Jump to content
HybridZ

flow difference 44mm intake valve vs. 42mm


madkaw

Recommended Posts

All the searching I've done on these cylinder heads and I've never read a direct comparison between the 2. How much flow are you leaving on the table by not going to the larger valve? Would building a stock bore L28 with a larger cam, headers, etc.. be undermined by the smaller valve?

 

I should add that I am staring at an MN47 head as I ask these questions. With the D shaped ports that narrow down to 31mm it makes me wonder if the larger valve is still beneficial.

Edited by madkaw
Link to comment
Share on other sites

Not sure how you can calculate overall flow from just this one number because technically the head will flow differently at all different RPM's and bigger valves are better suited to higher RPM action.

But in terms of actual size of the valve, when you calculate the surface area of a circle with a radius of 22mm (44mm valve) you get 1520 square mm. A 21mm radius (42mm valve) is 1385 square mm. This ends up being about 9% bigger in terms of area not just the radius or diameter because you are dealing with a circular object. Experienced engine builders would know much better than me, but I am assuming the valve stem itself is actually restricting flow on some small level and because the stems (or the guides) are probably the same size in both scenarios the difference in flow between a 44 and 42 mm valve might be even higher. 

Link to comment
Share on other sites

Mack is correct. The area of the opening when the valve is depressed in the shape of a cylinder. The area is equal to the lift times pi times the diameter. The increase in cylindrical area would be equal to 44/42 or 104.7%.

 

The smaller valve can easily be compensated for by a slightly higher lift.

Link to comment
Share on other sites

You're already shrouded with the stock sized valve.

 

Look at the way the valve moves into the bore, and it doesn't even start in the middle. As the valve advances down and into the bore, it gets closer to the bore wall the whole time.

 

Going to a bigger valve can show some gains IF the port is worked to support it AND you have 89mm+ bores. Want more than that? Move the valve centerline over so as the valve opens, the valve head moves AWAY from the bore.

 

It's not exactly trivial, but it's certainly doable.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...