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Chevelle454 · 08-10-2004, 10:52 AM

My compression isn't what I initially thought it was. I have 100 psi in cylinders 1 and 2, and 60 in the rest. I have an aftermarket cam(.440 lift, 266 dur.), would that affect it at all? I triple checked the valve timing when I assembled the engine on the stand. Would it run with only 60 psi? I don't want to reassemble the whole top end to have it not run anyway...

Chevelle454 · 08-10-2004, 12:23 PM

Ttt

JimKshortstep4x4 · 08-10-2004, 01:29 PM

60 psi is pretty low compression. I don't believe that it will run. Do you have the valves adjusted too tight?

Jim

dallas71 · 08-10-2004, 01:48 PM

I agree with JimKshortstep4x4, in that it sound like you valves are staying partially open....

Chevelle454 · 08-10-2004, 01:57 PM

No, they are not too tight.

I had all the valves ground and new valve seats put in. I might not have done them right, it was my first time rebuilding heads. I did it in school with the shop teacher's supervision. The seats might not be in at the right angle or something. I don't know what else it could be except for that.

I had to change the exhaust seats because the engine was made for leaded gas, so the seats and valves were pitted from running unleaded.

I cannot afford to do anymore work on my truck right now. I have college starting in a week and I have very limited funds. If there is any way I can get this thing to run without spending more money, that would be great.

Valve timing is correct, valves are not too tight, new pistons and rings, new head gaskets. I have no idea...

Longhorn Man · 08-10-2004, 03:53 PM

Did you have the throttle all the way open and a fully charged battery? Either of these conditions would cause real low compression readings.

elk hunter · 08-10-2004, 04:15 PM

Quote:

Originally Posted by Longhorn Man

Did you have the throttle all the way open and a fully charged battery? Either of these conditions would cause real low compression readings.

HUH?

Russell · 08-10-2004, 04:20 PM

60 PSI is bad?

Uhh, my highest is like 35...

Longhorn Man · 08-10-2004, 04:44 PM

To properly do a compression test, you must first ensure your battery is fully charged. When you are ready to crank the engine over, you must opent he throttle all the way...floor the gas.
These are needed for a correct compression reading when doing a compression test.

dubie · 08-10-2004, 04:48 PM

I have never done a compression test with the throttle all the way open. And Russell, if your truck only has 35psi per cylinder, you're in for some major troubles

BET68 · 08-10-2004, 05:22 PM

CK YOUR BOOKS, LONGHORN MAN IS RIGHT!!!

dallas71 · 08-10-2004, 05:33 PM

I guess we learn something everyday being on this board!!!

Grim Reaper · 08-10-2004, 05:39 PM

8:1 V8 if you have done the check right good compression sould be around 120psi by the theird compression stroke.

With that low of number I would supect throttle was not open and the rest of the motor still had the plugs in it. I burn both exhaust valves in a multi valve head. The one valve had a 1/4 inch notch in it and the other had a 1/8 and that would still pull 30 psi. If you did the test right you got no rings left.

Test procedure.
1: REMOVE ALL PLUGS
2: Disable ignition buy pulling plugs on coil.
3: put on a battery charger with boost feature.
Make sure the throttle is open so the engine can pull unrestricted air. I usualy open the throttle and put a Larger square shank screw driver in the carb to hold both the choke and the bottom butter flies open Just make sure to hold open while you insert and remove it and ease it closed on it.
5. Try to place the compression tester where it is visible while you crank. and crank till you get 3-4 compression strokes. You will see the gage jump each time you have a stroke. After the third stroke you should not see any increase of compression.

Shop manual should list what the acceptable range is. I just did this on my 86 4Runner with 195,000 miles on it. They concider bad on that motor as 128 or less. 172 is perfect. I pulled between 165 and 170 on all 4.

Most compression testers you can do a poo mans bleed down test with. They will have a 1/4 inch air disconnect in them. The end you screw into the cylinder has a scharader valve in it (same as your tire valve). If you remove that schrader valve you can hook it to your compressor and hear were your looksing compression. In the intak it's intake valves. In the tail pipe it's you exhaust vlave. listen in the oil fill for rings. You will hear a little hiss but if you hear a lot thats a substantial loss. When you put air to it the motor may want to spin. Idealy you want each cylinder at TDC but it's hard to do.

Longhorn Man · 08-10-2004, 05:47 PM

Grim...thanks. I was going to go through all that, but I was being called away from the computer when i was typing it.

elk hunter · 08-10-2004, 10:29 PM

so what youre saying is to properly start your engine, battery at full charge, pedal to the wood and crank? that way there is full compression to start correct? ive done hundreds of compression tests and never held it to the wood. i just crank till it quits building compression usually about 5 compression strokes. not calling anyone out, thats always worked for me. guess ive been doing it wrong all these years. youre never too old for an education. thats why i love the professors on this board!!!

68C15 · 08-10-2004, 10:44 PM

keep in mind that a cam with more duration (valve overlap) will have lower cranking compression. but once running the effective compression will increase dramatically as rpms increase. (this is due to the 'ram effect') also the first stroke of cranking test should be about 1/2 of total.
did you do the test with a starter turning the engine or use a wrench? if you used a starter I would be concerned about cylinders 3-8. try adjusting the valve clearance.

68C15 · 08-10-2004, 10:46 PM

BTW, as a general rule of thumb you need at least 100 psi of cranking compression for an engine to support combustion.

neonlarry · 08-10-2004, 11:07 PM

If you put the seats in crooked you would have noticed when you were lapping the valves you would not have gotten a complete lap on either the valve or the seat. JMO. If you did compression test correctly either the valves are too tight or wrong size rings for the bore.

Chevelle454 · 08-11-2004, 12:20 AM

OK, here's an update:

I took off the pass. side head and lapped the valves once again. This time I got 95 psi on #1, 90 psi on #3 and #5, and 60 psi on #7.

I have the intake off the motor. All the plugs are taken out. The pistons, rings, all bearings, cam, lifters, and oil pump are all brand new. The starter is new and the battery is fully charged. I think that maybe the valve seats were not ground enough. Hard to say for sure...

Here are my cam specs, it shouldn't lower compression a whole lot I wouldn't think...
Adv Dur Int 266 Exh 266 Dur @ .050
Int 210 Exh 210 Deg Lobe Sep 110
Valve Lift Int .440 Exh .440

Maybe around 90PSI is normal for the cam that's in there?

stllookn · 08-11-2004, 12:46 AM

Did you try a little oil in each cylinder too? This can help the rings seal especially if they are dry and you have not pre-oiled the engine. My tester brochure states 5 cycles to get full pressure with the throttle open.

67shortie · 08-11-2004, 04:50 AM

If everything is brand new I would suspect that the rings have not seated yet. Rings need a load on them and heat for them to seat correctly. If the engine was built right, get it back together and start it up, and run it for break in and see what you come up with then.

bigjimzlll · 08-11-2004, 09:02 AM

Quote:

Originally Posted by 67shortie

If everything is brand new I would suspect that the rings have not seated yet. Rings need a load on them and heat for them to seat correctly. If the engine was built right, get it back together and start it up, and run it for break in and see what you come up with then.

Bingo.....does not make any sence to do a compression check until its broke in

Chevelle454 · 08-11-2004, 10:08 AM

That I didn't know... So it might work then? I'll lap the other head again tonight and see if she runs!

Grim Reaper · 08-11-2004, 10:45 AM

Quote:

Originally Posted by elk hunter

so what youre saying is to properly start your engine, battery at full charge, pedal to the wood and crank? that way there is full compression to start correct? ive done hundreds of compression tests and never held it to the wood. i just crank till it quits building compression usually about 5 compression strokes. not calling anyone out, thats always worked for me. guess ive been doing it wrong all these years. youre never too old for an education. thats why i love the professors on this board!!!

START?

HELL NO! you don't want to run the motor.

You want to make sure it doesn't start. You want consitant RPM on every test and your going to see more variation of pressure at low rpm durring cranking then if you start the motor. The higher RPMs will hide that. You do want the throttle open so the cylinder your testing can get plenty of air. You want all the plugs out so that you have a steady RPM.

elk hunter · 08-11-2004, 12:02 PM

i understand... i was just trying to get a rise out of you. im a brake and suspension mechanic, but do all of my own wrenching(too proud to pay someone else). you are correct in your instructions for a proper reading. i would venture to say that at that low compression reading that there could be a ring problem or a cracked head? dont you need 75# to fire?

08-10-2004, 12:23 PM	#2
Chevelle454 Registered User Join Date: Aug 2003 Location: MN Posts: 380	Ttt __________________ 1972 C10 1972 C20

08-10-2004, 01:48 PM	#4
dallas71 Registered User Join Date: Mar 2004 Location: Corinth,TX Posts: 444	I agree with JimKshortstep4x4, in that it sound like you valves are staying partially open.... __________________ My 1966 LWB Truck My 1966 SWB Truck

08-10-2004, 04:20 PM	#8
Russell Professional Grade Join Date: May 2003 Location: Fort McMurray, Alberta Posts: 7,915	60 PSI is bad? Uhh, my highest is like 35... __________________ 1995 Chevrolet 2 Door Tahoe (6.6L LBZ Duramax / ZF6 / NP241 with 1 ton solid axle swap)

08-10-2004, 04:48 PM	#10
dubie Registered User Join Date: May 2000 Location: Flin Flon, Manitoba, Canada Posts: 8,853	I have never done a compression test with the throttle all the way open. And Russell, if your truck only has 35psi per cylinder, you're in for some major troubles __________________ My name's Tim and I'm a truckaholic My 56 Chevy shop truck build http://67-72chevytrucks.com/vboard/s...d.php?t=562795

08-10-2004, 05:22 PM	#11
BET68 Registered User Join Date: Mar 2004 Location: BREVARD NC Posts: 156	CK YOUR BOOKS, LONGHORN MAN IS RIGHT!!! __________________ NAMASTE B&E 1971 LONGHORN C-30 1969 LONGHORN C-20 1968 DUALLY 1970 STEP SIDE PROJECT 1972 CST C-20 1980 M-80 MALIBU FOR SALE 1974 NOVA FOR SALE:

08-10-2004, 10:52 AM	#1
Chevelle454 Registered User Join Date: Aug 2003 Location: MN Posts: 380	OK....hows this? My compression isn't what I initially thought it was. I have 100 psi in cylinders 1 and 2, and 60 in the rest. I have an aftermarket cam(.440 lift, 266 dur.), would that affect it at all? I triple checked the valve timing when I assembled the engine on the stand. Would it run with only 60 psi? I don't want to reassemble the whole top end to have it not run anyway... __________________ 1972 C10 1972 C20 Last edited by Chevelle454; 08-10-2004 at 12:26 PM.

08-10-2004, 01:29 PM	#3
JimKshortstep4x4 Senior Member Join Date: Jul 2001 Location: Muskegon,MI,USA Posts: 6,026	60 psi is pretty low compression. I don't believe that it will run. Do you have the valves adjusted too tight? Jim

08-10-2004, 01:57 PM	#5
Chevelle454 Registered User Join Date: Aug 2003 Location: MN Posts: 380	No, they are not too tight. I had all the valves ground and new valve seats put in. I might not have done them right, it was my first time rebuilding heads. I did it in school with the shop teacher's supervision. The seats might not be in at the right angle or something. I don't know what else it could be except for that. I had to change the exhaust seats because the engine was made for leaded gas, so the seats and valves were pitted from running unleaded. I cannot afford to do anymore work on my truck right now. I have college starting in a week and I have very limited funds. If there is any way I can get this thing to run without spending more money, that would be great. Valve timing is correct, valves are not too tight, new pistons and rings, new head gaskets. I have no idea... __________________ 1972 C10 1972 C20

08-10-2004, 03:53 PM	#6
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	Did you have the throttle all the way open and a fully charged battery? Either of these conditions would cause real low compression readings.

08-10-2004, 04:44 PM	#9
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	To properly do a compression test, you must first ensure your battery is fully charged. When you are ready to crank the engine over, you must opent he throttle all the way...floor the gas. These are needed for a correct compression reading when doing a compression test.

08-10-2004, 05:33 PM	#12
dallas71 Registered User Join Date: Mar 2004 Location: Corinth,TX Posts: 444	I guess we learn something everyday being on this board!!! __________________ My 1966 LWB Truck My 1966 SWB Truck

08-10-2004, 05:39 PM	#13
Grim Reaper Registered User Join Date: Jul 2001 Location: Atlanta GA Posts: 1,704	8:1 V8 if you have done the check right good compression sould be around 120psi by the theird compression stroke. With that low of number I would supect throttle was not open and the rest of the motor still had the plugs in it. I burn both exhaust valves in a multi valve head. The one valve had a 1/4 inch notch in it and the other had a 1/8 and that would still pull 30 psi. If you did the test right you got no rings left. Test procedure. 1: REMOVE ALL PLUGS 2: Disable ignition buy pulling plugs on coil. 3: put on a battery charger with boost feature. Make sure the throttle is open so the engine can pull unrestricted air. I usualy open the throttle and put a Larger square shank screw driver in the carb to hold both the choke and the bottom butter flies open Just make sure to hold open while you insert and remove it and ease it closed on it. 5. Try to place the compression tester where it is visible while you crank. and crank till you get 3-4 compression strokes. You will see the gage jump each time you have a stroke. After the third stroke you should not see any increase of compression. Shop manual should list what the acceptable range is. I just did this on my 86 4Runner with 195,000 miles on it. They concider bad on that motor as 128 or less. 172 is perfect. I pulled between 165 and 170 on all 4. Most compression testers you can do a poo mans bleed down test with. They will have a 1/4 inch air disconnect in them. The end you screw into the cylinder has a scharader valve in it (same as your tire valve). If you remove that schrader valve you can hook it to your compressor and hear were your looksing compression. In the intak it's intake valves. In the tail pipe it's you exhaust vlave. listen in the oil fill for rings. You will hear a little hiss but if you hear a lot thats a substantial loss. When you put air to it the motor may want to spin. Idealy you want each cylinder at TDC but it's hard to do. __________________ Grim-Reaper 70 Pontiac LeMans Sport Convertible, worlds longest resto in progress Looking for 71-72 2wd Blazer or Jimmy Project

08-10-2004, 05:47 PM	#14
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	Grim...thanks. I was going to go through all that, but I was being called away from the computer when i was typing it.

08-10-2004, 10:29 PM	#15
elk hunter Registered User Join Date: Dec 2003 Location: oregon city, or Posts: 164	so what youre saying is to properly start your engine, battery at full charge, pedal to the wood and crank? that way there is full compression to start correct? ive done hundreds of compression tests and never held it to the wood. i just crank till it quits building compression usually about 5 compression strokes. not calling anyone out, thats always worked for me. guess ive been doing it wrong all these years. youre never too old for an education. thats why i love the professors on this board!!! __________________ yeah, but how's it wheel?

08-10-2004, 10:44 PM	#16
68C15 blood type; Retumbo Join Date: Jan 2004 Location: next to my reloading bench Posts: 10,269	keep in mind that a cam with more duration (valve overlap) will have lower cranking compression. but once running the effective compression will increase dramatically as rpms increase. (this is due to the 'ram effect') also the first stroke of cranking test should be about 1/2 of total. did you do the test with a starter turning the engine or use a wrench? if you used a starter I would be concerned about cylinders 3-8. try adjusting the valve clearance. __________________ Man rule #77...if you own a 67 stepside with a caddy 472 you will never be in danger of loosing you man card



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08-10-2004, 10:46 PM	#17
68C15 blood type; Retumbo Join Date: Jan 2004 Location: next to my reloading bench Posts: 10,269	BTW, as a general rule of thumb you need at least 100 psi of cranking compression for an engine to support combustion. __________________ Man rule #77...if you own a 67 stepside with a caddy 472 you will never be in danger of loosing you man card

08-10-2004, 11:07 PM	#18
neonlarry Registered User Join Date: Feb 2002 Location: Vacaville, CA Posts: 2,745	If you put the seats in crooked you would have noticed when you were lapping the valves you would not have gotten a complete lap on either the valve or the seat. JMO. If you did compression test correctly either the valves are too tight or wrong size rings for the bore. __________________ 70 C/10 Light Red 350/TH350, HEI, Duals w/40 series Flows, 91 seat, LED taillights 99 Pontiac S/C GTP, SLP Ram Air hood, GMPP Konis & springs 95 Neon ACR, MP PCM, AFX UDP, 3.0 CAI

08-11-2004, 12:20 AM	#19
Chevelle454 Registered User Join Date: Aug 2003 Location: MN Posts: 380	OK, here's an update: I took off the pass. side head and lapped the valves once again. This time I got 95 psi on #1, 90 psi on #3 and #5, and 60 psi on #7. I have the intake off the motor. All the plugs are taken out. The pistons, rings, all bearings, cam, lifters, and oil pump are all brand new. The starter is new and the battery is fully charged. I think that maybe the valve seats were not ground enough. Hard to say for sure... Here are my cam specs, it shouldn't lower compression a whole lot I wouldn't think... Adv Dur Int 266 Exh 266 Dur @ .050 Int 210 Exh 210 Deg Lobe Sep 110 Valve Lift Int .440 Exh .440 Maybe around 90PSI is normal for the cam that's in there? __________________ 1972 C10 1972 C20

08-11-2004, 12:46 AM	#20
stllookn Saving 1 truck at a time! Join Date: Aug 2001 Location: Kent, WA Posts: 6,465	Did you try a little oil in each cylinder too? This can help the rings seal especially if they are dry and you have not pre-oiled the engine. My tester brochure states 5 cycles to get full pressure with the throttle open. __________________ '68 C20 Longhorn 50th Anniversary 400/TH400 '68 C20 Longhorn 50th Anniversary 468/TH400w/buckets '72 C20 Halfhorn (Longhorn w/o cab and front clip) '69 Flxible Cruiser (look up ugly in the dictionary)

08-11-2004, 04:50 AM	#21
67shortie Green club Join Date: May 2002 Location: Central CA Posts: 1,059	If everything is brand new I would suspect that the rings have not seated yet. Rings need a load on them and heat for them to seat correctly. If the engine was built right, get it back together and start it up, and run it for break in and see what you come up with then. __________________ 71 short/fleet ECE 4/6 drop, LQ4/700r4/3.42 gears 70 SS396 Chevelle 427/200-4R/3.73posi 61 Lincoln Continental, black on black Son you are going to drive me to drinkin if you don't stop drivin that hot rod lincoln

08-11-2004, 10:08 AM	#23
Chevelle454 Registered User Join Date: Aug 2003 Location: MN Posts: 380	That I didn't know... So it might work then? I'll lap the other head again tonight and see if she runs! __________________ 1972 C10 1972 C20

08-11-2004, 12:02 PM	#25
elk hunter Registered User Join Date: Dec 2003 Location: oregon city, or Posts: 164	i understand... i was just trying to get a rise out of you. im a brake and suspension mechanic, but do all of my own wrenching(too proud to pay someone else). you are correct in your instructions for a proper reading. i would venture to say that at that low compression reading that there could be a ring problem or a cracked head? dont you need 75# to fire? __________________ yeah, but how's it wheel?

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