400 engine, weird pistons, strange object attached to cam! - Pontiac GTO Forum
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post #1 of 9 (permalink) Old 08-08-2018, 08:13 PM Thread Starter
 
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400 engine, weird pistons, strange object attached to cam!

Recently bought a 1970 lemans completely disassembled. Car was in very rough shape but came with a 400 and a turbo 400. Apparently I was at least the 4th owner of this forgotten project. Clearly hadn't been on the road in at least 25 years. The body ended up being scrapped but yielded up some parts. The 400 was dressed with an hei distributor, headers, Mickey Thompson valve covers, edelbrock p4b. Ripped it apart today and found the inside of the engine spotless. My only questions was about this weird thing on the front of the cam, timing oiler maybe? It's very off centerAnd also the pistons have I machined divet in the center of the valve reliefs. Never seen this before. Pistons are machined aluminum. The block is a 69' 9790071, letters XH on front of block. Heads are 62's December of 68. Seems to measure stock 4.12 bore, stock 3.75 stroke. Anyway information about the pistons or cup shaped object on the front on the cam would be greatly appreciated.
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post #2 of 9 (permalink) Old 08-08-2018, 08:54 PM
 
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Divet in piston is just the piston's design - looks like a factory cast piston.

Round cup is the fuel pump eccentric. The arm on the fuel pump rides on top and as you can see, it is offset like a cam, so it causes the fuel pump arm to move up and down thus pumping the fuel.
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post #3 of 9 (permalink) Old 08-08-2018, 11:18 PM Thread Starter
 
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Thanks for the info. Was hoping someone could identify the piston, wanted to be definite on the compression ratio seems to be a lot of variance in the supposed compression ratio with a 62 head, I've heard as high as 10.6:1, I'm coming up with 9.6 :1. If my info is correct. Still need to cc the heads and take the finer measurements to verify these "internet" numbers I used.
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post #4 of 9 (permalink) Old 08-09-2018, 07:31 AM
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Originally Posted by 1970PA4lifelemans View Post
... 62 head, I've heard as high as 10.6:1, I'm coming up with 9.6 :1. If my info is correct. Still need to cc the heads and take the finer measurements to verify these "internet" numbers I used.
Which is why you always take specific measurements yourself. the chamber volumes on Factory heads are known to have varied significantly, plus you have no way of knowing if work may have been done previously on the engine that could have changed everything, such as having the heads cut. For what it's worth, those look like factory pistons. They will have about 6 cc's of clearance volume in the dimples for the valves. To compute your compression ratio in addition to that value you need to know how far down in the bore the pistons are at top dead center so that you can calculate that part of the volume. You need to know the diameter of the opening in your head gaskets, you need to know the compressed thickness of your head gaskets, and you need to know the actual chamber volume in your heads. Once you have all those volumes calculating compression ratio is simply a matter of dividing the maximum volume (total volume when the piston is at bottom dead center) by the minimum volume (total volume at top dead center). And in case you didn't know, that 10.75 to 1 compression ratio that you will find quoted in factory literature for these heads was for blueprinted heads. That means heads that have been cut down so that their volumes match the engineering specs for the engine, not the usual volume for the heads as cast from the factory. This was a game that all manufacturers played back at that time to be competitive in NHRA Stock class racing. The tolerances and clearances published for the "factory blueprint" would be much different from what was actually being produced in production engines. You may have heard the expression of having an engine blueprinted. That's what the term actually means. Re-machining the engine so that all the specs and clearances match the factory blueprint and not what was actually being rolled out of the factory. Blueprinted engines made more power because of that but they were also less suitable for street use. It was a game that all manufacturers played in order to make their blueprinted engines legal for NHRA stock competition.

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post #5 of 9 (permalink) Old 08-09-2018, 09:29 AM Thread Starter
 
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In your opinion would measuring one combustion chamber per head be sufficient or best to check all of them?
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post #6 of 9 (permalink) Old 08-09-2018, 09:58 AM
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In your opinion would measuring one combustion chamber per head be sufficient or best to check all of them?
My opinion? More information is always better than less information Unless something really unusual has been done to, or is going on with your heads it's not likely that they'll vary enough to make a difference. For me though, knowing for certain what I'm dealing with gives me peace of mind.

What could happen? Well, if your 62's have been run hard on heavy loads on unleaded fuel for a long time, there's a possibility of exhaust valve seat recession. If the heads of one or more valves have receeded into the head, that will change the volume enough to matter. You'll want to address that problem before using the heads. How will you know if you don't check?

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post #7 of 9 (permalink) Old 08-09-2018, 08:09 PM
 
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If you are just looking to get a general idea, then 1 chamber should be close enough. What is the point of checking them all? Make sure you get one chamber clean of carbon. Then see how many cc's you have. As Bear pointed out, there can be variables that can make each chamber different, but there is really no need to do all the chambers unless you were trying to equalize them - which you would do at the time of a head rebuild, not with the "old valves" or unknown condition of the valves & seats.

If you take a look at the online Wallace compression calculator, you will see all the values you need to determine actual (static) compression of your present engine. But once you change any of these variables, you may be changing compression. You probably have about 10-to-1 compression.

What is it that you are looking to do or accomplish?
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post #8 of 9 (permalink) Old 08-10-2018, 11:36 AM Thread Starter
 
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The block and heads judging by the paint weren't originally assembled together. This thing had at least a minor overhaul. Cam and lifters look like New. There's no carbon on anything. Cam bearings are spotless. I'm trying to verify whomever assembled this motor doesn't have anything's messed up like overly high compression. Also this engine came from a 70 lemans and the heads definitely belong to a 69 gto. Block vin doesn't match my 70 lemans title. Dated a year to early also
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post #9 of 9 (permalink) Old 08-10-2018, 05:42 PM
 
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The block and heads judging by the paint weren't originally assembled together. This thing had at least a minor overhaul. Cam and lifters look like New. There's no carbon on anything. Cam bearings are spotless. I'm trying to verify whomever assembled this motor doesn't have anything's messed up like overly high compression. Also this engine came from a 70 lemans and the heads definitely belong to a 69 gto. Block vin doesn't match my 70 lemans title. Dated a year to early also
The compression on Pontiac engines is adjusted by the chamber cc's. So flat top pistons will be the same from shortblock to shortblock. Anything over about 9.0 compression with iron heads is "overly high" in my book if you plan on using pump gas. You can get way with a little more if using 93 octane. After about 9.5 compression you will need high octane racing gas or a mix and/or an octane booster.

This ALSO depends on your altitude. If you live at a higher altitude, the air is less dense and that's where you would then need the higher compression to compensate for the less dense air.
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