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1.3:1 rockers hurt top end rpm

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Builder611

Member
I have the following engine:
  • 196cc clone
  • Arc billet flywheel
  • Arc billet rod .010" longer
  • Stock head with .010" milled off for a combustion chamber size of 25cc
  • .009 head gasket
  • Champion 1.0:1 rockers
  • 5.18 push rods
  • 26 lb springs
  • BP2 cam
  • 35 deg ign timing
  • Stock carb milled to .615
  • .039 jet
  • .026 pilot jet
I can get this setup to about 9000 rpm on the dyno with good power and torque curves.

I tried some 1.3:1 rockers thinking that would help. Power and torque didn't go down, maybe went up slightly, but it won't rev over 8000 rpm on the dyno. Tired changing exh rocker back to 1.1:1 with no change. I checked the springs and they don't appear to be binding with the 1.3:1 rockers.

Any thoughts why it won't rev last 8000 rpm with the 1.3:1 rockers?
 
Intake valve float? Depending on cam profile, 26# springs may not be able to control the valve. Not only is lift enhanced, valve speed is also increased. Bounce after set down limits rpm, regardless of engine, from clone to SBC.
The intake is the heavier valve, based on its size. The cam timing in relation to the compression stroke make this a critical problem.

If you do an ABA test sequence, and the rpm returns, there is the problem.

The exhaust may bounce, but this happens during the intake stroke, so the losses are not as apparent.
 
The BP2 cam is suppose to have 26lb springs with 1:1 ratio rockers so it's reasonable to go with a stronger spring when changing to higher rocker ratios.
 
I'm shocked to see you can get 9000 to begin with.

Specs
Engine: 196cc OHV Clone, 212cc Hemi Predator
Recommended Class: Builder's Prepared
Target (MAX) RPM: 7000+
Spring Pressure (In / Ex): 26 lbs
Valve Lash (In/Ex): .003"
Ignition Timining: 32 Deg BTDC
 
Here is a dyno run with the 1:1 champion rockers. Do the curves look similar to what you guys are seeing on your engines? I calibrated it using an LO 206 with red/green & black slides and using the B&S power/torque curves to match the dyno numbers. I've had 3 different engines that are Builder Prepared engines and they all seem to come in around the same power. So it might not be accurate as far as magnitude, but seem consistent and repeatable.

dyno_capture.JPG
 
Here is the champion 1:1 rockers with new 26# springs that I ran tonight. I did shim the exh/intake spring .050" to see what that would do. Made 8830 RPM. I don't plan to gear it to run there. Just wanted to see what it would do. How do these curves compare to what you guys are seeing on your dynos?

dyno2.JPG
 
Try goin down a little on the main jet with the 1.3 . They may be opening the valve faster an allowin more fuel and causing the engine to hang at 8k because it cant burn that much
 
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