Assuming proper timing is pulled for the Shot size
Assuming Fuel system is working properly and consistantly
Assuming Nitrous is added progressively to avoid the initial torque spike.


Does anyone have information on the ring tolerance of a stock piston..
I take it that the main cause of the failure is the more violent explosion the nitrous causes in the combustion chamber...
Since among board members and my friends say its equally safe to run a 100 shot on a stock Fbody as it is to run a 100 shot on a heads/cam Fbody.

So power output increase isnt necessarly the cause.. it must be the way that power is achieved.

Links to comphrensive articles on this subject would also be helpful.

 

That's a complicated subject. Way beyond me to expain the theory of it. But here's real-world for you....

ANY power adder, be it boost or nitrous, throws extra heat at the piston. This heat goes quickly into the top ring, given a drag-race-type setup that doesn't see extended high RPMs.

more heat = more ring end gap needed

Most piston manufacturers recommend opening up the top ring end gap about .005-.007" for a 4" bore "street" supercharged or nitroused engine, once you clear away all the fluff and trivia.

I recommend you follow their recommendations at the very least. You don't want to know what happens if you butt a ring gap closed. Very little is left to salvage in the motor. It's much better to go a little too wide than to go tight and risk a catastrophe. You would thing that a much wider top ring gap would instantly equal high levels of blow-by but I haven't found that to be the case is the real world. When I went fom .025 to .032 on my current blower motor blow-by did not increase AT ALL from what I have been able to tell.

I would NOT assume that just becuase you are making your considerable power N/A right now that you can add 100+ HP worth of nitrous just as safely as you could on a stock motor. Added HP ALWAYS means extra heat. Maybe not in the vicious, violent way that boost or nitrous makes it, but it's still there, nonetheless.

 

detonation is the big issue with nitrous and any power adder. forced induction is a lot more prone to detonation with such high cylinder pressures. And detonation is NOT your friend- especially with brittle hyperutectic (sp?) pistons.

With that said we gapped my pistons SLIGHTLY larger for nitro added heat and no gap = boom.

gaining 100hp with a blower is not the same as gaing 100hp with heads and cam.

 

Huh? Air is air, it doesn't matter how it got there. The only possible difference I can think of is an inefficient, non-intercooled blower that's adding heat. Other than intake temp, 100hp is 100hp no matter how you got the air in there.

Or what did you mean by this?

 

What Damon said makes sense. I use JE pistons and follow their recommendations which may be found here http://www.jepistons.com/pdf/piston_instrc4032.pdf

I tend to go a little small within the recommended range because my piston crowns are ceramic coated. The coating results in less heat to the piston crown and hence to the ring package. That said, my last buildup seems to have too much blowby. I am going to go a bit narrower this year as well as using a different block which will only be 10 over rather than 40 over. I suspect bore distortion may also be an issue now that I am making more hp.

Rich Krause

 

I may be wrong- but i was always under the presumption that spraying a 75 shot created more "stress" for a lack of better words in the engine than say a CAI, catback, headers and rockers.

 

Actually, I think a 75 shot with Nitrous would be less stress than porting that achieved the same goal, since Nitrous has quite a cooling effect on the intake charge.

 

Rich, unless my information is wrong, I believe you are doing this backwards. With thermal coatings, less heat is absorbed by the piston crown, and more left in the chamber. As such, the rings expand more, not less. I believe the people I have had coat my stuff recommended the same, but it's been a year or so...

Also, about the post saying gap should be .005-.007; I think you mean per inch of bore. Typically, a 4.030 bore setup huffing a tad of bottle will be gaspped around .022/.024 top and bottom, respectively (yes, less gap on the top ring ).

If I'm wrong on any of the above, please let me know.

 

In theory, it makes no difference how you add power, but in practice things happen in a nitrous motor that complicate the equation. For example in my old carbed 306, the front two cylinders always ran leaner than the rest, and the rear two always fatter. I burned the electrodes off the fronts a couple times, and that has obvious implications... on a 175 shot.

Anyways, nitrous definately does work an engine more than an N/A setup, unless you spin the N/A setup harder... Theories aside, and whatnot.

 

Yes, per inch of bore. As far as the coating go, I am not sure at this point. I will talk to Swain Coatings before doing up the new rings.

Rich Krause

 

I would disagree with that, though I may definitely be missing something. My reasoning would be that when you use a power adder, say nitrous, you are having a large effect on the flame speed of the mixture, which is going to change how you produce that power.

The actual application of force to the piston during the power stroke can vary, while the hp/torque value is only going to be representative of the total applied.

Say with a nitrous motor you have increased flame speed greatly - you reduce timing, have a quick short burn, and then are done. This is going to cause higher peak cylinder pressures vs. a NA setup (where burn time may be longer)


Chris

 

To clarify my earlier post.....

When going to a forced induction/nitrous gap you will typically, under most circumstances, and with most brands of pistons, want to be .005-.007" larger than the same top ring gap spec for a N/A application.

For instance, using KB Hypereutectics, they want a ring gap of .026" for a typical 4" bore street performance engine, N/A. Same same but gapped for forced induction/nitrous they recommend .032" .032-.026 = .006" larger overall gap.

ALWAYS follow piston manufacturer's recommendations on piston ring gap NOT the ring manufacturer's recommendations. Piston design and material has a HUGE effect on how much heat gets thrown at the rings. Don't use my specs or anyone else's specs for YOUR engine. What works well on one could make another one go ka-boom or have massive leak-down problems.

Also, therefore, don't use old .xxx" of gap per inch of bore kinda calculations. They're ESTIMATES and don't take into account the specifics of piston design and material= especially the new stuff that didn't even exist years ago when those kinda formulas became widely popular. They'll get you close enough if you're rebuilding the family truckster's engine for hauling groceries but they're another old "rule of thumb" that can get you in a lot of trouble if you try to apply it to every combination you come across.

The differences can be SUBSTANTIAL.

Just using my above example with KB Hypereutectics do you notice that even the N/A recommended ring gap is a rather wide .026"? That's BIG compared to a stock cast piston that will usually be in the range of .018-.020". THink of how many people over the years never bothered to read KB's ring gap recommendations, isntalled them at .020, just like they had always done with other cast pistons, and then blew the motor to smithereens at half track after they added only a little 100-125 shot of nitrous to it!!! What do you think they said after that? I'll bet it's something like this: "Those damed Hypereutectic pistons are junk- they break if you try to use them with nirtous." Do you think they ever figured out that the fault was not with the piston but with THEIR assembly, having the rings .012" too tight for the intended application? Not likely. And so the pistons get blamed when there was nothing wrong with them, and no detonation may have even been present when the motor blew!

I've run KB Hypereutects many times. My current blower motor has been running around for a year with KBs in it. Several previous motors, as well as stuff I've built for others have used them and they're all motors that see regular nitrous use at the strip. The pistons work fine, and can take a LOT of punishment if you set them up right. Not saying I RECOMMEND them for a blower/mitrous application, but they WORK just fine if you know what you're doing and FOLLOW DIRECTIONS.

 

Rich, I'd be interested in what Swain says.

I knew as soon as I posted in this form I'd make a fool of meself . Actually, Chris, I have no scientific information regarding this, just old adages and rules-of-thumb. Too bad Chuck doesn't come by any more, cause I think you are correct about higher peak pressures. Aside from that I agree with what you said, and have had experiences that bear your advice.

Damon is right on with the last post, too. I've taken apart plenty of grenaded Hyper piston motors that had the crowns ripped off the piston, and the remaining rings were bent upwards, a telltale sign of end gap being too little. Anybody that assembles any engine without checking each ring in it's respective bore against the piston mfr instructions is rolling a very expensive pair of dice.