ring gaps on stock pistons for supercharging

[user-23911]

It's still going to make less HP per litre than a stock jap turbo engine.
There's plenty of examples of stock jap turbo engines making 100HP per cylinder without breaking the pistons. That's tuned "right".
On the other hand, there's far more of them blow up.......when tuned by deaf people relying on ears.
It's detonation that does it.

[user-23911]

This one is pretty impressive.
Now if stock ring gaps were insufficient and they butted and took the top off the piston......it would happen the very first time on the dyno at that power level.
It would break every piston.
There'd be lots of smoke and lots of blowby, the dipstick should be hitting the roof.

https://www.youtube.com/watch?v=YnS-nT6o4xA

So.....something's not right?

[roadrunner]

Engines generally don't destroy all 8 pistons at the same time, usually the cylinder that runs the highest temps will most likely fail before the others wether that be due to pre-ignition or insufficient ring end gap.

[F-BIRD'88]

It is detonation that causes the piston top and top ring to get very hot very fast
which causes the top ring to expand and lock up, lifting the top ring land.

Running on the street in the real world, in battle, where conditions are not ideal is a LOT different
than a single controled very short dyno test.

Yes its usually one piston that gives up. The larger gaps gives you some play room, before this happens.
The real difference in installed running at temp gap space and ring gap blow by IS VERY SMALL.

When you get a bit carried away, and say the ring momentarily lockes and scuffs the bore/ring face, the engine does not self heal.
Now its a real huffer.
This is not a computer controled engine with knock detection and timing control and boost control
and does not have a intercooler.
ALL the modern supercharged engines go into a reduced power, reduced timing mode and or limit boost
when ever operating condirions are not optimum and or detonation is detected.
They do not come even close to making their claimed rated power in that condition.
It is easy in real world use to put these engines in protection mode.
Hot eninge, hot heat soaked intercooler, poor pump fuel etc etc etc.
They do not repeat power once they get hot from repeated WOT runs. Casue the computer proetects the engine.

This engine does not have this computer control protection.
Thats why you want low less than mechanically optimism compression ratio and larger generous top ring end gaps.
Thats how you get stock cast piston engines to live on the street with a supercharger.
The supercharger makes the power. It will make a ton of power and not hurt the pistons as long as detonation is avoided.
You will not hear high speed detonation.
A simple set of "engine ears" listener headphones can help. Easy and cheap to DIY build.
Now you can hear detonation under WOT, while tuning.

DIY Engine Ears knock detector headphones.
http://www.autospeed.com/cms/article.html?&A=0353

image of listener
http://us1.webpublications.com.au/stati ... 53_8lo.jpg

I built one for under $100 using stuff from Radio Shack ("The Source", in Canada).

[user-23911]

Ears don't work.
You can't actually hear knock until it's enough to break a piston.

You know this once you've got proper electronic knock detection running alongside a microphone and amplifier.
The best way is to use the Jaycar kitset with a resonant knock sensor.

If you're running a V8 with a supercharger or turbo, no intercooler, you'll be limited to less than 15PSI boost with 8 to 1 CR and pump fuel, even then you'll probably get detonation.
Been there, done that 30+ years ago.

If you're running stock pistons, your CR will be too high. In that case you'll be limited to maybe about 7 PSI or boost with no intercooler before it detonates.
With any engine there's always one cylinder running a bit leaner than the others. That's the one that detonates first and breaks first.
BUT.....if it really WAS tight ring gaps that made them break, they'd all break all at the same time.
The best I've managed was to break 8 out of 8. 4 of them in 2 places, the other 4 in only 1 place. But that was years ago. I only break them 1 at a time these days.
Bigger ring gaps cause more blowby, get more oil into the cylinder, lower the octane......cause more detonation.

[F-BIRD'88]

Funny, in some 35 years I have only broke one piston. EVER.
(it was not on a supercharged motor)(but it was a cast piston)

Yes the cr is low, Thats what the 3 stack of head gaskets is for. Cr will be under 8:1.
Yes the boost will be under 15 psi. Thats the point. this engine will make a TON of horsepower at 10 psi blower boost.
It will meet or exceed the OP'ers power and performance goals.
These slightly larger top ring end gaps will not increase oil consumption or oil in chamber contamination.
The actual running ring blow by will be negligable- insignifgicantly increased.

But poor ring seal from scuffed rings will make it a "huffer" in short order.
(Slightly larger) Top ring end gap is not a meaningfull factor of oil consumption.

But scuffed rings/cylinders , is....

[F-BIRD'88]

The top ring end gap on the pistons in my street strip 406 SBC are larger than what this motor needs (It is set up for a bit of nitrous). It does not use oil. It does not have blowby. It makes MORE power than I ever expected it to.
It has some 10 years service on the dredded KB hypers that like and want a bit increased top ring end gap. (especially on nitrous and supercharged applciations).
None of the other motors that got these slightly increased top ring end gaps use oil or have blownby.
They all made very good power. None have failed in service.

This will not make the engine indestructable.

[benno318]

The Mahle suggestion is a tighter second?
Say it Int so! LOL

i wonder if mahle think that in use the top ring gap closes up and the 2nd effectively ends up wider??

[RevTheory]

The Mahle suggestion is a tighter second?
Say it Int so! LOL

i wonder if mahle think that in use the top ring gap closes up and the 2nd effectively ends up wider??

That's exactly what I was wondering. I was thinking that running .028 upper and second in a mildly boosted 4.03 bore would get the job done but I dunno...

[MadBill]

A lot of the current thinking on the ring gap subject is of course that the 2nd ring gap should be larger, to ensure no inter-ring pressure buildup. However if the top ring gap increased the correct amount to minimize the chance of butting under high load/piston temperatures, it will be proportionally smaller when it matters, so perhaps the 2nd ring should be gapped relative to what an NA top ring gap would be..

[Warp Speed]

Depends on where you get your "current thinking" from?

[Orr89rocz]

I run .0065x bore on top for my 400" sbc. .0067xbore on second ring.
Made no difference in power from what i can tell from .0056xbore i used to run. I opened up for more boost

Stock cast piston, no reason not to go wide. Rather have little bit of blowby than pinch off the top of the piston. Tune is def a key to gettin them to live but at somepoint, you will still have heat in chamber when making bigger power numbers. Only so much a tune can do. Power is cyl pressure and pressure is heat

[MadBill]

Depends on where you get your "current thinking" from?

The ongoing disagreement re the 'wider second gap' theory is why I said 'perhaps the 2nd ring should be gapped relative to what an NA top ring gap would be.. ' This wording covers all bets.

[Dan Timberlake]

The "wider second gap" recommendation sounds to me like a logical extension of the pretty popular low pressure = crankcase ring seal enhancing theories.
At least when rpms get high enough to start making ring inertia high enough to overcome sealing pressure.

piston ring max acceleration - vintage.jpg

What I can't make fit is use of Total Seal total seal rings in the second groove, at least in "high rpm" applications.

[MadBill]

...What I can't make fit is use of Total Seal total seal rings in the second groove, at least in "high rpm" applications.

Here's my theory:
1. TS develops and markets the Gapless® top ring.
2. The then-available metallurgy proves unable to cope with top ring conditions, resulting in unsatisfactory performance.
3. As a Bandaid, the Gapless® ring is moved to the second groove where conditions are more benign, ignoring the fact that if it works as designed this reduces or eliminates the pressure differential across the top ring, leading to premature flutter and loss of sealing.
4. Available metallurgy takes giant steps forward, allowing the return of the Gapless® ring to its logical top groove location.
5. The second groove Gapless® design continues to be sold as a loss leader and to save face re the preceding saga.