Ring Gap advice
 

I am rebuilding a 2001 VVT 1.8 L.
I am having trouble finding information on ring gap for the application, and am asking for advice.

I had it bored to 10 thou over, and will use new 9.5 compression Maza pistons, with Mazda over bore rings.
It has an 8 psi turbo, expecting to make 210 to 230 hp at wheels, and will use it in a wheel to wheel race car. (It did 230 hp with 10:1 compression ratio, but was belching oil, so I had to strip it down.)
Boost may go up a bit to get the higher hp rangen of 230 with lower compression ratio, to maximize power to weight in class.

Mazda 2001-5 manual (found on Mellons site) seems to only speak to ring gap for stock bore, and not even for the Mazdaspeed turbo engine.
The 99 engine manual does not mention a different ring gap for 10 thou over bored engine.

This link to guidance from Wiseco (scroll down FAQ page) indicates around an 18 thou ring gap for mild boost and given engine bore, but even for stock bore and stock NA it recommends 2 thou over Mazda max. And my understanding is Wiseco says its pistons and rings should not be compared to recommendations for stock OEM products. Plus when I used Total Seal rings, I noticed they recommended almost twice what Mazda does. So it seems it really does matter who made them, and what they say.

I am inclined to use the top end gap of Mazda's recommendation, does that seem about right?
Or should I be using Wiseco's calculation table?

Many thanks for any feedback.

For your application I would use forged pistons. The ring gap for OE rings for a turbo application should be increased as should the piston to wall. A good ring gap for the oe cast rings would be .016-.018” for the top and .018-.020” for the second. I would increase the ptw too .0025-.003”.

Agreed on these points. If you're committed to Mazda pistons, so be it, but they may not be as tolerant to "off nominal" conditions, such as a bad tune, malfunction, or some random bad fuel. LeoNA is correct that you typically run larger ring gap with forced induction or power adders.


100%. Piston material and design, ring material and design, application, etc, all factor. Typically, there is ring gap calculation based on bore, application, ring type, and ring position. As an example, I'm running NPR low tension rings. For "moderate turbo / nitrous" they recommended First Ring Gap = 0.0050" x cylinder bore diameter. For me 0.0050 X 3.090 = 0.01545. Second Ring Gap = First Ring Gap X 1.25 = (0.0193"). The oil rings... no tighter than 0.015"

Speaking to some folks who build other engines, they're comfortable with a little more ring gap than less when it comes to turbo motors, but getting the ring gap right and the geometry of the ring ends right is where the magic is.

I'm in the same boat. Trying to figure out what ring gap to run with OEM oversized pistons for a FI build. Did you end up following LeoNA's advice? Were you able to find any other info?

Good luck with the build!

Yes, I went with 16 and 18 thou gaps, but as the block was already bored I did not adjust the piston to wall distance.
Started it up yesterday, and now need to do the break in.