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of six cylinder, four cylinder, and other kinds of engines:
On searching on Google for the firing order of a Mercedes Benz, various things appeared which were nothing to do with it!
So, rather than put up with disinformation and misinformation I decided to create a page to tell other people suffering from the same problem what the answer is:
The firing order for four cylinder engines where the cylinders are in-line is 1243 or 1342.
Those are the only two possibilities I've ever found for inline four cylinder engines. In practice, it's almost always 1342, except for Ford* engines which are 1243.
For inline six cylinder engines such as the Mercedes Benz engine in question, it's typical to have a firing order of 153624. Other combinations are possible. But if you get stuck, you can eliminate various combinations that it can't be. For example, with the basic four cylinder, the order has to be of the form inner cylinder, outer cylinder.
That's why it can't be 1234. At some point cylinder 1 will fire, and the next one must be either 2 or 3, since it can't be 4. If the next one is 2, the one after that is 4, leaving 3 as the last. The only other order being 1 and then 3, meaning that it would then have to be 1,3,4,2. So, only two possibilities; 1243 and 1342. This is good logic, but remember that it only really applies to basic inline engines typical of the type where the crankshaft is only supported at the ends.
With six cylinder engines it's a bit more complex, but 123456 is out. In fact anything with 34 or 43 in it is out, as both middle cylinders would fire sequentially. See, engine firing order has to be arranged so it is as smooth as possible, spreading out the load. The business of load-spreading is not as simple as that, however, as there are resonances and other dynamic factors which have to be considered. Also, if the crankshaft as several bearings along its length, this improves the stress handling and makes the firing order less important relatively.
A special note: "in-line" means the cylinders are all in a row. For other configurations of cylinders, for example a flat four, boxer, or V type engine, the rules are different! I am reliably informed that Porsche/VW flat 4 engines have a firing order of 1432. This can still make sense, because the engine is arranged as two pairs of opposing cylinders, so the criteria for engine load balancing are quite different.
As with many things, this whole situation becomes much more complex when the field is expanded to include a wider variety of different engine configurations. V6, V8, flat 6, flat 12, delta, boxer, V at 180 degrees. In addition to the extra complexity of higher numbers of cylinders, there's also the fact that larger engine configurations have a variety of different cylinder numbering systems too! This page doesn't have a total solution to all engine firing order combinations, but it contains real information which gives clues to firing orders.
I'm sure some people will wish to add comments that are helpful. Together we can build an information resource, Generally a lot more constructive than just having annoying links to ebay which have absolutely no useful firing order content at all!
Right, so to sum it up so far: four cylinder inline engines are always 1342 or 1243. If it's Ford* then it's assumed to be 1243 and if not then 1342. Flat four engines are different.
* Regarding the 1243 firing order on the Ford, a helpful contributor has commented: "Only the Kent family of engines had the firing order 1243 (and their later derivatives i.e fiesta/Ka pushrod engines). All the OHC engines, zetec etc use 1342, as do the diesels".
Six cylinder engines have been found to sometimes be 153624, but there are other possibilities which are valid. Good sense prevailing, you can eliminate some of the possibilities. I hope some of this helps!
More ODD firing order possibilities:
Well what about the firing order for a three cylinder engine? It has to be 123 or 132. Those are the only possibilities. Two cylinder engines? There would only by one possibility, as the cylinders would have to fire alternately.
Incidentally, a firing order MUST include all the cylinders exactly once. And, it's reasonable to number the engine cylinders from 1 to n, however many cylinders there may be, and as no cylinder may be missed out, it is usual to include cylinder 1 first. If you know of any specific exceptions to the "1 first" convention I'd be interested to know.
So why not 1234 on a basic engine? Quite simply that would not be a good firing order. If you were designing an engine, there would be better choices you could make.
How about five cylinder firing order? It would have to be something like 14253 or 13524. A helpful person has written in to say "I have a 5 cylinder engine 2226 cm2 turbo engine from Audi S2 and the firing order is 1-2-4-5-3". Thanks!
The more cylinders, the more possibilities. Plus, with different cylinder arrangements and cylinder numbering systems, a greater range of possibilities exist.
You can tell me if you like. This page is open to expansion. Always learning. Thanks to ALL who have helped to improve it so far, but special thanks to those who have been nice about it and been constructive.
If you're looking at this page trying to work out what order to put the spark plug leads back on to your engine or some similar technical problem, here are a few extra items which might help:
* Engine cylinders are usually numbered from front to rear. That's "front of the engine" rather than "front of the vehicle".
* On engines which have an electric spark plug ignition system with a distributor cap, you can sometimes see cylinder numbers mentioned inside/on the distributor cap.
* Technical workshop manuals for the specific engine are the best source of information for finding the firing order. Also, little booklets called "driver's handbook" sometimes give the engine firing order and other data specifications.
* Apparently, the old rear engined Renaults (R8 R10 etc) had No 1 cylinder at the flywheel end unlike all other engines because the flywheel was nearest the front of the car.
* If you know the engine type, a search for it with "firing order" sometimes gives good results, but it's best to be as exact as possible with the engine model so as to eliminate those silly "buy it on eBay" things!
Some more relevant references:
SAE Library / Society of Automotive Engineers: www.sae.org
10W40 - People who are in the know: www.10W40.com - online auto-repair manuals (gone)
Update 2012: There's a much better article about this at http://en.wikipedia.org/wiki/Firing_order