Question: Do valves with neck down stems flow better than straight stem valves?

Straight stem (left) vs. neck down (right)

The common assumption is that valves with a smaller diameter where the valve stem meets the head of valve will flow more air. That assumption, based on our testing, is not necessarily true. Let’s look at data from a recent series of tests.

1.94″ intake valves are popular for many heads used in circle track applications such as the Dart SS, Chevy Vortec and Engine Quest cylinder heads. Recently, we performed an extensive series of tests with 11 different 1.94″ intake valves on 3 types of heads. The valves varied in margin thickness, stem-to-head radius and stem neck-down. For this series of tests we actually had a couple custom valves made with 7 & 8 degree radiuses. After hundreds of flow tests, we ranked the valves from best to worst.

Conclusions: What worked and what didn’t

1. The “neck down” stem valves were never the best flowing of the 11 valves we tested.   However, they always flowed in the top 5 of the 11 valves tested.
2. The best flowing valves in each of the 3 heads tested was a straight stem valve.
3. The custom 7 & 8 degree radius valves sucked cannel water.
4. If you do not have neck-down stem valves in your heads it probably is not worth it to change to a neck-down style valve. Actually, some tracks do not allow neck-down valves.
5. The Chevy 602 crate 1.94″ intake valve was the worst flowing valve.

Flow testing 11 different intake valves in 3 types of heads was a lot of flow testing.   What makes this type of testing fast and accurate are our flow bench Sliders.

With our Sliders, valves can be changed in a head without taking the head off the flow bench! Yes, by sliding the head bolted to a Slider all the way to the side of the flow stand, the valves can be changed without unbolting the head. Valves can be changed in a matter of seconds.

That brings us to the next “Air Flow: Fact or Fiction” question: Do 7 mm & 5/16″ valves flow better than 11/32″ stem valves?