How 4-cycle ATV Engines Work Discussion

[JacobSlabach]

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...

[Admin]

Good topic, but maybe rename it to "How 4-cycle ATV Engines Work Discussion"?

[JacobSlabach]

2 minutes ago, Admin said:

Good topic, but maybe rename it to "How 4-cycle ATV Engines Work Discussion"?

how do I do that?  lol didnt know that I could edit a thread after starting it...

[davefrombc]

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 February 15, 2019 by davefrombc

[JacobSlabach]

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?

[Admin]

2 hours ago, JacobSlabach said:

how do I do that?  lol didnt know that I could edit a thread after starting it...

All members can edit their content up to 5 minutes after posting. Our premium memberships come with the ability to edit content without restrictions. I've changed the topic title for you 🙂

[JacobSlabach]

k thanks👍👍

[davefrombc]

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. 

[JacobSlabach]

ok.  I understand what your saying.  Any input from other forum members would be great too!  Do you know of any chain-cam lawn mowers?  maybe 0-turns?  (just wondering haha)

[David Peterson]

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 February 17, 2019 by David Peterson

[JacobSlabach]

thanks for the great explanations guys!

[bchanfcb1987]

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.

 

[JacobSlabach]

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.