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Discussion Starter #1
Howdy all, I have been trolling this site for a while and have had a ton of help and even more questions. I am totally a novice when it comes to wrenching and I have pulled and gapped plugs, installed a distributor, fuel pump, water pump, tuned a carb (man this sucked) and am now getting ready to replace the oil pan in my '70 GTO all thanks to your guys' help.

I found out that the engine in my GTO is a '74 pontiac 400. My question is what is the difference between these 400s and the ones from earlier in pontiac's youth? I know that mine is a 2 bolt main and not a 4 and that the heads are completely different than anything 70 and prior resulting in lower power. All of the information I have gathered searching the darkest depths of the internet lead me to believe that, other than the heads and the fact that it's a 2 bolt main, there's nothing different in the guts of the block. Is this true or am I way off?

Obviously it must sound crazy to many of you that I don't know all of this information. I purchased my GTO in '02 while in high school and enlisted in the Marine Corps. the next year and had my uncle totally restore it for me while I was gone for 4 years and when I came back I really lost interest in all cars, but now that I purchased my first home it's back and in full effect.

I plan on posting some pics ASAP but the weather hasn't been working with me here in Indy!

Thanks for any information and I'll be posting any problems I have dropping my oil pan this weeknend.
 

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These are some "involved" questions. Basicly your info. is correct not much different, basic engine wise. Pontiac has had many design changes, on alot of different parts over there production years. First the 2 bolt 4 bolt thing... most 400's are not 4 bolts some are drilled for 4 bolts and still have 2 bolt caps. The heads on later models have larger combustion chamber sizes, resulting in lower compression ratios. The majority of changes are brackets, pulleys, and cable styles...that is where it gets real interesting. Pretty much from 1965-1981 you can interchange parts as long as you use the corresponding brackets, pulleys and cable styles. It can be very confusing and interesting to discover what works and what don't. Sorry if I confused you, it was not my intension. Stick with it you will learn.
 

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I found out that the engine in my GTO is a '74 pontiac 400. My question is what is the difference between these 400s and the ones from earlier in pontiac's youth? I know that mine is a 2 bolt main and not a 4 and that the heads are completely different than anything 70 and prior resulting in lower power. All of the information I have gathered searching the darkest depths of the internet lead me to believe that, other than the heads and the fact that it's a 2 bolt main, there's nothing different in the guts of the block. Is this true or am I way off?
Don't be afraid of a 2-bolt main engine. Pontiacs aren't ::cough spit:: chevys (thank goodness!) so the shortcomings and failings of the latter don't apply to the former. For example, Pontiac cranks are shorter in length and therefore more rigid and stronger, even the cast ones. I'm running a cast crank in my 461 (stroked 400) and it's going to be plenty strong for the horsepower level this motor's likely to ever see. I'm at 495 now and would need different heads and more cam to push it higher.

More important than the number of main bolts is the specific block casting number (6 or so digits on the passenger side rear of the block) - because different ones have differing amounts of metal thickness in the main webs - and that's where the strength comes from. It's also the reason that the 326-350-389-400 family blocks are generally stronger than the 421-428-455 family blocks - the 3" main journals result in more "meat" in this area than with the 3.5" main journals. When a heavily "leaned on" 455 block fails, it usually cracks up through one of the main bearing saddles. Depending on whose opinion you trust, a "good" 2-bolt small journal block with a quality cast crank is quite viable up to around 600-650 "or so" horespower.

Cylinder heads --- with the notable exception of the 1967 670's, most later model Pontiac combustion chambers are all very similar in shape. Where you're going to find most of the differences are in the chamber volume, intake/exhaust valve diameter, and screw-in vs. pressed in rocker studs. For performance applications you're going to want the 'big" valves i.e. 2.11" intakes and 1.77" exhausts. For strength you want screw-in rocker studs, however the factory screw-in "bottle neck" studs aren't that great either, especially with rowdy cams and heavier springs because they tend to break at the neck. The hot tip is to bite the bullet and have the heads machined to accept the larger/meatier 7/16" studs anyway, so it doesn't matter much what the head started out with --- however 'generally' the larger valve heads also usually had screw-in studs. Combustion chamber volume is one of the key determining factors for static compression ratio - so that's going to matter based on cubic inch displacement, cam profile, intended use, fuel type, etc. So first you've got to make all those decisions, then see what compression ratio you need to be at to meet those goals, then pick the combinatoin of chamber volume, piston, deck height, and head gasket that gets you there. I guess what I'm trying to say is that just because you may have small valve, pressed-in stud heads now -- that doesn't mean they're "bad". Depending on where you want to "get to" with the motor they might be ok with some minor modifications. Aside: always measure ("cc") the combustion chambers on any heads you're planning to use because Pontiac heads are known to vary quite a bit from the published factory specs. for example, the spec for #62 heads says they have 72cc chambers, but actual measurements for a specific head can vary from that by several cc's, and that makes a significant difference in compression ratio.)

Generally the right way to do it is to design your engine set up based on your goals, intended use, and budget --- then start putting together the combination that gets you there. I really enjoy doing that :party:

It helps to have a good engine modeling software package so you can "try" various combinations before you start spending money for parts, too.

Bear
 

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Bear you are dead on, although 1974 4X or 4C heads are quite possibly already drilled for screw in studs. Just need to replace the bottle necks as you stated
 

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Discussion Starter #5
As always I thank you guys for sharing your knowledge and expertise with me. Now that I have a place I can wrench all hours of the night I plan on really having some fun and have been putting stupid hours at work for some extra cash.

Thanks all and I'll be back with more questions I guarantee it!
 

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Discussion Starter #6
I'm not trying to head to the dragstrip or beat an '11 camaro by 4 door lengths. The only mods I currently have are hookers headers, holley street avenger carb (out of the box my ass), edelbrock intake, HEI ignition, full exhaust system and a new filter. I really don't think I want to do anything to it as the tires will chirp to my liking and it's been very dependable with some minor exceptions. In all honesty I kind of like when I get problems because I then have to spend my weekends drinking beer and wrenching on her and when I run into problems it's straight to GTO forums.
 

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Also Bear, the screw in studs are already 7/16 thread in the heads, its just the tops of the stock studs that are 3/8. You can install BBC screw in studs and poly locks just by unscrewing the stock ones and replacing them, no machining needed. As a general rule, though not always true, if it had a 4 barrel carb it will most often have screw in studs. Some two barrel 350 heads, like 46, have screw in studs too, but the 46 has small valves. Other than that, good info bro.

Just by swapping heads around you can go from 7:1 compression to 13:1 with flat top pistons on a 455, about 11.5:1 is about as high as you can go with milled small chamber heads on a 400, sometimes higher. Unless you are planning on running race gas or E85, stay under 9:1 so you can run the cheap gas. Most D ports flow essentially the same before porting, and after porting the casting number only matters for chamber size.

I am a big fan of the Qjet, next up is Holley in either 3310 or 4150, I am not a fan of the Avenger carbs, and have absolutely no affection for the Edelbrock carb. The Qjet takes some time to learn how to set up, but you will make the best power with great drivability, and use less fuel than with Holley and especially Edelbrock carbs.

Stay away from single plane intakes on a mild 400 like this, it wont help any and will load up while sitting at stop lights. Without ported heads there isnt much reason to use anything but a stock intake, or the Performer. With some ported heads you will see a bit of improvement with the RPM, but not always. Still the RPM is a good intake and really it doesnt hurt to use it.
 

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Discussion Starter #8
I had the original intake and carb and tossed it years ago. It was originally just a 2 barrel. I do like my carb now that I understand how to tune it and I luckily purchased a dual plane intake all those years ago. I have some problems to definitely work out this winter. I really need to fix the oil pan as it's leaking and the tranny just started leaking pretty bad in the past few days. I am hoping I can get to both of them and fix it without having to putt the engine but we'll see.
 
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