[David Jones]

I did a cold compression test on my 455 some months back and was satisfied that all the numbers were within a few lbs of each other, right at 140. I was chasing a smoke issue. There was a bit of curiosity involved too since the history of the shortblock wasn't completely clear.
All is well, the engine runs great, uses no oil and does not smoke....replacing the stem seals and getting the PCV system in correct functional order did the trick.

Now I want to do a hot compression test....just because.

Warm it up completely, pull all the plugs, disable the fuel and ignition systems, lock the carb wide open and proceed.

My question is how does the cam affect the numbers? Not just my cam, but in general, how does a cam change the numbers on a compression test?

I have a Comp hyd roller (230/236) on a 110. It has approx .510-.520 lift with 1.50 rockers and I run Crower solid roller lifters set at .006 cold. This is a completely factory 73 model short block. Just as it left mother Pontiac. 6X-8 heads, 97cc.

[72LuxuryLeMansLa.]

Originally Posted by David Jones

I did a cold compression test on my 455 some months back and was satisfied that all the numbers were within a few lbs of each other, right at 140. I was chasing a smoke issue. There was a bit of curiosity involved too since the history of the shortblock wasn't completely clear.
All is well, the engine runs great, uses no oil and does not smoke....replacing the stem seals and getting the PCV system in correct functional order did the trick.

Now I want to do a hot compression test....just because.

Warm it up completely, pull all the plugs, disable the fuel and ignition systems, lock the carb wide open and proceed.

My question is how does the cam affect the numbers? Not just my cam, but in general, how does a cam change the numbers on a compression test?

I have a Comp hyd roller (230/236) on a 110. It has approx .510-.520 lift with 1.50 rockers and I run Crower solid roller lifters set at .006 cold. This is a completely factory 73 model short block. Just as it left mother Pontiac. 6X-8 heads, 97cc.

Fairly simple deal; the engine CANNOT begin to make any compression until the intake valve closes. Once all valves are closed and the piston rises toward TDC it builds psi. Late intake closing equals less psi........Rhoads type lifters change the intake closing until they pump up around 3000rpm or so.

[David Jones]

Thank you Karl, but I'm still a bit lost on this.

Think pre-school with your explanation....with examples.

[72LuxuryLeMansLa.]

Originally Posted by David Jones

Thank you Karl, but I'm still a bit lost on this.

Think pre-school with your explanation....with examples.

Do you have an understanding of the four cycles the engine goes through?


"Event 1 - Intake valves opening

The camshaft opens the intake valve, and the piston moves down the cylinder. As the pressure drops in the cylinder, the air starts moving past the intake valve to fill the cylinder. This period of the engine cycle is known as the intake stroke.

Event 2 - Intake valves closing

At some point, usually after the piston reaches the bottom of the intake stroke, the intake valve closes. The piston moves up the cylinder, beginning the compression stroke and compressing the fuel/air mixture within. At some point, usually before the piston reaches the top of the compression stroke, the spark plug ignites the mixture, causing it to burn and expand rapidly. The crankshaft has rotated once at this point.

Event 3 - Exhaust valves opening

After the piston reaches the top of the compression stroke, pressure from the burning, expanding mixture pushes the piston back down the cylinder. The exhaust valve starts to open, usually before the piston is all the way down, allowing some of the burnt gasses to exit the cylinder. This is commonly referred to as the blowdown phase. The piston begins to move back up, forcing the rest of the hot gas out of the cylinder.

Event 4 - Exhaust valves closing

As the piston moves back up the cylinder, the exhaust valve remains open, usually until slightly after the piston reaches the top of the cylinder. We refer to this as the exhaust stroke. As the piston reaches the top again, the intake valve begins to open again, often before the exhaust valve is fully closed, and the whole cycle begins anew. The period when both valves are open simultaneously is referred to as “overlap.” The crankshaft has now gone around twice.

In a nutshell, here’s how it all happens.

Overlap
Intake stroke
Compression stroke
Blowdown
Exhaust stroke
Repeat"

http://hiperfautotech.blogspot.com/2...le-engine.html

[PONTIAC DUDE]

LSA along with actual compression ratings can really effect a compression test.

[David Jones]

I've got that part Karl. The basics.
Transferring those events to specific cams.....maybe specific examples of a early closing and late closing, that's what I'm unsure of. What is considered early/late?

[David Jones]

Originally Posted by PONTIAC DUDE

LSA along with actual compression ratings can really effect a compression test.

How? What does what?

My engine has approx 9.25-1 and my cam is your Butcher Special.

[android 211]

Generally, short cam; higher cranking compression. Long duration cam, reduced cranking compression. High compression; higher cranking compression. Low compression, lower cranking compression. LSA - tight, higher cranking compression. Wide - lower cranking compression.

[Stan Weiss]

This graph show a computer simulation of cranking compression for the same engine with 4 different compression ratios. You can see where each crosses the 180 PSI line there is line that runs down to the bottom where it show intake valve close in degrees ABDC (needed).

Stan

[David Jones]

Ummmm, not sure how this relates. I see the trend lower with various compression ratios but those numbers seem somewhat higher than what I've seen or read about. I thought a really good number was lower than 200.

[old66tiger]

Lots of variables. While I can't tell you what yours should pull, my 10.7:1 462 pulled the following numbers after about 1000 miles with old faithful installed at 108

2 - 175/180
4 - 175/180
6 - 170/175
8 - 180/185

1 - 180/185
3 - 180/185
5 - 180/185
7 - 180/185

According to cliff, at a true static of 10.7 to 1, and the cam installed around 108-109ICL, it should be around 175-185psi

You could loosen up the #1 intake and exhaust rocker and spin it a few times and see how your compression changes. That would be a neat experiment.

[David Jones]

Originally Posted by old66tiger

Lots of variables. While I can't tell you what yours should pull, my 10.7:1 462 pulled the following numbers after about 1000 miles with old faithful installed at 108

2 - 175/180
4 - 175/180
6 - 170/175
8 - 180/185

1 - 180/185
3 - 180/185
5 - 180/185
7 - 180/185

According to cliff, at a true static of 10.7 to 1, and the cam installed around 108-109ICL, it should be around 175-185psi

You could loosen up the #1 intake and exhaust rocker and spin it a few times and see how your compression changes. That would be a neat experiment.

Where would those numbers be at 9.25-1 with the same cam? Cut on a 110 versus the 112?

[GOAT WHORE]

Originally Posted by David Jones

Where would those numbers be at 9.25-1 with the same cam? Cut on a 110 versus the 112?

David, you can go to Wallace racing and get rough ideas of what it would be using one of their calculators.

[GOAT WHORE]

Or ask Karl, he has the daddy engine analyzer

[David Jones]

GW, I looked on Wallace but didn't really see one that fit. Which one?

[GOAT WHORE]

http://www.wallaceracing.com/dynamic-cr.php
Try this one.
You need a more advanced program like Karl has to put in perameters like lsa.

[72LuxuryLeMansLa.]

Originally Posted by David Jones

GW, I looked on Wallace but didn't really see one that fit. Which one?

David,

I ran a 455 @ 9.25:1SCR with the Butcher Special on a 110ICL and 110LSA using solid rollers with a .006" lash and the Engine Analyzer thinks you should have a theoretical Cranking Compression of 175PSI.

[David Jones]

Thank you Karl.

I ran these numbers on Wallace Racings dynamic compression calculator and it said 165.44
There is not a camshaft detail provision on the calculator.
9.25-1 compression
6.625 rod
4.15 bore
4.21 stroke
Intake valve closes at 45 abdc
0 boost
600ft elevation

[72LuxuryLeMansLa.]

Originally Posted by David Jones

Thank you Karl.

I ran these numbers on Wallace Racings dynamic compression calculator and it said 165.44
There is not a camshaft detail provision on the calculator.
6.625 rod
4.15 bore
4.21 stroke
Intake valve closes at 45 abdc
0 boost
600ft elevation

Engine Analyzer is using both the Advertised and .050" numbers to make this calculation. Is your cam installed on the 110°ICL?

[David Jones]

I think part of my struggle has been terminology. Dynamic. Static. Cranking. And just plain old compression ratio. I obviously need to dive into what does what.