Valvetrain geometry, proper setup

[Brian Boyer]

So i am very early on with building my Lifan, literally in the "this time next winter" stage. I plan on running my 420 close to stock all this first season and next November or so doing a teardown& build.

I dont have a crap ton of questions but i'm not going to lie, the off standard ratio rocker arms do throw me for a loop.

In my NOOB mindset i have it in my head that changing the rocker arms to do what a cam should is a jerry rig. (And i'm willing to listen to my ideas being wrong)

So how about this~ i'll describe the cam i intend to run, and you guys can fill in the needed info from there.


I never want to build this engine to exceed 7,000 rpm,.. (Hell really 6,500 is more a reasonable high end sustained rpm i could build the motor to survive at).. So i really only want to discuss PROPER VALVETRAIN GEOMETRY~ i wnt to use the right combination of parts for obvious reasons.

Here is the cam i will be using~ The 306TQ

http://www.nrracing.com/mobile/Produ...ode=390CAM-306

In my GX/Clone NOOB mind's eye i'm wanting to retain the stock ratio rocker arms, if not retain the Lifan rocker arms.. I will be switching to the chromoly pushrods. I'm uncertain if a pushrod guideplate change is necessary if i try to stay fairly conservative with the valvetrain.

I'm on the fence on the compression increase~ i'm thinking i'm going to go the longer ARC rod route/ kill two birds with one stone,.. And retain my factory head. The other option is ordering a built Honda head from NR Racing.

(I have no plans on valve size changes if i stay with the Lifan head)

I'm obviously going to need to swap valvesprings~ i guess i'll call NR Racing for guidance there.


I guess to summarize~ if i'm already going to swap cams,.. Theres no need to change rocker arm ratio correct?,.. I mean just choose the cam that does what you want to do corect?

[Brian Boyer]

I'm researching and a small increased ratio in the arm sounds like a good thing, and too much,.. Potentially a very very bad thing.

What is the stock Lifan 420's rocker arm ratio?, anyone know?,.. Its essentially as close as a GX clone as you'll see~ i'll look at stock GX390 rockers on NR.

[Brian Boyer]

After some research last night it looks like there are benefits to be had by changing the rockers to a higher ratio rocker but it requires purchasing guideplates as well, i'm thinking i'll just stay with the Lifan standard rockers and do al the camshaft duties with the camshaft.

[sho305]

It depends on the cam, since the stock cam is pretty low lift using higher ratio rockers gives you that much more lift on the same cam. You get more lift but same duration, so its not a huge change but every little bit can help right. It may help more with a high duration and low lift cam, like the cheater cams karts use, as the cam is lacking the lift it could have. Lets say you run 1:1 rockers and swap to 1.3:1 like I have. So if my cam has .100 lift (to make it easy) now I get .130 lift (+.030). But I only get that at the peak of lift, at .050 I only get .065 (+.015) not as much change, so you really only get full 30% more at max lift for a moment at peak, since the valve spends most of its time opening and closing.

So if you put in a high lift cam you will get that % ratio higher lift yet from the high ratio rockers, could it be too much? Will it cause valve float at lower rpm than you thought due to higher valve speed? How is your valve clearance to piston? The stock cam is so small these are rarely factors, but with a larger aftermarket cam they are. A larger cam may not be intended to use with high ratio rockers. It might have 50% more lift, a significant increase.

The big issue with rocker arms is the rpm you will run, if you run more rpm you will need a better arm period. The forces multiply as rpm climb and stock arms are not made to do huge rpm and more lift that creates more speed in the movement on top of more rpm. I would ask someone like NR they know of these things, or another engine builder. Same with the pushrods they want to bend at more rpm, more lift is more work for them also, the more HD springs you will have to use also put more force on them.

I don't know what rpm the rockers are good for. Another thing is kart racers run WOT high rpm all the time while most GC don't, so while it needs to not float the valves the valvetrain might not need to be as robust....unless you run roads at top speed often for example.

Usually when you talk about valvetrain geometry it is referencing the angle of the rocker to valve. You change the pushrod length to set this usually. If you cut the head it changes, usually gasket changes don't make enough difference to cause a problem. If you cut the head .100" your pushrod just got .100 longer than it was and now the rocker is hitting the valve stem at an angle. You have to make sure you are not bottoming the spring (coil bind) at full lift with a huge cam, issues like that. If the angle is off it can change the lift you get at the valve and cause other problems. But these engines are simple and the kart guys have been all over all this stuff, they know the proper combinations. Just trying to help, keep us up to date on your build! That cam looks pretty interesting for GC use.

They say the stock clone flywheel is only good for 6K rpm.

[Brian Boyer]

Originally Posted by sho305

It depends on the cam, since the stock cam is pretty low lift using higher ratio rockers gives you that much more lift on the same cam. You get more lift but same duration, so its not a huge change but every little bit can help right. It may help more with a high duration and low lift cam, like the cheater cams karts use, as the cam is lacking the lift it could have. Lets say you run 1:1 rockers and swap to 1.3:1 like I have. So if my cam has .100 lift (to make it easy) now I get .130 lift (+.030). But I only get that at the peak of lift, at .050 I only get .065 (+.015) not as much change, so you really only get full 30% more at max lift for a moment at peak, since the valve spends most of its time opening and closing.

So if you put in a high lift cam you will get that % ratio higher lift yet from the high ratio rockers, could it be too much? Will it cause valve float at lower rpm than you thought due to higher valve speed? How is your valve clearance to piston? The stock cam is so small these are rarely factors, but with a larger aftermarket cam they are. A larger cam may not be intended to use with high ratio rockers. It might have 50% more lift, a significant increase.

The big issue with rocker arms is the rpm you will run, if you run more rpm you will need a better arm period. The forces multiply as rpm climb and stock arms are not made to do huge rpm and more lift that creates more speed in the movement on top of more rpm. I would ask someone like NR they know of these things, or another engine builder. Same with the pushrods they want to bend at more rpm, more lift is more work for them also, the more HD springs you will have to use also put more force on them.

I don't know what rpm the rockers are good for. Another thing is kart racers run WOT high rpm all the time while most GC don't, so while it needs to not float the valves the valvetrain might not need to be as robust....unless you run roads at top speed often for example.

Usually when you talk about valvetrain geometry it is referencing the angle of the rocker to valve. You change the pushrod length to set this usually. If you cut the head it changes, usually gasket changes don't make enough difference to cause a problem. If you cut the head .100" your pushrod just got .100 longer than it was and now the rocker is hitting the valve stem at an angle. You have to make sure you are not bottoming the spring (coil bind) at full lift with a huge cam, issues like that. If the angle is off it can change the lift you get at the valve and cause other problems. But these engines are simple and the kart guys have been all over all this stuff, they know the proper combinations. Just trying to help, keep us up to date on your build! That cam looks pretty interesting for GC use.

They say the stock clone flywheel is only good for 6K rpm.

I'm the odd duck mud motor guy hanging out on a golf cart forum LOL, simply because this is where the 420 gossip is. That cam will work for what i'm doing but not necessarily in a GC.

Thanks for your input i had forgotten that milling the head would change the pushrod length needed, thats makes sense. (Just one more "x" in the longer ARC rod coloumn vs milling the head)

Yep i'm leaning more all cam & no higher ratio rocker,.. More reserach to be done obviously.

I'm willing to push the flywheel a little~ personally i think the " 'sploding" flywheel is pretty overblown,.. But we'll see.

[sho305]

Get the cam that does what you need! Some cams you don't want high ratio rockers and others it works great with....the cam maker/seller should tell you all about that.

Pushrod length thing, you just have to keep it in mind or get the rods you can cut to size, get it right and never worry about it again.

The flywheel I don't know. I suspect clone quality is much better now and maybe its not an issue but I don't know. I don't want to find out. Likely its only going to happen if you run 6K or better a lot. When you get to that power level you might want a light flywheel for other reasons anyway. You can dig for a honda flywheel. I once read you don't want the clearance to the coil real tight if you run high rpm lol.

[Brian Boyer]

Originally Posted by sho305

Get the cam that does what you need! Some cams you don't want high ratio rockers and others it works great with....the cam maker/seller should tell you all about that.

Pushrod length thing, you just have to keep it in mind or get the rods you can cut to size, get it right and never worry about it again.

The flywheel I don't know. I suspect clone quality is much better now and maybe its not an issue but I don't know. I don't want to find out. Likely its only going to happen if you run 6K or better a lot. When you get to that power level you might want a light flywheel for other reasons anyway. You can dig for a honda flywheel. I once read you don't want the clearance to the coil real tight if you run high rpm lol.

Mine's (eventually) going to exist at 6,500 for extended periods, and my coil clearance is .020", she'll either survive or she wont, LOL,.. May just find out this spring.

(I will be either built to, or propoed down to achieve 6,500, prop pitch on my manufacturer's mud motor kits are not selectable,.. They are ALLLLL 6" pitch,.. Only thing you can change is diameter~ so i'll build as much as i can by spring as much as a poor sap can anyway~ if that doesnt get me there, its prop swapping time,.. $12.50 per prop change/ no biggie)

Well let me clarify~ if i can swing the rod& springs.., i need the longer ARC rod and springs hopefully before spring.

[Brian Boyer]

I have all kinds of grand plans,.. All much more expensive than going the VC route but oh well that ship has sailed.