flexi New Reader
3/23/14 4:12 p.m.

Have a miata vvt head opened up and noticed that none of the valves have the classic 3 angle grind recommended by the FSM. Is this ok? There's just a one angle grind on both the valve and head. Is there any advantage to a single 45 degree cut? This will be for a turbocharged application.

Take to a machinist to fix, leave it and lap it, or look for a new head? I've never done a head before so I'm not quite sure what is ok or not.


mr2peak HalfDork
3/23/14 5:00 p.m.

3 angle should make more power. It's all about the $.

Zomby Woof
Zomby Woof PowerDork
3/23/14 5:15 p.m.

The other angles may be there, but you may not be able to see them, or recognize them. Multi angles are better for flow. You don't need it.

flexi New Reader
3/23/14 6:22 p.m.

In reply to Zomby Woof: There are vestiges of the angles on the head. Only 1 angle grind on the valves - even when held up to the light.

How much benefit might there be going with a 3 angle? Is it worth the $$$ given that there is a t3/t4 pushing air through the intake?

iceracer PowerDork
3/23/14 6:31 p.m.

it's just one of those things you do when doing a valve job.

How much benefit can be debated.

HappyAndy SuperDork
3/23/14 8:41 p.m.

I thought the way it was done is that there is one angle ground on the valve (the part that moves) and two or more angles ground, or cut, into the valve seat. I've never heard of more than one ground surface one the valve.

Also, if I understand correctly, most OEMs only do a two angle grind on the seats, but three or more is preferred for performance builds.

I've even heard of a seven angle valve job, but I have a hard time believing that is really beneficial. (Maybe on a giant big-block?)

Streetwiseguy UberDork
3/23/14 11:08 p.m.

I think it would be pretty rare to have more than one angle ground on the face of the valve. Seat, yes.

oldeskewltoy Dork
3/23/14 11:23 p.m.

typically... the 3 angles are all on the valve seat.

A single grind on the valve is to allow a seat to seat fit. A second cut on the valve is commonly referred to as a back cut

A 3 angle valve job with a back cut will usually run about $200-$400... and is something I recommend. Another point... often times the valve seat will be at one angle, and the valve ground at a slightly different angle... for example, the view above the contact surface of the seat was ground to a 45 degree angle, while the valve is ground to a 44.5 degree angle...

Edit: as far as NEED... as long as you have a good seal between the valve, and the seat - your fine. This includes a simple re-lap, to 2 surface grind(like you have), to a 3 angle, to a 3 angle with single back cut, to a multi angle with radius back cut.... Each is a step fancier, but all are designed to seal the valve and seat

wbjones UltimaDork
3/24/14 6:50 a.m.

noob here, how do multi angle cuts improve hp/flow …

bentwrench Reader
3/24/14 6:58 a.m.

Only three angles???? Maybe on the valves, the seats can easily use 5 angles.

The valve job does not end at the seat, it is best to blend the flow into the adjoining surfaces.

z31maniac UltimaDork
3/24/14 7:56 a.m.
wbjones wrote: noob here, how do multi angle cuts improve hp/flow …

SWAG coming up, more bends = less angle change between the surfaces = less disruption to the air = better flow?

Streetwiseguy UberDork
3/24/14 8:12 a.m.

Also a smaller surface , so higher pressure on the seat, so less likely to retain carbon umder it causing trouble.

Zomby Woof
Zomby Woof PowerDork
3/24/14 8:24 a.m.

When we do a valve job, we do the typical 3 angles on the seat, and 3 angles on the valve. The 45, a back cut above the 45, and we relieve the transition edge from the face to the margin. We've measured substantial flow increases on the flow bench with just the last one.

On the exhaust, you want lot's of contact on the 45 to carry heat away, especially in a turbo application. The 45 can be narrow (better for flow) on the intake side.

WonkoTheSane GRM+ Memberand Reader
3/24/14 9:13 a.m.
wbjones wrote: noob here, how do multi angle cuts improve hp/flow …

The idea here is from fluid dynamics studies/experiments (at the speeds air is moving through the head, it's effectively a liquid for all practical calculation purposes). Basically, you have a tube (port) that opens into a large chamber, and the "Corner" is 90*. The more viscus a liquid is, the harder it is to get it to "turn" so it has trouble filling the chamber and effectively getting out of the port. Putting a radius on the edge of the port makes it flow into the chamber better.

I forget the exact formula, but the gist of the equation is that you can only turn air 15* at a time over X distance (distance changes as the viscosity & velocity of the liquid increases).

So, by having multiple angles, you're just making it more efficient to fill that chamber. Better efficiency means more air & fuel can get into the chamber, it's going to be less turbulent and have better atomization, so you lose less energy just filling up your cylinder. A 7 angle valve job will be better than 1 angle, but from a practical stand point, on am street/mild-race engine, you're not going to see too much difference with anything better than a 3. That said, I wouldn't get less than a 3 angle, either.

After that, you're onto the actual design of the angles, which is another huge tangent, but Zomby Woof summed it up nicely :)

oldeskewltoy Dork
3/24/14 11:27 a.m.
wbjones wrote: noob here, how do multi angle cuts improve hp/flow …

simplest way.... envision a velocity stack in 2 dimensions... in essence the back cut valve and the throat cut angle[s] act similarly to how a velocity stack works...

Leafy Reader
3/24/14 11:45 a.m.

I've also heard of radius valve jobs, where there are no angles, just a smooth radius. Obviously the smaller the angled bit the valve actually sits on the faster the seat is going to wear out and the faster you're going to have to re-adjust lash and no longer have the designed seat shape.

wbjones UltimaDork
3/24/14 1:11 p.m.

thanks Zomby Woof, WonkoTheSane, and oldeskewltoy … when the head needs to come off, guess that's how I'll go

flexi New Reader
3/24/14 7:41 p.m.

Thanks. Above discussion was quite informative. Appreciate the explanations.

chestertiger New Reader
3/24/14 7:46 p.m.

You can do it yourself if you're really good with a Dremel... loljk. Lapping always works well and is free.

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