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jfoldbar

larger pistons

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im curios about engine piston sizes and why it is done.

i have 2 vehicles. 1 is a toyata landcruiser 4.2 diesel strait 6. its about 25 yrs old. i have recently bought a new isuzu truck with a 5.2 liter 4 cyl. 

something  i wonder is why are the truck makers seeming to go for larger pistons than 20 years ago. so why do isuzu have a 4 cyl motor when a 5.2 liter could easily be a 6 cyl. 

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Larger piston sizes in 4 cylinder engines mean peak power at relatively low revolutions which is preferable for trucks.

Mr Steed, from the original 'Avengers' TV show, drove a supercharged 4.5 liter inline 4 cylinder Bentley with revolutions limited to 4,000. That car won the Le Mans 24 hour race in 1928.

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well, i can tell you that my landcruiser has much more power at low revs than my isuzu. the isuzu lags until it gets to about 1500rpm. the cruiser has power at 600rpm.

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On 4/6/2019 at 11:38 PM, jfoldbar said:

im curios about engine piston sizes and why it is done.

i have 2 vehicles. 1 is a toyata landcruiser 4.2 diesel strait 6. its about 25 yrs old. i have recently bought a new isuzu truck with a 5.2 liter 4 cyl. 

something  i wonder is why are the truck makers seeming to go for larger pistons than 20 years ago. so why do isuzu have a 4 cyl motor when a 5.2 liter could easily be a 6 cyl. 

Well, years ago some manufactures started with one type of motor (gasoline for example) and then due to consumer demand went looking for a diesel engine that would fit into the existing model with as few additional modifications to the vehicle as possible. While others at the time of the vehicle's development had designed their chassis' so that either type would be offered without needing to compromise on the engine or performance for the sake of cost and convenience during manufacturing. Your Toyota was built with a very large engine compartment, big enough for a gasoline V8 that made installing that strait 6 diesel very easy. The manufactures are now trying more than ever to increase the vehicle's fuel mileage and a 4 cylinder engine weighs considerably less than a 6 or 8 cyl., while its smaller engine compartment allows a shorter chassis which itself weighs less. They also want the customer to have as much usable space in the cabin as possible but will work to avoid extending the vehicle's front end design to accommodate it, in the past those longer engines on smaller chassis' were usually extended into the cab by reworking the bulkhead/firewall that in turn reduced the front passenger's floor space.   

On 4/7/2019 at 1:33 AM, jfoldbar said:

well, i can tell you that my landcruiser has much more power at low revs than my isuzu. the isuzu lags until it gets to about 1500rpm. the cruiser has power at 600rpm.

Your Isuzu being a turbocharged engine takes more time to spool up to where its power band is, you are probably also experiencing what is called turbo lag. Your older Toyota is probably not supercharged and has the lower torque range and quicker pickup that the two additional cylinders provide. In the end, your Isuzu is considerably more efficient than that old Toyota and that is what most consumers are after when all things are considered together.

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arc.

thanks for the detailed answer. the shorter engine is totally plausible.  while the 5.2L 4 cyl is much bigger in size, it is a bit shorter in length.  its more cube shaped rather than long like the 6 cyl.

not sure bout the turbo spooling up. my isuzu is variable vane turbo which starts spooling up at about 800rpm. my landcruiser has an aftermarket old school turbo that doesnt really spool up till about 1200 rpm. it is however a very low psi turbo which hardly increase the power at all.

UD had a truck about 10 years ago, which is almost the same specs as my isuzu, except its a 6cyl instead of a 4 cyl, and it is noticibly more responsive below 1200rpm.

 

i wonder if its something like, less pistons means less rings, which means less end gap. less injectors. less valves.  different ratio of heat loss. simpler fuel delivery.

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21 hours ago, jfoldbar said:

 UD had a truck about 10 years ago, which is almost the same specs as my isuzu, except its a 6cyl instead of a 4 cyl, and it is noticibly more responsive below 1200rpm.

i wonder if its something like, less pistons means less rings, which means less end gap. less injectors. less valves.  different ratio of heat loss. simpler fuel delivery.

The additional 2 cylinders would make a considerable improvement in responsiveness, a cylinder under combustion is not only working to move the vehicle but must also help move the other pistons through their combustion cycle also. Have you ever heard the throttle response of a Ferrari V12 :-)  Goosebumps! 

Diesel engines produce much higher mechanical forces on their components like connecting rods, crankshafts, and their associated bearings and even the main bearing housings machined into the engine block. Each cylinder requires valves and camshaft lobes to operate them. By reducing the number of cylinders you reduce the number of associated components and design elements that could eventually fail, thus increasing the engines reliability through reducing its complexity. Your engine is a wonderful example of engineering. You should appreciate it for its thoughtful design, it will likely last many more years than older designs if properly maintained. I drive a diesel crane truck at work that is almost 20 years old and it has been a wonderful engine. When they first told me of the brand of engine it was equipped with when they first ordered the truck, I was rather concerned due to my familiarity with the earlier models some 30 years earlier (1970's). But the engineering had progressed so far by then that my earlier concerns were no longer relevant. It's even likely these newer engine blocks have alloys unavailable when your Toyota was built.

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