Low compression
07-18-2008, 09:01 PM
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Low compression So I took the car in to get tuned today and they said they cannot tune it yet due to the low compression. I was confused at why I only have 143psi on an 11.2:1 motor After I ajusted the valves I still only have 185 PSI and that is still low. Any thoughts guys?
07-18-2008, 09:56 PM
#2
How long did you run it with the valves missadjusted? Maybe it caused a little damage????? Just an idea, not even sure if that is possible.
Cam plays into cranking compression but that is still low, my Roady wagon cranked 195-200 at 180K miles and the sedan with the heads/cam package and similar static compression to yours cranks closer to 230psi.
Cam plays into cranking compression but that is still low, my Roady wagon cranked 195-200 at 180K miles and the sedan with the heads/cam package and similar static compression to yours cranks closer to 230psi.
07-20-2008, 05:33 AM
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I agree with Dwayne, that is pretty low.
Valves would have to have been REAL tight to have caused P/V contact on any LT1 and especially with that 280 XFI cam.
Do a leak down and see where the compression is going.
past the valves - pull the heads
past the rings - try and get them to seat and hope the cylinder wall finish and rings are compatable as well as cylinder walls and piston clearance being correct.
Probably all good and not trying to scare you, but these are the things to check to find the problem instead of ignoring the problem. This way you can at least rule out problems with the valves and bottom end leakage.
Maybe rings have not seated well enough. Check with engine builder with leak down results and see what he says. Get with him on what oil you should run until they seat (not synthetic) and how they should be driven to seat well. Everyone has their own recipe.
If the car is running real rich, stop driving it until this is fixed as you are just washing away the cross hatch (basically wet sanding the cylinders) and the rings will NEVER seal.
If the leak down checks good, it could be the cams effect on dynamic compression due to a poor design or not degreed properly. If using the XFI 280 cam, it has a 69 IVC so you "should" be fine if degreed properly (cam card calls for 108 ICL).
I just noticed TFS heads in sig . . . . . any idea about what chamber size they were milled down to? If they were not milled a good bit, this is ALOT of the problem.
What deck ht and head gasket thickness are you running also?
Even though the pistons are 11.2 to 1, is that based on YOUR set up with your chamber size, deck ht and head gasket thickness and bore diameter?
Could explain the low cranking pressure.
Give us this compression info and we can help you more.
Lloyd
Valves would have to have been REAL tight to have caused P/V contact on any LT1 and especially with that 280 XFI cam.
Do a leak down and see where the compression is going.
past the valves - pull the heads
past the rings - try and get them to seat and hope the cylinder wall finish and rings are compatable as well as cylinder walls and piston clearance being correct.
Probably all good and not trying to scare you, but these are the things to check to find the problem instead of ignoring the problem. This way you can at least rule out problems with the valves and bottom end leakage.
Maybe rings have not seated well enough. Check with engine builder with leak down results and see what he says. Get with him on what oil you should run until they seat (not synthetic) and how they should be driven to seat well. Everyone has their own recipe.
If the car is running real rich, stop driving it until this is fixed as you are just washing away the cross hatch (basically wet sanding the cylinders) and the rings will NEVER seal.
If the leak down checks good, it could be the cams effect on dynamic compression due to a poor design or not degreed properly. If using the XFI 280 cam, it has a 69 IVC so you "should" be fine if degreed properly (cam card calls for 108 ICL).
I just noticed TFS heads in sig . . . . . any idea about what chamber size they were milled down to? If they were not milled a good bit, this is ALOT of the problem.
What deck ht and head gasket thickness are you running also?
Even though the pistons are 11.2 to 1, is that based on YOUR set up with your chamber size, deck ht and head gasket thickness and bore diameter?
Could explain the low cranking pressure.
Give us this compression info and we can help you more.
Lloyd
07-20-2008, 12:54 PM
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The Heads are down to 58cc, Pistons have 2cc dome. deck is 0.022, gasket is 0.049, 4.030 bore and stock stroke. And the car is actualy running a tad lean.
07-20-2008, 08:04 PM
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So I got to working on it and did a leakdown test and found that the valves were still out of adjustment but im still only getting 190psi when cranking.
