Question : Compression PSI

[Maverick H1L]

Can someone tell me about what compression pressure, in PSI, I should expect to see on my gauge at roughly 11.4:1 CR? I'm trying to set my valves using max pressure.

I know my stock 3.1, except for cylinder #6, was around 160-165 per cylinder...

[geoffinbc]

Depends on the cam more than static compression ratio. But with all other things the same a higher ratio will obviously make more cranking pressure. So if you have not changed anything other than compression by say using a thinner gaskets or a smaller piston dish then you should see nearly 200PSI I would think. Also consider warm VS cold engine and old VS new rings

[Maverick H1L]

Only thing I did to the 3.1 short block was put new (crank, not cam) bearings, seals, and oil pump pickup in it...

I refuse to be breaking in a new cam while trying to tune my MSII...

Head gaskets are stock 3.1 iron head gaskets.
Stock 3.1 RWD iron head pistons (12cc).
CR jumping from about 8.5: to 11.4:1.

I'm thinking over 200 PSI, but I'm no expert...

[trotterlg]

If you have iron heads on it then I doubt you have 11 to one. Iron head pistons in an anluminum head engine gets you about 12 to one. What do you have? Larry

[trotterlg]

On anothrer note, I have never heard of a procedure where you set your valves based on cylinder pressure. Hope you don't break something. Larry

[geoffinbc]

Agreed. You would have to have solid lifters in order to do that anyway. Can you give us a better idea of what you have built?

[Maverick H1L]

3.1 RWD/3100 hybrid. Only change to the upper end other than the heads and intake is the 1.6:1 rocker arms.
I've set valves this way on my 2.8. Get it close using standard X turns after zero play and then get max compression pressure.
Doesn't matter right now anyways since my compression gauge is AWOL at the moment.

:edit: On a side note, we run 14mm plugs (thread size) right?

[trotterlg]

On my 3100 the rockers are set by turning the rocker bolts all the way down. That is with the stock roller cam, if you have a flat tappet cam then you will need differetn length pushrods, the exhaust from a stock 3100 can be used on the intake valves but you will need different ones for your exhaust. In any case you should have bled the lifters down before you put them in the engine. I am still not sure what you have. Sounds like iron head block and pistons with aluminum heads? If this is the case you are probably closer to 12 to one CR. Larry

[Maverick H1L]

I'm not using standard 3100 rockers... Mine are adjustable (roller tip rockers for an SBC). As in early 3100 heads (pre-96 or whatever). Or, if those used a torque spec for the nuts (like 2.2s do), that would help a lot. I would have used the standard 3x00 post-96 (or whenever) rocker assemblies but one went AWOL somewhere along the line between moving parts here there and everywhere (not even sure I got all of them from the place I bought the top end from... I know I was missing a couple dozen bolts and I don't think the rockers were bolted to the heads when I picked the parts up).

Lifters and cam were never removed from the engine block. Only the crank. But the rear cam cover was resealed.

I've checked a couple different CR calculators and the average is 11.44:1.

[trotterlg]

OK, that is what is on mine. Proper way to set them is, with the lifters bled down, cylinder at TDC compression stroke, take out the slack, then turn them down about 1 1/2 or two turns. That is is, nothing else to do. I have never heard of doing it with the compression reading, and it will do nothing but get you a bad setting on the lifter. You need to get the lifter piston in the middle of it's travel, so cylinder compression has nothing to do with the rocker setting. Which pushrods did you use? Larry

[Maverick H1L]

See, I'm not really trusting that X turns past no play method. I've still got play (not vertically in the pushrod but throughout the rocker arm) and I'm already down almost 2 turns on the lifter.
Which is why I was asking about an average compression PSI for 11.4:1 compression... To see roughly what I should be seeing and shooting for.
Again, not that it matters now since my gauge has gone AWOL (even though I had it right next to the fuel pressure gauge which I can find in seconds...).
I was hoping to see if I could get the valves set before my replacement damper comes in (TCE machined it wrong for the DIS trigger wheel notches, so I needed to get a replacement to notch myself), so I don't have to guess and pray that I'm not floating valves.
Pushrods were ordered through WOT-TECH.

[trotterlg]

Until the lifter is pumped up hard with oil you will always have movement in the rocker arm and pushrod, it is only being held tight by a tiny little spring under the piston in the lifter. After that lifter is pumped up all that play will go away. You really need the piston in the lifter to be in the center of it's travel, this is how it is designed to work, too close to the bottom or at the bottom and it will hold the valve open and probably burn it, if it is really far down you may hit the valve with the piston, too loose and you will hear it clicking because the piston (in the lifter) cannot take up the slack in that lifter/pushrod/rocker/valve system. Larry

[geoffinbc]

Agreed. If you are using SBC conversion studs then 3/4 turn past zero lash is the ideal setting. Stock metric studs have a finer thread pitch an work well at 1turn past zero lash. COMP cams has a good walkthrough on how to best position your cam for each lifter you are adjusting. Remember that a few degrees less or more is not an issue. That's the beauty of hydraulic lifters.

[Maverick H1L]

Well, I followed the factory method... 1.5 turns after 0 lash.
I got a replacement damper and machined it right for an external ignition trigger (TCE didn't machine the original right) and it's firing, but it's not working right.
Seems like it's trying but I'm either losing compression or I don't have enough fuel. It's hitting but mostly missing. I'm going to go get a compression gauge either today or tomorrow (since I can't find mine and it's driving me nuts!).
I'm NOT using SBC conversion studs. The rockers have been mounted using the pivot balls from my 3.1 on my factory 3.1 valve studs.
trotterlg, I noticed you have a for sale ad... Do you by chance have pushrod guides? (yes, I currently have guides installed EXCEPT for on the exhaust valves on 1, 2, 5, and 6, which currently have the non-adjustable rockers on them due to the fact that the guides I have were cut apart, as they were from my 3.1 iron heads, and those guides don't fit in those positions on the heads due to the fact that the intake is in the way. I'm thinking those rockers might be floating the valves)

[Maverick H1L]

Okay, I've been fighting with it for 3 hours...

Cylinders 1 and 3: 210 PSI
Cylinder 5: 160 PSI (couple things to check there other than the valve lash...)
Entire other side of the engine is 160 PSI (cylinder 4) and lower... I'm hoping the head isn't warped. The gaskets are both brand new FelPro. All valve seals brand new. Brand new head bolts. The heads were cleaned and checked for cracks before being reassembled... No cracks in the chambers on the cyl 2,4,6 head (driver's side). I'm going to remove all of the rockers and make sure I got the head bolts tight first.