Quantcast
Jump to content


How 4-cycle ATV Engines Work Discussion


Recommended Posts

in this thread, I want to discuss how 4-cycle atv engines work- what the different types are...  how they are set up and which you prefer.  To start off, are the cams on ALL atv 4-strokes powered by a cam chain?  I know that on say a lawn mower or logsplitter engine, the cam is turned by the tappet, pushrods, and valve rockers.  Which is better and why?  Chain-driven cams or pushrod-driven cams?  Are the pushrods just an old version and everything is transforming to chains or the other way around...or not at all?  Just looking for some explanation, history, and downsides/upsides...

Link to comment
Share on other sites

Cams are driven  off the crankshaft, whether by gear or  chain and sprockets. Push rods  and rockers are driven  off the cam and in turn  open  and close the valves (  in an  overhead valve motor).  In block cams ( used in valve in block motors,  mostly  lawn mower  motors and other small engines) are geared to the crank and drive the valves directly or in the case of overhead valves ,by means of push rods and rocker arms. Overhead cam motors (used in the vast majority of ATVs ) are normally driven  by a chain and sprockets. Overhead cams usually operate the valves by either a rocker or adjustable tappet  between the  cam  and valve.

Edited by davefrombc
Link to comment
Share on other sites

  • Admin changed the title to How 4-cycle ATV Engines Work Discussion
2 hours ago, JacobSlabach said:

ok, so what I'm hearing you say is a OHC (overhead cam) engine has the chain driven cam above the valves, and the OHV (overhead valve) engines have the cam in the crankcase and pushrods operating the rocker arms...?  So why would atvs have mostly OHC engines and general purposes engines (lawn mower etc.) be OHV?

Most of the more inexpensive  lawnmowers are "flat head".  Their valves are in the block and the motors are less efficient  because of it , but they are  cheaper to  produce. I  haven't pulled any of the  newer utility overhead valve motors apart to see if they are push rod or overhead cam.  They are more efficient  and develop more power , but at greater expense to  manufacture. ATVs have mostly overhead cams because they are  more  efficient  and can easily be  made to rev much higher because the push rods  and rocker  arms are eliminated.  There is less mass to keep  in reciprocal  motion with  overhead cam actuated valves. Less mass -  easier to  move - and less chance of valve float at  higher  RPMs.  The motors can easily  produce  more power  per cc in an  overhead cam  motor  than  in a push rod  engine. 

  • Like 1
Link to comment
Share on other sites

Over head cams can be a higher reving engine, less vibration. Less valve floating at higher rpm. Fewer moving parts too. A lot of European racing engines use the overhead cam designs. They are long distance, high reving engines.  Bad part is in (what is called interference engines). Valves and pistons can hit each other due to clearance problems if and when the chain or belts break. Things go boom faster when things fail. In block cams the gears use horsepower to drive them, The valves can and do float at high rpm, due to the long distance between the cam lobs and valves. Yes the springs do help close the valves but the pressure from the rockers, push rods and lifters has to be gone. The rocker arm nuts also have a slight tendency to back off (loosen) on in cam blocks after a while and require periodical adjusting in high reving engines. Over head cam is a better performance engine and smaller size over all too (lighter weight). 

The small engines world (lawn and garden) has started going to the cam in block over head valves with push rods configurations. Yes over rev one of them and you drop a push rod or bend it. Some have also gone to hydraulic lifters but have problems due to lack of oil changes. Thicker oil doesn't drain quick enough for the lifter and keeps the valve open too long. either shutting down the engine or bending rods. Over head cams use the head body for a bearing, trash and too thick oil will cause cams to seize real quick. So they went to in block cams. 

Even more bad news is they are adding more and more electrical stuff to these engines. Take you typical automotive engine cut it down to 2 cylinders and leave the rest.  If you don't know how that stuff works it's best left alone. I don't want anyone to think I'm bragging but I do hold certs in both automotive and small engine repair. Have also raced automotive. My certs don't hang on the wall they are  in a binder on a shelf.

 

Edited by David Peterson
  • Like 1
Link to comment
Share on other sites

  • 1 year later...

An ATV can come in many engine sizes, so when considering the purchase of a vehicle for a family member, their ability to handle the power needs to be addressed. Giving a younger member of the family an ATV with more power then they can handle is a mistake, and can be very dangerous. If the operator is under the age of 14 then it is recommended to keep engine power under 70cc. But if they are adults, then whatever engine size they are comfortable handling, and one that can carry the designated load, will be fine.

 

Link to comment
Share on other sites

7 hours ago, bchanfcb1987 said:

An ATV can come in many engine sizes, so when considering the purchase of a vehicle for a family member, their ability to handle the power needs to be addressed. Giving a younger member of the family an ATV with more power then they can handle is a mistake, and can be very dangerous. If the operator is under the age of 14 then it is recommended to keep engine power under 70cc. But if they are adults, then whatever engine size they are comfortable handling, and one that can carry the designated load, will be fine.

 

great advice and is similar to what ive said.  i think the parents know what their kids can and cant handle.. i was 13 when i started riding atvs and i learned on a sportsman 500cc.  at 14 i bought a kawasaki bayou 300 and now at 17 ive owned over 15 atvs and currently have two scrambler 850s.  the key is to have a governor on that throttle thumb and know the machines limits.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Similar Forum Topics

    • By Ajmboy
      OK, so I'm a boxing fan...anyone watch last night's fight between Manny Pacquiao & Ricky Hatton?

      The PacMan is just too fast and too accurate. Floyd Mayweather just announced yesterday that he's coming out of retirement to fight Marquez in July. Maybe we'll get a December fight between Pretty Boy and the PacMan?
      So, Manny has convincingly beaten up DeLaHoya (just announced his retirement from boxing) and now Hatton...
    • By 1998Zuki250
      Looking to see if anyone has any input on ATV Tires? 1998 Suzuki quadrunner LTF250
    • By Otter9099
      I plan on using my ATV to hunt. What have you found to be the best method for carrying your rifle that is secure and allows removing quickly to pursue an animal?
    • By Suzukiquad
      If you had to choose one mod that you couldn't live without, or had to recommend one what would it be? 
    • By mehim
      Hi everyone. 
      Let me explain the thread title. 
      My husband and I are very active in the nudist community, and we have stayed at many and most of the mainstream nudist and clothing optional campgrounds in the USA. We are now approaching our retirement and would like to start a podcast to share those off the road / backwoods / little known camping areas where nudity is allowed. We have purchased a 5th wheel camping trailer with a rear garage for the ATV and now need some good reports on the one to buy. 
      We are most interested in a side by side unit but would love to get your thoughts. 
      Feel free to stop by and say Hi and ask anything you would like to better understand what we are looking for in an offroad unit. 
      Thanks for reading and hope to hear from you. 

×
×
  • Create New...