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Discussion Starter #1
I have a 69 GTO with a 400 eng & auto trans
the engine is stock except for headers.
the guy i bought it from said that he had the engine rebuilt about a year ago with stock parts it was bored 30 over with new rings & bearings and a valve job done.
thats about it.
the numbers that i have found on the engine are as follows : exhaust port #'s on heads are 62 also the letters G DN on heads.
#'s on the front of the block are : 04 72 964 X H
and on the passenger side are : A 149 6
Is this a 69 400 Engine for sure ? and my next question is after a lot of research about camshafts it looks like the Comp cam XE 268H is my best choice and if so will it work great with my stock springs ,auto trans & stock converter.
the car does not have power brakes.
I plan on changing the intake when the cam is changed.
thanks for any help,ideas and comments.
 

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Since it's a 69, if that's the original engine there'll be a partial vin on the front of the block down low on the passenger side.
Here:


Also, check the block date code and block casting code:


If you have a 69 block (the date code above will tell us for sure), that XH on the front indicates it's out of a Grand Prix, but otherwise will be equipped the same as the GTO engine (which would have been coded YS).

You already have the best available intake for a moderate street Pontiac. You can save weight with an aluminum manifold, but you'll give up power to do it.

The XE268 is a decent cam in a 400, but you can't use stock springs with it. You'll also have to replace the factory rocker balls/nuts with fully adjustable pieces (poly locks), and if it were me while I was at it, I'd replace the factory bottleneck rocker studs with stronger 7/16" ones. Factory studs have been known to break at the bottleneck, especially with "stronger than stock" valve train components.

Since the heads are going to have to come off anyway (to have the spring seats cut for taller valve springs), be sure and cc the chambers and check piston deck clearance. Assuming all the dimensions are "nominal", that motor's going to be just shy of 9.6:1 compression which (in my opinion) is a bit much for 93 octane fuel and iron heads. Since you've got manual brakes and don't have to worry about vacuum to run the booster, I'd be tempted to go with a little more cam or maybe a wider LSA (or both) to try to manage cylinder pressure and stay away from detonation. You're going to want to make sure you cooling system is top notch, and double sure it never runs lean.

Bear
 

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Also, the XE268 grind cam is designed to build a lot of cylinder pressure early....which is why it works so well in nine to one engines. Not a cam to use in a 10 to one engine at all, IMO. You'd do better with a 068 grind Pontiac cam, which will bleed off a little initial cylinder pressure, not add it. Cylinder pressure+ high compression= detonation. How does it run now? Usually, a sharp tune on a factory set up will outperform a mish-mash of aftermarket parts that are not in in tune with each other. The stock intake, if a Quadrajet, is superior to aftermarket manifolds, for instance. So is the Quadrajet carb. I think a sharp tune: carb dialed in, distributor advance curve calibrated, the right torque converter, and the right gears will let you car scream. I knew a guy who kept changing cams until I showed him that the choke block- out linkage on his carb was keeping his carb's rear barrels from opening at all. It's the little stuff, all "just right" that adds up to big performance.
 

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:agree Some things to keep in mind with regard to compression ratio. What you really care about is dynamic cylinder pressure. Cam choice (intake duration, lobe separation angle) as well as how it's installed (straight up, advanced, retarded) has an effect on how much actual pressure is produced in the cylinder. Thing is, it's not an exact science because so many things have an effect: condition of your cooling system, actual shape of the combustion chamber including the piston top and edges (pointy edges tend to produced hot spots that tend to cause detonation), fuel mixture (lean is bad), cylinder head material, chamber "quench" area, and ignition timing - just to name a few. There are some "calculators" out there on various web sites that will help you approximate dynamic cylinder pressure if you know the details of the engine, cam profile, and how the cam is installed (installed retarded = later intake closing event = less cylinder pressure), but that's really all they're good for - approximations.

By closely controlling all those things, there are people who can and do successfully "push the limits" on compression and get away with it. However, unless you're building a race engine where every single HP matters, my opinion is that it just "ain't worth it". My 461 at 9.9:1 compression (with aluminum heads, so it's actually a little "low") is making probably 530 hp "or so". If I were to push that right to the ragged edge for aluminum, say around 10.5:1 - I'd only pick up about 8HP. On a street engine, to me, it just isn't worth walking the razor's edge all the time just for 8HP, especially since the car like it sits already runs high 11's.

Everyone gets to make their own choices though :D

Bear
 
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