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MG MGB Technical - Cylinder head porting

Upon comparing an intake/exhaust gasket to my cylinder head I noticed that the exhaust hole in the gasket is much larger than the exhaust port on the head.
-Should there be any problem with matching the port size on the head to the gasket size?
-Will I run into coolant or oil passages?
Thanks,
Robert
Robert

Robert-
The exhaust portsd on the MGB head are almost oversize to begin with. you enlarge them without radically increasing the amount of exhaust gases escaping through them (as in a race engine), the engine will suffer from loss of scavenging effect. If the engine is stock, leave them alone.
Steve S.

Robert-
Don't enlarge the exhaust port to match the gasket.
1)The push-rods pass nearby and you don't want to break through into that. 2) This is not the area where flow is that limited. At the entry into the port, the guide bosses severely limit the exhaust flow, and that is where most people reshape to successfully gain improvement.
You can polish with a flap wheel and take off any roughness left from casting. If you are interested in any serious results you should read Peter Burgess' book on "how to Power-tune MGB engine". FRom here you can either try the daunting task of modifying the head yourself (for seriously involved home mechanic) or buy a re-worked head from Peter Burgess in UK or from Sean Brown in Oregon.
If you are just trying to take care of a few odds and ends, I recommend matching your exhaust manifold openings to the head openings. Due to luck of the draw, you may or may not have a good fit. I had to enlarge the manifold opening for one opening (it was offset), clean up signicant extra casting material on another, and round out the corners on all three. The stock gasket is an approximate fit, and I wouldn't use it a guide for any modification on my engine.
Tom
Tom Lennon

Most of the extra head material is at the top of the port, if this were removed this would raise the port height and should result in a higher flow rate. Most of the exhaust gas will be along the top of the port since it's initial direction is up. Correct? No?
Are there any oil passages or coolant passages above the ports?
I have a copy of David Vizard's book on tuning the "A" series engine(yes it is a different engine but they are very similar)

Robert
Robert

Robert-
Yes, much of the exhaust does tend to follow the larger radius curve of the port. Yes, there are cooling passages above the exhaust ports. Improving exhaust flow in the ports is relatively pointless unless you improve the flow in the intake ports. If you don't want the extra expense of professional porting, remove your old valve guides,
use a Dremel tool with a #80 grit and then a #120 grit sander to gently smooth the existing port contours. A mirror finish is not only unnecessary, but is actually undesirable as it will lead to the fuel condensing on the port walls and a consequent loss of power. Have installed at equal depth lead-free tolerant three-angle nickel-chrome seats and three-angle 214N alloy austenitic stainless steel valves with tuftrided stems and stellite tips (don't panic, they're common and not very expensive), tapered valve guides (be sure to use manganese silicon bronze guides on at least the exhaust valves to help get rid of the extra heat that always comes with extra power), a set of the highly superior Fel-Pro umbrella-style valve stem seals (FelPro stem seal # SS 70373 - for Chevy Vega 4 cyl 140, '86-92 Ford 351W ) and APT Part # 70373), 1 1/2" SU carburetors with richer needles, 6" diameter X 3 1/4" deep K&N aircleaners, and a 1 3/4" Peco exhaust system. Be sure to carefully blend the port to the seat. This will get you started with a relatively small investment and you'll be both surprised and impressed at the improvement.
Steve S.

Robert,

We don't even enlarge the exhaust ports like that on racing engines. In fact, on the flow bench I have actually added modeling clay until the port was 84% of it's original (stock) cross sectional size before I saw a reduction in the flow rate, and that was not what I would call high-tech sculpting either. The intent with the exercise was mostly to see whether a "plate" added to the exterior of the head could be used favorably in speeding up the exiting exhaust flow.

There is another problem with enlarging the exhaust ports like that too. As they are, they allow some measure of reversion avoidance. If you match them exactly to the header, you will find the tendency for this to be reduced dramatically. And I will say that reversion on either inlet or exhaust will "soften" the power output of these engines noticeably.

Another story, We were confronted by a racing head (WE had not previously ported) that was in need of some re-work. The previous "specialist" who had modified this head had indeed matched the ports to the gasket. He had also done a lot of other questionable acts in an attempt to gain power, while leaving the most important areas untouched (thankfully for us).

We considered the head salvageable and went to work making everything as good as possible. In order to impart some measure of reversion resistance, we cut straight back along the gasket line (which the port had already been cut to) about 1/4" to form a very sharp step and this was done on both the top and bottom of the port. There were some other tricks used on the exhaust ports, but the upshot of the event was that the head performed significantly better than before. We have also been known to make "ditch" cuts on the manifold face inside the gasket line, but outside the port. This forms a very sharp "tooth" the reversing flow is faced with, and the effects are noticeable.

If you have Vizard's book, I suggest you read the part about port matching (and specifically the uses of mis-matching the exhausts) and then re-consider what you intend to achieve. I think you will find that David is thinking along the same lines we are...

Sean
Sean Brown

Sean-
You have so many interesting facts!! Just when does reversion show itself? I never thought that opening the flow from head to manifold could be deleterious. I did not enlarge my exhaust ports, but I did remove all the restrictions the cast iron manifold presented.
Tom
Tom Lennon

Tom,

I'm sure you did fine. Opening up the exhaust manifold as you described is good practice. Making an effective step for the reversing exhaust gasses to overcome is the goal.

Since flow in an engine will "pulse" at a frequency dependent on many things, reversion is likely to occur any time the ports are not "tuned" to these related factors.

It is generally assumed that the ports are best tuned at peak torque RPM, and that this is where peak volumetric efficiency occurs. At that time, it can be assumed that no reversion is happening.

However, the torque peak is a culmination of all the component's various individual peaks, so the exhaust ports themselves may be more efficient (and "in tune") at a slightly different RPM than that which you see as your peak torque RPM. The same goes for inlet ports etc.

As engine speeds go higher than a particular engine is designed for, there is not so much a reversion process as simply inadequate charge filling and exhausting. So as you probably already know, that is why racing engines have longer period camshafts and ports designed for higher RPM use. It is also why they idle so poorly!

Sean
Sean Brown

Sean- Thanks for your answer. I am more than happy with the engine I just rebuilt...it wants to rev, has a broad power range, and seems very quick. In fact I feel like I have to hold back when driving...
Many folks have disparaged the 4 cyl, saying one could never get adequate power, only the V-8 would do. I'm very happy with the results. But, I can never learn too much, especially as I'm ready to start another engine.
Tom
Tom Lennon

This thread was discussed between 04/06/2003 and 06/06/2003

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