Hello, I know I'm new here, but I swear I have been looking through these forums since 2017 when I got my Miata, just never had a serious enough question to make an account. I'm kind of in a dilemma right now choosing forged pistons. I am in the process of rebuilding my engine and don't really know what compression ratio to run. I currently have a 2001 VVT motor already out of my NB2 chassis. I want to be able to boost the miata however I would like the car be as stock spec as possible so that it can run on the stock ecu (CA Smog or if I end up selling the car, so yeah basically a super healthy & bulletproof stock VVT motor). I was thinking of going the Wiseco 10.5:1 pistons, but do you think the 300hp path I’m going towards will be a spiraling road, or is it possible? Are there any limits I should be worried about going boost with stock compression ratio 10.5:1 with forged pistons and rods? Could I go past 300hp or is 300hp a reach? ALSO to note, when I finish breaking in the motor (probably months after), and decide to go boost, I will go MS3.

 

Is this a troll post? You aren't going to get near 300 with a stock ecu.

 

I think I get his question, he wants to build his bottom end for boost, but wants be able to yank it all off when he needs a California smog check (every 2 years?).

My understanding is that stock CR on a VVT motor is 10:1, so your question is will the extra half a point of compression perturb the factory ECU. So I'm new around here and don't know everything about all the different years of miatas, but my understanding is all the NA/NB cars (except the mazdapseed) are made to run on regular 87. If that assumption is correct, I don't think half a point of compression is going to make so much more knock that it throws a code, however I'd throw in premium fuel as insurance. So no real issue as far as running the car as factory with the 10.5:1 pistons. Just leave everything else but the rotating assembly stock and your factory ECU should never know the difference (ie, don't port the head, don't put cams in it, don't change the coolant temp, etc)

Now, as for turboing the car that CR is going to present some challenges. Again, I'm new and not an expert by any means, but my understanding is that the BP engine are significantly knock limited when it comes to making power with forced induction. If you're planning to run E85, you're golden. If you want to run Cali 91 octane, you're probably going to have a better time with a lower compression ratio.

I'd love to help you with all the little gotchas of building a BP, but I don't know them yet. But good luck with your project.

 

The VVT cars are 91 or better. I am not sure if the extra 0.5 would be enough to cause knock on 91 though. I would stick with stock CR and other things, but as long as you put all the stock sensors/injectors back in I see no reason you can't do rods, rings, pistons, and run on the stock ECU for smog.

 

If you plan to turbo and run pump gas, I think the more prudent question would be, “can I drop CR down to 9.5 or 9.0 and run OK on stock ECU.”