At 10:28 AM 12/7/03 -0500, Dean Tetterton wrote:
>Well Jack, I guess everyone else is like me and don't know enough about the
>subject to comment. Without
>a flow bench it is hard to know what you are doing. I was hoping others
>would chime in and I could learn
>a little something or get even more confused on the subject.
>
>The thing I have noticed in looking at manifolds and heads, is that the late
>TR3 and early TR4 heads are too big to begin
>with.
I've sawed apart a number of heads and compared the actual shape of the
intake ports. What I've been able to determine is that the only difference
in the overall shape of the intake ports is, as you point out, the diameter
at the face where the manifold bolts on. The larger ports were created with
a machining operation, and that larger diameter extends just a little ways
into the port. It's that small area next to the manifold face that is the
culprit, hence, if you fill that in, the heads are equivalent. I think
we've thrown away a lot of very good heads -- but I'd like to hear from
anybody who has seen something other than this. Conversely, I've
experimented with flow of ports where the only change is to continually
increase that taper, thinking that If I could do that and then put on the
appropriate manifold, I'd have a killer head. Guess what? It flows LESS
this way.
> To get a small decrease in the area from the carb to valve it would
>require making the head port smaller where the
>intake attaches. This is what they did with the late TR4 and 4A head, thus
>making it the head of choice. I guess when
>we run out of 4A type heads we can start milling the early one's and putting
>in a sleeve to decrease the size.
I agree, or you could gamble and do it with epoxy.
> I would guess that the biggest improvement could be made in the shape of
>the port closest to the valve. Smooth out the
>long and short side of the turn.
I've seen a number of approaches to this in the heads I've taken off of
engines and measured. I strongly recommend against any grinding on the
short radius. A couple of the pros do alter the short side radius, but it
is a risky approach without a flow bench because you can very easily
decrease rather than increase flow.
> I have mixed feeling about shorting the
>valve guide where it enters the port. Something needs
>to be done but if the guide is shortened, flush with the top, will the
>valve run hotter because it can't transfer the heat to the head as well?
I've shortened them and I've left them long, and have seen no difference in
valve life or guide life. It makes sense that an engine that will expect to
see many miles of use would wear out its guides faster, but on race engines
that have proper valve train geometry, I have seen no difference.
If I were going to build a head and did not have access to a flow bench,
here's what I would do:
1. Use the late head
2. enlarge the diameter at the manifold face to 1-9/16". No larger.
3. Don't do any grinding on the short radius.
4. Do enlarge the valve pocket below the valve per Kas's book, either by
machining or by hand. The outside wall of the port is quite thick and there
is little danger of going through it.
5. Have your machine shop give you a competition valve grind -- 3 angle.
This one change has a bigger effect on flow than any of the other
individual changes (by themselves).
The perplexing thing about porting is that when you think you have figured
out where those little air molecules want to go, and you come up with that
magic tweak, it almost never works. If you're going to do a lot of
grinding, you just have to measure it, or otherwise you end up with less
and not more flow.
One final comment on the business of a constantly decreasing diameter.
Let's say you have a one foot long tube, constant diameter, and you measure
the flow through it. Now let's say that you machine this tube so that it is
larger at the inlet end by only .012 inch, and you measure the flow. You
find that the tapered tube, slight as it is, flows more air. Fascinating, huh?
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