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#1
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Compression Test Results Question
I compression tested my 455. Stock 76 with 6H heads; 7.6 CR. Idle vacuum is about 16".
All but two cylinders were between 125 and 135 psi. Cylinders 4 & 6 were both down about 15 psi; 110 psi (#4) and 115 psi (#6). Since these two are adjacent, is this a sign of a bad head gasket bleeding between the two cylinders? I don’t see any water or oil smoke in the exhaust, and I am not losing any coolant.
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69 Firebird Conv, 350 HO w/3 on the tree swapped to Tremec 5 Speed - Sold 72 GP, 400 Auto - Sold 09 G8 GXP M6 Orig Owner - Sold 76 Trans Am 455, 4 Speed - Current |
#2
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If you are able, you might squirter some oil in all the cylinders and see if the readings change any, especially on low cylinders.
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1977 Black Trans Am 180 HP Auto, essentially base model T/A. I'm the original owner, purchased May 7, 1977. Shut it off Shut it off Buddy, I just shut your Prius down... |
#3
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Take out all the plugs and then do a test on 4 or 6, if you still get the same pressure readings then the head gasket is intact, real simple!
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Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs! And he was not talking about 1/8 or 1/4 mile ETs! 1) 1940s 100% silver 4 cup tea server set. Two dry rotted 14 x 10 Micky Thompson slicks. 1) un-mailed in gift coupon from a 1972 box of corn flakes. Two pairs of brown leather flip flops, never seen more then 2 mph. Education is what your left with once you forget things! |
#4
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Take out all the plugs and then do a test on 4 or 6, if you still get the same pressure readings then the head gasket is intact, real simple!
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Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs! And he was not talking about 1/8 or 1/4 mile ETs! 1) 1940s 100% silver 4 cup tea server set. Two dry rotted 14 x 10 Micky Thompson slicks. 1) un-mailed in gift coupon from a 1972 box of corn flakes. Two pairs of brown leather flip flops, never seen more then 2 mph. Education is what your left with once you forget things! |
#5
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A leak down test is really the way yo go.
This will tell you is it’s rings or if the valve job is going away in either of those low cylinders. If it’s valve related I would try a get a set of 6X-8 heads to freshen up and bolt on. These would reduce the chamber volume by some 20 CCs and pick you up 15 Hp and likely over 20 ft lbs of torque.
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Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs! And he was not talking about 1/8 or 1/4 mile ETs! 1) 1940s 100% silver 4 cup tea server set. Two dry rotted 14 x 10 Micky Thompson slicks. 1) un-mailed in gift coupon from a 1972 box of corn flakes. Two pairs of brown leather flip flops, never seen more then 2 mph. Education is what your left with once you forget things! |
#6
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Re-TQ them headbolts. Re-measure, then go for a test-beat.
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The Following User Says Thank You to Half-Inch Stud For This Useful Post: | ||
#7
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If a head gasket is burnt through enough to make for a compression loss do you really think that retorting the bolts is going to solve that issue?
If it does then you have magic going on there! Let’s use some logic here. Here’s the symptoms of a bad head gasket between two cylinders. With only one plug in ( your choice) when you crank the motor for a compression test you are then asking one cylinder to pump up twice the volume of CID. So if one of your known good cylinders is at say 140, then a one cylinder trying to pump up itself and another is only going to make at best 70 PSi during a compression test.
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Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs! And he was not talking about 1/8 or 1/4 mile ETs! 1) 1940s 100% silver 4 cup tea server set. Two dry rotted 14 x 10 Micky Thompson slicks. 1) un-mailed in gift coupon from a 1972 box of corn flakes. Two pairs of brown leather flip flops, never seen more then 2 mph. Education is what your left with once you forget things! |
#8
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Thanks for the help. @steve25, I had all the plugs out. I have a set of 6X-4's I am planning on putting on. I was testing to decide whether I have to machine the block before starting with any upgrades.
@77trashcan, I'll probably do the oil trick next weekend.
__________________
69 Firebird Conv, 350 HO w/3 on the tree swapped to Tremec 5 Speed - Sold 72 GP, 400 Auto - Sold 09 G8 GXP M6 Orig Owner - Sold 76 Trans Am 455, 4 Speed - Current |
#9
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Dropping on a set of -4s if your motor is still running its stock 067 cam might make for a pin monster if you don’t stuff in a bigger cam.
__________________
Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs! And he was not talking about 1/8 or 1/4 mile ETs! 1) 1940s 100% silver 4 cup tea server set. Two dry rotted 14 x 10 Micky Thompson slicks. 1) un-mailed in gift coupon from a 1972 box of corn flakes. Two pairs of brown leather flip flops, never seen more then 2 mph. Education is what your left with once you forget things! |
#10
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Not keeping the stock cam. Still undecided though. Maybe an 068 with 1.65's or a 2802. Want it very streetable.
__________________
69 Firebird Conv, 350 HO w/3 on the tree swapped to Tremec 5 Speed - Sold 72 GP, 400 Auto - Sold 09 G8 GXP M6 Orig Owner - Sold 76 Trans Am 455, 4 Speed - Current |
#11
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Quote:
Otherwise you would be darn correct. |
The Following User Says Thank You to Half-Inch Stud For This Useful Post: | ||
#12
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Quote:
There was a time when GM spec was "worst cylinder at 75% of best cylinder, minimum 100 psi" or something similar...but that's more to avoid warranty claims than to actually have a properly-running engine. It's like saying a quart to 700 miles is "normal" oil consumption. I'm used to seeing ~150-ish on stock V-8 older engines. This is very altitude-dependent (higher altitude means lower compression pressure) as well as being dependent on the speed the engine cranks, camshaft, and a dozen other variables. Have you verified the gauge on your compression tester for accuracy/repeatability? Nobody seems to check their test equipment for accuracy. Quote:
Quote:
Compression pressure goes up because you added incompressible oil, not because the rings are actually sealing better. That's my experiences, anyway. Quote:
As said--a cylinder leakdown test is in order. |
The Following User Says Thank You to Schurkey For This Useful Post: | ||
#13
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Thanks Schurkey. It's a brand new tester, first time use. I haven't calibrated it.
I was also a little disappointed at the low psi numbers. Though, considering the stock heads are 124cc and the engine had a 7.6 cr when new, I wasn't too surprised. A leakdown will definitely answer more questions, but I'm thinking I should still clean up the block before putting the new heads on. Much better to do it now, then find out after doing the upgrades. Either way, the budget is going to get blown.
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69 Firebird Conv, 350 HO w/3 on the tree swapped to Tremec 5 Speed - Sold 72 GP, 400 Auto - Sold 09 G8 GXP M6 Orig Owner - Sold 76 Trans Am 455, 4 Speed - Current |
#14
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Quote:
Given the state of Chinese tool-making, I'd plug it into an air compressor and see if the tool pressure gauge matches the compressor regulator pressure gauge. Or test a couple of compression testers against each other and the regulator gauge. MOST (Not all) compression testers with a quick-coupler in the hose use an Industrial Interchange coupler and plug (Milton "M"). A union (or tee) with a coupler plug suitable for your air compressor hose, and an Industrial Interchange (Milton "M") coupler plug or two is all you need. Last edited by Schurkey; 05-30-2023 at 06:27 PM. |
#15
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Quote:
I also suggest a leak down test. |
#16
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Was the engine fully warmed up before you did the test? If not, warm it up and try it again.
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http://www.machdevelopment.com/album...775/527566.htm |
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