Converting cylinder PSI to engine compression ratio?
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From: Sioux Center, Iowa
Converting cylinder PSI to engine compression ratio? I've done a few searches on here and google but haven't found anything that tells you after compression checking your engine how to convert that to your compression ratio. The reason I ask is I bought my car with the ported LS6 heads and was told they were milled too but I don't know how much or if at all. The 2 cylinders I checked on a cold engine in 75* temperature were both almost dead on 150psi. Is there any way I can figure out my compression ratio from that on an LS1 or some kind of online calculator for psi vs. engine bore this?
I cranked the motor 5 turns but my buddy watching the gauge said by the 3rd crank it pegged 150psi and didn't move the next 2 cranks.
I cranked the motor 5 turns but my buddy watching the gauge said by the 3rd crank it pegged 150psi and didn't move the next 2 cranks.
The problem is the camshaft.....a small cam at 11.0 to 1 might crank 190 PSI or more.....a larger cam might be 170 ish (same static CR).
I tell you this....if all your cylinders crank 150 PSI you have low compression and/or too big a cam.
Most of the optimized N/A builds I design crank 190 - 210 PSI.....if I see less than 190 I feel I left some power under the table. Although most of my builds feature reasonably sized cams....nothing over the top. BIG cam stuff you might see 170-180 if its set up properly (static CR in the mid to high 11's) but 150 is definitely light on cranking compression regardless of the situation.
You may want to borrow a buddy's gauge and see if it reads the same.
Hope this helps...
-Tony
I tell you this....if all your cylinders crank 150 PSI you have low compression and/or too big a cam.
Most of the optimized N/A builds I design crank 190 - 210 PSI.....if I see less than 190 I feel I left some power under the table. Although most of my builds feature reasonably sized cams....nothing over the top. BIG cam stuff you might see 170-180 if its set up properly (static CR in the mid to high 11's) but 150 is definitely light on cranking compression regardless of the situation.
You may want to borrow a buddy's gauge and see if it reads the same.
Hope this helps...
-Tony
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Yeah I think there are more variables than just my psi reading. I am also at about 1350 feet above sea level, i've heard that makes a difference as well. The guy I got the compression checker from runs an old carb'd chevy 350 with around 12:1 compression heads on race fuel, he said 150 is also about what he does so I didn't know whats typical or LS1's but I guess I could have to compare to other LS1's at my elevation and checked in the same weather both being cold or warm engines to really get an idea.
Yeah I think there are more variables than just my psi reading. I am also at about 1350 feet above sea level, i've heard that makes a difference as well. The guy I got the compression checker from runs an old carb'd chevy 350 with around 12:1 compression heads on race fuel, he said 150 is also about what he does so I didn't know whats typical or LS1's but I guess I could have to compare to other LS1's at my elevation and checked in the same weather both being cold or warm engines to really get an idea.
How big is the cam in your engine....and Im at 1300 feet of elevation btw....all my builds are in the 190 - 210 PSI range.
Did you check every hole and did you also use a leak down tester out of curiosity?
-Tony
My engine has a static compression ratio of 11:1 with a dynamic compression ratio of 8.9:1. 122,000 mile stock bottom end LS1 I checked all cylinders with 3 different gauges. The cheap gauge read 210 PSI while the snap on gauges read from 220-230 PSI. It all depends on your intake valve closing point of your cam, static compression, rings and the gauge you use. My buddies 04 GTO T-rex cam only LS1 cranked 175 and it only has a dynamic compression ratio of 7.6:1. This may also be a part of the reason my car makes so much power under the curve; 365 rwtq @ 3000 rpm and 399 rwtq @ 3800 rpm.
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From: Sioux Center, Iowa
My cam is a 228/228 .588/.588 112+2 My motor runs great though I mean it dyno'd the numbers in my sig on a Mustang dyno that is typically about 9-10% less than dynojets in the area so I am sitting around 420rwhp/390rwtq which is decent for my mods. I just was wondering a way to tell people what my compression ratio was when they ask. I have LS6 heads which come in at 10.5 stock but was wondering if any milling had been done as well by my previous owner. With my car being lower than most people are reporting in here and my dyno numbers being nothing crazy I am guessing they are most likely not milled.
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Rassie LS2 I have a LS2 NA motor that I installed a K1 crank and conrods (4" stroke).Wisico+5cc pistons.(4.030)LS3 ported heads (821 casting) and intake.Standard 90mm TB. Zero piston deck clearance. COMP Cams Ultra Pro Magnum Rocker Arms 1678-16 (1.8 ration). Manley NexTek Valve Spring and Retainer Kits 26362134KS (Dubbel valve springs). Long headers with 76mm pipes. OTR Air intake.VCM 532 camDuration215/223Lobe Separation@ 112Valve Inlet.583" Exhaust.583"
What is your estimated HP and PSI on my motor?. I am still running her in .
Thanks appreciate
What is your estimated HP and PSI on my motor?. I am still running her in .
Thanks appreciate
