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Zero lash on AKRA Plate engines

I'm on an inertia dyno . When I start at 150 degrees , make 3 pulls back to back, power goes down as temp increases. Heat soak, lash increase, valve train instability, increase deck height? Where is the power lose coming from?
 
RCJ said:
I'm on an inertia dyno . When I start at 150 degrees , make 3 pulls back to back, power goes down as temp increases. Heat soak, lash increase, valve train instability, increase deck height? Where is the power lose coming from?
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I have seem papers, published by SAE, from Yamaha, documenting the difference between air cooled and water cooled 2 cycle engines. It's substantial.
I myself have seen, with my own eyes, on the dyno, testing a McCullough, the power loss as the engine warms up. It's substantial.
I've experimented with heating the engine and then, very quickly, honing it before it cooled. There was a definite improvement in compression coming off the track and testing it. Everybody remembers how we used to test the compression, don't you think. You'd grab the starter cup and turn it over by hand.
 
Do the expansion rates due to heat change on a seasoned engine vs a new engine ? If so would the difference be enough to worry about ? Probably over thinking this but after recently switching to clones I am learning.
 
RCJ said:
I'm on an inertia dyno . When I start at 150 degrees , make 3 pulls back to back, power goes down as temp increases. Heat soak, lash increase, valve train instability, increase deck height? Where is the power lose coming from?
Click to expand...
Check your A/F Ratio - I'd be willing to bet it dropped on pull 2 and 3......
 
Just so I understand!!! Most times when I heat metals, they expand, get bigger. So this is a deal where maybe the pedestal is rising as heated or something similar to that. Does aluminum expand more than steel? I dont have a dyno at this point, but I trust that you guys are telling the truth. Just trying to understand WHY.
 
T-roy said:
Just so I understand!!! Most times when I heat metals, they expand, get bigger. So this is a deal where maybe the pedestal is rising as heated or something similar to that. Does aluminum expand more than steel? I dont have a dyno at this point, but I trust that you guys are telling the truth. Just trying to understand WHY.
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Yes, Aluminum expands much more rapidly than steel! The head and block grow real quick and the steel pushrods and valves grow much slower.
 
Over the years here is what I have learned. Over time the adjustment will change each time you pull the valve cover. You need to re adjust the valves. If you stay on top of the adjustment process the ball and rocker begin to mate ( wear themselves in to each other ) and adjustment is no longer a necessity. Now, while the engine is operating under performance conditions the growth on a valve that is set at zero will grow to .003/.004 when checked when hot. When the engine cools down to ambient temperature the valves are back to normal settings. This is only on a rocker and ball assembly that has gone through the readjustment process as the engine is breaking in.
 
RCJ said:
Where is the power lose coming from?
Click to expand...

Remember fellas, we are not doing this to gain valve lift. We are doing this to control the "noise" in the valve train. The net result for those of you without dynos is that the amount the HP falls off as the engine gets hot, 2nd, 3rd, and even 4th pull is less than if the valve lash was bigger. If you set it just right you wont lose any power as you build engine temp. Those of you with dynos can "see" this phenomenon and easily test for it. Test the same engine back to back. Set the valve lash to .003-.004" make 3 pulls. Set the valve lash to .001 - .002" when the head temp is >375°F. Start both series of dyno pulls at the same starting temp. Change nothing else and observe the results.

My guess after all these years, is the power losses are not from a a diminishing valve lift but rather increasing noise, harmonics, bounce and float that occurs as valve lash grows. The slight pre-load on the valve spring suppresses the tendency of the valve train to chatter reducing bounce at valve closing. In short, the tight lash clearance grows into a "hot" clearance of .001-.002". This simply does a better job of controlling float & harmonics at high rpm than an engine that has .005" or more "hot" clearance. Harmonics man, harmonics.

You can argue with me, but you can test it for yourself.

Go to this thread, and start reading around post #43. This was 5 years ago and lead to a valve train / spring setup that kicks ass.
https://4cycle.com/karting/threads/take-a-guess-how-much-hp-gain-with-illegal-springs.54338/page-3
 
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