A common question regarding carnitine is what is the difference between L-Carnitine and Acetyl-L-Carnitine? (For ease of reading in this article, Acetyl-l-carnitine will be referred to as ALCAR)
WHAT IS L-CARNITINE?
L-Carnitine is the naturally occurring form of carnitine found in the human body. It plays a role in transporting fatty acids to the mitochondria to be used for energy.
The "L" in L-carnitine simply refers to what the atoms in the molecule physically look like: it indicates that L-carnitine is the biologically active form of carnitine found in nature and used by the body.
L-Carnitine is a key component of the body's energy production from fats, especially during periods of prolonged physical activity or when glucose (sugar) levels are low.
ALCAR Exerts Benefits on the Brain and Nervous System
The addition of the acetyl group on carnitine allows ALCAR to cross the blood-brain barrier and be absorbed into the brain.  ALCAR then enhances energy production in the brain and nervous system by improving the functioning of our cellular “furnaces”, the mitochondria, helping them to use oxygen to produce energy, and protecting from free-radical damage, thereby preventing cellular degeneration.
ALCAR not only increases production of an important mitochondrial membrane constituent called cardiolipin, (which functions like the insulation on a live wire), it also supports memory and alertness by aiding in the production of neurotransmitters such as acetylcholine and dopamine.     This makes it appropriate consideration for anyone looking to improve overall brain status and performance, or to reduce aging in general via “mitochondrial maintenance”.
L-CARNITINE vs. ALCAR
The "acetyl" in Acetyl-L-Carnitine refers to the presence of an acetyl group (CH3CO) in the molecular structure of the compound.
Unlike L-Carnitine, ALCAR (Acetyl-L-Carnitine) is special because it can enter the brain. Once inside, it boosts energy production and shields the brain from damage, which helps prevent aging-related issues.
Protection from toxicity… Benefits for depression and diseases of the nervous system (Parkinson’s, Alzheimer’s, neuropathy)
Due, in part, to its ability to boost neurotransmitter production, several studies have noted significant positive results for the effects of ALCAR on depression.  Additionally, ALCAR can protect our brain from the damaging effects of toxins, including excess alcohol.  In another study, primates given a substance called MPTP, which normally damages brain cells and induces Parkinson’s disease, were completely spared these effects if they were treated with ALCAR first.  
ALCAR has been scrutinized for its application in Parkinson’s disease. 
ALCAR has also been shown to slow progression of dementia and/or Alzheimer’s disease, and improve performance for these patients on test challenges, as well as being helpful for preventing and/or improving diabetic neuropathy.    Fibromyalgia is another condition that has been shown to respond positively to ALCAR. 
BENEFITS OF ALCAR
ALCAR has various health benefits. It can shield the brain from toxins such as alcohol, and has shown promise in preventing Parkinson's disease in studies with primates.
ALCAR may also slow down dementia and Alzheimer's disease progression, improve cognitive performance, and help with conditions like diabetic neuropathy and fibromyalgia.
ALCAR has synergistic anti-aging effects with Alpha Lipoic Acid (and other mitochondrial antioxidants like Coenzyme Q10, and N-Acetyl-Cysteine, etc.)
In some of the most popularized studies on ALCAR, Bruce Ames et al administered a combination of alpha-lipoic acid and ALCAR to old rats and reversed many signs of aging, attributed to reversal of aging of their mitochondria. The results were improved cognition, energy, and physical appearance. It is the protection of the mitochondria and of its function that has prompted Ames and other researchers to suggest ALCAR and alpha-lipoic acid for the prevention of cancer.  
In studies, a mix of ALCAR and Alpha Lipoic Acid reversed signs of aging in old rats, improving their thinking, energy, and looks. This combo is believed to protect mitochondria (the energy centers of our cells) and may even help prevent cancer, according to some researchers.
Is ALCAR better than L-Carnitine, and How Much Should I Take?
L-carnitine is quite poorly absorbed…(estimates are from 14 to 22%). Due to the acetyl group, ALCAR is absorbed into the bloodstream somewhat better, but absorption is still limited.  For this reason, you will absorb more of either form if you divide dosing up through the day. But I still do not think ALCAR is better than L-carnitine, per se.
I believe that eventually L-carnitine will be found to be more active in the periphery tissues of the body, and for this reason, it is L-carnitine I recommend if someone is looking to mobilize fat, and/or for weight loss.
But if it is energy, or brain and nervous system benefits that you are looking for, definitely try ALCAR, as carnitine has to be in its esterified (eg. acetyl) form to fulfill its role in the mitochondria.
As always, you have to go with the result, which no study can 100% predict for you. For example, though out of the two I have more often taken ALCAR, I would have to say I notice a greater effect on my mood and sense of well-being, (tendency to laugh, etc.), when I add L-carnitine.
Though I have not found any studies that specifically look at the issue, due to transferase enzymes that move acetyl groups around, presumably either form can be converted into the other, and the body will convert as needed, though the ability to do this decreases with age
As for dosing, though some studies have used up to 4 grams of ALCAR a day, most are in the 1-3 gram range, (i.e. 1000-3000mg). One gram taken twice a day is normally what I recommend for either form, but more may be appropriate for a more serious problem. Though it is often referred to (some say incorrectly) as an amino acid, as mentioned earlier, carnitine is actually much closer in form to a B-vitamin. Therefore, though it is usually recommended to be taken on an empty stomach, the benefit is not lost if taken with food. (This also applies to alpha-lipoic acid.)
L-CARNITINE FOR FAT LOSS, ALCAR FOR BRAIN POWER
If you want to mobilize fat or focus on weight loss, L-carnitine may suit your purposes better. For energy, brain, and nervous system benefits, ALCAR is a good choice as it is absorbed better by the brain.
Generally, a recommended dose is around 1-3 grams (1000-3000mg) per day, taken in divided doses with or without food. More may be suitable for specific situations: consult your trusted wellness practitioner.
Hope this Helps!