overbore/undersized rings to allow for heat?

[knowtwodrugs]

So on most racing engines, a little bit of play is allowed in the cylinders to allow for inevitable heat expansion.  Would this solve the problem of turbo-ing a N/A engine?  If not solve it, at least help?

[arb]

So on most racing engines, a little bit of play is allowed in the cylinders to allow for inevitable heat expansion.  Would this solve the problem of turbo-ing a N/A engine?  If not solve it, at least help?

Not sure I understand the "problem" - I turbo's my NA and put it in a Caravan. There was no issue with the lower end. Yes, on all engines there are required "clearance" between all moving parts. Pistons and rings are no exception. In some cases, such as an air-cooled aircraft engine, when they get too cold like in the depths of some Michigan winter nights, they can loose this clearance all together and you do not start them without a pre-heat. Follow the clearance specs for your NA engine and you will be OK with adding a turbo and even boosting quite a bit (with an inter-cooler if you like your turbo )

[knowtwodrugs]

I was asking because all of the naysayers against turbo-ing a n/a engine seem to use the oil-squirters and pistons seizing as the primary reason against the turbo.

[arb]

For sure oil squirters are a good thing. I'm only boosting to 9 psi max, I have a Summit Racing air / oil cooler and use 100% synthetic oil so I don't have any issue with my NA engine being boosted. Plus, I have a bit of an intercooler that the stock turbos don't from the factory, so heat is not an issue.

[arb]

Turboing an n/a will not necessarily result in higher temps, just the potential for them.  Typically up to 15 psi, the lower fuel to air ratio will result in lower temps provided fueling is not increased significantly.  Increasing piston to bore clearance or ring gap is not the right approach, IMO as it is just placing the engine out of spec sooner.   

My option as well :-)  Let us not forget, our lovely engine is a "compression ignition" engine, so increased gaps at the piston will only allow more blow-by gases and does not help compression.

[R.O.R-2.0]

making a looser engine is like building it half worn out already..

it wont run right if it has more than .001 clearance between the piston and cylinder..

[rabbitman]

In some cases, such as an air-cooled aircraft engine, when they get too cold like in the depths of some Michigan winter nights, they can loose this clearance all together and you do not start them without a pre-heat.

Actually, as long as they can crank they'll start. Back in the old days they'd land and drain the oil so they could keep it warm like in a cabin and next morning they'd pour it back in a fire up. Back when people where tough they did this down to -65F, flying at about -40F and colder is avoided these days.

[sdwarf36]

making a looser engine is like building it half worn out already..

it wont run right if it has more than .001 clearance between the piston and cylinder..

 Wow-who taught you that?   Care to back that up with some science? Dyno results?-pictures?

 The oil squirters shoot oil at the underside of the piston-for cooling. Too hot aluminum + 23 to 1 could make the top of the piston soft-eventualy letting it sag. But things would have to be really extreme for it to actually happen.

[R.O.R-2.0]

well, a diesel calls for .001 clearance between cylinder and piston..

.004 being the wear limit..

why would you want to build your engine with the bores already half way out of spec?

(just so you know, someone on this forum used that term on me before, about building an engine half ways worn out)

[sdwarf36]

The amount of piston to wall clearance is determined by the piston manufacturer based a a few things-piston shape-how the pistons made-cast-forged--silica content of the aluminum are a few of them. The amount of heat the engine makes IN NORMAL CONDITIONS determines how much expansion needs to accounted for. Luckily, our cylinder walls are really thick-so that helps keeping things in place. Also the very thick amount of meat above the ring keeps excess heat from getting to the skirts. So the engineers can say .001 is a safe number for quiet ( ) long lasting service.
 But what happens when we double the boost? Or overheat it once or twice? Or put our foot to the floor before things are fully warm? Scuffed skirts.  Any extra clearance is just extra safety for the things the engineers didnt plan for. I'm not saying starting off with .005 is needed, but an extra 1/2 thou or so wont hurt a thing. The .004 service limit doesnt mean that it won't run at .0041--it means the engineers figure by the time you've worn .003 out of the cylinder, you shouldn't be just putting the motor back together the way you found it on dis-assembly. (and by then your ring end gap would probably be getting pretty big--knocking the ability to hold compression well.)
 The ME motor I started with had .008! piston to wall clearance on teardown-and it was still running. And I bet many of us driving around with 300k miles on our motors that we think are running fine are in similar shape.