[steve25]

John one thing that comes into play air flow wise to some degree or another on Heads like these / ours with paired Intake ports is the Coriolis effect like the way water swirls down a drain.

As you look at the intake flange of the head 2 of the ports produce swirl at curtain valve lifts that work with the rotation of the Coriolis effect and the other 2 ports fight it and as a result flow less total air.

Good levels of swirl is a great plus to have on your side for good combustion all the time but especially on low port velocity type ports, but it can also cut into total air flow numbers even if the port is letting the air swirl the way it naturally wants to!

Intake ports that produce reverse swirl may need to delt with in regards to the shape of them if that highly matching the flow volume of the other ports is needed.

[steve25]

Here's another bit of rework.

This time around I lowered the bolt boss to roof kick in a good amount , widened the roof path around that lick in which made it less of a pinch in and blended in the dime size dip in the port roof.

Note that we lost a small little bit of flow with these two changes!

The first cfm numbers here are where I left off at in my last report/ post and the second row of numbers are from this rework.

The drop in flow from this work is due to the loss of velocity. The amount of air passing out of the valve bowl right now in terms of valve size and Throat area does not need this added port area.

For flow tes #4 I installed once again a 1.66" valve , but this valve has a minor 35 degree back cut and the stem necks down from a stock .341" to .324".


Flow# 3 @28"
.050--.100--.200---.300---.400---.450---.500---.550--- .600---.650---.700
-22.5--44.2--91.5--128.4--153.5--161.3-166.2--174----177-----180
-21.5--42.6--90.3--127.6--151.3--160.3--166.2--171.2-177-----181

Flow test#4 @28"
-24--44.7--98---132------156.4--163.5----170.2--176---182--185.9--188.9

[steve25]

Over the weekend I did a valve job to go up from the stock 1.66" valve to 1.77".
I applyed a quick 3 angle valve job and here's the results.

Note that the stock Throat size of 1.534" is still in place and nothing else has been done at this point, in fact the port is still just ruff cut and the valve new 3 angle valve job still needs to be tweaked.

Note also that this 1.77" valve has no back cut so my next post will be a flow test of that mod.

Flow@28"
.050--.100--.200---.300---.400---.450---.500---.550---.600---.650---.700
22----50.2--110.4--144.6--166.2-174----180----185.9--189.9--191.8--194.8
Flow change / gain over last flow test in post # 62.
-2)---+7.6--+12.4-+12.6--+14.9-+11--+9.8----+9.9---+3.9--+5.9--+5.8

[steve25]

Here's the before and after Exh flow test of a valve with a nice back cut and a bit of a different back angle.

.050--.100--.200---.300---.400---.450---.500---.550---.600---.650---.700
22-----50.2--110.4-144.6--166.2-174----180---185.9---189.9--191.9-194.8
Flow with different modded valve.
26.7---59.2-116.6--150.9--168.2-174----179---183-----187.9--189.9-191.8

[steve25]

More work on the Exh center port here of these 1968 casting number 15 heads.

Knowing in advance what was going to happen here I went back to work and spent some 35 minutes grinding out the whole air injection boss, reworking the roof more around the head bolt boss kick out and then blending all of that ruff cut work back in with a fine cut Burr, but with no polish yet.

The before and after flow numbers show what I knew would happen which can be used as a lession as to how the valve to Throat percentage and the average port area interact with each other.

The first 2 photos are where I left off at in the last post with a light clean up polish being done and some roof and side wall work done.

Note that the air injection boss is still there , but just taken down some into a combination bullet nose/ triangle shape.

The last 2 photos are of last nights remove the boss fully and blend in work.

Note that this test was done without a back cut 1.77" valve so the low to upper mid flow numbers are down.

The other interesting thing to note about where this port is now is that with its stock 1.66" valve there was 92% valve to Throat ratio, now with the 1.77" valve using that same 1.535" main Throat size the percentage has dropped down to 86.7%.

This is the main reason why my 35 minutes of grinding work last night produced little if any flow gain!

My next test I will post will be the flow change with just getting a back cut valve in here, and after that I will enlarge the main and then the sub Throat in the valve bowl.

Flow test from last night.
.050--.100--.200---.300---.400---.450---.500---.550---.600---.650---.700
23.5---51---114.4--146.6--167.9-173.8--177.8--181.7-185.7-188.6---190.6

[steve25]

Back now with a flow test of that same center Exh port but with a back cut valve as on the right in the picture.

Here are the flow numbers showing the low lift gain, the rest of the flow numbers from 300" lift on up where Unchanged from the previous flow test.

.050"---.100"---.200"---.300"
29.2------67-----121.4---150.5
Flow gain over post # 65
5.7-------16------10--------3.9

If your running RA type better breathing Exh Manifolds fat low lift numbers like this will make for no need to run a duel pattern Cam and you can then reap the benefits of making more cylinder pressure , but with less compression!

[steve25]

Here's another flow test showing the results of some 8 minutes of total rework time including the recutting / tweak of the valve job.

Just to note again this is a rework of a center Exh port on a 1968 casting number 15 head and all flow numbers are at 28" .

In photo 1 you will see how at the start of the short turn on the Intake valve side of the bowl I made that entry point wider and flared it back into the side wall.

Note that I left rest of that shoulder that runs back to the rear wall of the bowl in place ( note second photo ) as removing that will only server to decrease low lift flow which I have been fighting to keep up while gaining higher lift flow.

Note that the second photo is of the opposite Exh port, so the side that I am referencing would be on the left side of the short turn.

The second guick mod I made was to somewhat Bullet nose over the valve guide on the cylinder wall side.

It could use more rework then what I did but until I get new guide liners in the head I will not rework it any more.

The third change I made was to deepen the 60 degree bottom cut which in turn took the width of the main 45 degree seat down from 100" to near .080".
Note here also that as posted previously the valve job is still undersize for the 1.77" valve used.

Once again this was done to meat left for a correction valve job once the new guides are in, and this also means flow numbers will go up.

.050--.100---.200---.300---.400---.450---.500---.550---.600---.650---.700
29-----67.5---122.5--153.5--171.2-178---183----185.9--190.8--192.8--196.8
Flow gain over last flow test.
=-----.5-------1.1-----3------3.8----5.7-----6.7-----5.6----7.1-----6.1-----6.2
Flow gain over stock stock center Exh port with stock 1.66" valve.
6.6----23.7----32.6---33----21.7---21.6----21.7---19.3---23.6----24.6----28.6

My next change / mod to flow test will be to fully blend and polish out the whole port, and after that I may gasket match the top of the Exh port at the flange to a stock steel gasket, but we'll see!

The way the center Exh ports in stock and even some ported D port heads lag behind flow wise from .400" on up I can not to this day understand why some Cam company does not offer some Cans that have .030" more lift on the Exh lobes for the center Exh ports?

[steve25]

Hi all!
I want to take a bit of a deture here but really it all ties in with the post I have made so far about porting the center Exh port on this # 15 casting.

The picture below is a formed trace of the center Exh port short turn on the heads noted, it is a trace of the center run of the short turn.

The black line would be the floor of the chamber, the vertical section being the end of the short turn at the crown.

Note how straight and tall the stock short turn is of the 5 and 6 series heads of the small chamber size.

This height of short makes possible big high lift flow numbers along with great low lift numbers since you have the needed meat when going up the the 1.77" valve size to form the needed Venturi to get great flow numbers top to bottom!

Even on the center Exh ports when fitted with a 1.77" valve that is back cut I am talking well over 200 cfm @ .600" lift , and killer numbers of 160 cfm or better at only .300" lift with numbers in the high 120s at .200" lift!

For those who have been following this string and its rework of this number 15 casting I am not done yet going for greater high lift numbers that I may or may not get with more work ( in the mid 190s now) but in reagards to low lift numbers I think it's over , we will see !

If I have some time before my next post on reworking the # 15 Exh port I will dig out a few more heads and post up pictures of there Exh port short turn traces.

[455-4+1]

Picture ? 😁😁😁

[steve25]

Dam, sorry they where here when I previewed the post!

Ok here are those stock and ported short turn traces.

[steve25]

So here's the other thing I wanted to throw into the mix yesterday with my photo of the short turn trace's .
Here's what I idealized Exh port should look like.
Now let me ask this, in terms of our stock iron heads and how we have to deal with our bent and twisted up center Exh ports and also how much deeper the chamber is in the post 1970 how can we incorporate most of the best features of this idealized port to get the best average numbers we can?

Should you ask this Exh port with a 1.60" valve passed 300 cfm@28" depression!

[steve25]

Yesterday I was in the middle of correction on my last post when my iPad battery gave up the Ghost!

The Exh port in that picture I posted flowed 300 cfm with a header tube bolted on and 260 cfm with just the open port.

[steve25]

Here's the last of the Exh work I will be doing on this center Exh port.

Yesterday I spend 35 minutes blending out and then fully polishing the whole Exh port, was it worth it,you be the judge?

One thing I must say about these last flow numbers is that what I call my ruff Burr cut wall finish is way better then some other folks in terms of smoothness.

So someone else polishing out there ruff grinding work would see a greater flow gain then what I got here, but then again there ruff cut non blended and polished flow numbers would be somewhat less then mine also.

Here's the results.

The second row of numbers are the flow gain with the blend & polish work, andthe third row of numbers are the Exh flow gains with a 1.730" 45 degree tube on the Exh port.

.050---.100---.200---.300---.400---.450--- .500---.550---.600---.650---.700
29.2----68.6---120---154.2--172.8--178.8--185.7--187.6--191.6-196.5--207
---------------------------------------------------2.7----1.7----.8-----3.7------10.2
-----------------------------------------187----195----197.9---201---206----217

The big jump in flow numbers from .650" lift to 700" is due to a combination of my rework of cornor of the chamber and the valve being lifted high enough to clear the lip of the chamber.

This test was done with the same back cut valve as in post 67.

I also tweaked the valve job and brought the 45 degree seat OD out to 1.758 and then recut the bottom and top cuts , but as I knew would happen I gainded nothing flow wise because the main high lift flow restriction is still the 1.535" Throat size that's in place

[neill]

Hi Steve, thanks for posting all of this, its great info! Is it important to match the short turn radius of the exhaust to the bowl size like the intake? And will/would you increase the height of the short turn for more exhaust flow at this time? also you have mentioned a 15* cut above the 30 intake seat as being bad for flow. Is this why we see Heart shaped chambers with what appear to be 30*- 45* cuts around the valves forming the sides of the chambers?

[steve25]

Neill in a day or two I will hand draw and post up some pictures to help you see what goes on in iron Head Pontiac Exh ports and show how the many different short turns effect what you can improve or not and in turn what you can get out of them.

A lot of the aftermarket Heart shaped chambers have a floor that's near to a 30 degree angle and this is good as it helps to unshroud the valves at low lift without cutting into the valve to Throat percentage of whatever valve your looking at, be it Intake or exh.

What I mean by this is that say we where reworking a aftermarket head for max flow we would then want to get up to a 91 percent valve to Throat ratio.

Part of the concideration as to what will allow us to get there is how many angles we wil use for the valve job, hence the more angles the less of a Throat percentage we can use.

Hence having the chamber floor act as the 30 degree valve job top cut then having the main 45 degree seat be the first angle on the chambers seat incert we have gained back some .030" to .040" that we can now use to get the Throat ratio up.

Just keep in mind that high speed/ high velocity air ( as when the valve is at low lift) can not make its way around anything tighter then 15 degree angle , and this is in terms of dry air!
Now air at the standard weights in at .075 lb per cfm, but now in terms of the Intake side let's give it more mass by adding in some fuel , so now high speed / high velocity air may only be able to make its way around a 18 degree angle if you follow what I am saying here?

Good to hear from you Neill!

[Traveler]

Steve

I have been studying this thread on porting 1968 #15 Pontiac heads. I have a flow bench and have been trying to replicate your results. I have been slow and remove minimal material, then retest. I am trying to get the best results I can with the 1.96 valves. I have kind of hit a wall now. Here are my last two tests.

Lift in inches-----------------.05-------.10--------.20------- .30-------.40-------.45-------.50-------.55-------.60

CFM@28-----Test #1--------37.7------69.1------133.6------173.8-----199.2---------------220.1

---------------- Test#2--------35.7-----68.8------126.9------171.3-----195.7-----202.2----212.9------218.-----1221.9

As you can see I lost airflow. The difference in test #2 is that I widened the port floor and widened the short turn radius, I also removed the tail on the valve guide boss. I left the boss itself as I figured the valve needed this support. The boss now looks like a ski ramp. I think this is what hurt the flow. Looking at your pictures it appears you almost completely removed the boss. Are you going to install new valve guides? If you are, what guides do you plan on using, cast iron or bronze? I was looking at the PEP bronze performance guides but I know nothing about them. Cast iron guide are inexpensive. I saw where they don’t recommend stainless valves with cast iron because of galling. The Melling 2.11 valves are inexpensive on Rock Auto but they don’t look like they would flow as good as a performance valve like Ferrea.

I plan on running an 068 cam, a Q-jet and long branch manifolds on a 1965 389. How much flow do I really need? I know the valve angle is different on the later heads. At .408 lift I should be OK, right? Will I benefit from 1.65 rockers or is that overkill? I was targeting 250 CFM at .500 lift or lower. Thanks for the thread it has been a great help

Phil

[steve25]

Phil this is a picture perfect example of the real world compared to all the BS in tons of Magazines over the decades of how doing a simple bowl blend and short turn blend job is a guaranteed air flow pick me up!
When I first got my flow bench I was dumbfounded also for a while in regards to what my rework results where showing!

Here's one very important thing to keep in mind when porting any head, it's the fact that up to a certain amount of valve lift the level of air flow is totally controlled by the Venturi form created by the seat and back side form/ shape of the valve and the angles to the seat area and the seat approach area at the end of the valve bowl .

The rule of Thumb on this is that lift wise it's a factor of .18 of your valve size, so for a 1.96" valve the two areas I just posted about above will have near total control of the amount of air flow happening up to .352" lift in your case, or for a 2.11" valve up to .379" lift.

Your flow drop off is due to the air speed you have lost mainly due to the work you have done to the short turn, in other words you have lost air pressure you had in the runner that was pressurizing the valve bowl and keeping that air speed high.

This is by no means uncommon when you port a head , as the flow numbers will jump around a lot!

Right now you have the short turn shaped and sized to handle more air flow then you have at the crown / high point / window area of the short turn , what you need to do now is get that added air flow to take place which will once again pressurize the valve bowl and get the air speed back up.

Have you made any templates to guide your porting work like I show at times in my post on this #15 head you have put back up?

It's also very important that at low lift ( .050" to .100" ) you can replicate the amount the valve is open for flow test to .002" to .001" inch because that can make for a 2 to 3 cfm change at times!

Your spring you use for testing the Intake side of the head needs to be. Strong enough to keep the flow bench from sucking the valve open more then your setting at low lifts.
Sometimes to this day I find myself falling into this pit so always double check your low lift numbers to see if they repeat themselves once you get to the end of a flow test.

We will likley need to talk by phone to get you lined up right on getting your cfm goal .

One thing I need to ask is are you able with your flow bench to test a 15" up to .300" lift atleast?

[steve25]

Phil have you made any progress as to getting some of the air flow back from .100" to .200"?