Cranking compression variances - Camaro5 Chevy Camaro Forum / Camaro ZL1, SS and V6 Forums

Batmanntexas · 07-17-2021, 05:07 PM

So, during my trip to Camaro Fest, I felt the car was down on power a little bit. I drove it back home and let it sit for a few days before I got around to looking at it.

Today, I performed a compression check and got the following readings:

CYL #1 = 190psi CYL #2 = 175psi
CYL #3 = 200psi CYL #4 = 185psi
CYL #5 = 175psi CYL #6 = 175psi
CYL #7 = 170psi CYL #8 = 200psi

Engine specs are:
Stock rotating assembly (4.065" x 3.622")
Custom Cam (227/243 115LSA 110 ICL)
ported & decked stock heads (can't remember how much was cut, maybe .020")
LS9 head gaskets

Using some online calculators (using stock chamber volume, 70cc), the calculator indicated I should have around 190psi gauge pressure, with about 210psi actual cranking pressure.

Other than 'feeling' down on power, the car doesn't exhibit any symptoms, i.e. doesn't smoke, consume oil, run hot, etc.

What do you guys think?

-Scott

Vicster · 07-17-2021, 06:08 PM

Did you try doing the gimp mode (low octane) reset? Pull these fuses for 5 to 10 minutes...

What WILL happen with the fuse pull is your volatile memory will be reset. This includes long term + short term fuel trims, knock retard, etc... but what is important in this case is the little parameter that determines which spark table to use will reset...meaning if you were running from the low octane tables before (safe + slow mode), after the fuse pull it will force the computer to try the high octane table again.

morepowerjoe · 07-18-2021, 02:47 AM

Idk. 200-175 psi is a pretty big variance. But I think your ok for now but if it gets any worse I would start looking towards a rebuild possibly. How many miles do you have?

larry_g · 07-18-2021, 06:51 AM

General rule of thumb is +/- 10% so you are close to within variance. Squirt a little oil in the low cylinder and test it again. If there is a noticable increase, it's likely the rings.

Sent from my Pixel 5 using Tapatalk

G8One2 · 07-18-2021, 08:51 AM

Watch the first compression pulse on the gauge. It should jump immediately over 90 psi on that first pulse. That would tell you your valves have a good seal. If it doesnt, then your valves arent sealing as well as they could. Squirting oil in there as mentioned, will bring the overall compression up if the rings are worn.

Car Nation Videos · 07-18-2021, 10:31 AM

It sounds like the rings aren’t getting a good seal. Try what the guy above said with the oil and run the test again. Might be time for a rebuild.

Batmanntexas · 07-18-2021, 07:05 PM

Quote:

Originally Posted by Vicster

Did you try doing the gimp mode (low octane) reset? Pull these fuses for 5 to 10 minutes...

What WILL happen with the fuse pull is your volatile memory will be reset. This includes long term + short term fuel trims, knock retard, etc... but what is important in this case is the little parameter that determines which spark table to use will reset...meaning if you were running from the low octane tables before (safe + slow mode), after the fuse pull it will force the computer to try the high octane table again.

It's been tuned, and it is blown, so it only gets 93 or E85. Both the low octane and high octane spark tables are the same.

-Scott

Batmanntexas · 07-18-2021, 07:09 PM

Quote:

Originally Posted by morepowerjoe

Idk. 200-175 psi is a pretty big variance. But I think your ok for now but if it gets any worse I would start looking towards a rebuild possibly. How many miles do you have?

I'm sitting around 80K miles. Yeah, I'm leaning towards the rebuild option.

G8One2 · 07-18-2021, 07:10 PM

Excuse my ignorance here.... but what does octane, gimp mode and fuse pulls have anything to do with a compression test?

Batmanntexas · 07-18-2021, 07:12 PM

Quote:

Originally Posted by G8One2

Watch the first compression pulse on the gauge. It should jump immediately over 90 psi on that first pulse. That would tell you your valves have a good seal. If it doesnt, then your valves arent sealing as well as they could. Squirting oil in there as mentioned, will bring the overall compression up if the rings are worn.

Good info. I will test the low cylinders again and see what is what.

Thanks,
Scott

Aqua Blue RS/SS · 07-18-2021, 08:57 PM

You might try a leak down test. That will help diagnose things a lot better. It could be as simple as a couple leaky valve seats. I build engines for a living and this is what I would do next in your shoes.

morepowerjoe · 07-19-2021, 12:39 AM

Quote:

Originally Posted by Batmanntexas

I'm sitting around 80K miles. Yeah, I'm leaning towards the rebuild option.

Do you do a lot of racing or just around town short pulls?

Badbubba · 07-19-2021, 07:20 AM

Did you open your throttle body and bypass valve completely for the compression test? As well as disable the fuel injectors? I've read that if you hold the gas pedal down, it disables the injectors. Sort of like a flooded condition. Never did it though. I'd pull the plugs at the injectors.

Silverado60 · 07-19-2021, 11:18 AM

I'm not saying it's good or bad but that's the same numbers I had on my car sbe 30k miles. When I done the leakdown the cylinders with lower compression did show a higher rate but were still within acceptable ranges.
I do think it's the exhaust valves on those cylinders. We'll hopefully see this winter when I do the heads

07-18-2021, 08:57 PM	#11
Aqua Blue RS/SS Drives: ABM #93 Join Date: Oct 2018 Location: Lotaburger Posts: 2,720	You might try a leak down test. That will help diagnose things a lot better. It could be as simple as a couple leaky valve seats. I build engines for a living and this is what I would do next in your shoes. __________________ ABM #2399 2SS/RS:SOLD ABM #93 2SS/RS Black Rally stripes, Titanium Interior 4,000 miles: GM GFX side skirts and diffuser waiting on paint, GM dovetail, GM heritage, RPI ZL1 style splitter. ‘87 IROC-Z Iroc blue. all original unmolested with 50K miles.

07-19-2021, 07:20 AM	#13
Badbubba GrINned & JUICEd Drives: 15STI, 10Expy, 10 2SS/RS M6, 47KB1 Join Date: Nov 2009 Location: Pittsburgh, PA Posts: 1,247	Did you open your throttle body and bypass valve completely for the compression test? As well as disable the fuel injectors? I've read that if you hold the gas pedal down, it disables the injectors. Sort of like a flooded condition. Never did it though. I'd pull the plugs at the injectors. __________________ GPI SS3 Cam & stuff,N.O. Plate(dedicated fuel tank,progressive,fuel hose upgrade,remote opener,heater,console switches),LNC2000,CTAP,ARH 1 7/8" LTs/gutted cats/X pipe/mufflers,Jet Hot coated LTs,GPI HPT Tune,CAI, Scoop,Rx Catch Can,Gates heater hose relocation, Remote clutch & auto brake res,Metco loop,Momentum Tunnel Brace,TSW Nurb 18",555 & 05,Pfadt trailing arms & toe rods,Autometer WB,Fesler Pillar Pod,LED strips (int),Lloyds floor & trunk mats,LED DRL/front turn & license plate,DRL harness,VLEDs tail lights,Plasticoat Bowties. Ord: Oct 5, 2009 Rec: Nov 19, 2009 13.17@112.6 stock

07-19-2021, 11:18 AM	#14
Silverado60 Drives: 2010 camaro 2SS/RS M6 Join Date: Sep 2014 Location: Pa Posts: 238	I'm not saying it's good or bad but that's the same numbers I had on my car sbe 30k miles. When I done the leakdown the cylinders with lower compression did show a higher rate but were still within acceptable ranges. I do think it's the exhaust valves on those cylinders. We'll hopefully see this winter when I do the heads __________________ Texas speed F-35 cam, rollmaster timing set, melling hp pump, ls2 chain damper, fluid damper 25% under drive, Che rocker bushings, tsp .660 lift valve springs, BBK CAI Amzoil filter with Phastek ww relocation and scoop, Gmpp axle back, Speed engineering 1 7/8" LT's no cats, Peak speed shop ported tb, NGK tr6, MSD wires, Amsoil Zrod wix, heritage grill, 1.25" BMR springs, MRR 228, Nitto Nt05,dyno tuned by complete street performance, elite eng catch can, clean side separator, Hurst shifter, Zl1 addon wickerbill

07-17-2021, 05:07 PM	#1
Batmanntexas Drives: 2012 Camaro SS (2SS) Join Date: Aug 2012 Location: Stafford, VA Posts: 1,375 View My Garage	Cranking compression variances So, during my trip to Camaro Fest, I felt the car was down on power a little bit. I drove it back home and let it sit for a few days before I got around to looking at it. Today, I performed a compression check and got the following readings: CYL #1 = 190psi CYL #2 = 175psi CYL #3 = 200psi CYL #4 = 185psi CYL #5 = 175psi CYL #6 = 175psi CYL #7 = 170psi CYL #8 = 200psi Engine specs are: Stock rotating assembly (4.065" x 3.622") Custom Cam (227/243 115LSA 110 ICL) ported & decked stock heads (can't remember how much was cut, maybe .020") LS9 head gaskets Using some online calculators (using stock chamber volume, 70cc), the calculator indicated I should have around 190psi gauge pressure, with about 210psi actual cranking pressure. Other than 'feeling' down on power, the car doesn't exhibit any symptoms, i.e. doesn't smoke, consume oil, run hot, etc. What do you guys think? -Scott __________________ 745RWHP/634RWTQ on 93 @ 10psi 806/676 on E85 @ 10psi COTW 9/8/14

07-17-2021, 06:08 PM	#2
Vicster Drives: 13' ZL1 Join Date: Jan 2013 Location: . Posts: 396 View My Garage	Reset Did you try doing the gimp mode (low octane) reset? Pull these fuses for 5 to 10 minutes... What WILL happen with the fuse pull is your volatile memory will be reset. This includes long term + short term fuel trims, knock retard, etc... but what is important in this case is the little parameter that determines which spark table to use will reset...meaning if you were running from the low octane tables before (safe + slow mode), after the fuse pull it will force the computer to try the high octane table again. Attached Images

07-18-2021, 02:47 AM	#3
morepowerjoe Drives: 2012 Chevrolet Camaro l99 Join Date: Aug 2020 Location: Austin, TX Posts: 2,142	Idk. 200-175 psi is a pretty big variance. But I think your ok for now but if it gets any worse I would start looking towards a rebuild possibly. How many miles do you have? __________________ DSS FX Forged Pistons, Eagle ESP L19 Forged Rods, ARP Head and Main Studs, TSP PRC 260 CNC Ported Heads milled .030, Cometic .040 Head Gaskets, GPI SS3 VVT Camshaft, GM Performance Racing Lifters, CHE Trunnions , Molly Pushrods, Melling 10355HV Oil Pump, Cold Air Inductions CAI, Fast LSXR Intake Manifold, Nick Williams 103 TB, TSP 1 7/8" Longtubes with 3" TSP Exhaust, ZL1 Fuel Pump, Mishimoto Radiator, Mishimoto Oil Cooler, Tru Cool 40k Tranny Cooler, 3600 Circle D Converter, Fluid Dynamics Balancer Build Thread https://www.camaro5.com/forums/showthread.php?t=609817

07-18-2021, 06:51 AM	#4
larry_g Drives: 2014 1SS/1LE Join Date: Nov 2015 Location: Louisville, KY Posts: 828	General rule of thumb is +/- 10% so you are close to within variance. Squirt a little oil in the low cylinder and test it again. If there is a noticable increase, it's likely the rings. Sent from my Pixel 5 using Tapatalk __________________ 830 WHP 700 WTQ, Whipple 2.9 @ 13lbs Boost, Flex Fuel Tune by Blevins Tuning, LSA SBE, 3.5 GripTec, ATI LS3 Damper, AFR 260cc Mongoose, ARP Studs, Yella Terra Ultralites, Johnson 2110R, BTR PDS Stage 3, Melling 10355, Improved Racing Baffle, ID1050X, DSX Triple Fuel Pump, RotoFab CAI, NW 103mm TB, Stainless Works 1-7/8" LTH to 3" Speed Eng w/ X-pipe to NPP, Powermaster 9509 XS, Detroit Speed Motor Mounts, MGW, Monster LT1-S, ZL1 Calipers, DBA Rotors & Pads, Bilstein B6 Struts & Shocks, BMR 1.25" Drop Springs & Adj Rear Bar, Pegasus Solid Bushings, Z/28 Wheels, other misc parts.

07-18-2021, 08:51 AM	#5
G8One2 Drives: 2014 Camaro 2SS Join Date: Feb 2015 Location: Metro Detroit Posts: 492	Watch the first compression pulse on the gauge. It should jump immediately over 90 psi on that first pulse. That would tell you your valves have a good seal. If it doesnt, then your valves arent sealing as well as they could. Squirting oil in there as mentioned, will bring the overall compression up if the rings are worn.

07-18-2021, 10:31 AM	#6
Car Nation Videos YouTube-Car Nation Videos Drives: 2010 Chevrolet Camaro Join Date: Nov 2019 Location: Florida Posts: 246	It sounds like the rings aren’t getting a good seal. Try what the guy above said with the oil and run the test again. Might be time for a rebuild.

07-18-2021, 07:10 PM	#9
G8One2 Drives: 2014 Camaro 2SS Join Date: Feb 2015 Location: Metro Detroit Posts: 492	Excuse my ignorance here.... but what does octane, gimp mode and fuse pulls have anything to do with a compression test?