Valve float on a datalog

[Adger Smith]

It's not valve float, but the intake valve blown off or held off the seat by intake pressure before the cylinder pressure pushes the valve shut on the compression cycle. Then normal combustion takes place. I should have been more clear on my comment about 02 readings. I've never had any 02 sensors on an engine that had this problem, so I don't know how 02's would respond. I do think they would be normal because the problem with the valve off the seat occurs during the compression event & before combustion, not during overlap. It is Ex Valve Float during overlap that will mess so much with 02 readings.
I think Ed was on somethng with the A/F ratios. I've seen the same thing with a BB blown gas engine I did quite a lot of development work on. I called it Fat when it made good/best power.
My thoughts are you might be looking at two problems. Intake spring pressure and being lean. Don't be shy to address both or others. It is human nature to look for one problem, not a couple. Good luck
One other thought: What is the timing curve look like when & after the boost comes in?

[dart440]

Thanks for your help. The timing is at 20 degrees and steady when it happens. I will try richening up next time. The valve spring info makes alot of sense, and agrees with what my go-to guy was suggesting. Guess I'll be ordering up some springs soon.

Dave

[buzzinhalfdozen]

Ok, 20 degrees of timing with 12 PSI of boost? Again I'm certainly no expert on boosted engines but seems high to me, certainly at 12.5 AFR, have you ever looked at the plugs on this thing? While I'm all in on the weak valve spring theory, maybe as Adger had eluded to you have a few issues instead on merely one. BTW is this a new engine, I mean fresh engine and have you spoken to the engine builder? Who engine controller are you using? Joe

[dart440]

The plugs look good. The engine builder.....that would be me! Yes its a fresh build

[buzzinhalfdozen]

Ok, then can we assume you assembled the heads and if so what did you use for springs, XXX's lbs. @ XXX installed height. This alone should indicate whether you have enough spring on it thus solving 1 mystery.Has this engine been tuned on either an engine or chassis dyno or did you start with a base map? Joe

[ss3011]

Which brand is your ECM ? What rpm limits are in the calibration ? Silly me, but I ran my engine on the dyno with a low rpm chip in the MSD, took a few pulls to figure that out. Opps !

[dart440]

I'm using a MS3x for ecm. The rev limiter is set for 7000 rpm. No dyno time (yet)
The valve springs are stock 100lbs@ 1.7" and 325lbs @ 1.5"
Valve lift is .550"

[Mike Taylor 3601]

I'm no expert on boosted stuff,or anything else as far as that goes,but in your description you said 5800 was like hitting a wall, I would check springs first. n/a engines will act like that if intake valves are leaking,blows compression back up runner,basically blows fuel back up runner,messes up carb signal,etc
usaully they will get a certain rpm and act like has a governor,had one once on dyno would get to 6500 and shut off like you killed ign.
If you are blowing valves open could act the same as leaking valves.
I'm gonna say that if valves are blowing open you could add fuel and A/F ratio would probaly not change like it should,it probaly would'nt richen as much as it should.
Mike Taylor 3601