Piston Ring Gaps

[DatRat]

I am interested in hearing what your thoughts are regarding the #2 compression ring end gap. 

 

RETHINKING RING GAPS
The old school philosophy (my philosophy until now) of engine building said the end gaps on second compression rings could be tighter because the number two ring is not exposed to as much heat as the top ring. The new school of engine building says it's better to open up the second ring gap a bit so pressure doesn't buildup between the rings and cause the top ring to lose its seal at high rpm. The result is better compression, better piston cooling and reduced oil consumption. Any pressure that builds up between the rings will blow down into the crankcase, keeping oil out from between the rings.

 

Doing some more research, I found the following talking about Perfect Circle Piston Rings in a Clevite catalog:  Most of the second ring gap recommendations are larger than the top rings. Recent testing has proven that a larger second ring gap increases the top ring's stability allowing for a better seal. This larger "escape" path prevents inter-ring pressure from building up and lifting the top ring off the piston allowing combustion to get by. Many engine builders have reported lower blow-by and horsepower gains at the upper RPM ranges with the wider second ring gaps. Also, almost every new car made is using this inter-ring pressure reduction method to lower blow-by and emissions and to increase engine output.

 

One last thing, the information above on the piston rings is from articles discussing foreign engine builds.  Have any of you folks gone with a larger # 2 end gap compared to the top ring end gap?

 

Do you see any problems with the New School philosophy?

 

Thanks in advance for your thoughts.

 

 

[letitsnow]

I've always made the 2nd ring gap larger, and I've never seen any ill effects, and all my stuff seems to make power as good or better than expected.

 

On my L6, my pistons(and consequently my bore) were .003 over a standard size, meaning my end gaps were .009 larger. I still put it together because I was in a hurry, leakdown was about 5% higher than I'd like for a new engine, but it wasn't a fresh bore, so that may have made a difference too. It still ran great and is one of the higher dynoing stock turbos that I've seen on here.

 

My little story is to demonstrate my opinion that as long as the gaps aren't absurd, in either direction, that it doesn't make that much difference. If you're racing in a very restrictive class like IT, that goes out the window, chase power anywhere you can find it.

Edited February 20, 2013 by letitsnow

[nash542001]

I researched this too when rebuilding my engine and came up with the same conclusion. Wider gap on the 2nd compression ring. So I gapped according to the new school. Did not have a baseline to compare to though. Talked to a pro-drag guy who knew his stuff and he said it was better, but said that they would drill a small hole in the top of the piston and have the gasses come out the top ring land, I think it was called gas porting but this is said only to be effective above 7000rpm.

[DatRat]

It does seem the consensus is leaning toward running a larger 2nd ring gap.  Do you guys recall your end gap clearances on the top ring and the second ring? I'm interested in the gap differences between the top ring and the second ring.

[letitsnow]

I don't recall mine, other than about .009 over the reference that I was using.

[ozconnection]

I researched this too when rebuilding my engine and came up with the same conclusion. Wider gap on the 2nd compression ring. So I gapped according to the new school. Did not have a baseline to compare to though. Talked to a pro-drag guy who knew his stuff and he said it was better, but said that they would drill a small hole in the top of the piston and have the gasses come out the top ring land, I think it was called gas porting but this is said only to be effective above 7000rpm.

 

Drag engines are refreshed regularly. Those little holes would fill with carbon very quickly and on a street car would be useless in no time.

[clarkspeed]

I gapped mine to the Ross piston specs. My normal engine builder said it was way too much.