[steve25]

I have over the long weekend spent a bunch of time porting up a 6X-8 head to try and get the highest level of flow up to .450" of lift with the minimum port volume increase and also with out any of the normal flow losses that can take place below .450" lift with certain rework changes / valve bowl and short turn bend work .

I don't know why after all of these years with these heads as opposed to tons of others I have worked on I took a long hard look at the shrouding done to the Intake valve by the far deeper chambers these heads have as opposed to the pre 1971 heads.

These 6X heads have a shallow side chamber depth of .477" to a high comp heads chamber depth of .338" or darn close to a 1/4" deeper and greater level of shrouding going on.

The deep side of the 6X head checks in at 1.070" to to the high comp heads .949", so once again darn close to 1/4" of greater shrouding taking place.

Noting this difference I set out to try and make up for this in regards to helping the Intake flow out.

Here's the ported flow numbers before the unshrouding work in the first column and then after in the second, followed by the flow gain column.

Note how in the 3rd photo you loose sight of the lower right section of the Intake valve seat area as compared to the 4th photo of the unshrouded work done to the chamber.

The amount of flow gain picked up by the mod will pick up the overall HP of the motor by 12 to 16 HP overall , so it's a good way to fly once you Mill the head a little to make up for the 3 CC increase in chamber volume.

This is also a dam good example why it pays even with aftermarket heads no less factory heads to run a high comp head and use a dished piston to get your static compression ratio you need so to keep shrouding to a minimum.


.050". 40.6 42.3. +1.7

,100". 77.4. 79. +1.6

.200". 138. 139.5. +1.5

.300". 178.6. 181.2. +2.6

.400". 206.2 211.6 +5.4

.450". 214.3. 220 +5.7

.500". 218.9. 222.4. +4.7

[1968GTO421]

Good work, Steve. A lot of food for thought about these heads.

[steve25]

Your welcome and thanks!

One important thing that I failed to note about the mod is that the near sharp edge needs to remain when you grind in the different shape as that edge acts like a wet fuel shear point on that side of the chamber.

[694.1]

Fascinating!

[dmac]

3 questions-
1. Is low lift exhaust flow on these heads any issue? Thinking biggest possible intake valve, if low lift exhaust is sufficient

2. Do you think that maybe maximizing flow at mid lift would lead to a different chamber shape or different porting or unshrouding?

3. And finally, would maximizing mid lift flow increase total flow through the valve when open since total flow is what the engine actually sees?

[JLMounce]

Originally Posted by steve25

Your welcome and thanks!

One important thing that I failed to note about the mod is that the near sharp edge needs to remain when you grind in the different shape as that edge acts like a wet fuel shear point on that side of the chamber.

Can you elaborate on the fuel shear subject. I've never heard about this, so just looking to learn.

[steve25]

Dmac, Exh flow numbers can veri greatly from D port iron casting type to D port iron casting type in terms of differences in low lift and high lift flow between them, and not only due to valve size.

This is most noticeable in the high comp iron castings where there are at least 3 different kind of short turn shapes , this also makes for differing amounts of port area and can limit how much high lift flow you can get out of a given Exh port when reworking it.

A classic example of this in the factory round port head for the SD455 head which has less Exh short turn height in it then any other round port iron head, the result is that a factory D port 48 casting or 12 or 13 casting can be max ported to flow bigger high lift numbers then the SD head.

When Pontiac came to making the 4 , 5 and 6 castings they finally settled on a standard type of short turn and thank the Lord is nice and tall, so even though it starts off with less Exh port area then any of the previous D port casting they can still be reworked to flow well over 200 cfm when fitted with a 1.77" valve.

Just note that on the hugely deep chambers of the 5 and 6 series heads ( like the 6H casting ) machined for the 455 that Pontiac had nowhere lesse to go CC wise but deeper to get the needed 121 CC chamber volume , so these castings have far less short turn height in them also!

It's always a great idea in any type of head to max out the mid lift Intake flow in regards to the total amount of valve lift you are running, and yes a bigger valve provides more curtain area and in theory can provid more flow at any mid lift flow point .

So for example if you where running a Cam of 500" lift you would want to rework the head to provide the most Intake flow by like .450" lift.
One thing to keep in mind here is the near rock solid .18D rule for Intake valves.

This rule means that for a 2.11" valve for example the air flow up to .379" lift is totally controlled by the valve angles in the head and the angles used on the valve , how shrouded the chamber is,and inturn any minor porting work in this lift range does not produce a good flow increase for the time spent.

Also many home porters working without the aid of a flow bench end up with a head that flows less at lifts up to .400" then what the head did stock!!
Talk about shooting yourself in the foot!

In short gaining more Intake flow in your chosen Cams mid lift range provides the same effect as adding more duration , but without having to deal with increasing the motors torque curve up higher in the rpm range, so you get to not only keep your motors low speed power but also have more of it due to greater Intake port velocity for any given point of rpm.

There's only so much you can do chamber wise in regards to improving the mid lift flow of a low comp head once you get to that tipping point of not being able to recover the needed compression due to the CC increase from the rework.

This why I posted that if your doing a limited compression build from scratch you will be far better off using a shallow chamber high comp head ( be it iron or Aluminum) and running a dished piston.

JL, wet flow is when fuel falls out of the air and fuel mixture when a air mass slows down to make a turn.
This is most common around the roof of the valve bowl and on the push rod wall just before the valve bowl.

When enough fuel does this it turns into a small stream coming out of the open valve on the plug side of the chamber.
The factory combination of that change in chamber wall angle and the resulting near sharp lip left by such acts on wet flow to shread it back up into a combustible form , not just a stream of unburnable gas.

A secong good effect is that limiting excess wet flow means your not washing needed lube oil off of the cylinder wall on that side of the Bore or washing out the rings themselves!

[JLMounce]

Thanks, for the explanation!

[steve25]

Your welcome Jason!

Also in regards to wet fuel flow issues the finish of a Intake port or Manifold runners, and especially Intake Manifold Plenum floors should never be left polished as that is guaranteed to promote wet fuel flow problems .

The added very small flow gain from polishing a Intake port and or Manifold runner as seen on a flow test and the power that may seem like it will add will be more then offset on the running motor by wet fuel issue every time!

When I port a Intake port on a iron head I do polish the area that I have reworked to get smooth transistions , but after that I go back in and go over the whole port with a fine cut Burr and leave what amounts to a 80 grit finish in the port .

[dmac]

Thank you. I am slowly gathering what I need including knowledge for a 9.5 to 10:1 cr that I want to make power in the driving range- like a truck motor that makes tons of torque down low in rpm range and at the same time keep valve train away from high lift that obviously is harder on cam, springs, rocker, etc.

[steve25]

One thing to keep in mind Dmac is the impact of Intake air flow gains thru the same size port area or a area that is a little larger then stock.

For instance if you have done some Intake porting work on a big valve D port head and your rework of the port has kicked the flow up from a stock average peak of 205 cfm up to 217 cfm then that's a 5.8 percent increase in flow.

If this flow gain came to be from only a 3 percent increase in the minimum port area then you have picked up the port velocity by 2.8 percent.

This translates in a 2.8% increase in torque but at 2.8% lower rpm then where the torque peak was stock, and this ties in with the motors peak HP number that will be greater due to the flow gain, yet come in at 2.8% less rpm then stock.

This percent of change will make a 455 feel like a 468 cid motor all on its own!

The great thing about a iron Pontiac head or the 190 CC port volume or less Aluminum heads with there big 2.11" Intake valve is that when they are ported right you will always have this greater level of velocity gain then you have in the amount of minimum port area, so this allows you to have your Cake and eat it too!

Another way to look at this is that in this example with that 2.8% gain in port velocity you can then if you want to run a Cam duration of 2.8% greater then you where running and yet keep the amount of low speed torque you had, plus pick up more Hp.

[steve25]

Here's the difference in shrouding between a iron 6X head with the Intake valve at .500" lift and the next shot of a high comp head with the valve at that same .500" lift.
There's quite a difference as you can see!

In the shot of the high comp head the seat area of the valve is already above the deck of the head even before .500" lift comes around.

[Half-Inch Stud]

Steve25, that was good.

Valve curtain AREA is a very good "nozzle trait" to optimize: too much shrouding (Low-Comp heads) could use a little opening, whereas High-comp heads are already close. Too little shrouding leads to the 0.66 discharge coefficient for "a hole in a plate".

I've been buggy about slight adjustments with the Intake zone nead the CYL divider.

However, Overall i find the pushrod buldge to be the CSA Chokepoint.

[steve25]

Lol!
So exactly how does the valve curtain area change with valve shrouding if that's what your implying here?

[pmd400]

Originally Posted by steve25

Here's the difference in shrouding between a iron 6X head with the Intake valve at .500" lift and the next shot of a high comp head with the valve at that same .500" lift.
There's quite a difference as you can see!

In the shot of the high comp head the seat area of the valve is already above the deck of the head even before .500" lift comes around.

How much flow is there around the short turn and out that side of the valve? I understand the vast majority of flow occurs on the opposite side of the valve. So decreasing shrouding on the spark plug side of the chamber would halve a much greater impact on flow. Am I incorrect?

[steve25]

Right you are!

And this is why with the 1967 670 casting with its still basically closed chamber shape porting then to what is a maxed out 245 cfm out of them then looking at that same level of work on a 68 and up casting would be netting you at least 20 cfm more!!

The air mass passing its way thru the valve bowl finds the tight chamber side as being restricted,so as flow rates go up as the lift goes up the main mass of sir wants to make a Bee line for the center of the cylinder since that is the least restricted area to dump into.

One of the factors in amount that this takes place and at what lift it starts to shift to this trend at is the Heads valve inclination angle and the shrouding of the chamber you may be dealing with.

I could not do anything about the 6X heads 14 degree ( which is pretty good as is) valve inclination angle , but I could reduce the shrouding in that chamber area I reworked.

I will post up more detailed pictures and more info for you on Monday .

[Cliff R]

The 5C and 6X castings also have a "double pass" on the exhaust past the valve guide bosses. Most of the early castings only allow flow past one side of the valve guide boss. Despite the pretty large boss cast into the 5C and 6X castings on the roof of the exhaust port past the valve guide they still flow very well. I've always removed the boss past in the roof of the exhaust port and enlarged them for the larger 1.77" exhaust valves but not sure how much improvement in flow this provides as I don't have a flow bench here.

I'd also note that folks often throw rocks at the smaller valve 2bbl and "low" performance heads. A few years ago we were preparing a 455 for one of the Firebird's headed for the Tri Power Nat's at Norwalk that we were going to race. We had custom ported heads and cam coming from Dave at SD but they weren't going to show up in time to make the race. The shortblock was finished aside from the cam. I decided to just throw a set of heads on it and cam off the shelf in the parts room to make the Norwalk race. I had half a dozen cams to choose from, but the only set of heads that were going to make the grade without guides and a bunch of work were a set of #46 castings from a 1969 428 360hp engine.

I cleaned up the heads small valve heads, converted them to screw in studs, ground the seats (45 degree intake), ground the valves, installed better springs (Crower 68404's), and finished the assembly. There was no porting done to the heads anyplace, not even a "clean-up" or gasket match. The cams I had to choose from that I thought would work well with this combo were a Comp XE284, Comp 290H, and Crower 60213. We chose the 60213, 308/314, 248/256, 108LSA. I didn't even degree the cam just put it in "dot to dot". We topped the engine with an Edelbrock RPM intake, 1" spacer, quickly prepared a 1978 Olds Q-jet and stock HEI for it. Nothing fancy just recalibrated the carb and put a 10 degree (20 at the crank) curve in the HEI's mechanical advance with a positive stop, all-in at 3000rpm's.

We finished the engine and got it in place just days before the Norwalk race. Was expecting maybe low to mid 12's out of the combo, 1981 FB with a TH350, Continental converter, and 4.10 gears with 30" tall DOT's. It was very hot/humid that year at Norwalk. Despite the heat the car went 11.80-11.90's right off the trailer and ran there very consistently all weekend. Despite the somewhat miss-matched combination of parts we were very nicely rewarded with a pretty strong running engine despite having stone stock small valve heads on it. I've actually seen folks put much better heads on similar builds with roller cams in them and not run any quicker at that race weight with similar drive train parts......FWIW......Cliff

[steve25]

I know Cliff!
We see it all the time Cliff, even on this web site no less at the track!
All new big buck high power parts people Bolt on during a build when a well thought out stock rebuilt high comp iron head 400 with the right Cam with 3.90 gears in. 3600 lb car can easily dive into the 12s!

[Half-Inch Stud]

So, as-cast 46 small valve heads got no valve curtain problems. No pushrod bulge problems. And an infallable short-turn radius.

[steve25]

How it is your taking about the curtain area of any given size valve and amount of valve lift to produce such, and a nozzle effect that can be had with Exh port shapes and sizes in the same sentence has me baffled!
Also my comments and flow test posted up here are in regards to the 5 and 6 series heads , not a casting number 46 Head.