If you need examples do more research. Your on the right track but there’s a lot more to it.
My take on you post was that there is no downside to an intake valve back cut and you did not note any of the possible other issues in your initial post.
My point was to note the potential down side of it especially in a racing application. If a well designed port and back cut valve are used, the potential of dilution becomes more possible and a loss of low end usually results with that. It’s another case of flow bench “progress” not quite being reality. That means among other things the camshaft design becomes more critical as you note in your response to my post.
In short- like everything else the whole design has to be understood and the package put together with parts that exploits its advantages and minimizes the negatives.
Not sure who can view this 17 year old post from Larry Meaux on
SpeedTalk.com so I’ll copy and paste his comments. I’d would suggest you read anything he writes. Also look at Don Terrill’s book The Horsepower Chain and pay attention to the areas regarding cylinder blow down, the intake cycle, and camshaft overlap.
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- Post Tue Jul 24, 2007 2:47 pm
the "Engine Masters" Summer 2007 issue
on Pages 74-81 just about "mirrors"
my Dyno experience with increasing Low-Lift Flow
or the effects of Low-Lift Flow on bottom end RPM Torque output.
In that series of Tests...they used different Cyl Heads....
and basically came up with same results as i found.
in my Dyno Tests..i used same exact Head,
didn't change the casting or CSA, or Volume CC's to any extent,
just increased Low-Lift Flow.
2-Valve Heads
So far my Tests show increasing Low-Lift Flow always
"Hurts" Torque "below" the RPM point of Peak TQ
and sometimes increases top-end HP
...just the opposite of what most people think.
most will "equate" increases in Low-Lift Flow = increased bottom-end
or low RPM Torque......but i've never seen this to be True yet,
even with a smaller duration Cam like in the series of Engine Master tests.
The 2 Cyl Heads=> AFR and the TricFlow 's
had the best .100" Lift Flow to .300" + Flow
but the TQ output was not as good or any better at 2700 RPM to 3500
as the worst low-lift Dart Head
(especially at 2700 RPM)
i wonder what was happening below 2700 RPM ??
along with gas mileage MPG ?
the 2 worst Low_lift Heads (Dart + Edelbrock)
made as good or more low RPM TQ.
the better test would have to change Low-Lift Flow on the same exact casting one at a time..and repeat those Tests studying Low_lift Flow effects.
There are times you can increase Low_lift Flow then use an even shorter
duration Cam (like 4-Valve designs)
and Pickup HP and TQ at low to midrange RPM,
but usually always looose top end HP cutting down duration to achieve that.
Also Heads with CSA Choke problems respond favorably to increased
Low to Mid-Lift Flow so far in my Tests.....but overall the best method
is to fix the Choke ...and not rely on Low-Lift Flow so much.
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