I'm working on building an sb 400 this winter and the two parts I haven't settled on are pistons and rings. This question is about the rings.

I thought I was sure I wanted Total Seal rings for reasons of efficincy, and power, but I have noticed some comments about them not being stong enough. I am looking for about 400 hp and I don't forsee ever wanting a blower or nitrous. What do I need to know about these rings?

The other feature I want is for the top ring to be as close to the top of the piston as possible, for reduced crevice volume, and reduced emmissions. But I have also noticed comments that this can cause the top of the piston to crack, or the top ring to break due to it's close proximity tto the heat of combustion. This I don't get at all.

Here's why

I have a snowmobile that has a two stroke, 800cc engine that is pumping out 160hp. The top ring of each piston has a cross section that looks like an "L", with the raised part of the "L" facing up. This top ring is located so close to the top of the piston, that the raised "L" part of the ring acctually extends slightly higher than the edge of the domed piston crown itself! How is that for no crevice volume! So this makes me wonder; how can this engine push 200 hp/liter, with the top ring totally exposed to the combustion, and we have potential piston and ring breakage at far less than 100 hp/liter? Plus, the snomo engine doesn't benefit from the extra two cycles to allow greater cooling. And I don't run this thing easy either. I use it for highmarking here in Utah, and typical use includes 30 - 45 seconds at WOT (200hp/liter), then a closed throttle coast to the bottom of a bowl, a U-turn, and full throttle again untill it won't climb another inch. Sometimes this goes on for 20 minutes straight, and I've been running it in this configuration for 3 years w/o a tear down.

So back to the question; why can't car parts take the same abuse?

P.S. I run Wiseco Ultra Lite, forged pistons in the snomo.

[This message has been edited by Tom 400 CFI (edited February 15, 2001).]

 

Size difference.

The diameter of the piston and thermal expansion rates are where the difference comes in.

Also if you want to do a fair comparison of power per liter, then use torque/liter instead of horsepower/liter.
Horsepower is calculated based on engine speed (rpm), where torque is the actual measured power output of the engine.

ODB

 

Well, whats the ring gap (cold) on the snomo? If you increase the gap, you can run them higher up in the piston without problems. Theres other factors too like sizes, materials used, oiling, etc.

 

Some good points, but the snomo engine makes 106 lb/ft of torque at 7900 RPM. That is 132 lb/ft per liter. That would be 750 lb/ft in a 350! The ring end gap in a machine with a 2.83" bore(my machine) is .012-.018"

So what about the Total Seal rings? Are my theories sound? How close to the piston crown can I take these or any rings?

BTW, thanks for the responses.

 

So we're all on the same page, a 2-stroke makes 1.5 times more hp than a 4-stroke for the same displacement and specifications. A 4-stroke makes more overall torque over a broader (and lower) range. Not to mention an 800cc is a triple IIRC.

Now, your 2-stroke uses completely different clearances. When the piston expands against the cylinder wall the raised part of the ring becomes a load bearing surface and is more like a sleave than a ring. That's part of the reason you can cold sieze one.

Total seal rings should be fine and will keep your leakdown rate around 1-3% vs. 8-10% for normal gapped rings. Just get a standard piston in the oversize and CR you want (don't go ordering funny ring locations). If the engine is kept in tune and taken care of it should burn clean enough for most inspections.

[This message has been edited by 87RS402 (edited February 15, 2001).]

 

Thank you. And your right my 800 IS a triple, but some of the new ones are twins.

 

Tom, In 12 years being around my buddies motorcycle salvage and repair shop I've learned a few things. Maybe one of these days I'll show you how to cook bacon and scrambled eggs on the exhaust of that triple.

 

Right on, man!

 

moving the rings closer to the top of the piston is asking for trouble in an automotive engine. (i am going to guess that you do not put 12k miles a year on your snowmobile......)

As for using the total seal piston rings, my vote is... GO for it. having less than 1% leakdown sounds like a good plan to me. (consider that GM feels that up to 25% is "acceptable".........) this is power that you can get back from your engine. right now, it is being blown into the crankcase, and the thru the PCV system. Reducing crankcase pressure, and increasing combustion efficiency always makes good sense to me....

 

No I don't run 12,000 mi /yr, but I do have a machine that has 12,000 mi on the origional engine. (Not the one that makes 200 hp/ liter though!)

I do agree with you about the efficiency of gapless rings, however, which is why I'm so interested in them.

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'83 Trans Am. 400 CID, 224/234 crane cam, Summit aluminum roller rockers, hand ported intake, home bored 2.09" (53mm) throttle bodies, MSD 454 injectors(75 lb/hr), Holley 255LPH fuel pump, custum modded FPR w/Vacuum port added, Edelbrock TES headers, empty cat, SLP 3" stainless steel cat back, stock ECM & chip. Borg/Warner T-5 World Class, 12" Corvette rotors and clipers, GTA 16" wheels, South Side Machine subframe connectors, 1LE sway bars, 3.45 ring and pinion, Alpine sound.
Best E.T. 13.532
Best MPH 102.24