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FWIW all those compression numbers seem low, with #2 obviously having the biggest issue. MY S2 was around 205-210psi across all four cylinders the last time I checked it. Might be a good idea to have the whole thing refreshed at this point.
Static compression ratio of 10.9:1. Effective stroke is 3.03 inches. Your dynamic compression ratio is 9.68:1 . Your dynamic cranking pressure is 209.36PSI.
I think the soft lifters may be reducing valve lift, which would have an impact on the compression ratio. If the valves aren't staying open for the full sweep of the piston, then the cylinders will never fill to full capacity to reach maximum cylinder pressure.
I found the slightest hairline of a hairline of a hairline scratch at the top of cylinder #2. It is only about 1" long, will slightly catch my fingernail, but is sooooo light I am not worried about it. It is odd though, and seems to have come from a single sweep of the piston. I don't think it is ring, or ring lands, related, and I don't think it is something still stuck in the piston. I'll get closer pics, and measurements, up soon so you can see how my cylinders look after 167k miles. Overall, they appear fine.
Camelman I wear L or XL gloves. Surprised how big these pistons are! You can't see the scratch in the #2 cylinder wall in this pic. It is on the left side of this pic, but is too small to make out. #4 seems to have experienced moisture ingress. #2 and #3 are obviously covered in carbon, which I think is a byproduct of low compression from soft lifters. #1 is okay.
Static compression ratio of 10.9:1. Effective stroke is 3.03 inches. Your dynamic compression ratio is 9.68:1 . Your dynamic cranking pressure is 209.36PSI.
Your compression #'s are spot on.
T
Thanks, you taught me something today.
Is there a fixed relationship between HP and Compression? Like, I understand that you can still get the motor to start/run with low compression, but is there a formula for X psi = Y hp loss? Probably not as cut and dry as that, but what's the relationship like?
Is there a fixed relationship between HP and Compression? Like, I understand that you can still get the motor to start/run with low compression, but is there a formula for X psi = Y hp loss? Probably not as cut and dry as that, but what's the relationship like?
Yeah, it's complicated and changes are intertwined.
An engine with less than 7.5:1 dynamic compression will have low torque at low rpms due to low cylinder pressure and lower BHP at higher rpms because the incoming charge air's ram effect is lost if the intake valve closes too late.
You have to compensate by raising static compression if you close the intake valve later so in that sense, compression is directly related to making torque and power.
Intake valve closure at 50+ degrees past BDC seems extreme, like it's taking away a lot of the piston travel's ability to compress as compared to the valve being closed right at BDC but you have to keep in mind that the piston is parked in the cylinder for a fair time during TDC and BDC.
I pulled the head and everything looks good. 7/16 lifters were soft though, so I’m thinking it’s time to clean them all.
Will install all new rubber seals in the head and valve cover along with new bolts. The big question is if I send the head off for new valve guides, etc by Lindsey Racing (or other), or just clean it and reassemble. I had to drill out some cam cover bolts, so will definitely need a cleaning. A full refresh will be about $1600 including shipping to Lindsey.
A refresh would include new valve guides and seals.
Car has 167k miles.
Camelman
Did you inspect the head gasket thoroughly.?
The cylinder sealing ring specifically...?
Regarding the above picture, did you clean the pistons any before taking this shot? I'm amazed at how clean they look at 160+ US miles. Mine were caked with carbon
Regarding the above picture, did you clean the pistons any before taking this shot? I'm amazed at how clean they look at 160+ US miles. Mine were caked with carbon
I didn’t clean them. The front and rear are pretty clean, but the center two are caked with carbon. I think the cylinder head pic tells a better story than this pic regarding carbon buildup. The rear cylinder appears to have had some water ingress, probably from coolant. Water will clean carbon off quickly. I didn’t see any obvious signs of leakage around the head gasket though.
This kind of advice is exactly why I subscribe to Rennlist. It just doesn't get any better.
My compression tester also has an 18" hose going from the spark plug hole to my gauge, and the pressure relief valve is at the gauge. This configuration effectively adds the volume of the hose to the volume of the cylinder, which reduces the overall compression ratio while the gauge is attached. The result is that I will never get a perfectly accurate compression reading with this gauge, so I can only use it for cylinder-to-cylinder comparison purposes.
Here's a good writeup on the issue from a motorcycle forum, where the additional volume of the hose has an even greater impact on absolute measurement.
I cleaned the lifters last night. They wouldn't come apart fully like I have seen in a number of videos, so I left the center "button" assembled, filled a plastic cup with paint thinner, and dropped the button into the cup. Then, I repeatedly pressed on the check valve with a small screwdriver to cycle paint thinner into the cavity, and squeeze out the old oil and varnish. The old stuff come out quickly, and the buttons quickly pumped up. I then turned each button upside down, and pressed on the check valve again to drain it. As a final check, I filled the button with compressed air, and then pressed on the check valve again to see if the button was holding pressure. If I heard a little "whoosh", then I figured the button was clean. If no "whoosh", then I'd cycle more paint thinner through before trying again.
I'm still waiting to hear about the cylinder head and valves since I'm not 100% sure on why I lost compression in cylinder #3. I am fairly certain I need a new catalytic converter though since the one I have overheated enough to glow like a Canadian palm tree (outdoor porch heater). The engine barely ran after the cat overheated. I also don't know why the cat overheated, so I'll have to dig into that too.
Well, that was quick. I heard back from the machine shop and they found a lot of corrosion on the head immediately around the cylinders, and in line with the fire rings. That will allow pressure to leak out of the cylinders. I'm going to inspect the head on Monday and make a determination on what to do. The shop suggested welding up the holes and decking the head. It looks like the head is a simple A356 or 357 aluminum, so I think that's what I'll do.
03-20-2019, 12:46 PM
