Enlarging PRP doesn't always help

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With that engine, the car has gone 10.80@123, at 3250lbs.
I won’t give too much away, but the peak intake flow of the heads is 200cfm.
You can use your favorite performance calculator to figure the HP.

Apologies for the thread sidetrack.
 
273 said:
My guess is the csa around the ssr will be the ultimate cork since it can only be modified so much, and design the rest the port to be in balance with that
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In a way I guess that's right. Area, especially width at the SSR is what is needed. Since the velocity over the SS is so high, flow separates and the effective area at the throat is what gets cut off. Interestingly enough, every time I try to make the throat larger, flow goes down. This particular head has 50 degree seats and 88.5% throat. Conventional wisdom says a 90-92% throat would flow more. I have not yet been able to make that work.

Two possible reasons come to mind:
1. I know that flow separation is what is shutting down flow. Does the smaller throat reduce the separation?

2. Throat sizes are made from varying the angles and widths of each angle. It's amazing the difference a few thousandths can make. Maybe I don't have the right combination of angle and width for the wider throat.
 
PRH said:
With that engine, the car has gone 10.80@123, at 3250lbs.
I won’t give too much away, but the peak intake flow of the heads is 200cfm.
You can use your favorite performance calculator to figure the HP.

Apologies for the thread sidetrack.
Click to expand...
Not a sidetrack at all. Many of those stock class heads are the perfect example of what we're discussing.
 
Since much of my learning curve for porting evolved around messing with 906/915 heads, and “what worked for me” on those heads included a fair amount of SSR reshaping, so when I had flow regression issues at higher lifts on other heads, one of the first things I would do is some tweaking of the SSR……..and more often than not, it would show improvement.
Often not 100%, but it was usually “better”.

I’ve spoken to other porters who don’t mess with the SSR until the last thing.
But I usually start rubbing on it earlier in the process than that.
 
PRH said:
Since much of my learning curve for porting evolved around messing with 906/915 heads, and “what worked for me” on those heads included a fair amount of SSR reshaping, so when I had flow regression issues at higher lifts on other heads, one of the first things I would do is some tweaking of the SSR……..and more often than not, it would show improvement.
Often not 100%, but it was usually “better”.

I’ve spoken to other porters who don’t mess with the SSR until the last thing.
But I usually start rubbing on it earlier in the process than that.
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It's certainly the magic area. I find the center of the SSR easier to visualize. It's the corners that I've yet to figure out. So many options for shaping them.
 
There have been discussions of SSR work in/on various magazine articles/forums thru the years.

And I’ve seen where some are having luck with the approach to the turn having a more rounded shape(as opposed to flatter).
I can’t think of a single time where I’ve lowered the center of the floor as it approaches the turn, to create some of that “rounding” of the floor, and had it end up as a flow gain.
It’s never worked for me.

I’m not saying I haven’t lowered the floor, I’m saying I haven’t purposely lowered the center of the floor, more than the sides, in an effort to create a rounder approach to the SSR.
On 906/915 heads, I lower the floor at the SSR a fair amount……..and I do it mostly by feel, with confirmation that it’s good from the bench.
 
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The ssr is still a bit of a mystery to me, I get the basic idea what your supposed to do to it but still seems like a bit of a black art, the rest seems like there's straightforward strategies, throat and bowl to valve ratios, csa of the port opening, pinch, port volume to cfm etc.. That should get you in the ballpark.
 
That should get you in the ballpark.
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Depending in what you’re working on, without a proper form on the SSR……you could still be leaving a bunch of flow on the table.

With many stock heads(and some aftermarket heads too), when fitted with oversized valves and the bowls enlarged accordingly………the overall SSR form actually gets worse.
 
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Here’s an example of a reworked SSR vs one where the entire bowl was generously enlarged, including the SSR side of the bowl, but the form over the SSR was not addressed.

2 different 906 heads with 2.08” valves

Quite a bit more bowl work on the one without the SSR done, along with it having a valve job done where the seating angle(45) extends out past the edge of the valve, and has a quite narrow area of contact with the valve itself.

The one with the SSR rework has a std 3 angle seat.

The below .300 lift flow for the big bowl head was surprisingly good, but the lack of SSR rework came into play from .300 and up.

A- big bowl
B- reworked SSR

Flow at 28”
Lift—— A/B
.100—74/65
.200–153/137
.300–204/202
.400–237/251
.450–239/256
.500–240/258
.550–237/262
.600–232/258
 
PRH said:
Depending in what you’re working on, without a proper form on the SSR……you could still be leaving a bunch of flow on the table.

With many stock heads(and some aftermarket heads too), when fitted with oversized valves and the bowls enlarged accordingly………the overall SSR form actually gets worse.
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Ahh, the bowl. Mystery area number two. Conventional wisdom says big bowls slow the air down to help it get around the turn. It also creates a large reservoir of high pressure air that is waiting to get into the cylinder the second the valve cracks open. The theory says that its good for torque.

On the other hand, a large bowl on a head with flow separation issues is not a good thing. If the bowl is larger in area than the apex area, the head has diverging(expanding) flow across the turn. Expansion of the air is a contributor to separation. Converging and accelerating the air helps prevent separation. Decelerating air efficiently is difficult, especially if the air is already in the process of turning.

This is where the dyno or track is really needed. Not going to answer this question with the flow bench.
 
PRH said:
Here’s an example of a reworked SSR vs one where the entire bowl was generously enlarged, including the SSR side of the bowl, but the form over the SSR was not addressed.

2 different 906 heads with 2.08” valves

Quite a bit more bowl work on the one without the SSR done, along with it having a valve job done where the seating angle(45) extends out past the edge of the valve, and has a quite narrow area of contact with the valve itself.

The one with the SSR rework has a std 3 angle seat.

The below .300 lift flow for the big bowl head was surprisingly good, but the lack of SSR rework came into play from .300 and up.

A- big bowl
B- reworked SSR

Flow at 28”
Lift—— A/B
.100—74/65
.200–153/137
.300–204/202
.400–237/251
.450–239/256
.500–240/258
.550–237/262
.600–232/258
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The valve job really helped the first one. My experience is the port has very little effect until 0.300-0.400 lift.
 
For some builds, you have to ask yourself……..do you really want more flow in the areas of the flow curve where the piston is going the wrong way?

My experience is the port has very little effect until 0.300-0.400 lift.
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As a general rule, I’d agree with that.
 
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PRH said:
For some builds, you have to ask yourself……..do you really want more flow in the areas of the flow curve where the piston is going the wrong way?
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Is that not one of the advantages of 50 deg seats? Less low lift flow but more high lift flow. Maybe the intake duration can be increased a little due to the this? That's not the real reason I started playing with it though. I saw the 50 degree seat as 5 degrees less of a turn on the short side and 5 degrees less of a combustion chamber trough for the air to climb out of on the long side. The 5 degree steeper top cut also creates a natural anti-reversion dam on the short side during high lift flow separation - as much as 5-7 cfm gain in my testing.
 
In my limited testing of changing only the valve job from a 45 to a 50, the results were less lower/mid-lift flow, and no gains, or even a slight loss at the higher lifts.

Keep in mind these were tests done at the customers request, on a head that had to be usable after the testing was done.
So, pretty limited.
But I saw no magic from the 50.
I’m not saying it doesn’t work, but I can see where it would take actual R&D to be able to exploit it.
 
PRH said:
In my limited testing of changing only the valve job from a 45 to a 50, the results were less lower/mid-lift flow, and no gains, or even a slight loss at the higher lifts.

Keep in mind these were tests done at the customers request, on a head that had to be usable after the testing was done.
So, pretty limited.
But I saw no magic from the 50.
I’m not saying it doesn’t work, but I can see where it would take actual R&D to be able to exploit it.
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The one data point that I have that I have confirmed several times is this. On the SM head with 50 degree seats, changing the top angle from 39 degrees to 45 degrees ON THE SHORT SIDE ONLY is worth 5-7 cfm because of the anti-reversion. I have only done this test with clay in the chamber due to the obvious difficulty of cutting a 39 deg top cut on 2/3 of the perimeter and a 45 degree top cut on the other 1/3.

Otherwise, with just a 50/39 vs a 45/35 I have seen a small low lift loss, but moderate gains in mid and high lift flow.
 
Is that with the seat at the “std” height, or after you’ve lowered the valve in the chamber .090”?
 
Thanks for taking so much time to answer questions. You and PBR are both a blessing to a lot of people. Interaction and information like this is hard to come by.
 
PRH said:
Is that with the seat at the “std” height, or after you’ve lowered the valve in the chamber .090”?
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Good observation. All of my steep top angle testing is with the valves sunk in the head. So that means both the 50 and the 45 were sunk. The additional shrouding of the steep top cut hurts low lift flow but helps on the top end (I think).
 
Glad you’re getting something out of it.

Frankly, you done some fairly elaborate testing yourself…….that I’ve gotten some nice info from.

Like mapping the flow around the perimeter of the valve on the chamber side.
 
The additional shrouding of the steep top cut hurts low lift flow but helps on the top end (I think).
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I just wonder how much help the steep top angle would be if it was that much shorter…….by not sinking the valve.
The transition area from the seating angle to the chamber wall wouldn’t be as “optimal”.
 
I feel that when it comes to porting, it’s good to let people go down their own path. That way there’s a good possibility of them discovering something you(me) may not have even thought about.

As an example, in the earlier stages of your exploits with the SM testing, you blew that chamber wall out……before it seemed like you did much of anything on the SSR.
Totally the opposite of how I would have done it.

I’m not saying your way is better or worse, just different.
And your results lead you in other directions of things you wanted to test.

I’m enjoying your journey.
 
PRH said:
I just wonder how much help the steep top angle would be if it was that much shorter…….by not sinking the valve.
The transition area from the seating angle to the chamber wall wouldn’t be as “optimal”.
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I believe you are correct. I looked at the SM (15 deg top cut) and the Edelbrock (stupid 0 deg top cut) and since I can't weld worth a dime sinking the valves seem like the best option for testing. Look at any modern (racing) chamber and they all have wide top cuts. It's got to be good for something.
 
PRH said:
I feel that when it comes to porting, it’s good to let people go down their own path. That way there’s a good possibility of them discovering something you(me) may not have even thought about.

As an example, in the earlier stages of your exploits with the SM testing, you blew that chamber wall out……before it seemed like you did much of anything on the SSR.
Totally the opposite of how I would have done it.

I’m not saying your way is better or worse, just different.
And your results lead you in other directions of things you wanted to test.

I’m enjoying your journey.
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Many of the things I've done seemed right at the time, but didn't go as planned. Thanks for having the wisdom to let me fail. Sort of like grandparenting. You can sit back, watch it all, get a good chuckle then give advice. Thanks again.
 
Since I hadn’t ever gone about it in the order you did, I wasn’t really qualified to say you were going about it the “wrong” way.

As for the 50* testing……. That SM head probably still has a few untouched chambers to experiment on……. Without going “all in” to .090” deep in one shot.
 
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