It's been a while since I have done anything with the engine.
With the lower speed misfire and poor performance the engine builder took the engine back and did a few things.
We installed a new MSD distributor to rule out any ignition issues.
Back onto the engine dyno and rewarded with pretty good even power up to 6,000 RPM.
Peak HP = 257 @ 5,800 RPM. Peak torque = 270 @ 4200 RPM. (we are not tuning for power, just mechanical issues)
The problem is when we hit 6050 RPM! All hell breaks loose!
It's as if a crude rev limiter kicks in and a destructive misfire event occurs.
I accept the fact that 500 RPM past peak HP the power curve is dropping but the engine should still be able to run fairly well.
Here are a couple of links to show what is going on. The one from the control room is using my phone as a camera, but listen carefully as the tach hits 6K. (Headphones help)
The second video shows misfire through the carb and then the timing marks. View from control room carb view and timing marks view
Here is a short summary that was sent to Comp Cams to get their input.
This engine (chev I6 250cid) was on the dyno last year, and had very erratic curves, and the engine would not rev up clean after about 4800 rpm. Saw that the valve tips were getting a "star" pattern after only a few pulls, so the decision was made to increase spring rate and pressure. Went to an Isky295D spring (140 - 360 pressure, up from 120-330). This helped a bit, gaining about 400 RPM. Seemed to like near zero lash (hyd cam).
Engine went back to the builder, and a tried a few things for round 2.
Limited the hyd plunger travel down to about .025 (running the plunger down about .010).
Swap to a dual LS type 1.290 OD PSI spring, 160 - 390... in an effort to change the harmonics from a single with damper type spring.
Put it on the dyno yesterday and the curves look much smoother, power is up about 20, and engine runs clean to 6050 - 6100 RPM, and then hits a wall. Take a rocker off after two pulls and see a pattern again on the tips, tho faint, but it is only after very little time.
Rockers are Harland Sharp 1.70
Pushrods are Manton 3/8 OD x .120 wall (right around 10.125 OAL)
Lifters are (what I assume to be) the comp hard bottom hyd lifters, now with limited plunger movement to about .025.
Engine is being run NA right now (will be turbocharged once in the car.. just proving engine now).
ANy thoughts come to mind here? Attaching a few pics and the cam card. Cam (X 8754-12) was purchased by the customer and supplied to the engine shop that built it.
On the graph, the blue is HP and green is TQ, it falls off right after 6000 rpm on that pull.
The pic of the valve tip is after just two pulls,
Appreciate any ideas you can come up with,
I don't know how to post pictures here but I can email them to anyone who is curious to see them.
My engine builder is saying replace the cam but the dyno guy says he is not convinced that cam will cause this event.
Maybe I am being unrealistic expecting this engine to rev to 6,500?
Regardless, no boost for baby just yet.
I welcomes anyone's input to the problem.