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Wondering what compression i should be looking for in a 400 with a 68cam. i will be taking cold readings as the carb has been sent off for rebuild. any info will be greatly appreciated.
 

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Discussion Starter #2
Readings

1-150
2-150
3-160
4-180
5-150
6-150
7-155
8-60

Looks like #8 is bad. 3and 4 slightly higher.
any comments or observation? need all the help I can get.:confused
 

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Wondering what compression i should be looking for in a 400 with a 68cam. i will be taking cold readings as the carb has been sent off for rebuild. any info will be greatly appreciated.
From the 1968 Service Manual.

Compression test needs to be done warm engine and all spark plugs removed. Spin engine over through 5 compression cycles before doing test.

8.2 - 9.2 compression engines - regular fuel. 150-170 PSI @ 155-165 RPM's
10.5 - 10.75 compression engines - premium fuel. 185-210 PSI @ 155-165 RPM's

Note: Low compression pressures on HO 400, HO 428, & Ram Air engines are not a valid indication of engine condition. Due to the long valve overlap period with camshafts used in these engines, compression readings at cranking speeds as low as 120 PSI are considered normal.

No cylinder should be less than 80% of the highest cylinder reading.

Number 8 does look bad.

:thumbsup:
 

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Thanks Jim, Just sent the Carb out to Cliff to be redone I guess i will be pulling the engine now, I hope its not the block.
 

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Thanks Jim, Just sent the Carb out to Cliff to be redone I guess i will be pulling the engine now, I hope its not the block.


Na, don't get so negative. Probably just worn out rings or even a bad valve. Good excuse to freshen up the engine and get it running like new again. Even IF you had a bad cylinder, they can be sleeved so it can be saved. Stay positive. :thumbsup:
 

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Thanks, I needed some positive news. having it refreshed is on the list, it just moved to the top.
 

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Thanks, I needed some positive news. having it refreshed is on the list, it just moved to the top.
You did not say how many miles on the engine or why you decided to do a compression check.

Was it burning oil or smoking out a tail pipe? Running poorly/missing?

You may want to also pull a valve cover to make sure the rockers have not loosened up on that bad cylinder. I am thinking if a valve is not opening fully, your readings may be affected. Once purchased a '68 GTO that the guy thought the engine was bad. It ran, but poorly. Pulled a valve cover and one of the rocker arms has loosened up and gone sideways off the valve. No damage, just put it back on and tightened it up and that car would catch rubber on a hard 1st to 2nd shift of the TH-400. Next time I saw him I told him about it and I could see the regret in his eyes 'cause he thought he was selling a car with a bad engine - cheap.

So, you may want to check a few things just to be sure, could even be a worn cam not opening the valves fully. Otherwise, you need to do the rebuild.
 

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She has been running good, getting second gear rubber on Saturday. Just had the time to do a test on her after i pulled the carb off. exhaust has been clean only smoke was from the tires.
 

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the milage is a bit of a mystery i would guess around 150,000. I will pull valve cover tomorrow and take a look and see what ti find. Have started to look for a good machine shop in Maine. I am actually looking forward to getting the running gear back up to spec. I will have to make my mid up on weather to put dished pistons in the engine so it will run on pump gas. Have you done this? How big of a reduction of power would there be?
 

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the milage is a bit of a mystery i would guess around 150,000. I will pull valve cover tomorrow and take a look and see what ti find. Have started to look for a good machine shop in Maine. I am actually looking forward to getting the running gear back up to spec. I will have to make my mid up on weather to put dished pistons in the engine so it will run on pump gas. Have you done this? How big of a reduction of power would there be?
I just did this, dished forged pistons and forged rods 30 over on the bore.
Compression ratio is 9.5-1 and the goat runs like a scalded ape on pump gas.
The old 335 HP was tired and now it is around 370 with the cam.
It will smoke the tire going into second with the TH-400 no problem in fact I really have to baby it so it doesn't chirp on the shift.
My experience with different engines is if you just do the top end it puts pressure on the old tired bottom end and you end up doing the whole engine anyway.
Go big or go home.
:grin2:
 

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I just did this, dished forged pistons and forged rods 30 over on the bore.
Compression ratio is 9.5-1 and the goat runs like a scalded ape on pump gas.
The old 335 HP was tired and now it is around 370 with the cam.
It will smoke the tire going into second with the TH-400 no problem in fact I really have to baby it so it doesn't chirp on the shift.
My experience with different engines is if you just do the top end it puts pressure on the old tired bottom end and you end up doing the whole engine anyway.
Go big or go home.
:grin2:
What cam did you use? I have a 68cam (ys 1968 code engine) with a th400 and 3.36 gears (ZG code rear end) . this is all stock. would I need to change the cam and gears? I would like to keep it as stock as possible, but still smoke the tires.
 

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You did not say how many miles on the engine or why you decided to do a compression check.

Was it burning oil or smoking out a tail pipe? Running poorly/missing?

You may want to also pull a valve cover to make sure the rockers have not loosened up on that bad cylinder. I am thinking if a valve is not opening fully, your readings may be affected. Once purchased a '68 GTO that the guy thought the engine was bad. It ran, but poorly. Pulled a valve cover and one of the rocker arms has loosened up and gone sideways off the valve. No damage, just put it back on and tightened it up and that car would catch rubber on a hard 1st to 2nd shift of the TH-400. Next time I saw him I told him about it and I could see the regret in his eyes 'cause he thought he was selling a car with a bad engine - cheap.

So, you may want to check a few things just to be sure, could even be a worn cam not opening the valves fully. Otherwise, you need to do the rebuild.
Hi JIM, just a quick update, I poured some mystery oil in the cylinder and let it sit over night blew it out and tested it. the pressure came up to 180psi. There must have been a stuck ring. thanks for all the advise.
 

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Hi JIM, just a quick update, I poured some mystery oil in the cylinder and let it sit over night blew it out and tested it. the pressure came up to 180psi. There must have been a stuck ring. thanks for all the advise.

My Opinion. 150,000 original miles on an engine of the 1960's is much. They are not today's engines that have better fuel management which in turn gives longer mileage. With a carb engine, most of the ring/cylinder wear comes from the carb during the warm-up/idle time. The choke is closed and the carb runs rich which washes the cylinder walls down of lubricating oil.

When you add a squirt or two of oil into a cylinder and then re-test the compression, the oil acts as a ring seal and the compression WILL come up - so it is an indicator that your rings are worn or cylinders worn. Read this: Compression Wet Test Procedure: Results Easily Explained

That said, your engine mileage & the oil test would indicate to me that the engine is in need of a rebuild. And, at that mileage, if the oil/filter was not changed regularly and the engine saw any hard action, I would not trust the rod bearings at this point. Rod bearing failure are one of the killers of a high mileage Pontiac engine. :yesnod:
 

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I still plan on the rebuild, it just made me feel better knowing the block wasn't the cause. still have to deal with the high compression engine vs pump gas problem, so a rebuild is in the near future. will be going with dished pistons hope to drop the compression down to 9.5 and go .30 over on the bore.
 

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I still plan on the rebuild, it just made me feel better knowing the block wasn't the cause. still have to deal with the high compression engine vs pump gas problem, so a rebuild is in the near future. will be going with dished pistons hope to drop the compression down to 9.5 and go .30 over on the bore.
OK, good. Didn't want you to think the engine just became "good to go" and then something happened.

Lowered compression is the way to go. I will also suggest, as will most evryone here, that you purchase a set of forged connecing rods to replace your original rods. They are stronger and by the time you rebuild the stock rods & add the better ARP rod bolts, for a few dollars more you get a much better rod that you will not have to worry about and can take the upper RPM's if you choose to build a nice stout engine that'll spin a few RPM's. Don't know how current these prices are, but $298 is not bad. http://butlerperformance.com/c-1234812-engine-components-internal-rods-5140-forged-replacement-rods.html

Plenty of info on the forum that should help in deciding on how you want to go with your engine with regards to performance level - stock, modified, or all out. :thumbsup:
 

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"...Don't know how current these prices are, but $298 is not bad..."


I think you can buy those rods directly from RPM for $288 shipped.

SAE 5140 STOCK ROD

But, at the time of this post their site is showing that they are out of stock.

They also sell H-beam rods which are lighter and stronger, for $400 shipped. They are showing to have some bushed H-beams, in stock--but are out of stock on the press-pin H-beams.

SAE 4340 STEEL H-BEAM

Eagle H-beams are a little higher.

http://www.cnc-motorsports.com/eagle-crs6625p3d-h-beam-connecting-rods.html

http://www.ebay.com/itm/Eagle-Specialty-Prod-Connecting-Rods-ESP-4340-H-Beam-Cap-Screw-Bushed-Pontiac-V8/391401903454?_trksid=p2047675.c100005.m1851&_trkparms=aid=222007&algo=SIC.MBE&ao=1&asc=20160617090737&meid=cf936d5f88084c36aba96b55af48c8e8&pid=100005&rk=4&rkt=6&sd=161863242926

Icon makes a nice dish piston for a 400.

https://www.rpmmachine.com/shop/index.php?l=product_detail&p=6218

http://www.cnc-motorsports.com/icon-ic891-030-forged-dish-pistons-4-150-bore.html
 

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Thanks for all the info guys. Time to do my home work and get a package put together. I am leaning toward a sturdy stock like set up as it is a driver and i don't race but still like to smoke the tires. Once I have settled on a package i will post it and have the updates on the progression.
 

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Thanks for all the info guys. Time to do my home work and get a package put together. I am leaning toward a sturdy stock like set up as it is a driver and i don't race but still like to smoke the tires. Once I have settled on a package i will post it and have the updates on the progression.

Sounds like a plan. You'll want to go with the forged rods & forged pistons. However, the Hypereutectic pistons are also a good alternative for a street build - it is better than a cast piston and not quite as rugged as a forged. I used these on my brother's 360 Mopar build knowing how he drives and uses the car - cruises with it with the occasional light up the tires. Mid price range as well. Like anything else, lot of opinions on hypereutectic pistons.

Keith Black offers these: https://www.summitracing.com/parts/uem-kb346-030/overview/make/pontiac These have a 17 cc dish which with stock 72cc heads/.040 head gaskets, puts your compression about 9.4 which is a good number to shoot for on pump gas.

Here is the Wallace Compression Calculator which can be of help to you in figuring out the numbers to get your compression between 9.0-9.5 for iron heads & 91 octane pump gas. Deck height on most Pontiacs is .0200 down in the bore. Compression Ratio Calculator - Wallace Racing
 
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