Hi fen !

Dimetrans:

This drive appeared first in the "M87" - or "Time Police"-cycle, that means in the issues between 300 and 399, years 2430 till 2435 AD.
It was not used or invented by Terrans, but by Haluters, Ulebs, Beasts and the "KdZ" (="Konstrukteure des Zentrums", ="Constructors of the Centre") including their warrior-race, the Dumfries. The drive depends on the use and knowledge of the Paratron-technology, which was superior to terran technology at this time. Later, in the thousand years between issues 399 and 400, this technology became also used by Terrans.
This drive could only be used for inter-galactic distances. The flight consisted of a jump into hyperspace and back, but contrary to the old transition-hyper-drive, there was no dematerialization and the "jump" takes a few seconds till minutes.
The major disadvantage of this drive was that the flight was only possible from the center of one galaxy to the center of the target galaxy.
As there is not known much about the centers of far galaxies normally, the use of the dimetrans-drive sometimes ended in a desaster for the ships.

Dimesexta:

In issue 450 (year 3437 AD) the new flag-ship of the Solar alliance, the MARCO POLO, was the first ship equipped with this drive (additional to the Linear-light-drive).
This drive is a further, now terran, development of the above Dimetrans-drive. It is also used only for long-distance flights.
In contrary to the Dimetrans-drive, it did not make a jump into hyperspace, but it was a flight similar to the Linear-drive, but in the zone "between" the fifth and the sixth dimension instead.
Because the engines for this were very expensive, only very few ships in the course of PR-history were equipped with it. (They also needed a special, 6-dimensional oscillating material, made out of the 5-dimensional oscillating crystal Howalgonium. The "manufactoring" of this material was extremely time- and energy-consuming and also the handling was very dangerous.
If I remember right, they sometimes needed the assistance of the mutant Ribald Corello to build and/or handle it).
The velocity it reached first was many trillions times light-speed, but then "the degrader" :-) hit it too (I think this was already happening to the Linear-drive), and so, for example, the official given speed for the Dimesexta -drive of the SOL was 4.8 billions over light.
(To compare: The speed of the Linear-drive of the SOL was given with 5.12 millions over light.)
The Dimesexta-drive finally disappeared out of the series with #1000 without any good explanation. (That was the Degrader to the top !)

Septadim, Transferdim:

According to the lexicon, those were the drives of the SOL and the BASE, respectively.
But I know for sure that the SOL was equipped with a Dimesexta-drive initially and the Septadim-drive was only possible with a mysterious device of the Keloskers, one race of the Council of the Seven, which was on board of the SOL for some years.
Also, I dont remember that the Transferdim-drive was mentioned anywhere in the series.
I recently looked up some issues concerning the BASE and didn't find any hint on this drive, but a weak hint on a Dimesexta-drive instead.
By the way, in a scene where the BASE used the linear-light-drive, it was said that it flew with 11.2 millions fold (over-) light-speed.

Metagrav:

Since issue 1007, year 424 NGC, metagrav-engines build the standard and also one-and-only drive of galactic space-ships.
The metagrav-drive is an over-light and a sub-light drive too. So, it replaced the impulse- or corpuscular-engines for sublight acceleration and also the linear-drive (and unfortunately also all the faster drives too).
When it was introduced, it reached speeds up to 1.8 billion over-light (this was not given explicitly, but you can calculate it from the time the BASE needed to reach the Frost-Ruby, (around 6 days!] which was 30 million lightyears away).
In the 1200's, the degrader occured once again and limited the speed to 50 million times the speed of light. Since then, the drive undergoes a constant development and has now reached 80 million speed-over-light.
According to the lexicon, 2 billion times the speed of light is the theoretical maximum of this drive.
The sub-light accelerations reach 1200 km/sec^2 now.

The principle was developped theoretically by Payne Hamiller, based on the experiences terrans sampled with the technology of the Laren, the Wyngers and the Orbiters.
But it took another 400 years to really construct the usable engines.
(The development of this drive went along with the development of the new power supply system, the Hypertrop, where ships get the energy they need directly from hyperspace!)
The functional principle is that an artificial gravity-center is projected in front of the ship and the vessel "falls" towards it then. The gravity-center is often called the Virtual-G- point or the Hamiller-Point.
The advantage is, that there are no more forces applying on the ship's structure, because it is always
in the free fall ! And in a kind of boot-strap effect, the G-point moves always away from the ship, so acceleration goes on so long as the G-point is projected.
To acchieve over-light speed, the virtual-G-point is intensified until it became a pseudo- (or virtual-) black-hole, the so-called metagrav-vortex.
This black-hole-like thing now remains stationary and finally the ship falls into it and, protected by the so-called Grigoroff-shield, enters hyper-space.
Once in hyper-space, the velocity or direction cannot be changed, only a premature "fall-back" into the normal space can be done when something is going wrong.
At the begin of the use of metagrav-engines, the Grigoroff-shield sometimes malfunctioned and the ships were stranded in strange universes !

There are many problems coming up with the use of strong gravitational fields ( when ships fly close together or when coming in the proximity of a planet)
So, because there was no satisfying explanation in the series, readers are discussing it often in the newsgroups. One possibility is that the gravity-field is non-homogenous and anisotropic.
Another is, as the name virtual-g-point implies, that in reality it is only a force-field around the ship which has the same effect like a gravitional field with the G-point as the source.

Enerpsi:

This drive appeared in issues from 1250 until 1350. (That's the second half of the "Chronofossils/Vironautes"-cycle and the "Walkers of the Net"-cycle.)
The races of Estartu's sphere of influence, which is located a 40 million lightyears away in the Virgo-Cluster of galaxies and consists of 12 galaxies, used it.
The maximum speed was at least twice the one of a metagrav.
(But it was only a super-light-drive)
The function was a bit metaphysic :-).
In hyperspace, there exist streams of psionic energy, in its entirety called the psionic net. The drive allowed to use these streams, each stream was called a cord (or: trace ?).
It turned out later, that the drive worked only in the sphere the cosmonucleotid Dorifer is responsible for.
The reason was, that a 50,000 years ago, as Estartu "went" away to the strange universe Tarkan, Dorifer changed a natural-law's constant, the so-called psi-constant.
When the galaxy Hangay materialized then in our universe, the change was undone and the enerpsi-drive did not work anymore.

Finally, a note to the degrader:
Naturally, that is not a thing or event appearing in the series, but it's the name the PR-fen (at least the ones discussung in the newsgroup) gave to the "phenomenon" that much technology becomes poor in the course of time.