[grivera]

I’m looking at a car with a fresh 400/461 stroker, 1968 #16 heads, HR cam with hydraulic lifters. I’m still waiting on the cam specs - duration supposed to be in the 236/242 range. The owner doesn’t know all the details on the build- but said the pistons were ordered with a dish to arrive at 9.6 compression.
I used an inspection camera via a spark plug hole and from what I can see they do appear to be dished.

Cranking compression numbers are 150-155# on all cylinders except #6 which is 175#. I ran the test twice on two different days and arrived at same results. Tests were done with engine at full temperature, all spark plugs removed and the throttle fully open.

Two questions -

Does the cranking compression of 150-155 indicate what dynamic compression is? Or if static compression is actually around 9.6?

What would cause the one cylinder to pump at 175? I thought maybe the lifter(s) on #6 cylinder are bleeding down after turning engine off? I pulled the valve cover and while cranking the rockers move normally.

[70gtojosh]

I think it may be hard to determine what the actual compression ratio is based off a cranking compression test. I can tell you my 400 has 175 psi cranking and it is 10 to 1 compression ratio. I would do a leak down test to see if that would give any clues as to why you have 1 cylinder higher than the rest.

[tom s]

Looks like 1 cyc about right and 7 low.Tom

[srmmmm]

Sounds like the engine is not broken in yet and the rings have not fully seated. Try squirting a bit of oil in one of the lower reading cylinders and see if the compression changes. I remember helping my dad do compression checks when he owned my 70 Esprit with the 8.8 CR 350 2 barrel. He wrote each cylinder's number on the underside of the hood in white chalk. (One of those great Dad/Son bonding memories) All the cylinders came in at 145-155. The lesson of the day included remembering to keep you foot on the gas pedal to the floor throughout the whole process so you wouldn't flood the engine and that at cranking rpm, the volume and momentum of air in the intake path can force enough air in the cylinder to give you cranking pressures that create the impression of a higher compression ratio than what you actually have.

Thanks Dad!

[grivera]

Originally Posted by tom s

Looks like 1 cyc about right and 7 low.Tom

are you saying based on info provided about the build, the cranking compression should be around 175#?

[Stan Weiss]

Originally Posted by grivera

I’m looking at a car with a fresh 400/461 stroker, 1968 #16 heads, HR cam with hydraulic lifters. I’m still waiting on the cam specs - duration supposed to be in the 236/242 range. The owner doesn’t know all the details on the build- but said the pistons were ordered with a dish to arrive at 9.6 compression.
I used an inspection camera via a spark plug hole and from what I can see they do appear to be dished.

Cranking compression numbers are 150-155# on all cylinders except #6 which is 175#. I ran the test twice on two different days and arrived at same results. Tests were done with engine at full temperature, all spark plugs removed and the throttle fully open.

Two questions -

Does the cranking compression of 150-155 indicate what dynamic compression is? Or if static compression is actually around 9.6?

What would cause the one cylinder to pump at 175? I thought maybe the lifter(s) on #6 cylinder are bleeding down after turning engine off? I pulled the valve cover and while cranking the rockers move normally.

Yes, it is possible to calculate SCR from cranking compression psi and IVC. The problem is coming up with IVC. IVC is just that the Intake Valve closing point.

Stan

[grivera]

Originally Posted by srmmmm

Sounds like the engine is not broken in yet and the rings have not fully seated. Try squirting a bit of oil in one of the lower reading cylinders and see if the compression changes. I remember helping my dad do compression checks when he owned my 70 Esprit with the 8.8 CR 350 2 barrel. He wrote each cylinder's number on the underside of the hood in white chalk. (One of those great Dad/Son bonding memories) All the cylinders came in at 145-155. The lesson of the day included remembering to keep you foot on the gas pedal to the floor throughout the whole process so you wouldn't flood the engine and that at cranking rpm, the volume and momentum of air in the intake path can force enough air in the cylinder to give you cranking pressures that create the impression of a higher compression ratio than what you actually have.

Thanks Dad!

Thanks for the detailed post - and your dad sounds like a smart fella.

So the owner is a friend and he had another friend of his do the build - he says the engine has several hundred miles on it, which I characterized as “fresh” but based on your post. The builder would have done all break in and seating of rings.

[Schurkey]

I do not understand the mathematics of calculating compression ratio from cranking compression pressures. A guy I trust says it can be done. To me, there's too many variables to have reasonable accuracy. But I've been wrong before.

Cranking speed, valve and ring leakage, and intake air heating (and thus expanding) in the manifold--and then again in the cylinder from compression itself--would all affect the pressure results.

Similarly, squirting oil into the cylinder to "seal the rings" makes little sense to me. MAYBE, on an upright in-line engine, that doesn't have a dished piston--where the oil stands a chance of flowing all the way around the rings/cylinder. But when the cylinders are leaned-over, all the oil will pool at the low side, or in the dish. The compression pressure goes up because the oil isn't compressible, you've effectively reduced the chamber CC--but you're not "sealing" the rings because the oil isn't going all the way around the piston/cylinder to "wet" the top ring. I think folks get false ring-seal results from "wet" compression tests. The compression pressure goes up, but not because the leaking rings are now sealed.


I quit blocking the throttle "wide open" decades ago. Throwing the throttle onto the high step of the fast-idle cam has achieved the same results for me, with less effort.

Don't forget to test your pressure gauge(s) for accuracy once in awhile. I plug mine into the shop air compressor, and make sure that the pressure gauge on the regulator is in agreement with the compression tester gauge.

One cylinder shows "extra" compression? I'd be looking at carbon build-up in that cylinder. Perhaps fuel distribution is uneven, and that cylinder runs rich. Or there's an oil-consumption problem that has left oil residue/ash in the cylinder. There's other reasons for higher-than-expected cranking compression pressure, but excess deposits in the chamber is the first thing I'd be thinking of.

[tom s]

I have built a lot of 9.5 CR iron headed 450ish CI with pretty close to that duration and are usually around 180 pumping compression.Tom

[Stan Weiss]

Originally Posted by Schurkey

I do not understand the mathematics of calculating compression ratio from cranking compression pressures. A guy I trust says it can be done. To me, there's too many variables to have reasonable accuracy. But I've been wrong before.

Cranking speed, valve and ring leakage, and intake air heating (and thus expanding) in the manifold--and then again in the cylinder from compression itself--would all affect the pressure results.

Similarly, squirting oil into the cylinder to "seal the rings" makes little sense to me. MAYBE, on an upright in-line engine, that doesn't have a dished piston--where the oil stands a chance of flowing all the way around the rings/cylinder. But when the cylinders are leaned-over, all the oil will pool at the low side, or in the dish. The compression pressure goes up because the oil isn't compressible, you've effectively reduced the chamber CC--but you're not "sealing" the rings because the oil isn't going all the way around the piston/cylinder to "wet" the top ring. I think folks get false ring-seal results from "wet" compression tests. The compression pressure goes up, but not because the leaking rings are now sealed.


I quit blocking the throttle "wide open" decades ago. Throwing the throttle onto the high step of the fast-idle cam has achieved the same results for me, with less effort.

Don't forget to test your pressure gauge(s) for accuracy once in awhile. I plug mine into the shop air compressor, and make sure that the pressure gauge on the regulator is in agreement with the compression tester gauge.

One cylinder shows "extra" compression? I'd be looking at carbon build-up in that cylinder. Perhaps fuel distribution is uneven, and that cylinder runs rich. Or there's an oil-consumption problem that has left oil residue/ash in the cylinder. There's other reasons for higher-than-expected cranking compression pressure, but excess deposits in the chamber is the first thing I'd be thinking of.

That is why weather (temp and BP) should be part of the calculation input, and the results should be different for the same engine at sea level then it is for Denver.

Stan

[grivera]

Thank you

I plan to do a leak-down test tomorrow and report the results.

[Stan Weiss]

Bore = 4.155" - Stroke = 4.25" - Rod = 6.625"
Temp = 60 degrees - BP = 29.92 - % Hum = 0

Graph #1 uses 152.5 PSI

Graph #2 uses 175 PSI

Stan

[grivera]

Stan - how do I determine the x-axis?

[Formulas]

you COULD have a intake lobe going away on the high cylinder

a reduced intake lobe from wear will close the valve earlier but lift the valve enough to fill the cylinder at cranking speed

obviously if the entire lobe is MIA cranking compression will be way low

a way to verify lobe integrity is just remove a few rocker arms and compare lift at the pushrod with a dial indicator to take lifter bleed down out of the equasion by eliminating valve spring force

[grivera]

Formulas- It’s aRoller cam - I realize it could still happen though but miles are quite low. The car runs strong and does have a minor lifter tick on the even cylinder bank. I realize ticking HR lifters are common these days.

Also, a bit more info from the owner. He bought the car with the 461. The prior owner thrashed on the car some evident by the rest of the build. Based on this, he had the engine disassembled which revealed broken piston rings on at least one cylinder and what his engine builder described as signs of detonation.

The builder looked everything over and reassembled using new pistons, rings, bearings, HR cam, HR lifters There is no mention of machine work being done but the piston dish was reportedly spec’d for 9.6 considering the #16 heads. Butler spec’d the cam, which I believe is 236/242 duration on a 112 LSA but waiting for confirmation and lift numbers.

My reason for checking the cranking compression is due to the heads used and not being able to speak to the engine builder.

I assume a dingleberry hone was used to break any glaze on the cylinder walls but can’t confirm. The builder is reportedly a seasoned home mechanic with lots of Pontiac experience. The quality of the work done on the car otherwise supports this but obviously the most important piece of the puzzle is the details on build and what prep work done.

The builder also installed a Tremek TKO600 trans, redid interior added Vintage Air and detailed engine compartment among other things. He also dialed engine in (in chassis) and the car was delivered to the current owner ready to cruise.


Hope this helps with a virtual analysis.

[Stan Weiss]

Originally Posted by grivera

Stan - how do I determine the x-axis?

You need a positive piston stop to find TDC. Set you degree wheel to zero and then use a dial indicator on your intake valve to find IVC.

Stan

[grivera]

Ok - just got the cam specs:

Butler Performance COMP Custom Cam- BP8023SP
(HR) 282/288, 230/236, .510/.521, 114LS with 1.5 ratio rockers

The engine has 1.5 Harland Sharp rockers

Here are the leak-down test results done with engine at full temperature, all plugs out and each cylinder tested at TDC. Left hand gauge at 90 PSI

1 = 86 psi
8= 84 psi
4= 85 psi
3= 85 psi
6= 84 psi (this is the cylinder with high compression)
5= 85 psi
7= 84 psi
2 = 84 psi

[Formulas]

sorry didnt catch that its a roller

[Ram Air IV Jack]

Originally Posted by grivera

Ok - just got the cam specs:

Butler Performance COMP Custom Cam- BP8023SP
(HR) 282/288, 230/236, .510/.521, 114LS with 1.5 ratio rockers

The engine has 1.5 Harland Sharp rockers

Here are the leak-down test results done with engine at full temperature, all plugs out and each cylinder tested at TDC. Left hand gauge at 90 PSI

1 = 86 psi
8= 84 psi
4= 85 psi
3= 85 psi
6= 84 psi (this is the cylinder with high compression)
5= 85 psi
7= 84 psi
2 = 84 psi

Long overlap cams will cause lower compression check readings. Further, the Pontiac Service manual I have reads that the compression readings should be uniform within 75%-80% of the highest reading which your engine shows. Many things can affect compression readings but the method you used to check compression is what the books always recommended. I went back and checked my Motor and Chilton's manuals which are written from factory service manuals, and they all say pretty much the same thing. Remember, these manuals were written by factory engineers and are sources to trust. You will get many, many opinions here that will probably confuse you. I think using trusted sources is the best way to answer questions you have so do your own research!!!