Questions about ports that stall, and cam lift.

[Stan Weiss]

Hi Stan
Here's a screenshot of the flow chart.

RHS AMS.jpg

Kevin,
As Mike talked about it would be nice to have heard the heads on the flow bench. Looking at this flow sheet I would say that the heads weren't turbulent and would not have a problem if they were mine of running .700" or more lift.

Stan

[skinny z]

Hi Stan
Here's a screenshot of the flow chart.

RHS AMS.jpg

Kevin,
As Mike talked about it would be nice to have heard the heads on the flow bench. Looking at this flow sheet I would say that the heads weren't turbulent and would not have a problem if they were mine of running .700" or more lift.

Stan

Thanks for that Stan.
Once I've taken out the abrupt edges around the valves and back cut the intakes, I may get them back on a flow bench if for nothing other than having another bit of data (and to see if there were any improvements). Maybe I could hang around and have a listen for myself.
Thanks again.

[ClassAct]

So Classact. I take it that I should then just listen to you and not what I was taught at a Darin Morgan / BES class that I attended ?

I don’t care what you do.

[HQM383]

Scroll down to “notes on airflow”

https://jonkaaseracingengines.com/shop/ ... der-heads/

Thanks for that, great insights. A statement from a very smart guy from a now defunct forum looks at it the same but from a different perspective

“please consider thinking about an engine in this way. Do not think of an engine as an Air pump. That is a very inhibiting way to discover or understand the factors in an engine.
I think its better to think of an engine as a machine to raise the energy level of a fuel to as high as possible prior to initiating controlled combustion. That sentence really needs thinking about. If you approach an engine that way it opens a whole new way of thinking about whats going on inside a cylinder.

If you design an air pump engine you can have no regard for what the fuel condition in the cylinder is. You will just concentrate on pumping air. The more air you pump, the better you think it will be.”

[1980RS]

What we found on a SBC Vortec head was one little area like Joe Mondello and David Vizzard found. David clayed a small area in the chamber and the Intake port that stalled @ .550 now pulled all the way up to .650 after that with a slight loss at lower lifts. For me it was an eye opener to learn that. Now I just need to find a way to weld that area and fix it for good.

[Walter R. Malik]

I'm seeing about a 3% drop from .500" to .600" (254 to 246) .
It does rebound approaching .700" though but that's far and beyond the .575" target suggested.
Might not be worth thinking about.
Thanks.

Is this a stock GM head by chance?
This is something we run into with classes that have to run stock, unported heads. Many of the Chevy heads would back up after .500". The more successful engine builders would work on the valve job, until they moved the point the port flow started decreasing, to above ,550" lift.

Not stock but RHS' version of a Vortec replacement. 2.02/1.60, 170cc. Since bought new there's been some bowl work and cleaned up the exhaust but nothing major. 5 angle valve job all done by a top end shop.
I had one cylinder flowed (4.03" fixture) and am working with those numbers.
There's obviously more that could be done but my investment in these heads is about at an end. A little chamber work, measure everything up and then order a cam.

It's the subject of a back cut on the intake that caught my attention (and thanks to gregsdart for the loan of the thread). Something I had done when I was running Vortecs although I understand that's more to do with what happens below .200" rather than where this port stalls. That said, I'm pretty sure my new intake valves are as not back cut. Something I'll have to address.

Thanks again.

A lot of times, you will see a slight back-up around that lift because the outer edge of the valve is now at the point where it is entering at the bore top edge which is inside and smaller than the combustion chamber wall. A slight bore notch or chamfer at that point sometimes alleviates that issue.

[mag2555]

So Classact. I take it that I should then just listen to you and not what I was taught at a Darin Morgan / BES class that I attended ?

I don’t care what you do.

And I consider anything you replay to about my post as misinformation!

[skinny z]

What we found on a SBC Vortec head....David clayed a small area in the chamber .

I saw that vid. Very interesting.
What I'd like to know is how that translates to the RHS version ( I think I'm one of the only people on the planet currently running them). With a 2.02 intake and a revised chamber, any reworking would be a whole new ball game.

[ClassAct]

So Classact. I take it that I should then just listen to you and not what I was taught at a Darin Morgan / BES class that I attended ?

I don’t care what you do.

And I consider anything you replay to about my post as misinformation!

I wasn’t responding to YOUR post. I was responding to the OP. Get over yourself. Maybe go actually test something instead of just repeating what you hear. When a port is backing up, it isn’t always a bad thing because the flow bench will lie to you. Jon Kaase said that if you read the link.

I stand by what I posted. If a back cut increases reverse flow it WILL lose power.

Simple as that.

[Walter R. Malik]

I stand by what I posted. If a back cut increases reverse flow it WILL lose power.

Simple as that.

Let's qualify that ... what I have witnessed is IF that back-flow increases MORE than the air flow in the correct direction has increased from that back-cut, the engine will lose power.

Most people won't even go there when the air flow increases but, the difference between the two is what to look for here.

[BobbyB]

I stand by what I posted. If a back cut increases reverse flow it WILL lose power.

Simple as that.

Let's qualify that ... what I have witnessed is IF that back-flow increases MORE than the air flow in the correct direction has increased from that back-cut, the engine will lose power.

Most people won't even go there when the air flow increases but, the difference between the two is what to look for here.

How much does the overlap of the cam affect the power gain/loss? In other words, does the backcut tend to hurt race engines more than street strip engines?

[ClassAct]

I stand by what I posted. If a back cut increases reverse flow it WILL lose power.

Simple as that.

Let's qualify that ... what I have witnessed is IF that back-flow increases MORE than the air flow in the correct direction has increased from that back-cut, the engine will lose power.

Most people won't even go there when the air flow increases but, the difference between the two is what to look for here.

Yeah, I should have been more clear. The gain in reverse flow can’t be equal to or greater than the increase in forward flow or it will lose power. Every time.

I have posted this exact thing in this forum many times, and it’s the same professional arguer who claims I post bad information. He’s in love with the flow bench and anything it says.

Since I’ve tested it more times than I care to admit, and I spent way too much money to prove it out but if I’m worried about reversion I always use a 50* seat. In fact, if I’m doing the heads and I can get the 50 on the valve or the customer will buy valves they all get a 50* seat. But that goes against what most guys say. I trust my testing.

[steve cowan]

I stand by what I posted. If a back cut increases reverse flow it WILL lose power.

Simple as that.

Let's qualify that ... what I have witnessed is IF that back-flow increases MORE than the air flow in the correct direction has increased from that back-cut, the engine will lose power.

Most people won't even go there when the air flow increases but, the difference between the two is what to look for here.

How much does the overlap of the cam affect the power gain/loss? In other words, does the backcut tend to hurt race engines more than street strip engines?

Like most things it depends
There is no clear cut answer.
The problem I see and I am as guilty as anyone-
We read something or watch a high end engine guy on the google machine and think we need to follow.
As difficult as it can be we need to test our own combinations or be lucky enough to have people willing to share trends of there findings. I think most including myself would be better of learning how an engine works dynamically but I really doubt very few people actually know

[juuhanaa]

I stand by what I posted. If a back cut increases reverse flow it WILL lose power.

Simple as that.

Let's qualify that ... what I have witnessed is IF that back-flow increases MORE than the air flow in the correct direction has increased from that back-cut, the engine will lose power.

Most people won't even go there when the air flow increases but, the difference between the two is what to look for here.

Yeah, I should have been more clear. The gain in reverse flow can’t be equal to or greater than the increase in forward flow or it will lose power. Every time.

I have posted this exact thing in this forum many times, and it’s the same professional arguer who claims I post bad information. He’s in love with the flow bench and anything it says.

Since I’ve tested it more times than I care to admit, and I spent way too much money to prove it out but if I’m worried about reversion I always use a 50* seat. In fact, if I’m doing the heads and I can get the 50 on the valve or the customer will buy valves they all get a 50* seat. But that goes against what most guys say. I trust my testing.

Interesting. Im trying to get understanding about this, so little help wouldn't hurt

I would like to ask is the 50 seat to get more area under the valve/bigger throat which in turns flows less reverse? Or is it something how different seat angles affects with the chamber and how you handle the flow reversion?



-juhana

[gregsdart]

As i read through this discusion the thought occured to me that a cam with a lot higher lift will also most often have more duration at each step of valve lift. That more agressive profile i assume would offset lost flow from a head ported to gain flow up high at the expense of some mid range? I also assume this may most likely be true ONLY in certain circumstances, like restrictive heads that need help, like most 60s stock performance heads.