Let's Talk Turbo Sizing, Boost and Valve Float

[garrittpwl]

Okay so a little about turbos, a little about valve spring seat pressure, a little about displacement and a little about my issues.

Too Long Not Gonna Read Preface; I'm trying to find out if I'm floating the valves or if my turbo is still too small....

Start with the details, turbo built 5.4, e85, +1mm valves etc etc.

I've got a second hand turbo kit with a 70/65 .68 AR turbo that I ran last year, car made 573/610 at 5200rpms, but the graph looks like an ecoboost after 5200 rpms. Didn't matter if 7psi or 14psi, 5200 was the limit on power, although I could continue to rev the engine to 6500.

picture of said graph at the bottom of this post. (note the 469 pull was 7psi and the 573 was 14psi. They both max hp at 5200 which corresponds with where the MAF stops climbing).

Fast forward to 2025; I installed a gen1 VSR 78/75 .96AR turbo to see if I could get some more RPMS out of it (got it stupid cheap). Haven't had a dyno log on it but per the MAF and datalog on some mexico pulls, the MAF isn't reading any more counts after about 55/5600rpm, regardless of boost level.

I don't think it's valve float (unfortunately I'm not sure the exact spring thats in it as they came second hand as well). I was told they're comp beehives but idk. machine shop seemed to think the seat pressure was around 100lbs back when I built it, and honestly I've never thought about it again until now. I know they're not factory springs as I'm running .550 lift cams without binding.

Now I also know the math behind calculating valve surface area and the PSI against it. But my thought is that if it truly was valve float, the more I turn up the boost, the less and less RPM the car would turn before the issue arises.

My thought is that even with the upgrade on the turbo, I'm still not running a big enough turbine/AR.


What are yall's thoughts?

 

[Snorky]

Is that the graph with the 7875? If so I would think it's valve float. Probably should have valve springs that are 130# seat pressure. Also when do you reach full boost with that turbo? What maf are you running? Is the Maf big enough? Could the maf be pegging?

 

[95opal]

/\ my first thought was like above MAF pegged. If that is indeed your issue I have your solution BNIB.

 

[96blak54]

Lifters begin to pump up within that rpm range. Remember,lifter and valve spring are a harmonized dance of dominance with cam lobe ramp rates as rpm increase, so does psi to the lashers, causing them to "bleed slower". Oil weights affect lashers bleed rate and the wrong oil weight will cause them to hydraulic. This oil psi at high rpm is no more than 20psi. If you believe its mechanical like valve spring pressure, I would suggest solid lashers before springs. Heck, if youre running anything other than synthetic normal 5w-20/30 oil, the lasher bleed and hydraulic action will not be correct. Even 5w-20 "high milage" oil screws with their purpose

 

[garrittpwl]

Is that the graph with the 7875? If so I would think it's valve float. Probably should have valve springs that are 130# seat pressure. Also when do you reach full boost with that turbo? What maf are you running? Is the Maf big enough? Could the maf be pegging?

dyno graph is with the 70/65 .68 AR.

/\ my first thought was like above MAF pegged. If that is indeed your issue I have your solution BNIB.

That is the MAF I'm running (HPX-E). Running SCT Tuner, load % doesnt exceed 2.0 so I'm not maxing the maf.

Lifters begin to pump up within that rpm range. Remember,lifter and valve spring are a harmonized dance of dominance with cam lobe ramp rates as rpm increase, so does psi to the lashers, causing them to "bleed slower". Oil weights affect lashers bleed rate and the wrong oil weight will cause them to hydraulic. This oil psi at high rpm is no more than 20psi. If you believe its mechanical like valve spring pressure, I would suggest solid lashers before springs. Heck, if youre running anything other than synthetic normal 5w-20/30 oil, the lasher bleed and hydraulic action will not be correct. Even 5w-20 "high milage" oil screws with their purpose

Car runs on rotella t4 15-40. Ford GT Exhaust lash adjusters and followers (on both intake and exhaust valves). The only reason I think its NOT valve float is because it does it at the same RPM regardless of boost. Theoretically if I double the boost, it should lose max RPM (if boost was causing valve float). You can see here in the datalog from the dyno pull that the RPM(grey) continues to climb without issue while the MAF(red) slowly stops flowing. At the same time that the MAF stops increasing the Load(upper orange) falls as the rpms increase. I really think my issue here is airflow and the engine is getting choked in the upper RPMS. No clean dyno pull for the 78/75 .96 but if feels like its doing the exact same thing but at 5600 instead of 5200. AFR stays constant at 10.5-11 during the pulls as well.

 

[96blak54]

15-40 oil weight?

Change the oil to 5w-20 synthetic just for shits, see what you get. Modular engine oil weights are set to the lash adjusters..... not bearing clearances, not piston wall clearances and not overhead cam aspects.......lash adjusters are tiny hydraulic actuators and their character changes to the weight of the oil. Thick oil will pump them up off the seats. Try changing the oil and let the oil cycle a few rounds of rpm's to get into the lashers

 

[garrittpwl]

15-40 oil weight?

Change the oil to 5w-20 synthetic just for shits, see what you get. Modular engine oil weights are set to the lash adjusters..... not bearing clearances, not piston wall clearances and not overhead cam aspects.......lash adjusters are tiny hydraulic actuators and their character changes to the weight of the oil. Thick oil will pump them up off the seats. Try changing the oil and let the oil cycle a few rounds of rpm's to get into the lashers

By that thought, should I run 5w-50? As that's what ford specs for the 05/06 GT and that's what lash adjusters and followers I'm running.

 

[96blak54]

Yeah give it a try.
Running the thinner 5w-20/30 may help with flushing out the heavy stuff thats been running. It wont hurt anything and will only reduce the hydraulic resistance(I like to refer as lifter bleed)

 

[96blak54]

And if changing the oil to 5w-20 showed differences then you can say "hey man, you dont know chit"

 

[garrittpwl]

And if changing the oil to 5w-20 showed differences then you can say "hey man, you dont know chit"

I ran 5-20 when I broke this thing in years ago. its had rotella ever since. makes me nervous changing it lol

 

[Snorky]

I ran 5-20 when I broke this thing in years ago. its had rotella ever since. makes me nervous changing it lol

Don't be scurrred

 

[lwarrior1016]

I ran 5-20 when I broke this thing in years ago. its had rotella ever since. makes me nervous changing it lol

Why does that make you nervous?

 

[garrittpwl]

Why does that make you nervous?

I've always been taught not to mix oils.

 

[lwarrior1016]

I've always been taught not to mix oils.

Really? That’s kind of odd. So you’ve never switched oil brands?

If you’re changing weight of oil, there will be residual of the old weight and it will get mixed in with the new stuff. Next change will have less and so on.

I’ve never heard that, but I can tell you from personal experience, it doesn’t hurt anything.

Now if you mix 3 quarts of 20 and 3 quarts of 50, you will change the characteristics of the oil. But it would still work.

 

[garrittpwl]

Really? That’s kind of odd. So you’ve never switched oil brands?

If you’re changing weight of oil, there will be residual of the old weight and it will get mixed in with the new stuff. Next change will have less and so on.

I’ve never heard that, but I can tell you from personal experience, it doesn’t hurt anything.

Now if you mix 3 quarts of 20 and 3 quarts of 50, you will change the characteristics of the oil. But it would still work.

I've only switched oil brands in this engine after break in. Otherwise this engine has only ever been ran on rotella t4.

 

[lwarrior1016]

I've only switched oil brands in this engine after break in. Otherwise this engine has only ever been ran on rotella t4.

What would happen if you took the car to an oil change shop and they used a different oil?

 

[96blak54]

There are many youtube vids where people swap oil out for ketchup or vegetable oil and then run the crap out of it. Surprisingly the engine sounds good for miles and runs well, but eventually craps out.