Supertech "hi flow" vs "standard flow" intake valves
 

Valves for the Mazda Miata from HorsepowerFreaks

Towards the bottom, note there is a "std flo" and "hi flo" intake valve for the 1.8.

Anyone know the difference?

At face value it looks pretty obvious. The diameter is larger on the hi-flow. I'm curious if it might have to do with extra length too.

I did the right thing and called 'em.

Nope, it's not the diameter.

The hi-flow valves have an back-cut face and undercut stem. This is what WE want. The std-flow don't have those, and are used for racing classes which don't allow "modified" valves.

The shorter length stems are an option on the +1 valves, because they tend to sink in the seat; these will typically require shorter stems so that the shims required are close to standard.

BTW they make shim under bucket lifters too. List price $22 each.

Novel concept around here! Great information to have.

High flow valves are back cut. The standard flow valves are stock replacement. You obviously want high flow. They come in a lot of different lengths, I don't know which are correct.

do you have any picture to explain the difference? i just bought a set of manley +1 valves and i wonder if those are back cut as well?

Back cut
http://home.comcast.net/~v.navarrett...es/Backcut.jpg

Undercut stem:
http://www.fourseasontuning.com/parts/109.099.jpg

22/ea shim under lifters is a ripoff (probably rebranded MMS). You can get those from miataroadster for $200 for a complete set of 16. Then buy 6mm lash caps for a domestic for dirt cheap (<$20) and grind them.

I prefer the fatter, lower flowing valve because I've seen so many under-cut valves fail that I'm afraid and alone, in the cold.

Undercut stems are usually not recommended on exhaust valves.

Are the failures you've seen on exhaust valves?

it bothers me how long i've had both undercut valves and shim-under buckets sitting in my basement.

I only have generalizations, not hard facts. NEVER let facts get in the way of a good rant!!!

These valves will decrease spool by 1000 rpm.

Most serious engines will have undercut exhaust valves. They are very fragile, and usually get bent from careless handling of the cylinder head with cams still installed. I learned that the hard way. I have also gotten brand new valves that were slightly bent from improper shipping.