2.5 Four vs 2.5 Five ? Decision? Or would you buy a 3 Cylinder car?


Asked by Aug 15, 2015 at 09:18 AM about the 2010 Subaru Outback 2.5i Limited

Question type: General

2.5 Five Cylinder cars have the same displacement as 2.5 Fours only smaller .
Do you think they may work harder under load conditions to produce torque and run
slightly hotter than the larger Fours and and not as balanced as the H4 Subaru engine?   
Recently VW has decided to change from their Five cylinder engines back to a 2.0 Turbo.  
SUBARU has been perfecting and using the boxer engines H4 and H6 for many years.   
Acura Vigor was another Five cylinder car but it didn't last long either.   What's with these
odd cylinder configurations and is there a future for them?

8 Answers


the future for the 5 cylinders appears to be rather uncertain due to the cost for the manufacturer to produce these engines. While many five cyl engines are terrific due to excellent balance and superior displacement, the manufacturers have worked very hard to reduce engine size and even engine compartment size in order to reduce drags and weight. Also, the trend for the car manufacturer is make standards engine worldwide. The VW 2.0L turbo in US is a perfect example of a engine designed for European market with superior gas mileage.


Do Five cylinder inline engines cost more than Subaru boxer engines with two head that are married together?


And, tell me why a Five cylinder inline motor would have more balance than either an H4 or H6 boxer?


Toitoine, see this picture, also, I've been informed by someone else on the forum that a 2.5 litre Four is more likely to run cooler and more efficiently than a 2.5 litre Five with smaller cylinders. They said it is more likely to breathe easier.


Toitoine, see this picture and description of the inherent boxer engine design.


Mark, Yes, 5 cylinders inline cost more to produce that boxer engine due to lack of general demand. Overhead cost is definitively a factor on production decision. Regarding the balance, an inline 5 cyl will be automatically more balanced than a 4 cyl boxer because the boxer has a slight out of line cylinders, that's the concept of this engine. There is nothing with it but this is a mechanical fact. Vibration can be an issue if not properly equipped with the right transmission. Finally, a Boxer runs cooler, not because of the engine design but solely based on the engine space inside the frame. More space there is, more airflow will be. Tighter the space, more heat is trapped and therefore less cooling capacity. This is known for the Porsche vs other Boxer such as Subaru Outback.


Toitoine, OK, so, you're saying it's the scarcity of the five cylinders that make them more expensive to build. Take a look at this picture attached here discussing the boxer engines. As you know, these engines don't require counter balance weights unlike Inline engines. What do mean by an inline Five will be "automatically more balanced ", please explain why you think it is. And, what do you mean by the "vibration can be an issue without the right transmission ", what transmission are you talking about? Yes, I understand that the boxer H4 is slightly offset causing a slight rocking motion, but since it has a similar design powering the crankshaft, it seems that it would have a more natural motion.


This may come a little late but torque is generally the result of piston speed. For example: two engines with the same displacment but one with larger (or more) pistons...the other with smaller pistons attains equal displacment by travelling further. If both engines are operating at the same rpm the smaller piston must travel farther. Simplified: envision a rope with two knots tied in it. One tied at one meter from your hand and the other at two meters. When you sling it around they have identical rpm but the knot at two meters travels farther. Generally torque is relative to stroke and more cylinders provide a smoother running engine. My old two cylinder John Deere tractor attains tremendous torque from 348cid and 7 inches of stroke but if I'm going to town I want my eight cylinder with 3 inches:)

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