Should I rebuild

[Stevo-vz_berlina]

So I think for now I’ll just change the lifters again, I’m thinking of doing the oil pump to. And what do you fellas think about cam timing? Do you think it could be just out maybe

 

[losh1971]

Since the heads are coming off it would be worth getting the valves and seats ground and a skim.

 

[Skylarking]

I‘m no expert but I thought the crank compression pressures seen during testing depend on the installed cam. That means with a factory cam you can reference the factory pressure specs but you can’t use factory numbers if you have a cam with big overlap as that impacts the pressures seen.

Probably much more important to look at % variation between cylinders and your variation looks rather good @ an odd 6% (max). It’s said if variation is more than 10% things need to be checked more closely.

 

[losh1971]

I have a cam and still got about 185 to 190 across the board. I could be wrong but I'd be thinking that valves likely need a grind on the OPs engine.

 

[07GTS]

dynamic compression only starts when the intake valve is closed, later close is less dynamic compression, static compression is based on full closed stroke of movement

 

[07GTS]

I have a cam and still got about 185 to 190 across the board. I could be wrong but I'd be thinking that valves likely need a grind on the OPs engine.

valve lapping is pretty easy to do if u can get the valves out, just need to shine a torch in the ports and see if there is any light shining between the valve/port if so give it a lap

 

[Skylarking]

dynamic compression only starts when the intake valve is closed, later close is less dynamic compression, static compression is based on full closed stroke of movement

Maybe I’m misunderstanding the terminology but I’ve understood with intake or exhaust valves open the compression during a compression test will be zero, even as piston move up. Only when intake and exhaust valves are closed will compression start to build within the cylinder. The point within the cylinder where the piston starts to compress the air is dependant on the valve timing.

During a comp test, I’ve understood that lots of overlap will allow the piston to move further up the piston before both valves close so the volume of air trapped within the cylinder at that point will be less than a standard factory cam without much overlap (which will trap more air volume). So big cam = less air trapped in the cylinder = lower compression.

As such, I’ve understood that compression readings during a compression test may be different for different cams (and everything else being equal).

Is this understanding wrong as I’m not sure who you were replying to.

(I’m not across dynamic compression as I haven’t looked into it in detail, but I’ve understood such needs combustion pressures and a running engine to achieve?

In any case 6% variation is a good outcome.

 

[07GTS]

Maybe I’m misunderstanding the terminology but I’ve understood with intake or exhaust valves open the compression during a compression test will be zero, even as piston move up. Only when intake and exhaust valves are closed will compression start to build within the cylinder. The point within the cylinder where the piston starts to compress the air is dependant on the valve timing.

During a comp test, I’ve understood that lots of overlap will allow the piston to move further up the piston before both valves close so the volume of air trapped within the cylinder at that point will be less than a standard factory cam without much overlap (which will trap more air volume). So big cam = less air trapped in the cylinder = lower compression.

As such, I’ve understood that compression readings during a compression test may be different for different cams (and everything else being equal).

Is this understanding wrong as I’m not sure who you were replying to.

(I’m not across dynamic compression as I haven’t looked into it in detail, but I’ve understood such needs combustion pressures and a running engine to achieve?

In any case 6% variation is a good outcome.

yea as long as they are even or close to they are fine as there are alot of variables, sounds like u were one event early before compression the overlap event is on the exhaust close and intake open so dosnt affect the compression stroke ''kinda'' at the slow cranking speeds it may affect the airflow getting in as can allow more some from exhaust and some intake for the short duration, exhaust usually is closed very soon after TDC at start of intake stroke, as the compression cant start until the intake valve is close the piston is already moving up so some of the stroke is lost pushing airflow back out the intake during cranking

dynamic is the compression caused by the valve events, stroke and boost if used http://www.wallaceracing.com/dynamic-cr.php

 

[Frank500]

Maybe I’m misunderstanding the terminology but I’ve understood with intake or exhaust valves open the compression during a compression test will be zero, even as piston move up. Only when intake and exhaust valves are closed will compression start to build within the cylinder. The point within the cylinder where the piston starts to compress the air is dependant on the valve timing.

During a comp test, I’ve understood that lots of overlap will allow the piston to move further up the piston before both valves close so the volume of air trapped within the cylinder at that point will be less than a standard factory cam without much overlap (which will trap more air volume). So big cam = less air trapped in the cylinder = lower compression.

As such, I’ve understood that compression readings during a compression test may be different for different cams (and everything else being equal).

Is this understanding wrong as I’m not sure who you were replying to.

(I’m not across dynamic compression as I haven’t looked into it in detail, but I’ve understood such needs combustion pressures and a running engine to achieve?

In any case 6% variation is a good outcome.

Everything you said is correct with reference to a compression test.

Big cam = less air trapped in the cylinder = lower compression readings in the compression test.

 

[mirrabucca]

And one more point. Make sure the trottle is at WOT. No point in doing a comp test if the engine can't draw in any air.
On that, any one know if removing the air filter makes any difference? Is the "factory" number done with the filter in place?