I don't understand a claim made by Jafro's machinist in this video (around 26:00):

Still from video

(Jafro inspects undercut valve)

Machinist: You drive it on the street too, don't you?

Jafro: Mhmm

Machinist: If [the valve face] is a little bit off the edge it's because it's seeing street duty.
If all it was was a track queen, we'd try to move it aaaaaalllll the way to the edge. But always keep that street in mind.

I don't get it. Why is pushing the valve face all the way to the edge a bad idea if the engine will see street duty, but fine if it's meant to see track use only?

  • can I ask, what is your definition of 'street duty'?
    – axa
    Apr 18, 2017 at 4:14
  • @axa to me the difference between street duty and track duty boils down to engine load and speed. Street duty will see mean that an engine spends a fair amount of time in low-load/low-speed operation
    – Zaid
    Apr 18, 2017 at 20:10
  • 1
    agreed. I think the mech just chose an odd way to say if running on the street you don't want compromise reliability...
    – axa
    Apr 19, 2017 at 6:20
  • I'd have thought that street use is less taxing on the valves though, in which case the valves would last longer if they see street duty
    – Zaid
    Apr 19, 2017 at 6:25
  • 1
    I'm going through a similar situation now, cut valves, decked the head, and failure at 1000 miles of 'street duty'... Im saying I just rebuilt and I have to go through this again... but if you have a vehicle that only sees the track then chances are you going to tear it down for whatever reason way before these valves give you trouble...
    – axa
    Apr 19, 2017 at 6:32

3 Answers 3


having dabbled in valve work including back cutting, Ive heard that reducing edges helps air flow. if these seating faces are closer to the edge then at least in theory be more aerodynamic and less of an obstacle, and perhaps receding deeper in to the valve seat while closed would be some kind of benefit, in theory...

so keeping these faces away from the edge will increase​s durability, especially in respect to heat, therefore I imagine that he judges the street vehicle would want durability over performance. especially for as in my opinion/experiance, the durability takes a big hit to this perceived performance gain.

i could be wrong, just my opinion and interpretation of what you posted

  • Also fwiw, as a real life example you can see my other post, what thinning up valves may have possibly contributed to: i.stack.imgur.com/i3Ri6.jpg here you can see a uniform decrease in size of the valve head as heat ate it away. it got smaller and smaller and recessed further into the seat
    – axa
    Apr 17, 2017 at 16:17
  • Jafro replied
    – Zaid
    Apr 19, 2017 at 20:11

Removing material from the valve stem makes it weaker. If you remove too much material and it becomes too weak, the valve breaks off and typically wedges itself between the piston and the head, causing the car's power to decrease by 100 percent and dollars to rapidly leave your wallet.

On a race engine which gets torn down frequently, a few extra hp worth of flow might be worth the price of making your valves a inspect/replace item. On a street car, you aren't going to be frequently inspecting the valves, so you really need to err on the side of caution to avoid expensive catastrophes.

One other consideration with engine setups for the street is that you aren't constrained by the rules of a racing class, so you can get extra head flow from forced induction and nitrous on a street car rather than trying to squeeze power from a rules compliant engine. This is why you see spec miata guys paying 20k USD for an engine that makes makes 150 horsepower while the street guys are throwing together reliable 250+whp builds for under 5k USD. It isn't that making 150hp from a miata engine is hard, it's that it's hard to do without breaking the rules of the racing series.

Keep in mind that there is no guarantee that undercutting a valve gives any power at all, since flow through the head is determined by a LOT of variables, the size and shape of the valves being only a small part of the big picture.

  • What you say makes sense, although I have to wonder if durability/reliability is guaranteed to take a hit when a valve is undercut to the extent that it becomes noticeable. In other words, is it possible to move the valve face all the way to the edge without it suffering from the streetability issue that the machinist alluded to in the video, and why?
    – Zaid
    Apr 17, 2017 at 16:33
  • 1
    @Jim - Read this article, if you would, please: stangtv.com/tech-stories/engine/… ... while this is from a forum, the explanation comes from Ferrea. Apr 17, 2017 at 18:18
  • I wouldn't quite guarantee that you'll take a performance hit- the valve stem might be massively over-engineered (why add the weight for this?)- but my guess is that the valve stems are probably as thick as they need to be for strength with a margin to allow reliability- shrinking the diameter to let more air flow around the stem sacrifices that safety margin for increased flow.
    – Jim W
    Apr 17, 2017 at 20:07
  • The article pretty much says that some engines respond better to some tradeoffs than others- for example they only recommended undercutting for certain poorly flowing heads. But these might happen to be applications that don't require the full strength of the stem anyway... probably because they're poor flowing heads with low rpm potential.
    – Jim W
    Apr 17, 2017 at 20:09
  • er that was reliability hit in the post above, not performance hit
    – Jim W
    Apr 17, 2017 at 20:12

I decided to ask Jafro in the comments section:

Around 26:00 your machinist talks about keeping the face off the edge since the engine's going to see street duty. Why does he say that? Does the longevity of the valve take a hit if the face is pushed right to the edge?

Here's his reply:

Yes, that's exactly right. The extra edge around the seat serves as a heat sink. If you've got a red-hot glowing valve slamming shut 3500 times per minute (valves operate at half the frequency of the crank), they stay together longer if they're not slamming shut on their edges

So according to Jafro, cutting the valve face so that it's right on the edge has the effect of reducing the useful life of the valve, regardless of whether it sees street use or track use.

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