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Engine Help ...455

13K views 46 replies 7 participants last post by  BearGFR 
#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?
 
#7 ·
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...
 
#8 ·
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
 
#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.
 
#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.
 
#12 ·
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
 
#14 · (Edited)
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.
 
#17 ·
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
 
#22 ·
Awesome. Those are some of the better D-port heads. They have screw-in rocker studs and the larger valves (2.11 intake, 1.77 exhaust).
Factory nominal chamber size is 94 cc's, but don't take that to the bank. We don't know if they've ever been millled/machined, and also Pontiac heads are known to sometimes vary quite a bit from the factory spec. There are other details about the engine we'd have to know in order to calculate the true compression ratio and the only way to get those measurements would be to tear it down and start measuring things.
However, if we make some ASSUMPTIONS about the engine, we can probably get reasonably close.
If we assume:
* It's a standard bore 455 (never been bored) - bore size:4.150
* It's not running a stroker crank. stroke length: 4.210
* It has 'nominal' deck clearance (pistons are .020 "down the hole")
* It's running standard head gaskets (4.160 bore, .042 compressed thickness)
* It has flat top pistons with 6 cc's of clearance in the valve reliefs
* The heads are 'factory nominal' with 94 cc chambers

If we make those assumptions, then the engine has 9.20:1 static compression ratio.
That's just about perfect for an iron-headed street engine that can run and stay healthy on 93 octane pump gas.

Wish we knew the specs on the cam...

That 600 cfm Edebrock carb is a little small though, especially if the motor has any "cam" in it at all. It'd be a lot happier with 750+.

Sounds to me like you've got a very good start there on a good street engine, and whoever built it at least knew a little bit about what they were doing.

Just so you can see how small changes can affect things. If we change just two assumptions, namely if
1) The block has been zero-decked (pistons are .000 "down the hole)
2) The block has been bored .030 oversize so that bore=4.180

That bumps the compression ratio up to almost 9.7:1 (9.660) which (again in my opinion) is starting to get into the danger zone for detonation on pump gas with iron heads.

Bear
 
#23 ·
Awesome. I hope that is all correct. I feel better about the engine now. I do want to change to a more aggressive cam, which a 9.2:1 CR will allow.
one of my plans involves taking the engine out, and apart, cleaning it all up, repainting, and reinstalling. We can take more measurements or do upgrades at that time.

Thanks Bear for helping...I may have more questions for you later.
 
#25 ·
There's a whole lot that goes into a cam choice. You've got to think about how you plan to drive the car, what RPM range it's going to "live in" most of the time. What transmission and rear gear are you running, do you have vacuum operated accessories like power brakes and/or factory a/c, etc.

For a "mild" to moderate street engine build using mostly what you already have, I'd say power in the 400-450 range (at the flywheel) is easily attainable at reasonable cost while retaining good manners. If your pockets are deeper and your tolerance for maintenance needs and more "attitude" is higher, and you're willing to sacrifice some street manners, then you can get a lot more. Like A.J. Foyt once said, "Speed is just a question of money. How fast do you want to go?" :D

Bear
 
#26 ·
im not a racer, but i love to go fast on occasion. it may sound silly but i just want a good sounding lope. I love having a car that is a true muscle / street machine. I need low end performance since I will do almost always street driving. I do want to dyno the car one day and would love to be in the 400-450 range.
Power Brakes: Yes
A/C : Yes, after market.
Power Steering: Yes
Am i going to have to change all the lifters with the cam??
 
#30 ·
What kind of transmission, and what's your rear axle ratio?

Keeping your power brakes working means you need at least 15" of vacuum at idle, and that's a minimum. Or - you'll have to either add an auxiliary vacuum pump (noisy) or convert to a hydroboost brake system (expensive).

Keep in mind a 455 is NOT a high rpm engine. 5500 is going to be your upper limit - in fact with factory rods I'd recommend keeping it at 5000 or below. Going to a cam that makes peak power above that would be dangerous and a waste of money, unless you also install a GOOD set of forged H-beam rods but even then, if you're realistic how often is the engine going to see that kind of rpm with the driving you'll be doing? Probably not often - so you'd be sacrificing torque down low, where you'll be spending most of your time, to gain torque up high, where you rarely go.... doesn't sound like a wise move to me. :D

All a cam change really does is move the point where the engine's peak volumetric efficiency occurs to a different rpm range - higher or lower depending on the cam. Remember HP = (torque * rpm) / 5252, so if you move the torque peak to a higher rpm, you get more power --- but you don't get any more torque. But since there's no such thing as a free lunch, if you make the motor more efficient at a higher rpm, it's going to be LESS efficient at a lower rpm (less torque). That lopey, rumpity rump idle everyone associates with a hot motor? What's really going on there is the darn thing is so inefficient at low rpm it can barely stay running, much less make any power down there.

Here's a great video that illustrates what I'm talking about. It's a little contest between a sport bike that makes 180 or so HP way up at 9000 rpm and an old Harley that barely makes half that much horsepower, but has some torque way down low.

http://www.liveleak.com/ll_embed?f=4a88a935f5f0


Bear
 
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