Have we really gone from the race track to the dyno now to flow bench comparisons?
01-14-2005, 09:06 PM
#1
Have we really gone from the race track to the dyno now to flow bench comparisons? I can't thing of anything more useless than a comparison of two heads max flow over .6" at 28 inches when the there is less than a 10% gain over .5" or even .4" lift.
You are only there a for a small fraction of the time the intake valve is open, and then centered around 106 degrees after TDC.
Keep in mind that the intake valve is open around .15-.20" at TDC more or less depending on the cam. And .4" 30-40deg after TDC. It is the area under the curve that counts. And that is at a lot less vacuum than 28 inches.
Exhausts are even funnier. When they open there is something like 150psi in the cylinder. By the time the exhaust stroke starts most (nearly all) of the exhaust has already left the party.
So, in the interest of non-confusing the many forum members and lurkers who are new to modifications (if they have an LSx they have one of the best platforms for modifications) lets not confuse everyone with the price of tea in China (another one of my mother's expressions; I guess I'm just a momma's boy
)
My 2¢. Why do I think I am about to be flamed?
David
You are only there a for a small fraction of the time the intake valve is open, and then centered around 106 degrees after TDC.
Keep in mind that the intake valve is open around .15-.20" at TDC more or less depending on the cam. And .4" 30-40deg after TDC. It is the area under the curve that counts. And that is at a lot less vacuum than 28 inches.
Exhausts are even funnier. When they open there is something like 150psi in the cylinder. By the time the exhaust stroke starts most (nearly all) of the exhaust has already left the party.
So, in the interest of non-confusing the many forum members and lurkers who are new to modifications (if they have an LSx they have one of the best platforms for modifications) lets not confuse everyone with the price of tea in China (another one of my mother's expressions; I guess I'm just a momma's boy
)My 2¢. Why do I think I am about to be flamed?
David
01-15-2005, 10:14 AM
#2
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IMO,flow bench numbers are like dynos.a tool to quantify the results of your modifications.while important,they don't tell the whole story.just because a head flows 300cfm doesn't mean it's going to produce awesome power.
01-15-2005, 01:11 PM
#4
That was an interesting thread. Here is the link: cylinder head discussion
However, the topic is different. In that thread the discussion is on the accuracy and conditions about the numbers vendors publish.
Here, the discussion is that we use the numbers, what ever they are, incorrectly. Flow at .4" is far more important than flow at .6". What we are looking at is the time spent at those flow numbers and what the actual flow rate from vacuum and momentum is at that time.
My contention is a head that flows 10cfrm more at .3 and .4, even at .5, and down 20-30cfm at .6 will outperform the higher peak head in virtually all circumstances.
David
However, the topic is different. In that thread the discussion is on the accuracy and conditions about the numbers vendors publish.
Here, the discussion is that we use the numbers, what ever they are, incorrectly. Flow at .4" is far more important than flow at .6". What we are looking at is the time spent at those flow numbers and what the actual flow rate from vacuum and momentum is at that time.
My contention is a head that flows 10cfrm more at .3 and .4, even at .5, and down 20-30cfm at .6 will outperform the higher peak head in virtually all circumstances.
David
01-15-2005, 01:18 PM
#5
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Originally Posted by DavidNJ
That was an interesting thread. Here is the link: cylinder head discussion
However, the topic is different. In that thread the discussion is on the accuracy and conditions about the numbers vendors publish.
Here, the discussion is that we use the numbers, what ever they are, incorrectly. Flow at .4" is far more important than flow at .6". What we are looking at is the time spent at those flow numbers and what the actual flow rate from vacuum and momentum is at that time.
My contention is a head that flows 10cfrm more at .3 and .4, even at .5, and down 20-30cfm at .6 will outperform the higher peak head in virtually all circumstances.
David
However, the topic is different. In that thread the discussion is on the accuracy and conditions about the numbers vendors publish.
Here, the discussion is that we use the numbers, what ever they are, incorrectly. Flow at .4" is far more important than flow at .6". What we are looking at is the time spent at those flow numbers and what the actual flow rate from vacuum and momentum is at that time.
My contention is a head that flows 10cfrm more at .3 and .4, even at .5, and down 20-30cfm at .6 will outperform the higher peak head in virtually all circumstances.
David
01-15-2005, 01:36 PM
#7
Trying to sell a set of heads based on low/mid lift numbers is much harder to do than using peak numbers. High flow numbers sell parts, period. If a vendor came out with a head that flowed 350cfm@.600 for $2000, how many do you think they'd sell? A ton. Most people would not pay attention to the runner volume required in getting a set of heads to flow that # or the low and mid lift numbers, and would get upset when there huge cammed, 350cfm headed car put down less HP than a 300cfm head and small cam setup.
Shawn
Shawn
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01-15-2005, 01:39 PM
#8
Closer. However the discussion talks about averages. Take this picture, I think you posted it:
And the text said: The discussion was almost there, however it still focused on the accuracy of the test. And average isn't the right number. Unless you multiply it my some approximation of flow dynamics. At TDC the valve is already way over .1" and the exhaust is still open. On the power stroke the are raising cylinder pressures that reduce flow. We can't close the valve quick enough.
I'd say .2-.5" valve lift numbers are what count on virtually all of the hydralic roller setups. Maybe an average in those ranges. The difference would be a much more significant 4.3% higher flow compared to the 1.5% higher flow reported. And compared with the 4-6% lower flow reported at .6".
David
And the text said:
Average flow through the ls6 intake on the PP was 168.70 while mine was 171.34. Now can you guys see why peak flow doesnt mean [expletive deleted] and why you must flow your head through an intake, whether it be ls1/ls6/lsx, to get an accurate view of your heads performance.
I'd say .2-.5" valve lift numbers are what count on virtually all of the hydralic roller setups. Maybe an average in those ranges. The difference would be a much more significant 4.3% higher flow compared to the 1.5% higher flow reported. And compared with the 4-6% lower flow reported at .6".
David
01-15-2005, 01:43 PM
#9
Originally Posted by SPANKY LS1
Most people would not pay attention to the runner volume required in getting a set of heads to flow that # or the low and mid lift numbers, and would get upset when there huge cammed, 350cfm headed car put down less HP than a 300cfm head and small cam setup.
01-16-2005, 01:20 AM
#10
I hate to say it, but this has been brought up many, many times before. Those of us that know about will have faster cars, the ones that want to buy the big .600" heads will go slower. Some people listen, but like Spanky said, most don't. Even after posts like the one JRP posted above, there will still be people that buy Stage 2 Patriots over FFHP stage 1's. Why, because people instantlly see "stage 1" and think it will not perform like a "stage 2". It's the people that buy the "stage 2" name that come to us wondering why there cars didn't perform like they would've thought. It sucks, but that's the way it is. Just be glad you understand.
