ThePass

iTrader: (3)

I'm tentatively gathering data for a tear down and rebuild of my motor which will be inevitable and so I want to have all the info researched and know the plan before that day comes.
This car is the uber-budget build, fast and reliable, things done "right" but nothing but the necessities - if the coolant isn't getting too hot at races, then I don't need a reroute sort of thing. So, consider that context in your responses, if it were absolutely necessary to have everything on the bottom end forged, I'd do it but it's not..
Anyways, I know I will be doing forged H-beam rods. Now to the pistons.. First off, I've heard mention of using automatic pistons and I can't find in any searches the actual compression ratio specs of one - can anyone help me out there?
I want to drop the compression some. Builds like Hustler's have convinced me that a marginal compression drop allows much more power from boost than is lost from lower compression. So, if auto pistons are a lower compression, sweet deal.
Now, on to the cost issue - assuming auto pistons are a lower compression and would work well (and yes I would get nice rings for them), I would have the engine guy do a slightly loose bore and go with those pistons and new rings with the H-beam rods, but are would the cost of the pistons + new rings + tri-coating (is this necessary?) add up to almost the cost of forged pistons which come with rings? Because if so, no sense in not getting the forged ones, but if it is a more affordable option, why not go with the coated auto pistons?

Anyways, discuss...
Thanks
-Ryan

Stein

iTrader: (46)

Interested in the responses as I have a 96 automatic motor at home. IIRC, compression is 9:1 instead of 9.5:1 for similar 5 speed motors.

Braineack

iTrader: (62)

I don't believe there was a difference in compression in auto vs 5-sp in a 1.8L.

94-95 have been rated at 8.8:1
96-97 9.0:1

Saml01

iTrader: (9)

I would get the higher 10:1 pistons. Better mid range. If you know how to tune you can be perfectly safe with a slightly higher compression.

I wouldnt bother with all that voodoo with coating pistons and stuff.

hustler

iTrader: (6)

I considered tri-coating 9.0:1 pistons and I was basically $100 shy of forged slugs. Think about the headroom and pizza mind.

I firmly believe that even on budget builds, "do it once and do it right" with the fundamentals or engine internals. Audi, mitsu, subaru, and toyota go with forged guts for a reason on their sportscars.

he probably wants to make some torque. Do you consider 4000rpm mid range? I'm making 252wtq there, that's roughly 275wtq on a dynojet. I consider that midrange. I've auto-x'd my car a few times too, and the powerband is fine. I wouldn't want boost to come on any higher, but never had a complaint yet.

y8s

iTrader: (8)

while sam doesn't subscribe to voodoo, I doo.

see a piston will die from detonation whether it's forged or cast. the coating provides a little thermal protection and reduces (some of the swain coatings) liklihood of detonation. here's your voodoo:

but swain is MOTHER EFFING PRICEY last time I looked.

still, it is good insurance.

but if you're going to be under 250 rwhp, you might not need forged or coated. what power level are you looking at?

Stein

iTrader: (46)

Brain, to counter your quote, here is a quote from the solomiata page so I don't know which one is correct:

"In 1996 the peak HP rating of the Miata 1.8 motor was raised to 133 hp from 128. This comes soley from the new for 1996 ODBII software having the ability to lean out the above 6000 rpm fuel curve. There have been rumors that there was a running change to the pistons to up the compression ratio to 9.5:1 during the 3/95 cutover, but I have not seen any documentation to prove it. "

My reference to the auto being lower compression may be rumor as well.

RotorNutFD3S

iTrader: (33)

If my research on pistons was correct, 1.6L Miatas were the only ones to have a different piston for automatic or manual configurations. I could find no evidence of this happening on the 1.8L.
I will also say that the SoloMiata quote, for once, is actually wrong about the compression ratio. My '95 R had the same pistons as the ones I pulled from a '96 (they have a slightly raised dome), however the '94s I pulled were completely flat topped and would be the ones rated by Mazda at 8.8:1. Also had a set of MSM pistons (same as '99-'00 - 9.5:1) and they had an even higher dome than the '95/'96 pistons (9.0:1).

Saml01

iTrader: (9)

Cmon man, seriously. Coating pistons on a street driven gas engine? Come on.

I can think of much better things to spend 200 bucks on.

Stein

iTrader: (46)

Interesting. Thanks for that. I need to check, as I would have sworn today that my 96 auto pistons are actually dished. I'll check tonight. The head is off of it.

RotorNutFD3S

iTrader: (33)

If that's the case, something interesting there for sure. Please post back with what you find.

You're just focusing on the top coat, most people go with a tri-coat option. If you're using OEM pistons, it can't be a bad idea to help them out in any way that you can. The top coat helps as y8s described, but the side skirts are coated in a dry-film lubricant, so less friction = less heat, and the undersides are coated to help shed the oil off faster, which helps keep as much oil in circulation as possible. I paid $140 ($35/piston) for the tri-coating with Swain, a justifiable cost in my book.

If you figure $440 for a set of Supertech forged pistons w/ rings from Belfab and then the machine shop charges you more to overbore the cylinders than just to hone them (3 of the ones I got prices from quoted $75-$150 more for an overbore). So say $515 at the least, possibly more depending on piston and labor.

So even if you're buying a new set of OEM pistons (about $120 for all 4), having the cylinders honed ($50-$75 depending on shop), you still come out at least $180 under the cost of going with oversized forged pistons, which is great for a budget like what the OP is aiming for.

I personally went with the tri-coated option as I didn't have to buy the pistons and I'm not searching for insane power numbers, just something to support a nice bump in power. Oh, and not to mention I was tired of hearing piston slap during warmup on the cars around here running forged pistons, even though everything is properly clearanced inside the engine. That wear and tear has to add up sooner or later.

y8s

iTrader: (8)

I would think a street driven motor with all those miles would be a BETTER reason to put the coatings in your motor.

LowBoostn

If this is a cost vs. preformance issue i dont see much preformance for the dollar. So i would spend the money somewhere else. Where? that depends on what u have and what else you need. The tuning of the ECU may net more power and torque. Is the injector duty cycle over 80%? then maybe bigger injectors. Boost toping out at 18psi @ 6400RPM and not making any more power past 6500RPM? Then maybe a bigger turbo. These are all if's and but's to where your cars at and where you expect it to be (HP level's).

Stein

iTrader: (46)

Well, I checked and I was all wet. My 96 auto pistons have a slight (maybe .020-.030") high dome in the center with about a 1/2" band of lower material around the periphery, so likely the same as stock.

ThePass

iTrader: (3)

As to my goals, right now I'm probably in the 200 whp/11 psi area which is right on the edge of what is generally accepted as the limit for preserving engine longevity.. I will probably not go above 250 whp but in the spirit of doing things properly the first time (with budget in mind), I am aiming to make everything good for 300 whp - the intercooler, the injectors/ecu, etc. all could do 300 if I chose to swap to a 19T down the road so when I crack open the motor I would like to close it back up with the ability to do 300, even if I may not actually push it that high.

The recommendation to go with higher compression pistons is one I will have to decline, I just don't feel comfortable with that - yes, I know y8s has pulled it off beautifully, but I want to create a car that is my daily and race rat e.g. something that has the highest probability of lasting for as long as possible and imo running high comp pistons on a moderately boosted motor, while possible with a good tune, leaves very little wiggle room for a slightly imperfect tune.

So.. you guys were saying the 1.8 pistons did not differ from manual to auto but that the 1.6 did... I have a 1.6 which is exactly why I am researching this - can anyone actually confirm or bring forward info about the 1.6 pistons from the auto? I'd love to find out that they are 8.8 or so...

Rotornut - thanks for that break down on what you found you could save by going with coated stockers instead of forged. If I can save close to $200 by doing a factory piston and the tri-coat over a forged, perhaps that w/ H-beam rods would be perfect for a budget 300 whp ceiling motor..

-Ryan

ThePass

iTrader: (3)

edited

Saml01

iTrader: (9)

You know why Chuck Norris named his cat after you? Because its a *****.


j/k

hustler

iTrader: (6)

I'm excited to see 1.6 guys going with the low comp / high-torque option. It works for subaru and mitsu pretty well, lol.

juxt3r

iTrader: (5)

hustler

iTrader: (6)

you live in Dallas...where is your discussion on high compression, superchargers, and avoiding stand alone?