[Jack Gifford]

FWIW- Valve inclination is 20 degrees in the early heads.

[TheSilverBuick]

I don't want to highjack your thread too much, but what did the unmodified chamber look like? I'm always looking for data on what may work on the Pontiac OHC chamber.

Here was a fun test I did several years back. My shop was like 20ºF and I sealed the bottom of the bore with a shop vac and foam, turned it on and rotated the cam through a few rotations (with checking springs holding the valves) and misted water into the intake runner.

[steve25]

Jack indeed its 20 degree, my mistake.

Photos for proof.

[steve25]

Silver Buick I think you’ve invented a new form of art!

You should have misted in different colors of tinted water.

Anyway here’s a photo of what the chamber looked like stock, it appears to be just a larger version of yours although I do not know what the valve inclination angle is in those OHC motors.

What is the intake seat angle on those heads, 30 or 45?

[694.1]

Regarding Silverbuick's post, the Late Great Dan Whitmore did a OHC head for me years ago.
It was basically a light clean-up and he related that the port & bowl design was clearly related to the 65-66 GTO heads.
Some years ago, I posted up some numbers & they should be around here someplace.
My notes should be within reach as well.
Someday, my 347 is on the list to warm up so this thread is of great interest.
Thanks Steve.

[TheSilverBuick]

The OHC literature says a 15º angle which matches up with a quick mock up. Valves are 1.92"/1.60" on a 3.875" bore. I believe this was a 1966 head so 30º seats.



For right or wrong, this is where I ended up and cc'd all the chambers to make sure they matched. Copying a professional's work but on a low compression chamber for my turbo engine.

[carcrazy]

I have a set of 1962 177 XO heads that I am locked in for a 425A motor so this has been a great thread. I'd really like to get as much out of them as I reasonably can. (yes, I understand that it makes no economic sense) What approach would everyone take?

[Jack Gifford]

I stayed with the '58 heads on the engine for my GMC. No port work, just smoothed the guides and the approach to the seats. Stock valve sizes. The intakes barely flowed 200 CFM @ .550" lift. I milled away the bathtub shape of the chambers and reduced some of the valve shrouding. I used D-shaped reliefs in the pistons (matching the new chamber shape), for 9.05:1- it's happy on 87 octane. Bore is 4.127" for 382 c.i. Not dynoed, but I'm very happy with the performance in the 3,800 pound truck. Compared with many other early Pontiacs I've built since 1959, I think the chamber shape is helpful.
The picture also shows the grooves which receive the 1/8" thick dividers to un-siamese the center exhaust ports, so the longbranch manifolds can be effective.

[steve25]

Carcrazy, the first thing I would try my damest to do before putting a grinding Burr into a intake port would be to get the intake manifold your going to use flow tested in terms of at least the shallow plenum side of the intake.

The shallow plenum side aways flows the least.

It’s only detrimental to any motors power curve to have greater intake port area and in turn greater flow numbers.
You at least want the intake runners to have the same amount of minimum port area as the intake port has before the short turn starts off.

It’s all of a 2 hour job and under 20 bucks in related items to take a Jig saw to a section of 3/4” plywood and make a manifold adapter plate that can then be clamped to the top of a cylinder head adapter on a flow bench

[steve25]

Jack it was a great idea to do away with open center exh ports.

I can’t believe that Pontiac revived that bad idea on some of the 1971 head castings!

[carcrazy]

I will be using the stock tri power intake on this build.

[b-man]

Great seeing the work you’re doing on the 1964 9773845 heads, used on many high compression 10.5:1 389s and 421s as well as the 2-barrel 8.6:1 326 engines.

Have you done any flow bench testing on the 1.92”/1.66” valve 9770716 1964 GTO 389/421 HO heads and if so how do those heads unported as cast compare with your ported 9773845 heads?

[steve25]

This evening I will dig out some flow sheets from 1996 on what I did on a casting # 76 for you.

The 76 and 716 are near duplicates of each other from what I have seen of them in stock form.

[steve25]

B-man here's some numbers for you on a 76 casting .

Here are the stock flow numbers (1). Bowl ported (2) fully ported with a a valve job I spent a lot of time tweaking and with a 1.94"I take valve with a 30 degree seat. (3) flow change from stock.with full port. (4)

----------(1)------(2)-------(3)-------(4)
.100"/////62.1///// 57.6///// 59.9//// -2.1

.200"///// 114.3///// 125.9///// 128.7/// +11.6

300"///// 160///// 175.7///// 195.7//// +35.7

400"///// 169.4///// 197.5//// 225.4/// + 56

450"///// 174///// 197.5///// 237/// +63

500"///// 177.7///// 197.5///// 240/// +62.3

550"///// 177.7///// 197.5///// 249///+71.3

.600"///// 177.7///// 197.7///// 251/// +73.3

On the bowl ported Intake I did trim up the valve guide boss pretty good

As I recall the stock Exh flow numbers came in at 150, pretty much what a stock 6X casting flow in terms of peak numbers.

[steve25]

Hi all!

I have just done what might be the final work to this center exh port unless I go up to a 1.62” valve and rework it for that, but if so that would be a few weeks away.


Today I took 15 minutes and with a 320 grit roll blended in every transition and got rid of all the ruff grinding work the port had.

I payed particular attention to the transition of the short turn to the floor at the crown not only in terms of port length, but width.

I was allowing the diameter of the sanding roll I was using to somewhat give me that radius for the corners on the floor.

I think that my polishing work in some areas might have added .015” to the port area, but I did not take the time to nail that down, but I know I would not be far off with that .015” number.

My flow numbers are as before with a back cut valve.

The first column is the previous flow numbers, next is the change with todays work and the third column is a retest at that lift but with a 1 3/4” tube on the end of the port.

Note that I started testing with the tube at .150” lift.

.050”/////27/////26.5

.100”/////51.3///// 53.3

.150”///// 76.9///// 78.6///// 80.7

.200”///// 101.3////// 108.6///// 118

.250”////// 125.5///// 134///// 147.5

.300”///// 144///// 154.4///// 165.3

.350”///// 157.5///// 164.3///// 177

.400”///// 163.4///// 169.2///// 184

.500”///// 169.4///// 175///// 190.8

If I do go up to a 1.62” valve I will get one with a under cut stem I think as I can always build it back up with tape to being close enough to 11/32” so I can make a comparison flow test.