drmike New Reader
Oct. 4, 2010 8:26 a.m.

I've recently finished a poor man's top end rebuild on my '94 Miata, and while I was in there, I did a street port and polish, along with port matching. I'd like to share what I learned, and I bet there are a lot of folks here who have their own experiences to share.

Or I'm wrong about some of that. So, do you want to see what I did?

BTW from what I've read, the best way to improve flow through a Miata head is to trade up for a '99 head and intake. Love to, man, maybe next time...

RossD Dork
Oct. 4, 2010 11:29 a.m.

Lets see some pictures.

RandyS Reader
Oct. 4, 2010 12:08 p.m.

"What I leaned" is don't touch the floor for a street/autocross head. Ruins flow.

pitbull113 New Reader
Oct. 4, 2010 3:18 p.m.
RandyS wrote: "What I leaned" is don't touch the floor for a street/autocross head. Ruins flow.

can you give more details.

drmike New Reader
Oct. 4, 2010 4:11 p.m.

Totally agree, I didn't touch the floors, either, other than to get them smooth. I'll get some pictures up this evening (stuck on the west coast, so it'll be a while).

oldeskewltoy Reader
Oct. 4, 2010 5:34 p.m.

I'd love to see some Miata 16V head porting, I currently do bowl work, and porting on 4AG heads

Here are a couple of my photos

Intake ports before and after... note nothing done to the horizontal part of the port, the bowls were blended, and the guide bosses minimized

Exhaust port in mid work, left side shows all the major work finished, right side shows untouched

Some of my bowl work - intakes

more - http://s79.photobucket.com/albums/j143/oldeskewltoy/MOmo/

drmike New Reader
Oct. 4, 2010 8:28 p.m.

Nice! Here are a few pictures of my work on the Miata head.

As this was a super-cheap top end rebuild, I didn't knock out the valve guides, so I had to work around them during the porting work. It's far from ideal, but good enough for my goals. This is a shot of the finished intake and exhaust port bowls. I cut them a little deeper, but I wanted to keep a smooth flowpath along the port ceiling down to the valve seat, so I didn't do a hack of a lot of cutting. I do a LOT more on MINI Cooper heads (first gen. BMW, not the old-school ones). Those ports are pretty nasty from the factory.

This is a view down the intake runner. As has been mentioned above, I worked almost entirely on the port ceiling. The port floor was only cleaned up, with no real cutting there. I did thin doen the septum a bit, but I didn't go too crazy. Honestly, for stock cams these ports are pretty big already, so I didn't want to decrease the air speed if possible.

It seems no matter how many pictures I take of a project, I want more. This time, I forgot to take pictures of the finished exhaust ports. They look about the same, though, with cleaned up runners and slightly deepened bowls.

One of the things I researched before cutting this head was valve shrouding. Given the lower compression built into the old BP05 engines, a bit of material removed from the combustion chamber isn't going to hurt. Besides, I had to deck this head 0.012 inches to get it flat again. So, I put the head gasket on the head and marked where the gasket laid on each combustion chamber.

Using the markings as a guide, I cut back the head around the corner of each port to unshroud the valves a bit. I did this for both the intake and the exhaust.

BTW This is my idea of a GREAT Saturday. Perfect weather, and a car to work on.

Anyway, even with the stock cams in place, the results are VERY evident on the butt dyno. The engine benefits from the work mainly at the bottom and the top; these mods have straightened up the torque curve a bit. Driveability around town is particularly nice, as the engine has a lot more off-idle torque, and the top end is a little more robust as well.

drmike New Reader
Oct. 5, 2010 7:32 a.m.

Anyone else got porting pics? Any head, any car, let's see 'em.

oldeskewltoy Reader
Oct. 5, 2010 7:29 p.m.

Dr Mike.... how long are the Miata intake ports? from the port divider, out to the manifold mounting surface.

drmike New Reader
Oct. 5, 2010 8:12 p.m.
oldeskewltoy wrote: Dr Mike.... how long are the Miata intake ports? from the port divider, out to the manifold mounting surface.

About two inches, maybe a bit more. I used the absurdly long carbide in the last picture to reach up there and thin out the divider. I had to be careful with the flapper wheels to keep from gouging the port with the tool's collet nut.

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