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Discussion Starter #1
I am trying to determine the compression ratio on my '69 GTO. It has a '74 trans am 455 engine. I saw 6X casted on the side of the heads.
Are 6X heads standard on the '74 455 or are they after market?
How do you measure the stroke?
 

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I like the wallaceracing.com website, lots of good pontiac info on coding heads and engine blocks. Can give you an idea of the various chamber volumes, etc. Without knowing "exaclty" what you have for crank, pistons, and chamber volumes, you can only guess on Compression ratio, unless you used a "Whistler" test or something similar. You would likely need to find a performance shop that would have the right equipment or a Whistler. We always had one at the local circle track tech barn for checking CR.

There may be other ways to find CR that I am missing...
 

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You do have to have some very accurate measurements to get an accurate CR (or access to some very high dollar test equipment).

Let me ask you, why are you looking for the information --- I mean, how does that figure into your plans?

It'd be good for you to visit some of the various sites that have information on how to decode all the various markings on Pontiac engines (Google "pontiac engine codes" for starters).
I recommend you start by positively identifying exactly what you've got there, then working from that point. By that I mean making sure it's a 455, what model year it is, what haeds are on it, etc.

We can help if you get stumped but it's more fun to figure it out on your own.

Bear
 

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Discussion Starter #9
im trying to just find out some information about my car. when i bought the car 7 months ago, i found th code on the engine. it matches a 74 trans am 455. The heads dont seem to be stock to the engine based on the numbers (D185 is not off a 74). So what could have gone into this thing? with thos heads, what kind of horsepower should I be getting? better than the 250 it advertises??? I love a mean sounding muscle car and would like to put a street/strip cam in it, but i need to know the CR.
I am slowly upgrading parts on it. Getting a HEI distributor and coil next month, getting rid of the points. Adding A/C..too damn hot in south texas...need some body work for rust issues. minor cosmetics interior (mainly just dash, rear tray, and headliner.
Its a great car. I just wanna be able to answer questions from peopl and myself about what kind of power it really has.
 

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Discussion Starter #11
I dont have it off hand, but when I found the block code/vin, i was able to find engine codes for pontiacs and it matched a '74 Trans Am 455. I was hoping for a SD, but it didnt say that. I will post the code on here when I find it again, I have it written down somewhere.
 

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For a positive id you really have to use all 3 pieces of information off the block, not just the 2 character code on the front. This is because Pontiac in some cases used the same front code in different years to mean different things.

Bear
 

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The second digit of the vin is the year. In this case, '72........

I believe the cast(block) code is "4"85428? 455 block.

Engine displacement was also cast in the side of the block.

Date code/block casting number.

2 character engine id next to the head.
 

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Ok... now we've got something.
C012 says the block was cast March 01, 1972 -- making it year model 1972.
485428 pegs it as a 455
YH for 1972 marks it as having originally been installed in a B-body (big car) with a TH-400 transmission, 2 barrel carburetor. (RPO code L-75) rated at 200 HP. It would have originally had either 7D4 (early production, no divider on center exhaust ports) heads or 7L4 heads. Both had smaller valves (1.96 intake, 1.77 exhaust) and pressed in rocker studs.

Don't despair though. It's still a 455, and with the different heads and (I assume) different carburetor that's on it now, it's making more power than that. Plus it most definitely has got some serious potential.

Next step is to get a positive ID on the heads. You've already said 6X, but there are two varieties of 6X: 6X-4 and 6X-8. Here's a photo of some 6X-4's


See that "4" down on the far right hand exhaust port? That's one of the ways you tell. Some of them had either a 4 and an 8 in smaller size on the "side" of the head sort of above the spark plugs. Knowing if they're -4's or -8's matters because the chamber sizes were different, making compression ratio different.

Bear
 

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Hmm.. never heard of a 6X-6.. check for smaller numbers on the side of the head, like on that small square flat face on that raised part between the spark plug hole and the head bolt hole on the left side of that photo I posted.

Here's another photo showing the location.


..and another link
http://www.pontiacstreetperformance.com/psp/6XheadID.html

Bear
 
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