Q: 400 Rebuild Suggestions
i have 1968 firebird i restored last year. I’m get ready to rebuild my 400 and looking tricks and tips for a pontiac motor. can you help me
A: The following is some helpful information on properly building a street strip Pontiac 400. If built properly, the engine will make 450+ HP with commonly available parts.
Start by boring the block .060 to get max cubes (413), hone to a #625 Sunnen finish with .003 piston clearance and minimum deck it. Leave, or add the scallops at the intake valve side of the bore to unshroud the valve. Make sure the main bearing saddle alignment is + -.001 or better. A 2 bolt block is ok providing you use studs. Remove *all* the casting flash in the lifter valley and re-tap/deburr the whole block. Don’t forget to tap the oil galleries for threaded plugs, drill an .042 hole in the right rear plug for dist gear cooling and chamfer the oil filter block and oil pump holes nice and big. Take your rotary file and elongate the oil return hole in the front of the block down to the valley floor for immediate drainage to the pan, there are others along the sides between the lifters if you feel energetic. Do *not* scrimp and try to save money on the block! You get what you pay for so use a competent machine shop!
When cleaning your freshly-machined block for assembly use *hot* water and dish soap, a *steel* rifle-cleaning rod will work great for the oil passages, a 12 ga. shotgun brush for the lifter galleries and a 9mm for all others. After cleaning soak the block with WD-40, wipe the cylinders, you will see more blackness on you *paper towls*, clean again with soap and hot water using a pressure nozzle. If you have an air compressor blow dry the block and coat the cylinders immediately with oil.
Get an early 350 crank, have it magnafluxed for cracks, heat treated (case hardened), shot-peened and straightened. Grind it -.010 on the mains, offset grind the rod journals .015 (more stroke), you’ll need to go -.020 undersize to get the offset. Cross-drill the crank and lightly chamfer the rod oil-holes, chamfer the main oil-holes in a tear-drop shape in the direction of rotation, just a small chamfer will do. Micro polish it, since it is heat-treated it will polish nicely. Run .002 clearance on the mains.
The reason for using the 350 crank is that it is lighter and has thinner crank throws. You can also use a late 400 crank as it is a similar casting. Avoid very late 400 cranks as they have a different flange. The lighter crank will reduce your rotating weight and rev faster under load. If you anticipate super-high rpm, you may want to “knife-edge” the crank for even less weight and less resistance.
Before installing the crank clean it just as you did the block, they are covered with powdered metal after grinding.
***To use this crank you must also do the following:
This is an area that I will hold back on the tricks slightly. If you *must* know more, private e-mail will do.
You will have to reduce the weight of the *forged* 400 pistons, you can machine some off the back-side of the dome, or machine or drill holes in the pin boss area from the bottom. Another way is to use thin walled tool steel pins, they are fairly inexpensive. Use moly-filled rings *only* with the Sunnen #625 finish. Ring end-gap is .014 top and 018 2nd.
You *must* upgrade your rods to at least the ’73-’74 SD or preferably Carrillo or any other reputable racing rod as you will be revving this thing to 7000+ rpm easily. If you use factory rods you will have to remove most of the small-end pad to get it light enough to balance correctly with the 350 crank. Run .002-.0025 clearance. If you can afford it, use racing bearings. Grooved uppers on the crank. Torque rods by stretch to +.005 to .008.
Use a Milodon 455 H.O. pump and tack-weld the pickup in place. I suggest removing the cover-plate phillips srews, loctite them and use an impact-driver to re-install them.
The stock ’65-’73 Pontiac 4/5 windage tray will work fine, if you can find a full-length one use it.
Take a cut-off disk and cut 2 square openings in the end troughs and 1 long slot in the bottom-center trough of the tray. Note the position of the end-pairs of rods above the tray, cut your square end-slots exactly below them. The two end slots should measure about 2.5″ x 2.5″ and the center slot 2.5″ x approx. 8″. When you make your end cuts on each slot, continue 1/4″ past the side cuts, this will allow you to bend a flange downward along both sides. You can set the tray on the edge of something and tap the flange down with a hammer, the metal is very soft so it bends easily. Find a piece of course perforated or expanded metal, cut it to size and form the same radius as the tray. Tack weld the 3 pieces in between the flanges and on the ends.
This modification will allow oil escaping from the crank to be blown directly into the pan and keep oil from splashing up onto the spinning crank, its good for 10-15 hp.
Call Bob Cook at Competition Cams and go over your proposed setup with him, he is very experienced with Pontiacs and helps many a racer with the proper cam, etc. He is *realistic* so be prepared, he won’t let you over-cam your engine, no matter how nasty you want it. His # is; 800 999-0853.
Any big valve, early Pontiac head will work as long as it has 2.11″ int and 1.77″ ex valves and screwed-in rocker studs. Exceptional D-port heads would be 16, 12, 13, 62 and 48. The 62’s and 16’s should be fairly easy to find. These heads have 72cc chambers and should yield a 9.7-10:1 compression ratio, which will allow you to run a fairly radical cam effectively.
Heads, more than anything are an area that will determine how much hp your engine makes. If your cores are rusty, remove the freeze-plugs and have them acid-dipped. Start by installing new bronze guides and hardened exhaust valve seats for use with unleaded gas. Since the seats will have to be blended into the port, now is a good time to do some porting.
On a street engine do *not* fully port the heads! You want some turbulence in the port to keep the fuel/air mixture atomized, thus keeping your engine from loading up. The best street port-job that will wake-up your Pontiac is simply to open up the “bowl” area under the valve and blend back into the port-runner. Try and keep each port relatively close in volume, don’t get carried away removing material! Just blend the seat into the bowl/runner and polish. Use some “Dykem” machinist’s dye, or if not available use spray paint around the intake ports, install an intake gasket and snap the plastic locators in place. Scribe a line on the head where the ports are mis-matched and open them up with a *large-diameter* rotary file and blend 3/4″-1″ into the port. Leave the gasket on during this procedure (taped down and numbered) to insure a perfect match.
Minimum mill the heads if necessary and do a good multi-angle valve job. The spring umbrellas can be discarded, make sure you use spring dampners to reduce friction and heat build-up in them. If possible find a 1 piece intake valve for peace of mind, at present I’m not aware of anyone making a 1 piece exhaust valve for a Pontiac.
A good valve cover to use is a late baffled cover. You can spot them in the boneyard by the “8” dimpled spot welds on the surface. These covers have “fingers” that channel oil onto the rocker-balls. The next-best would be the bolt-on baffles that came on the 455 H.O. and the like. Poly-Locks would interfere with these however, you would have to use lock-nuts and hardened washers.
Do not use factory head gaskets, they are too thick and will add several cc’s to your chamber volume. Do use head studs if you can afford them, especially if you are planning to run nitrous.
Use a torker or preferably, a Doug Nash. I have a friend who has a couple of ’em, and a 750 Holley with what ever thickness carb-spacer your hood will allow. Third choice would be a gasket-matched early stock manifold with the #7 runner opened up to relieve the throttle bracket bolt-boss protruding into the port.
Run an MSD #8563 distributor, 6AL box #6420, Blaster coil #8202 and Soft Touch Rev Control #8738 (or 2 Step Rev Control #8739). Use the biggest plug-wires you can find and stock heat-range AC plugs gapped at .035. Experiment with the plugs in each hole while the heads are off to get the electrode pointing down toward the piston on as many as possible. (or you can mark the plugs for later installation).
Timing curves vary with each application, a general rule is to keep it at 34-38 degrees total and don’t get wild on the street regardless of what you’ve read in the magazines. Getting it “all in” by 1800 rpm will only rattle and ping. Keep that figure at around 4000 rpm.
If you can find a decent set of 4-tube headers that aren’t a nightmare to install and maintain use them. I recommend 3-tube “Tri-Y” headers because of their ease of installation and room, they will also make lots of torque at a very low rpm. Always use a cross-over tube on the way back to the mufflers and slightly smaller tube at the muffler exit.
FLYWHEEL and BALANCING:
If you run a 4-speed use a “neutral” aluminum flywheel and internally balance the engine. That will give you a softer launch without a lot of tire spin and will rev quicker.
The stock Pontiac dampner works fine, it is a waste of money to replace it, just make *sure* you torque it to 160 ft. lbs.
It is better to build a “hot” engine and add mild nitrous, than to build a mediocre engine and cram it with the “happy juice” trying to make power. The result will surely be a pile of scrap iron, empty wallet and a severely bruised ego.
Well, these are the basics as I see it that will give you an 11 second car provided you have the chassis for it. If you run slicks, the right gears (4.33-4.88) and a race-prepared TH350 and have the Chassis extremely dialed in, I would put $$$ on a 10 sec run. 🙂
All this costs plenty of $$$, but take your time and shop prices, it will be worth it. This info is based on my many years of building Pontiac engines.
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Content last modified: January 16, 2014 at 7:05 am