Did you mean to say exhaust valve?
Why would low lift head flow hurt power?
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MOPAROFFICIAL
Oogliboogli
No but I do not and have never used a back cut on the exhaust. Never even tried it. I agree with what he says about tulips as well and would have added that you'd want to specific throat shape for that valve also. I have a lot of experience with stock iron heads and they are limited on the exhaust and what you end up with is going for balance between a good number and port speed/velocity. There's a natural tendency when you get really really good intake numbers to push closer to the ideal exh ratio and that can diminish the velocity/a potential reversion susceptibility with a low lift enhancing backcut if you go by this literature/theory. They don't like to do much over 200 CFM they like to perform in the 175-198/200 range. Casting vary.. but the thing is.. the intakes can flow in the 265-290's cfm quite routinely.
What does the fuel do when it hits the back of the valve with a back cut as opposed to with no back cut. That's a little more interesting to me at the moment because I'm not trying to use the wrong cam timing. Think about the back cut '30* or 33*' at overlap and its aimed, the exhaust. We collect data when we flow a head and there's different things to look at. That data is very important when choosing a cam and building an engine. Engines I would use a back cut on the intake would be in the range of idle to 5400.
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Very much so. As an another member says, it’s not all about flow numbers.
The most important thing a new guy to understand is where the heads starts and where it is afterwards not only in flow numbers but also what the port is doing and where as well as how the port has changed in size and shape. This also gets coupled with the valves, chamber, bore and stroke sizes and then, what the end user is looking to do and with what drivetrain in what car for whatever the purpose is.
Why don't you use a back cut on the intake valve and is that for certain applications or across the board?
Rat Bastid
Dunamis Metron
Well you guys that think a back-cut on an intake valve totally disagree with Darin Morgan’s thinking. His thinking is it will help in most applications and never hurt. Again I will say this is talking about the intake side.
MOPAROFFICIAL
Oogliboogli
I don't expect anyone to take what I say as gospel, I'm just sharing how I do it.
I have my way and reasons and others have theirs.
Theres always speculation and from different perspectives.
Running engine vs a bench arguements...always room for the "but"...
This might be useful or not...the only engines I've ever had blackened intake runners were also sporting back cut intake valves and purple shaft cams, single and dual plane.
Those back cut heads gained an average of 4cfm and as high as 7 cfm .200-.300 lift.
What's 4-7 cfm...8-13 hp...where? Reminds me of the power loss from a heavier oil or an inferior air cleaner...or a alt belt removed. Its there.. but it's not a lot.
I very much understand when a person or group prefers something to another that it puts into question others ways. This is not one of those instances.
To each his own. I believe while the bench says it helps.. that there are engines that can benefit more than others from the backcut.
The ferrea valves are a little thick @127.xx grams
The valves I use are 124 grams with a 12 degree u der head and under cut stems.
That's why the highest gain I've ever seen with them back cut is 7 cfm., average 4cfm. The video is showing you what different valves do of the same size w/bc's of diff kinds and or no bc. In every instance the back cut was used was just to squeeze a hair more out.
...on a chevy head...that flows terrible at .100 and .200 or so.
I have my way and reasons and others have theirs.
Theres always speculation and from different perspectives.
Running engine vs a bench arguements...always room for the "but"...
This might be useful or not...the only engines I've ever had blackened intake runners were also sporting back cut intake valves and purple shaft cams, single and dual plane.
Those back cut heads gained an average of 4cfm and as high as 7 cfm .200-.300 lift.
What's 4-7 cfm...8-13 hp...where? Reminds me of the power loss from a heavier oil or an inferior air cleaner...or a alt belt removed. Its there.. but it's not a lot.
I very much understand when a person or group prefers something to another that it puts into question others ways. This is not one of those instances.
To each his own. I believe while the bench says it helps.. that there are engines that can benefit more than others from the backcut.
The ferrea valves are a little thick @127.xx grams
The valves I use are 124 grams with a 12 degree u der head and under cut stems.
That's why the highest gain I've ever seen with them back cut is 7 cfm., average 4cfm. The video is showing you what different valves do of the same size w/bc's of diff kinds and or no bc. In every instance the back cut was used was just to squeeze a hair more out.
...on a chevy head...that flows terrible at .100 and .200 or so.
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Rat Bastid
Dunamis Metron
IIRC, he does the back cut when the shape of the valve is perfect for his valve job.
I can think of maybe 5 times in my career when I got to order valves with the shape I wanted, and yes a back cut on those valves helped forward flow and didn’t increase reverse flow.
Every single time I’ve tried a back cut on a shelf valve no matter what valve job I use (a couple of which I designed) reverse flow went through the roof.
Thats why I say I almost never use a back cut on either valve.
Rat Bastid
Dunamis Metron
Here is a link to a Larry Meaux interview. I had to laugh because I just read it (for the umpteenth time) back on May 13th.
In the article Larry discusses back cuts and other interesting things.
PipeMax Creator Larry Meaux on Race Engine Head Porting - Hot Rod Engine Tech
In the article Larry discusses back cuts and other interesting things.
PipeMax Creator Larry Meaux on Race Engine Head Porting - Hot Rod Engine Tech
MOPAROFFICIAL
Oogliboogli
I think fuel shear and relate it to making more average torque with a sharp lower angle than radiused to the 45 on the intake side as proven by darren Morgan and others. A cousin sister's brother's nephews uncle said he dyno'd and seen 20 foot pounds more average torque once he stopped blending it to the 45 and put a nice sharp lower angle. True story with humor injected.
what does the sharp angle do with the fuel when it hits it.
Let me take a leap and risk it just being my tunnel vision atm..
Quite possibly a back cut is a compromise with a slight increase in power to offset ..making it acceptable/desirable. Wish I could test
Not sure I fully understand all that but thanks for taking time to explain.
MOPAROFFICIAL
Oogliboogli
MOPAROFFICIAL
Oogliboogli
Duh.
"Are you building a street car, or an 8,000-rpm race engine? The head has to be tailored to what it's being used on, and if this is done properly, the higher-flowing head will almost always make more power"
I'm guilty of skimming through and missing this pos right here,
Could have saved time here..
As simple as what was just said is exactly what it means... the head that's right for the combination that flows more than a head that is right but doesn't flow as much will make more power...
No ******* **** Sherlock.
What's the ******* best head for any n/a combination? a head that's big enough for the flow needed and no bigger, same for the valve..
fuuuuuuuuuuik
Welcome back to kindergarten engine building everyone! What a misleading pile of car salesman pitched basics of cylinder head selection bullshit.
"Are you building a street car, or an 8,000-rpm race engine? The head has to be tailored to what it's being used on, and if this is done properly, the higher-flowing head will almost always make more power"
I'm guilty of skimming through and missing this pos right here,
Could have saved time here..
As simple as what was just said is exactly what it means... the head that's right for the combination that flows more than a head that is right but doesn't flow as much will make more power...
No ******* **** Sherlock.
What's the ******* best head for any n/a combination? a head that's big enough for the flow needed and no bigger, same for the valve..
fuuuuuuuuuuik
Welcome back to kindergarten engine building everyone! What a misleading pile of car salesman pitched basics of cylinder head selection bullshit.
Too much is being made of something so basic. 
MOPAROFFICIAL
Oogliboogli
It's the veering off that disguises it as such. Nothing wrong with the discussion as long as it isnt branching too far off into hypotheticals cause then the learning reader gets left behind as it cyclones into what ifs and perspective theoriesToo much is being made of something so basic.
Original question why would more low lift flow hurt power, .100 - .200 brought up by the guy at trick flow.
What I got so far is .100-.200 is where both valves are open during overlap and don't want the intake easily overcome to reversion.
What I got so far is .100-.200 is where both valves are open during overlap and don't want the intake easily overcome to reversion.
MOPAROFFICIAL
Oogliboogli
And it was successfully over analyzed, or analized in some posts..
Classic reasoning and therefore we have successfully complete the backflip.
Next we'll talk about low rpm/lift limited engines and why low lift low increases power....
finally crawling back in the womb of cam timing to be reborn ..lol
Rat Bastid
Dunamis Metron
MOPAROFFICIAL
Oogliboogli
Yep. Kind of what I like about rollers, they don't hang around below max lift very long
MOPAROFFICIAL
Oogliboogli
For something that's ported well ..a back cut is of least concern. I would say on an engine that doesn't get any port work , a back cut along witht a really good valve job is even more important
I think the main reason Darin Morgan preached about exhaust air speed so much was so you didn’t have to worry about reversion. At 325 plus foot per second it’s not hanging around in the port. Shape shape shape.
MOPAROFFICIAL
Oogliboogli
nothing like losing inertia trying to push slow exhaust column out.
"Don't youuu..forget about.." scavenge.
"Don't youuu..forget about.." scavenge.
Bingo, but not to large in pipe size.
The only time I have ever heard of a slower valve opening being used for a in serious N02 and high PSI supercharging.
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From Harold Brookshire. Sadly Harold is no longer with us and so there is no later......You cant fill the cylinder effectively if the the exhaust residual has somewhere to go and the greater the invitation for it to travel up the intake tract the less the cylinder fill will be.
Jon Kaase Tests Airflow Dynamics With His Finger
Maybe ask @Mattax what all that black stuff is in his intake manifold.
Heres what some one very knowledgable had to say about it:
"That inlet manifold looks to have large area runners, although I'm not familiar with the manifold. If they are large then thats not helping the situation. Large runner areas enable any reversion pulse to miss the incoming fuel charge. Its the mixing of the 2 that causes a collapse of the reversion, its a thermodynamic thing, the heat absorption of the fuel and cold air forces the reversion charge to stop progression and start the flow towards the cylinder slightly earlier than the piston alone would do. So when you have a cam thats opening the intake valve too early for the combustion progression of the cylinder a smaller runner intake can help. But its not fixing the real thing, the real thing is to do things that make the combustion go faster."
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