 11.08:1 MEASURED and if it rattles I'll bump it to 11.25:1 (already measured that with a thinner gasket).

Got one for a friend on the stand. It's at a MEASURED 10.6:1, just because we could.

I like to wipe the table clean.

 

[yellow rose]

OK I'll play. 150/14.7 = 10.20. Actual Static CR = 11.0:1, Dynamic 8.0:1. Someone said earlier this is supposed to be Dynamic CR but obviously that isn't even close. Before I had the motor rebuilt, it had 225 psi cranking. 225/14.7 = 15.3. Actual Static CR = 13.9:1. Dynamic unknown. Numbers obviously aren't close to anything and I wouldn't use this method. Mostly because it's just plain wrong. You have to understand that a compression test says nothing about what the differential in cylinder volume at TDC to BDC is. Without that, you won't know how to get dynamic CR either.

Let's take an example. Say you take a motor cranking 175 psi. It's 10:1 Static and 8:1 Dynamic (known and calculated values). Now let's change the camshaft with a later closing event on the intake valve, all other aspects of the camshaft the same. The engine now cranks 150 psi cylinder pressure because it's bleeding off some pressure before the valve closes. Is the Static CR changed now? No. The cylinder volume hasn't changed. Has the Dynamic CR changed? Absolutely. Now the engine is 10:1 Static and the Dynamic could now be, say, around 7.5:1 and more pump gas friendly. People tend to want to gravitate to the Static number to know whether something runs on pump gas or not. Static does not matter in the grand scheme of things. It's all about the Dynamic CR.

My point is, one cannot get Dynamic CR from a compression test. Dynamic is calculated from Static CR and camshaft intake valve closing event. A compression test won't give you the Static CR either to calculate Dynamic. So in all, you get no accurate information for CR from compression testing. There are several things compression tests are good for, but getting accurate CR's is NOT one of them. If you want to get a ball park figure by using the "14.7" method, be my guest. It may be ball park, but ball park in another county!

Pretty well summed it up.
You can run much more static than people think if you work out the combination.

But, just because you CAN, don't mean you SHOULD.

 

[T-N-T]

No way in hell to determine static or dynamic compression from cylinder pressure. No way, no how. Not yesterday, not today, not ever. The only way to determine true static compression ratio is by measuring.

Once static is determined, you can plug in your intake valve closing event into one of the many formulas and figure dynamic from there.

135 PSI is fairly low and in the stock range. If I had to hazard a guess, I'd say somewhere around 8-8.5:1 static, but again, there's no way to tell but measure.

Here is what I found using the SWAG method(scientific wild assed guess).

I would say your guess of about 8.0-8.5:1 is correct. Using this site Wallace Racing: Dynamic Compression Ratio Calculator by guessing his static at 8.3:1, entering his bore, stroke, rod length and using comp cams intake valve closing point for his cam is shows he should see 135psi at sea level. It shows his dynamic compression at 7.0:1. These numbers should be close but still just a guess.

 

[73Swinger18]

Here is what I found using the SWAG method(scientific wild assed guess).

I would say your guess of about 8.0-8.5:1 is correct. Using this site Wallace Racing: Dynamic Compression Ratio Calculator by guessing his static at 8.3:1, entering his bore, stroke, rod length and using comp cams intake valve closing point for his cam is shows he should see 135psi at sea level. It shows his dynamic compression at 7.0:1. These numbers should be close but still just a guess.

That's a fun calculator. Thanks.

 

[B3RE]

The VE of a running engine is gonna have a huge effect on the real cr.

Finally! Someone is close to the truth about DCR. It is not constant!