Forced Induction - second ring end gap smaller?




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lmt0705
01-23-2008, 06:08 PM
for an FI motot I always thought the end gap on the second ring was supposed to be larget than the top ring, the shop that built my motor(very well known and resptected on this site) set it up with .026 top and .018 on the second, I called and asked about it and they said that every engine builder does stuff a little different and thats what they have found to work. are they just feeding me a line of you know what or is there some truth to that, and maybe they found an end gap combo that works better than someone elses.by the way thay are NPR stainless chrome top with napier second, sizes are 1.2, 1.2, and 3mm


Phil99vette
01-23-2008, 06:20 PM
Every engine builder I talked to on my build said that the smaller 2nd ring is old technology. On turbo apps they found HP by opening up the 2nd ring gap. Kurt from W2W said they always open up the 2nd rings on FI builds.
Phil

lmt0705
01-23-2008, 06:27 PM
would it be any different on a supercharger? i wouldent think so


lmt0705
01-24-2008, 08:37 AM
anyone else have an opinion?

kp
01-24-2008, 10:31 AM
Honestly if you didnt know you wouldnt be worried. I have built many an over 1500hp engine with the 'old' way of a smaller 2nd ring gap on some pretty narrow rings befre people started doing it the 'new' way that made the power they should have.

I wouldnt lose sleep over it, next time ask before you get it done, dont just assume they will do it the way you want or use the accepted norm.

427
01-24-2008, 11:49 AM
Every body has a different opionion on this, and most important is both ways "work" without engine failure.
We use the larger second on all engines, not just FI, this is just our choice.
The idea starts with the fact that the second ring is a oil control ring, it is not meant to hold combustion. The top ring works on the pressure difference above and below it, and seals in two main places, the cylinder wall and the pistons ring land area. If pressure leaking by the top ring collects between the top and second ring, the top ring can not seal to the piston. Making the gap larger on the second is just a way to be sure that the pressure does not equalize. It is also possible that the direction change in the overlap cycle can be affected by these changes, helping to get the top ring sealed faster.


Kurt

kp
01-24-2008, 12:23 PM
Every body has a different opionion on this, and most important is both ways "work" without engine failure.
We use the larger second on all engines, not just FI, this is just our choice.
The idea starts with the fact that the second ring is a oil control ring, it is not meant to hold combustion. The top ring works on the pressure difference above and below it, and seals in two main places, the cylinder wall and the pistons ring land area. If pressure leaking by the top ring collects between the top and second ring, the top ring can not seal to the piston. Making the gap larger on the second is just a way to be sure that the pressure does not equalize. It is also possible that the direction change in the overlap cycle can be affected by these changes, helping to get the top ring sealed faster.


Kurt

I agree, the 'new' way makes perfect sense. I'm not pro-smaller second ring gap, even though it seemed odd when I first heard it about 15 years ago :) But I always ended up with larger 2nd gaps than recommended anyhow, them iron 2nd rings ground a lot easier than the top rings so maybe I was ahead of my time by complete accident lol.

Lonnies Performance
01-24-2008, 06:04 PM
The rumor that I heard was the top gap was larger due to that ring running hotter & therefore expanding more. This suposedly closes the gap up, making them closer to equal under operating conditions. That was the explanation I received well over 20 years ago. For what its worth, I how gap the 2nd ring larger.

chriswtx
01-24-2008, 06:10 PM
Gaped my second ring larger on my forged supercharged motor...Ring and piston technology has changed over the last 15 years.. Even the stock factory motors are gapped this way...:D

lmt0705
01-24-2008, 06:56 PM
could this cause any issues with my motor?

DeltaT
01-25-2008, 05:32 PM
Speed-Pro site recommends larger on the 2nd and talks briefly about why.

I used the Hellfire rings and using their recommendations ended up with equal sizes at .026" on both. A bit more conservative on the top ring, and just about in the middle of the FI range on the 2nd.

Is the motor still out of the car?

Jim

lmt0705
01-26-2008, 01:48 AM
yes, im not really keen on pulling it apart though, maybe I should have just built it myself again

helicoil
01-26-2008, 02:54 PM
Gaped my second ring larger on my forged supercharged motor...Ring and piston technology has changed over the last 15 years.. Even the stock factory motors are gapped this way...:D

Very true. OE's been doing it that way now for over 10 years. Will it make or break your engine, no.

Are you leaving it a little on the table, yes. It has been proven, the gains are usually north of 5000 RPM.

I wouldn't really call it new technology, but maybe new to some.

Standard rings out of the box will usually measure smaller on the second ring gap even from the manufacturers who recommend them to be equal or larger.
They must be filed, maybe the machine shop you used did not want to do that.

The reason for increasing the second gaps was explained pretty well in an above post.

DeltaT
01-26-2008, 03:53 PM
Well, now's the time to do it. Wouldn't be that bad. At least you need the gaps bigger instead of smaller!

Jim