[Ragtop Man]

Which AFB are you running ... is it a vintage version? Carb seems very under-rated for a street build IMHO, having suffered old used Holleys for many years on my Fords.

[GT182]

Weber made, manual choke AFB Comp Series. No complaints so far, runs great. Builder took it out to test it after he put the motor in and couldn't believe it ran as good as it does. Says there's more than 325hp coming from it. I don't race it so that's not important. Just burning Regular gas and getting to where I want to go and back is all that matters to me.

[b-man]

Originally Posted by Dick Boneske

Neither one of those tables lists cc's correctly. The '64 GTO heads were specified to cc at 72. The actual chamber size on '64 GTO's from the factory was 74-76 cc. The 421 HO heads were identical in valve size and casting number, but were bigger chambers to achieve 10.75:1 compression. None of the '64 421HO or GTO heads were 68 cc stock.

I'm in the middle of my 421 HO build and have had the heads cc'd.

Chamber ccs of the 9770716 421 HO heads before starting any work, with the original valves popped back in. The factory lists the spec for these chambers at 72ccs. Pretty close in size with only a .6cc volume variance, attesting to the excellent cylinder to cylinder compression ratio control provided by the fully machined chambers that were a quality feature built into all Pontiac V8s:

1. 74.2
3. 74.0
5. 73.8
7. 73.8

2. 74.0
4. 74.0
6. 73.6
8. 73.8



So rest assured both the 389 and 421 chamber sizes were identical. Heads that were factory spec'd as being 72ccs coming out to near 74ccs actual is to be expected, factory heads from no matter what year quite often measure larger than the factory numbers on chamber volume.

The compression ratio difference was made up using much larger and deeper valve reliefs in the 421 pistons, much like the 428 used a 12cc dished piston compared to the 400 with 6cc valve reliefs.

421 piston is shown on the left and 389 on the right, both examples are 1964 factory original standard-bore pistons. I haven't measured both pistons, but it's pretty easy to see a considerable amount of added volume provided by the 421 pistons. The 421 pistons having 10ccs compared to the 389 pistons having 4ccs wouldn't be too far-feteched.

[Brad_bb]

Nice b-man, Are you planning to reuse the pistons or?

[b-man]

Originally Posted by Brad_bb

Nice b-man, Are you planning to reuse the pistons or?

Both the 421 and 389 pistons were damaged by hammering them out of rusty bores, so enough of the pistons were damaged that each set became essentially unusable. With that said, using old pistons really can't be done that often anyway. The amount of bore wear on most of these old injuns makes new pistons pretty much mandatory. I photographed two of the pistons that didn't have hammered-on tops to show the valve reliefs here.

In keeping with the theme of this thread here is the best way to set up your 389 or 421 to run on pump gas, a set of dished pistons. The one pictured below is for my 421, custom-ordered with a 26cc dish to drop compression all the way down to 9.05:1.



Another forged dished 421 piston, from RaceTec with a 23cc dish. The valve reliefs on this piston will allow the use of both early 20-degree valve angle 389/421 1966 and earlier heads (1.92"/1.66" valves) as well as 1967 and later 14-degree heads (2.11"/1.77" valves).

[Ron Landis]

Originally Posted by Ragtop Man

Be sure to cc the heads you plan to use before you get out your wallet.

Pontiac published CR's that were higher than delivered, sometimes substantially to give sportsman NHRA teams a little more leeway at tech inspection. The chambers are typically a skosh bigger than listed.

The longer the exhaust valve cycle and the later the exhaust valve closing point, the more static cr an engine can tolerate on the same gas. But, that pushes the torque curve up higher in the rpm range, and can negate the best qualities of a Pontiac V-8.

A dished piston is definitely the best way to go, there are some different ways to achieve it, but deepening the relief under the valves is the best way to go; keep the deck clearance as tight as you can without banging the piston into the head.

Rule of thumb with a traditional Pontiac and bathtub chamber is to just move the decimal point on the static cr to find your ideal R+M/2 octane with typical ignition advance... 8.7 = 87 Octane, 10.0 = 100 octane, etc. As noted below, the lower heat retention of an aluminum head will raise the octane tolerance, too, by how much is open for discussion depending on cam timing.

Don't worry if you are a touch light on the static CR; it will let you put a little more spark lead into the engine. Also, be sure your carb tune is right on the money; if it is running too lean, or not calibrated for the transitions, it will rattle like a poltergeist.

and to clarify...the zero deck helps flame propagation which in turn makes for a more complete burn and reduces detonation. As far as aluminum heads...as a rule of thumb they afford you about one full point in compression as far as pump gas goes.

[Brad_bb]

Guys, this thread is a little old now. My engine is finished. I D-dished my pistons similar to what you show. I had Butler's do it. They ran the engine in on the stand. I picked it up a year ago and am just now about to paint and detail it properly.

[b-man]

Originally Posted by Brad_bb

Guys, this thread is a little old now. My engine is finished. I D-dished my pistons similar to what you show. I had Butler's do it. They ran the engine in on the stand. I picked it up a year ago and am just now about to paint and detail it properly.

I don't usually add replies to thread that are this old, however this thread is a good reference for others who are looking to accomplish the same thing.

One of the biggest mistakes I see are folks in the old car hobby getting their older high-compression engines 'rebuilt' without any thought as to how they're going to manage today's poor-quality pump fuel. The high cost and difficulty of finding adequate high-octane fuel leaves many of these cars stranded in the garage and the owners disillusioned, losing interest because the car just isn't user-friendly anymore.

Hopefully threads like this can help inform others about the best ways to get that engine into the proper shape for managing low-octane pump gas the first time around.

[aceaceca]

I went to 1966 heads 091's that yield 8.6 compression. Problem solved.From 1965 they are 75's and from 1964 they are 5437796. See Wallace racing cylinder head codes for others. Runs on regular and still has plenty of giddy up.

[b-man]

Originally Posted by aceaceca

I went to 1966 heads 091's that yield 8.6 compression. Problem solved.From 1965 they are 75's and from 1964 they are 5437796. See Wallace racing cylinder head codes for others. Runs on regular and still has plenty of giddy up.

Most GTO owners keeping their factory engines want to keep them intact, so swapping on non-GTO heads is not in the cards. No GTO owner at a car show or even just hanging out with his Pontiac buddies wants to pop the hood and see the wrong casting number heads on his or her 389.

True the '65 and '66 heads are all pretty much functionally identical with the exception of chamber ccs. The ports and valve sizes are all essentially the same.

However in the case of 1964 heads using anything but the 9770716 GTO heads would be a definite downgrade, only the '716' heads have the bigger valves (1.92"/1.66" compared to 1.88"/1.60") and decent ports.