Valve float on a datalog
 

Hi

Does anyone here know what valve float might look like on a datalog?
How would it affect the AFR? I am chasing a RPM wall (5800) and can see nothing on my logs, but am not really sure what to look for. I want to eliminate any mechanical possibilites, as the tune-up seems fine.

Thanks

Re: Valve float on a datalog
 

Not positive but would expect to see quite a bit of shift on the AFR, depending on what valves are floating, INT. or EXH. I've floated a valve or 2, and on my datalogger it looks much like I'm on the 2step, wasn't logging AFR at the time. What application, and what is your AFR doing...as well as what is your fuel and ignition doing (adding or removing fuel same for ignition) running in closed loop? 5800 sounds a bit low for valve float on most any engine, most completely stock engines will easily rev beyond that point.Again more info please.

Re: Valve float on a datalog
 

OK...you asked for it. Its a 5.7 Hemi with a 76mm turbo, running 12 psi boost. I know this combo doesn't belong here, but I also know that some of the smartest racers are on here so I am hopeing to tap into your brain trust. The AFR is pretty much flat at 12.5 when this occurs. The boost and MAP are also steady. I am in open loop.

Re: Valve float on a datalog
 

O, well I have no experience with turbo cars, however 12.5 sounds a bit on the lean side for 12 PSI of boost IMO. Have you tried richening the map in the area of your issue? I'll bet Ed Wright has tuned a hair dryer car or 2 hopefully he'll chime in here. Do you have any idea of what kind of valve spring pressure you have? Again on boosted stuff I'm merely guessing, however I'd be hard pressed to think you're in valve float at that engine speed. Do you have a map to run unboosted, you could revert to that map and pull off the charged air tube and try it.

Re: Valve float on a datalog
 

11.5-1 is safe. 12.0-1 is pushing it. I would never let one out the door at 12.0-1, certainly not 12.5-1. You can keep leaning it until it eats it's self, and power keeps going up.

Re: Valve float on a datalog
 

I've never had a blown engine float valves and seen AF ratio change. I have seen one with too little spring pressure that had blower pressure keeping the intake valve open too long. It was blowing the valve off the seat. It acted like the cam was too big. The intake did finally close, but way late and it made no power and did some stupid surges. With a large valve and 12 lbs boost you had better have way more spring than you think is normal. Do the math 12 PSI x area of the backside of the valve... that just gets you started back to holding the valve on the seat... I just did a little quick math and with a 2.2 valve you need a min of 46 additional lbs just to hold the valve on the seat. That is over your traditional seat pressure.

Re: Valve float on a datalog
 

I believe your O2 would show a big difference because of a incomplete combustion. Its been a long time since i have used a gas analyzer, but there would be alot of oxygen going out the headers.

Kirk

Re: Valve float on a datalog
 

I'll agree with Adger on this one.

My gut says AFR won't change during valve float.

The valve train would be noisier when valve is bouncing off its seat.

Re: Valve float on a datalog
 

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?

Re: Valve float on a datalog
 

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