head porting bang for the bux trix
 

What I learned, in rough order of bang for the buck/time

1) Of all the head porting trix the most bang for the buck is indeed to backcut the intake valves.
2) Next is to just pay extra for Supertech intake valves if the intake valves need replacing.
3) Next is a 3 angle job, esp. if you're doing a valve job already
4) Next is if the exh valves need replacing AND a 3 angle job is planned, pay extra and get +1mm Supertech exh valves.
(Rebello tells me that +1mm intake valves need to have extra bowl work to make em flow better than standard.)

After that some mad skillz are necessary to do it right:
5) Blending back of seat into throat
6) Other Bowl work
7) That ridge in the short side when the air makes a turn.
8) Then teardropping the guides.
9) After that is their top secret labor intensive stuff

The most common problem in medium and hi miles heads is leaky exhaust valves.
I've seen this on 4 miatas now.

how to test

Lay the head on its side and point the exhaust ports up. Pick the 2 or 3 cylinders whose valves are closed and the lobes are pointing away from the valves. Pour some kerosene or thin oil or spirits into the ports. Watch for weeping into the combustion chamber. I'll bet you a donut some or most of the exh valves will weep, and the intake valves are fine.

Turn cams to close other valves. Repeat test on them.

With leaky valves turn cam to max lift and look at valve seating mating surfaces. Mating surfaces should be a shiny unbroken ring. Look for telltalle little burn marks in this ring, especially on the exhaust valve. There's your leaks. If none, look for dirt that prevents seating, clean it out, repeat leak test.


If you get some weeping (but not dripping lol), the cheapass way to fix it is to lap the exhaust valves by hand yourself. This will improve your leakdown and increase the life of the valves. While your at it, check the guides by checking how much side to side play the exh valve heads have.

I like this post, especially the leak test. If upgrading to a 1.8 didn't yield such better results, I'd be doing this.

They completely left out blending the back of the seat into the port. Thats like the #1 cheap and "easy*" flow increase.


*easy assumes you have a steady hand and experience porting heads.

EDIT: To add more, it almost doesn't matter a whole lot for uber porting anyway. The stock lift on miata heads is so low that you will never see the flow ranges that you can achieve with anything really beyond a vj and some mild porting.

Does anyone have pictures to illustrate these concepts (bowl blending, blend the seat into the port, etc.)?

/n00b

What a timely article...I've been out in the garage with a Dremel every night this week.

+1 on what neo said about blending the valve seat with the bowl. I was very surprised to see how big that lip can be...it's almost as bad as a 2.5" gasket in a 3" exhaust pipe...

For bowl blending check Hakuna's page koto.

Other than that I'm just taking casting roughness out, and doing a little deshrouding in the combustion chamber.

I don't disagree with the generalizations listed above, but it would be helpful if there were some flow bench numbers for a 1.8L head that detailed the gains associated with the various steps. Does anyone have flow bench numbers (@ 28" water) showing the intake and exhaust flow of a stock 1.8 head at .100, .200, .300, .400 & .500 lift that they can post up??

.400 and def .500 are basically useless to the majority of miata owners for the pure fact that they will never invest the money to ever realize those flow ranges.

True dat on the absolute lift with standard valve train. For trending and seeing where the port stalls, it would be nice to see .350 and then .400. Numbers anyone???

You will find this community is to...frugal... with their money to get results like that. I've ported heads and seen gains, but even I didn't want to pay for flow benching. It's highly overpriced. The vast majority of users, even here where performance is king, have 0 headwork on their cars.

I like #1 since I just payed to have my valves back cut, and #3 since that was done as well.

How much did your machine shop charge to back the 8 intake valves?? Do you have before and after pictures???

Head diassemble, reassemble, 3 angle valve job and back cut valves both intake and exhaust $320 I do not have before and after pics of the valve work. I will have after pics later.

I would pay for pictures since I could do the job at work with a little guidance and measurements.

Too much to ask that you remove one valve and attack it with some calipers for me?

Here's a start:

This site is focused on SBF Ford wedge heads but provides an overview of the tools required for porting, shows how to make port molds, how to blend the valve seat to throat transition, how to streamline the valve guide etc.

diyPorting.com

Standard Abrasives DIY head porting guide (Very basic porting, mostly smoothing and gasket matching)...
http://www.pirate4x4.com/tech/billav...0Abrasives.pdf


Engine Blueprinting book by Rick Voeglin has a decent chapter on cylinder head prep which includes some porting basics.

Not to contradict the postings about head work.

What I ask myself when I read all the above is if this car will be running the quarter mile in 9 seconds or is it been prepped to be a dyno bitch to see if it reaches 600 hp etc or whatever.

What I am trying to say is I have seen plenty of cars with UNTOUCHED cylinder heads doing 10 and 11 seconds, of course the engine has forged rods and pistons. I sometimes think porting is over-rated and unless every last drop of HP is the goal there really is no need to even touch those cylinder heads.

Do not misinterpret what I am saying, hey if you got the money go ahead and get yourself a badass port work. I think much more can be attained with a custom made intake manifold, bigger throttle body, cam (integralcams.com) etc. IMO, I would not touch at all the cylinder head... just my two cents.

+1 I think most of us can just turn up the boost if we want a little more, instead of spending $$$$ on max efficiency

but you guys miss the point.

it's right there in the thread title.

"bang for the bux"

in other words, do them in the order posted if you're doing it on the cheap.

An internal combustion engine is just a glorified air pumping system. Total system throughput is only as good as the weakest link. What good is 250CFM of custom intake runner going to do on a cylinder head that only flows 180CFM on the intake port? Cam lift beyond where the port stalls is useless and a long duration cam on a doggy port yields a peaky power curve with no low end and mid range torque.

Burp
 

Tell that to all the people on the racetrack who are doing 10s and 11s on stock cylinder heads...