1.8 Rebuild: Piston Sizes + Other General Questions
 

Hi all! I'm in the process of doing my first 1.8 engine rebuild for a 1996 Miata and I'm hoping to get some additional advice to help along with the process.

Right now, I have the block torn down and at the machine shop. Here's the top two question I could use some help on right now.

1. The machine shop says I need to get "20 over" pistons.
- Does this mean that the machine shop is asking for 83.2mm pistons as the OEM size is 83mm for a 1996 1.8? He mentioned something about it being .50 extra that leads me to believe I need 83.5mm. I could use some advice here!
- What compression ratio do I choose? 8.8:1 compression ratio seems to be recommended, but I'd like some additional thoughts - will that one be fine?
- I was planning on ordering Supertech Pistons: https://fab9tuning.com/supertech-forged-pistons/ - any feedback on these?

2. The machine shop wants to know the maximum boost I ever intend to put on the engine to determine ring gap size.
They couldn't really give me any cons to saying "just 30 PSI" boost. They said it's better to go for the maximum amount of boost you'd ever intend to put on the engine, but I'm not understanding the disadvantages of always saying "30 PSI", even if I'll probably be WAY more conservative on my actual boost (10-12 PSI).

To help with any advice, my here's my build goals:
- It's my daily driver
- Reliability is top priority
- I have a 1996, so OBD emission inspection passing is a hard requirement (I'm unable to get my car "fake" passed for reasons...)
- Rebuilding engine without a turbo initially - so it needs to run stock/unmodified for awhile. However I want to make sure I set the engine up for reliability in the future for turbo.
- Aiming for around 250HP in power when I do add my first Turbo.

Engine Parts Planned (So Far)
- Crankshaft: OEM Mazda
- Main Bearings: ACL Race Main Bearings (Standard Size) - https://fab9tuning.com/acl-race-bearing-set/
- Thrust Washer/Bearing: Mazda OEM 01-05 (ZJY1-11-SJ0) - https://fab9tuning.com/01-05-thrust-washer/
- Main Studs: OEM Mazda
- Oil Pump: Boundary Engineering Stage II (No Shims) - https://fab9tuning.com/boundary-engi...let-oil-pumps/
- Harmonic Balancer: OEM Mazda
- Trigger Wheel: Not needed with OEM?
- Connecting Rods: Manley Performance H-Beam Rods - https://fab9tuning.com/manley-forged-h-beam-rods/
- Rod Bearings: King Race (Standard Size)
- Pistons: Supertech Forged Pistons - https://fab9tuning.com/supertech-forged-pistons/
- Piston Rings: Wiseco NPR XX Complete Ring Set - https://fab9tuning.com/wiseco-npr-xx-complete-ring-set/

For more info, here's my full part/build list: https://docs.google.com/spreadsheets...MSg/edit#gid=0

Thanks a ton for any help with this build!

20 over is .020 of an inch. You'll want to convert it to mm.

Buy some forged ebay rods and put a good clutch and pressure plate on it when you put it in. Add a coolant reroute while the engine is out. Use a Mazda headgasket. 8.4-8.8 compression is fine.

Thanks. So 0.020 in is 0.508mm. So that means I'd need to buy the 83.5mm sized pistons. Is there any major advantage in just going all the way to 84mm pistons and getting close to a 1.9 displacement?

For the compression ratio, the stock 9:1 compression should be fine for the pistons? I've seen a lot of posts recommending 8.6:1 but it's not entirely clear as to why.

When running pump gas the Miata will be detonation limited for the amount of power that you can make. Lowering the compression ratio allows you to run more timing or more boost or both making more power than if your engine had a higher compression ratio.

Only taking what is necessary from your cylinder liners allows you to rebuild the engine again later and possibly not have to throw away the block. This may prove useful to you if you ever crack a ring and leave gouges in the cylinder wall that require 1/2 millimeter over bore to remove next time.

Ok, so deducting from your post:

- I'll be fine just doing a stock 9:1 compression on the pistons. If I chose the lower 8.6:1, I can potentially get more power from pump gas by increasing timing/boost.
- I should just do the minimum boring on the cylinders, which would be the machine shops recommendation of 20 over. I'm hoping to not ever have to rebuild this engine - so am I missing out on anything specifically by not going to 1.9?

83.5 Is technically a 1.9L but not really( 1862cc which is over the 1850 limit to be called a 1.9L).

84 from 83.5 is half a percent gain. Not really much difference unless you need every last bit (N/A). Since you are overboring; you may as well go forged pistons and forged ebay rods. NPR replacement pistons are good of you are on a budget. Throw in some good bearings and drive it.

Slightly thicker cylinder walls will have less variation in temperature due to thickness variations between it and the cooling jacket, for what it's worth. What's the value in preserving the block? Well, they stopped manufacturing them 13 years ago so they may get harder to find one day. Join the 1.8 Preservation Society.

The Supertech pistons I was going to get only come in 8.6:1 or 9.5:1 compression ratios - not the stock 9:1 that I would normally get.

Is it better to go lower or higher than stock? I intend to run my car mostly on pump gas (no race gas). Would the lower 8.6:1 compression be better suited for my build?

You want 8.6 for pump gas.

Is there anything wrong with your stock pistons or cylinder bore that you need to overbore?
For 250hp. All you need is a hone, same pistons, new rings, bearings and Ebay rods. And you just saved $1000 and have a reliable 250-300hp engine.

exactly what I did.

If the machine shop recommends a "20 over" bore, then they might have seen something that can't be addressed with a hone. As others have said, don't go all the way to 84mm because that gives you some margin in case you have to do it again.

Definitely go lower compression, 8.6:1 will be ideal for pump gas. I just recently built my engine and I went with a 84.5mm bore, but I had already built the motor 8 years ago @ 84mm so I kinda had to go with 84.5 unless I wanted to use a different block. ACL bearings also are a good bet, OEM Headgasket will work well too. ARP head/main studs. I use a stock head, no need to get crazy with it if you are only revving to 7200rpm stock redline.

0.5 compression making your car feel sluggish? Hahaha! It's a Miata. It's already slower than a 4 cylinder Camry!

What will actually make it more sluggish will be giving up boost and/or timing that make you drop 30-40whp to keep from detonating. I understand this is new to you but try to picture the difference in losing 5whp at 2000 rpm versus 40whp from 4500 rpm to 6500 rpm.

The outgoing engine in my car was 9.5:1 and the new "built" motor is 8.5:1 and I literally could not tell any difference.

Thanks guys. I went ahead and ordered the 8.6:1 pistons. I know I'm being overly cautious, but I just want to make sure I don't put all this time/money into a build that has negative downsides (sluggish acceleration off the start would definitely be a negative for me) 5HP is like ~4% of the OEM power lol. I'm glad you haven't noticed a difference, that makes me feel better!

Just out of curiosity, I'm assuming if I wanted, I could have the machine shop deck the head surface if I really wanted to get back closer to 9:1. Probably not going to do it, but I'm just wondering if that's even a viable adjustment to make?

Are you dense or are you just not reading what's being posted? Lower compression is better on a turbocharged Miata engine. Do I need to repeat it? If you ask the question again the answer will still be the same.

1. Yes, I understand that lower compression is better on a turbocharged Miata.
2. I'm asking questions to better my understanding on how compression ratios can be adjusted (IE: decking the head).

Ultimately I went with your advice - but I still like to learn what is possible, even if it might not be ideal. Just FYI - I wasn't asking if it's a good idea to attempt to go back to 9:1 - I was asking if it's even possible (again, out of curiosity).

Then it is possible to Deck the head or the block or both but you can eventually run into clearance issues between the piston and the head. It is also possible to run a thicker head gasket or a head gasket spacer to increase the space.

I don't think you could even remove enough material to accomplish that. You'd also need adjustable cam gears since decking the head that far would really mess with your cam timing. You'd almost certainly have an interference engine at that point as well. All of that would be traded for running a compression ratio that will be worse for your application. It's a colossally stupid idea.

Everyone's advice was to go 8.6:1. You're now asking stupid questions about how to NOT run a 8.6:1 engine like was suggested. It's a sure fire way to annoy the people giving you advice.