What ring gap for boost?

[Pvklol17]

What ring gap do you guys recommend for boost? I checked the ford shop manual and it states the top ring to be at anywhere from .15-.25mm and the second ring .3-.55mm I was thinking about just setting the top rings at .25mm and secondary rings at .55mm. Using stock pistons and rings gen 2 coyote. What do you guys think any recommendations on ring gap?

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[sigintel]

Look at the ring gaps on the GT500?

If the peak temp the rings are going to see is 20% higher than stock (measured from 0K or -273.15C), then you need to leave room for 20% for expansion to avoid binding and breaking the ring land.
I would guess 0.32 - 0.38mm top, 0.62-0.68mm second... again just guessing

Sure OP knows this already, but just for discussion, my understanding is:
When the ring is exposed to combustion peak temperature, the expansion of the ring material is exactly a function of the instantaneous temperature. If the ring cant expand further it buckles and binds against the cylinder wall and effectively locks itself in the cylinder bore on the way to TDC, where subsequently, when the piston starts to move down, the ring lands slam against the ring bound in the bore to either A. free the ring or B. break the ring land to unbuckle the ring and free the ring.

 

[chops44]

My MPR rings are gapped top .024 and .026 second.

 

[engineermike]

Gt500 has the same top ring gap as the coyote: .008-.012. Ecoboost is about the same as well.

 

[sigintel]

Gt500 has the same top ring gap as the coyote: .008-.012. Ecoboost is about the same as well.

@chops44 assuming those were 0.023 in or .61mm thats sounds closer to what I woukd expect. Is that oil squirter delete setup?

Awesome info mike, was wondering.
0.012 is like what, .305mm? Thats surprisingly tight upper limit.

edit: gt500 numbers look like ~20% larger gaps on upper:
0.15 * 120% = 0.18mm
0.25mm * 120% = 0.30mm

The quench/height set different?
maybe 2x oil squirter flow?
More oil flow rod end?
Guessing gt500 w lower compression keeps the rings lower away from deck at TDC (and thus cooler). Hmmm. Temperature vs longevity?

 

[chops44]

@sigintel those are in inches and the motor does has the squirter block offs.

 

[Pvklol17]

Gt500 has the same top ring gap as the coyote: .008-.012. Ecoboost is about the same as well.

.12? Isn’t that kind of tight for boost?

 

[Pvklol17]

So can I gap the top ring to .25mm and secondary ring to .50mm and have it be fine? I’m more concerned about the secondary ring gap seems like it’s too much. Can anyone else confirm?

 

[engineermike]

Guessing gt500 w lower compression keeps the rings lower away from deck at TDC (and thus cooler). Hmmm. Temperature vs longevity?

This is a hotly debated subject, but the compression ratio and IAT determine the temperature of the charge at the end of the compression stroke. The difference between 11 or 12/1 and 9.5 is significant. I’d have to run the thermo model for accurate numbers but I believe the difference is at least 75 deg F.

 

[engineermike]

.12? Isn’t that kind of tight for boost?

I used to think so. But like I said, even on the highly boosted Ford GT the spec is seemingly tight at .008-.010.

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