How to pronounce to differ spatial from special?

The long 'a' in 'spatial' is a diphthong made by combining a short 'e', like the start of 'elephant', and a long 'e', like the start of 'eel'. Like other diphthongs, these two sounds should meld together quickly and smoothly so that they appear as a single sound.

The short 'e' in 'special', is not a diphthong and sounds like the short 'e' at the beginning of 'elephant'.

You could make a further distinction between the two words by pronouncing the 't' in 'spatial' as an 's' sound, producing 'SPAY-seal'. It will be understood by most people and make you sound like Jean-Luc Picard, captain of the Enterprise (Star Trek television show.)

In all North American English that I'm aware of, "special" [/ˈspɛʃəl/] or [spesh-uhl] has an "eh" sound like "speck", and "spatial" [/ˈspeɪʃəl/] has a long "a" sound, so it rhymes with facial, glacial, etc..

The reason why spatial is pronounced the way it is (/ˈspeɪʃəl/), like space (/ˈspeɪs/), is probably that both come from the same root.

In Medieval manuscripts (here I go again), c and t were often indistinguishable, i.e. visually identical; moreover, at some point they often came to be used interchangeably even where the letters were in fact distinguishable (though not universally so). That is probably why we use illogical c and t in some words; that is, the variation is surprising when you consider their etymology and inconsistent with similar words. Space and spatial (Latin spatium and spatialis), vicious and vitiate (Latin vitiosus and vitiare), independence and independent (Latin independentia and independent-), interstice and interstitial (Latin interstitium and interstitialis), etc.

Note that these changes probably occurred in the Latin and French words before English borrowed them. The fact that we have both t and c forms is probably partly due to the fact that we borrowed French words in several stages, between which changes from t to c and back may have occurred in French.

In the Renaissance, scholars realized that all the c's in such words had actually been t's as the Romans spelled them, and they mostly succeeded in bringing Latin spelling back in line with Roman practice. But they mostly failed in reversing pronunciation. The old international overlap of c and t is probably the cause of our modern pronunciation of -ti- as -ci- in words of Latin and French origin: we pronounce nation as [nay-shuhn] (as though it were nacion, common Medieval spelling), instead of [nay-ti-uhn], which it should be if we pronounced the t in a regular way.

Most modern European languages pronounce this -ti- in an unusual manner for similar reasons, based on Medieval Latin/French -ci-.

In Dutch actie, -tie- is pronounced [see] or [si]; actief has [tee] or [ti]; natie has [ts]; and nationaal can be either [ch] as in English "rich" or (usually) [sh] as in "posh".

German usually has [ts] (though it is usually spelled -zi- in German), which is how -ci- would normally be pronounced in German.

French nation etc. usually has something like [sy] (though spelled -ci- and -c- in many words).

Spanish usually spells c (as in nación) and pronounces it like normal -ci- (voiceless dental fricative in Castilian; something closer to [s] in most other dialects).

Italian spelling is usually highly phonetic, which is why they spell nazionale and pronounce it as ordinary z, something close to [ts] or [dz].

Spatial - Spay-tial

Special - Spe-tial

If you say "special" like "spshl" it'll sound a bit odd, but still be fairly close. The "e" is pronounced as a "schwa", the default noun someone without a tongue would make. Spatial has to be clearly enunciated, and the "a" would be elongated.