8.6, 9, or 9.5 CR for initial NA eventual Turbo build?
 

Hey Gents,

94 NA that has been loosing it's oomph and burning a fair bit of oil (1qt every 1800-2000 miles). Compression is looow, and leakdown wasn't good either. Planning a rebuild this winter, then an eventual ~225-250hp turbo build funds permitted within the following year. Car is a street car with eventual goals to do some basic track lapping at high-plains.

Plan on Supertech pistons/Weisco XX rings
Manley Rods
ARP Fasteners wherever possible
Freshening of my stock head with maybe stronger springs (Crower? Supertech singles?)
OEM gaskets and seals everywhere.

Basically the ubiquitous brick-house build that everyone knows.

Will be tuning on MS2PNP on pump gas (Denver has 91 octane at best, e85 is very sparse from where I usually am).

Big question is this : What Compression ratio should I choose for an engine that will eventually be turbo, but will live at least a year and 15K miles Naturally aspirated. Stick with the proven 8.6 and deal with the low response until turbo? Go with 9:1 and have a conservative tune in the future? 9.5 for N/A enjoyment then do a lower-boost build? Build higher-comp now, switch pistons come turbo time?

Everyone seems to recommend 8.6 for turbo, especially on pump gas. I'm mostly concerned on how crummy it would be in N/A trim while I recoup funds for turbo.

First car "build", will be first engine rebuild and first turbo. Prototypical :noob: but I'm loving every second teaching myself about these little cars!

Thanks in advance for any input!

Obligatory cat gif :
https://www.miataturbo.net/attachmen...ine=1408586479

n/a non built miata's are slow either way.
if the best you have is 91 and don't want to rebuild or change things later, 8.6 or 9.1 is the way to go.

If the car will spend 90% of its time at altitude, you can get away with a higher compression build, and just lower the boost a bit if you have to come down to sea level.

Just a thought,

Keith

Thanks for the inpt guys, I'm leaning towards the 9:1, but Twodoor's comment has me intrigued. Car will stay at elevation 100% of the time. More research on higher compression at elevation needed!

Pump gas = 8.6:1.

This. If you are not going to run E85 I will always recommend the 8.6:1 just so you have a margin of safety.

The VVT motor in my car was broken in N/A and raced once N/A with 8.6:1 pistons. It felt like a complete monster compared to the 85k original NA8 motor that was in the car. I think you're blowing how bad it will be before boost out of proportion.

So if you ever expect to run pump, basically you should stick with 8.6:1. The question I haven't seen answered, is whether that recommendation changes under the premise that you plan to run E85 a good portion of the time.

The bolded part to me reads that you will be running pump gas occasionally, which puts you back at your first sentence of running 8.6:1 pistons. To me if you will ever run pump gas the margin of safety far outweighs the few additional points of compression.

OP never said anything about running e85.

Bandid just threw that in there.

Do our engines not like high compression with boost? I ask, because stock EVO pistons are 8.8:1 and people run 25+ psi on gasoline with them without thinking twice... so to me a built engine running 9:1 would be fine for 20 psi on gasoline and substantially more boost on E85.

Are the Miata 1.8's more prone to detonation so we need lower compression?

Keith

Not more prone to detonation necessarily. Just a shitty old design that likes tons of timing. Tons of timing + high compression = no bueno.

Not that it's prone to detonation, but that it's detonation limited, if that makes sense.

It's all about the MBT.

You can run 20psi on pump gas on a 9.5:1 BP, but you're going to give up a bunch of power vs. running 20psi on pump gas on an 8.6:1 BP with all the timing.

They are ancient, and flow LESS than a single slammer D16 civic 1.6.
Evo's 4G have some serious ports/valves, makes the BP look cute.

I think high comp boosted 4 cyl is still relatively new, with plenty of resistance from the "old boys", and plenty of mixed results, and very very little testing/info on a BP specifically, so it's hard to say one way or the other.

Gotcha, mine will stay stock block unless I blow it up, but I am 100% E85 use, so it isn't an issue for me... just wondering for the guys who don't have access to E85.

Keith

Leafy you make a great point. A freshly rebuilt anything will feel better than the tired stocker I am running now. I guess I am just wondering if I was leaving anything on the table for the year or so I ran N/A without really hindering anything once the boost comes. I'm not made of money by any stretch so I'd like to do this right once.

My logic was : Stock 94-97 CR was 9:1 (IIRC),
stock bottom ends are good for up to 250wtq and ~250whp safely
Those numbers are the high-end of my goals
Ergo : use 9:1, or possibly 9.5:1 to achieve my goals. 9.5 for a bit better response.

But, you guys know a helluva lot more than I do! And I am overthinking all this! :hustler:

You are not leaving anything on the table that an additional pound or two of boost won't fix. Stop over thinking it and go with the 8.6:1 pistons.

Does this still hold with flex fuel sensor?
 

Does this argument change with a flex fuel sensor?

Reason I ask is I'd REALLY like to do a high compression e85 build but I want the ability to go cross country and fill up with 91 in a pinch.

Say I have a flex fuel sensor and create one tune for e85 that is full potential, but also have a 91 spark and fuel tune that has decent power but is safe.

With a functional flex fuel sensor and a properly gimped 91 octane tune would I be safe going high compression?

that doesn't change much, since the assumption during this whole thread was that the car would have different tunes for 91 and ethanol.

the bottom line still ends up with how much boosted miles the car would see on pump vs ethanol.

that really is the only question

darn it. :)

BTW- my assumption is a tank or two every year. The trip I have in mind is from Chicago to Colorado. In Illinois, Iowa, and Nebraska I have found either 93 octane or e85 everywhere. Colorado has a max of 91 octane with occasional e85. I am just worried what would happen if I was running say, 11:1 and had no choice but to put a partial or full tank of 91 in.

I am half willing to risk it because I am sure I will get good at finding e85 stations around my local area before I go on a trip and have to worry about learning that habit.

For a tank or 2 every year, I'd go 11:1 and before you leave flash a 91 map and run wastegate only, then just avoid E85 for those 2 tanks. When you get back, set everything back. Why even bother with a flex fuel sensor for 2 tanks? As long as you don't let anyone else ever fill up the car for you of course.