Can't increase spacing before section title

Question

I'm having troubles with increasing spacing between the header and section title. As suggested in titlesec package documentation, I added the following line to my preamble: \titlespacing{\section}{0pt}{4em}{3em}. If I understand correctly, it should provide 4em space before the section title and 3em space after it. However, it does not: that space before the title is clearly smaller, as you can see:

I figured one ugly workaround: if I write \vspace*{0em} right before the \section{...}, the gap increases, however this time it's definitely bigger than 4em:

The only way that could somehow solve my problem by far is to put there something like \vspace*{-40em}, but that's incredibly ugly and I would rather not do that by any means.

Google didn't help much. In fact, some results showed that people have issues with decreasing this size, but I didn't find much of a relevant info for my case.

The code I used for this example:

\documentclass[a4paper,14pt]{extarticle}
\usepackage[cm-default]{fontspec}
\usepackage{xunicode}
\usepackage{xltxtra}
\usepackage{lipsum}
\usepackage[left=3cm,top=2.5cm,right=1cm,bottom=1.5cm]{geometry}

\usepackage{fancyhdr}
\fancyhead{}
\fancyhead[CH]{header}
\addtolength{\headheight}{\baselineskip}
\fancyfoot{}
\pagestyle{fancy}

\usepackage[compact]{titlesec}
\titleformat{\section}[block]{\filcenter\Large\bfseries}{\thesection}{1ex}{\uppercase}
\titlespacing{\section}{0pt}{4em}{3em}

\begin{document}
    %\vspace*{0pt}
    \section{Lorem ipsum}
    \lipsum[1-3]
\end{document}

I would really appreciate any suggestions!

Answer 1

A possible approximation to the specifications (as usual made by people that show little knowledge of typography, as testified by the 14pt size) is as follows:

\documentclass[a4paper,14pt]{extarticle}
\usepackage{fontspec}
\usepackage{lipsum}
\usepackage[left=3cm,top=2.5cm,right=1cm,bottom=1.5cm,headsep=\baselineskip]{geometry}

\usepackage{fancyhdr}
\fancyhead{}
\fancyhead[C]{header}
\setlength{\headheight}{17pt}
\fancyfoot{}
\pagestyle{fancy}

\usepackage{etoolbox}
\newcommand{\sectionfix}{\newpage\vspace*{3\baselineskip}\nointerlineskip}

\usepackage[compact]{titlesec}
\titleformat{\section}[block]{\filcenter\Large\bfseries}{\thesection}{1ex}{\MakeUppercase}
\titlespacing{\section}{0pt}{0pt}{3em}

\preto{\section}{\sectionfix} % sections start a new page

\begin{document}
    \section{Lorem ipsum}
    \lipsum[1]
    \section{Lorem ipsum}
    \lipsum[1]
\end{document}

Notice that \MakeUppercase is better than \uppercase; the package xunicode is loaded automatically by fontspec, while xltxtra is not required nor recommended as it used to be (but you can load it if you want).

Answer 2

Based on the discussion between @Kona-chan and @egreg, especially the rhetorical:

I could write something like \vspace*{-\headsep} before every section, but it looks ugly to me—or is this perfectly fine?

I think what you may want to do is something like \renewcommand{\headsep}{1pt} after \begin{document}. That will alter all the sections that start a new page.

I agree with @egreg that it seems unattractive. But specs are specs.