89 Voyager 1/4 mile times are in!

[4 l-bodies]

I just make sure mine are within factory spec. Plain ol cast mahles and plain ol rings that come with them. Is that considered big or small as far as gap?

Asa,
Probably pretty small from factory. IIRC .010-.020 is factory top ring for turbo applications. But NEW specs are obviously on the tight side of tolerances. About .012 is what I see on most lower mileage motors. Far to tight on gap IMO for anything above stock boost levels.
Todd

[turbovanman²]

I wonder how many people with 2.2/2.5's break pistons because they have been given unjustifiable fear of large ring gaps....only to break the pistons (so what would have been so bad about a larger gap ). I would use the information from a broken engine to increase ring gaps where damage occurred.

I would run .002 to .004 larger than whatever broke AND have your 2nd ring gap larger than the top. Headgasket needs to be the fuse, not the pistons.

How many hypereutectic pistons break because of ring gaps too close? Quite a few I'll bet. I'm putting together a low boost, low cost 2.5 turbo using hypereutectic pistons for my parts runner 85 NY'er. I'll be running around .019 top ring gap.
I'll bet the C and D stock Mahle pistons break ring lands less than the A and B's because the ring gaps are bigger. Every thousands bigger diameter bore will be .00314 bigger gap.
Todd

My own personal opinion but I bet the odd one is caused by ring gap. Detonation is detonation, doesn't matter if the ring gap is big, small, etc. The piston gets rattled which causes the breakage, ring gap won't stop that. Now if the rings butted together due to lean, etc, then yeah, that will leave a mess. Also, if the rings are too tight, your going to gouge the cylinder wall. Any engine I popped a piston, have factory or larger ring gaps due to mass production rings.

Also, going to push the limits on a cast shortblock next year, it was rebuilt aprox 250K ago so the ring gap might be nice and loose.

[Shadow]

I would run .002 to .004 larger than whatever broke AND have your 2nd ring gap larger than the top. Headgasket needs to be the fuse, not the pistons.

X2, That's some Great advice right there!!!!!

[acannell]

Asa,
Probably pretty small from factory. IIRC .010-.020 is factory top ring for turbo applications. But NEW specs are obviously on the tight side of tolerances. About .012 is what I see on most lower mileage motors. Far to tight on gap IMO for anything above stock boost levels.
Todd

hmm I wonder if I've had any issues because of that...I know I was setting to FSM spec but I'm not sure where in those specs the gap ended up..I was just verifying the gap not setting it..so it could have ended up anywhere in the factory range

[Ondonti]

My own personal opinion but I bet the odd one is caused by ring gap. Detonation is detonation, doesn't matter if the ring gap is big, small, etc. The piston gets rattled which causes the breakage, ring gap won't stop that. Now if the rings butted together due to lean, etc, then yeah, that will leave a mess. Also, if the rings are too tight, your going to gouge the cylinder wall. Any engine I popped a piston, have factory or larger ring gaps due to mass production rings.

Also, going to push the limits on a cast shortblock next year, it was rebuilt aprox 250K ago so the ring gap might be nice and loose.

I say detonation but the factors leading to detonation tend to make rings grow with heat, butt, and the piston fail before other things fail when ring gaps are tight.

[Vigo]

Detonation is detonation, doesn't matter if the ring gap is big, small, etc. The piston gets rattled which causes the breakage, ring gap won't stop that.

The piston doesn't get 'shaken to death'. The pressure on the top ring breaks the ringland under it. If it were just the ring ends butting, it would break the area ABOVE the top ring before it broke the land BELOW it, because the time of the most heat and the most ring drag against the bore is when the piston is going DOWN on the power stroke. But most of the broken pistons ive ever pulled out of were only broke on the ring lands under the top ring with the upper part of the piston totally intact. Im sure there are many ways to crack a piston but all the ones ive cracked were due to the top rings transferring force into the ring land below a little TOO well. Making them leak off pressure faster with a bigger gap protects the piston to an extent.

[Ondonti]

The piston doesn't get 'shaken to death'. The pressure on the top ring breaks the ringland under it. If it were just the ring ends butting, it would break the area ABOVE the top ring before it broke the land BELOW it, because the time of the most heat and the most ring drag against the bore is when the piston is going DOWN on the power stroke. But most of the broken pistons ive ever pulled out of were only broke on the ring lands under the top ring with the upper part of the piston totally intact. Im sure there are many ways to crack a piston but all the ones ive cracked were due to the top rings transferring force into the ring land below a little TOO well. Making them leak off pressure faster with a bigger gap protects the piston to an extent.

I don't think this makes sense to me, is this a phenomenon in ANY motor? I think its just motors running much smaller 2nd ring gaps. Small 2nd ring gaps can cause fluttering of the top ring which would put the 2nd ring under ALL combustion duties and since 2nd rings are junkier metal they will expand aggressively and break the ringland when they butt. Seems pretty presumptuous to think 200hp motors with 50hp/cylinder are snapping off ringlands with pure brute combustion force, even if its detonation. if we were talking about a 600whp, 150hp/cylinder I might be more swayed but event that sounds oddball. Rods/mains/headgaskets should fail long before something like that happens.

Increasing your ring gap by .004 will not bleed off measurable combustion pressure. Keeping your 2nd gap large enough to prevent pressure from building between 1st and 2nd rings is very important. You set your top ring to be safe and your 2nd ring to never build pressure.

I have .025" top ring gaps on my daily driver btw. .029 2nd rings. I would not run this gap on an 8 valve unless I had failures with a smaller gap. I did this because a smaller gap caused problems with the ring material I have during a leanout. The larger gap already proved it would push coolant before butting on the dyno. Success.

Show me somebody having this problem with a proper aggressive turbocharged ringgap (not a factory gap or the gap suggested by someone who likes watching you fix stuff). A lot of the advice out there for building motors does not take into account how far people are pushing 8 valve pump gas motors stockish motors. I would use the "race build" ring gaps for an 8 valve, not streetcar gaps, if you like to push the limits.

I find it pretty basic logic to increase ring gaps if you break a ring land. Being scared of a slightly increased ring gap will keep you backed into a corner once you start having motor problems. Forged pistons handle heat better so you can't compare ring gaps between cast and forged piston motors. If you like running cast pistons and making too much HP, open up the gaps. People would probably find a lot more utility in the newer junkier cast pistons that are available with a big gap. If you don't like the facts, buy forged. Let me tell you, a slightly smaller ring gap will make you 0.00 more hp The main purpose of small ring gaps is emissions related, and that includes keeping your oil cleaner longer. All those motors that have a bunch of blowbye are not ring gap problems, those are ring seal problems.
----------------------------------
Measure top and 2nd rings. Chart them out. See if you can find any problems, or see how poorly the tech built your engine. I have seen a spread of .012 to .019 top ring in a Mitsubishi.
This applies to any engine.
If you had top ringland problems, do the rebuild with .002 to .004 larger TOP ring gap than the motor already had , even consider running slightly larger on any cylinders that you think run hotter, and then ignore all 2nd ring suggestions and run .002 to .004 larger than whatever you set your top ring to. Prepare for success. If it fails, bump it up again. If you choose a larger jump the first time, you probably won't have issues.
If you have 2nd ring issues, you might be able to get away with just increasing your 2nd ring gaps on the large side of adjustment and going for a consistent top ring gap.

If you had top rings like this and you break your 2nd ring gap,

.014
.017
.016
.018

Try .018 top, .022 (or larger) 2nd ring. I would not call it crazy to go .018 top and .024 2nd ring if you really wanted to be safe but maintain (for little reason) a small top gap.

If I had the motor above, I would probably skip to .020 top and .024+ 2nd ring. Prepared for increased abuse. If you overdo the gaps, its not going to hurt anything but oil changes.

The real question is, what is the issue you want to solve. Do you want increased engine abuse capacity, or MAYBE a tiny increase in power until it breaks again? People seem to constantly take a shot in the dark based on advice given by OEM for OEM HP levels or people who LIKE to sell you pistons.

[Vigo]

Increasing your ring gap by .004 will not bleed off measurable combustion pressure. Keeping your 2nd gap large enough to prevent pressure from building between 1st and 2nd rings is very important. You set your top ring to be safe and your 2nd ring to never build pressure.

I am not totally disagreeing with you but you are saying that the top gap will not flow much and then saying the 2nd ring will butt because of all the heat from the gases that flowed through the top ring gap? And equalized the pressure enough to cause the top ring to float? Even though the top ring has a smaller gap and direct contact with the combustion space? Because to me the only way the ring breaks off the land ABOVE it is if it butts and drags real hard against the bore while the piston is going down. Seems like the top ring would butt first and break off the top of the piston instead of the second ringland if the force were going in that direction.

I havent done any real research on this so im definitely open to learning.

[shackwrrr]

Weisco instructions put my ring gap at roughly .02 top and .025 bottom. I actually had to take .010 off the bottoms to get it where it needs to be. If someone would have just tossed the rings in as is the gap would have been super tight.

[A.J.]

Some piston manufacturers give different gaps for different usages. I can't remember what brand they were but the last set I installed had different specs for N/A, boost, and NO2. It was a boosted application and I have a piston ring filer and I went to the large end of the spec.

[jefo]

Well, I took the van to the strip with the refreshed motor running the stock computer, and Treadstone intercooler. Put down a 14.7 at 92mph at 13psi. My launches still need work (2.24 60ft).

Before the new intercooler, with everything else the same, it had a best of 15.1 at 89mph at 13psi.
Can't wait for a computer tune with +40's, finally open the 3" cutout, and more boost!

[Ondonti]

I am not totally disagreeing with you but you are saying that the top gap will not flow much and then saying the 2nd ring will butt because of all the heat from the gases that flowed through the top ring gap? And equalized the pressure enough to cause the top ring to float? Even though the top ring has a smaller gap and direct contact with the combustion space? Because to me the only way the ring breaks off the land ABOVE it is if it butts and drags real hard against the bore while the piston is going down. Seems like the top ring would butt first and break off the top of the piston instead of the second ringland if the force were going in that direction.

I havent done any real research on this so im definitely open to learning.

I didn't see this post.

The purpose of opening the 2nd ring gap is to prevent top ring flutter (top ring unseating from the bottom of the ringland). When the exhaust valve opens the top ring gets a lot of pressure off of it and the pressue differential could flip the wrong way for the top ring. Its going to be worse in high rpm high boost engines where the bleed off time is small with greater pressure/volume trying to escape through the gaps.
I don't know the science of it, I just know its standard fix for the issue and helping the top ring seal/seat improved heat transfer etc. If you did excessively flutter your top ring all heat would have to transfer through the 2nd ring. I know in cast top ring applications the top ring shatters when its fluttered hard.

I would call it a preventative measure that might not end up mattering but finding out you do have the problem is difficult to judge and expensive. Some racers probably went their whole lives suffering if they stuck to the standard old school build specs. I wonder what the old 1950's ferrari race cars (small displacement, high rpm) had for ring gap specs compared to the v8 muscle cars back then. Its possible old school really just means Murica

[jefo]

I dynoed the van recently on 13psi shown here: https://www.youtube.com/watch?v=2zNNguniUf8

It made 182whp and 256wtq The hp curve is pretty flat from 3500rpm to 5500 rpm. I can't wait to see how the curve will change when I raise the boost.

[Vigo]

Actually the torque peaks early and then falls steadily. The power line is what is flat because power is a factor of torque AND rpm, so even though torque is falling, rpm is rising and it ends up evening out.

If the torque line was steady you would see the power line consistently climbing. If you were actually still making ~250wtq @ 5500rpm you would have made ~260whp!

[jefo]

I think part of the reason the horsepower stayed flat (torque rapidly decreasing) was this dyno pull was done running with the stock minivan catback exhaust (cutout closed).
It has a ported swingvalve to 2.8", a 3" downpipe, to a 3" cutout and I expect when I open the cutout there will be a lot more top end power.

I'll have to wait to get a tune, as when I open it it creeps to cutout.

[Vigo]

That's possible. I once dyno'd a 2.5L with a 2.25" (actually necked smaller inside part of it) exhaust and hp peaked at 3700 when it should have been ~4800. I couldn't find anything else wrong to move the peak power down 1000 rpm (advanced cam etc) so i assume the tiny exhaust was what caused it.

[turbovanman²]

Those are some good numbers. Can't remember, auto or 5 speed?

Also, the stock heads and one piece intakes really shut these 2.5's down around the 5500 rpm range.

[jefo]

Its a 5 speed. If I blow another head gasket I'll have to get a ported head. The 1 piece I have is ported and the elbow is replace with a bigger tube (crazyeye intake), so the head is the main restriction. I want to do the PT lifter mod as well, as some of my lifters make noise, and hopefully it will give it some more top end power.

[jefo]

Took the van to the track tonight. FWD stage 3, 17psi, with stock cat-back exhaust to prevent creeping.

3 bar tune is on order from boostbutton with a staging limiter. This combined with an open 3" downpipe should yield some interesting results.

60ft: 2.254
1/8: 9.141
mph: 78.54
1/4: 14.177
mph: 97.06

[BIG PSI]

Took the van to the track tonight. FWD stage 3, 17psi, with stock cat-back exhaust to prevent creeping.

3 bar tune is on order from boostbutton with a staging limiter. This combined with an open 3" downpipe should yield some interesting results.

60ft: 2.254
1/8: 9.141
mph: 78.54
1/4: 14.177
mph: 97.06

WTG, do you have pictures of the Mini & its motor ?

Chuck