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Author Topic: 44 VS 42 valves that is...  (Read 298 times)
type149doug
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Posts: 26


« on: May 24, 2020, 00:10:10 am »

   In an ongoing debate within myself and overthinking most proven facts.  I'm asking your real world experiences.  I know that a 44mm valve will generally out flow a 42mm valve while a 42mm valve will provide more port velocity over a 44mm valve.  The engine I intend on assembling will be 84X94 with 5.5" rods.  I figure the shorter rods will keep piston speed up so the chambers should fill quickly.  Which makes me think that I'll leave something on the table by going 42mm.  I know that this revolves around the CAM!!!  I figure that an 86C would get me moving pretty good in a heavy GHIA but there are a few other cams that might be good to.  FK series or one of the SLR XRs.  If I go 42mm a longer duration cam would allow the velocity to fill the cylinder faster(I think)   which is where I want to be(once again, I think).  May I ask your opinions and why you feel you'd choose 44 over 42?
                 
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mikko k
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Posts: 104


« Reply #1 on: May 24, 2020, 06:32:49 am »

Too complicate thinking...

For that size engine, I would go to 48mm intake valve.
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PPRMicke
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« Reply #2 on: May 24, 2020, 09:03:14 am »

f you think of flow
So you can't just talk about the surface of the valve plate
What determines is how the valve seat is internal dimensions in mm
What is most important is what flow you have in it
You can get with a lot of work almost the same flow on 42-44 mm valve what will differentiate is on high lifts (14-16mm)
If you have numbers on your head you can calculate Hp
Many people mistakenly think which valve is best as 42-44 mm
When not looking at the design of the intake port and size
For info you can get more than 200 cfm @ 25 "on a 40 mm valve Then the valve seat is about 37 mm in the hole
/// M
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Torben Alstrup
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Posts: 710


« Reply #3 on: May 24, 2020, 13:25:10 pm »

f you think of flow
So you can't just talk about the surface of the valve plate
What determines is how the valve seat is internal dimensions in mm
What is most important is what flow you have in it
You can get with a lot of work almost the same flow on 42-44 mm valve what will differentiate is on high lifts (14-16mm)
If you have numbers on your head you can calculate Hp
Many people mistakenly think which valve is best as 42-44 mm
When not looking at the design of the intake port and size
For info you can get more than 200 cfm @ 25 "on a 40 mm valve Then the valve seat is about 37 mm in the hole
/// M
All true, but it somewhat depends on where you want the power, - in high rpm only or over a wider powerband. Next, a common mistake that many head porters do is focus solely on higher lift/flow which is really only good for weekend warriors and drag engines. By reversing a little back towards the old 88% rule you can make the port and seat flow significantly more at lower lift, which is what makes torque. A 40 mm valve with a 37 mm seat can flow incredible hugh numbers at high lift, but the flow at low air speed and low lift is typically rather awfull. Decrease the seat ID with just 1 mm or a little more decreases flow at high lift but increases flow at low lift. On 42-44 mm valves I have often been able to increase flow below 10 mm without sacrificing flow at high lift on street/super street cylinderheads. In fact the heads sometimes get more efficient at higher lift too, but that is very port bowl specific
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type149doug
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Posts: 26


« Reply #4 on: May 24, 2020, 15:45:15 pm »

Thanks for your input...  This got technical quick! 
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PPRMicke
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« Reply #5 on: May 24, 2020, 18:05:47 pm »

Type149 doug

If you want to delve a little more about flow I have a thread on a Swedish forum
Unfortunately, in Swedish there are some pictures away unfortunately
 But you can use Google translate
/// Micke
https://boxerville.se/forum/viewtopic.php?id=10359
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dangerous
Sr. Member
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Posts: 263


« Reply #6 on: May 26, 2020, 09:41:54 am »

There are a couple of different methods around,
for choosing an intake valve size in a normally aspirated combination.

I prefer the old 'minimum cross section' method/formula
and, in most engines, this minimum is preferred at the seat throat.

In nearly every engine this throat is within 2% of the "90% of the valve",
but there are exceptions.

Most VW engine builders pay little attention to this,
and the 'minimum' is in the manifold and/or carb venturi.
This can still work,
but explains why a lot of these engines' torque graphs lack at lower rpm,
and fall away so fast, even when larger duration cams are used.
(as well as an intake runner length that is wrong for where most people want their peak hp-rpm).

The other method, is to choose something much larger in port and throat, (valve size over 52% of the bore size),
and then optimise the cam events, to put some torque back into the curve in the desired-rpm-range.
This method is an evolution from what has been learnt
from multi valve intake systems, as well as large 2 valve 'unlimited rpm' racing engines .

Not many of us, (with our long pushrods and wet sump oiling)
can reliably use the rpm that would be required, to create the ideal air speed through the 'larger minimum',
but careful selection of valve events and runner length can fudge the curve back.

Of interest, most of the 'off the shelf' heads use the same seat-inside-diameter and port,
as the larger valve options,
which means the smaller valve option is wasted, but it can be improved with a suitable valve job,
appropriate cam events, and runner length.

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