10.5 Compression with Turbo.....issues?
 

Hi, I've got an opportunity to build a 1997 1.8 with a decent deal on forged rods, wiseco pistons and MS3Pro. I only have access to 91 octaine and it was suggested that this would be fine if I ran a flex fuel sensor and a program that adapted the tune to compensate for octaine levels. E85 is an option, but I don't want to run it 100% of the time and am not adding the complication of water or meth injection.

Does anyone have experience with this type of build? Is my margin of error very tight with this compression level and boost around 10 to 15 psi? Horsepower goal is probably 300 to 350, but want the ability to ramp up if I swap in a BMW transmission.

Thanks for the help and advice, appreciated.

F

Oh, also will probably run boundary gears in the oil pump, a coolant reroute, a mishimoto radiator and of course an intercooler. The rest, head/cams/valves/springs are all stock.

I like high comp myself. If you decide to go with 10.5:1 on 91, for safety I would setup the boost to be lower when on 91 and ramp up to your target as the available octane from E85 is measured. I think 10lbs on 91 octane is doable but tuning will have to be both correct and conservative. I would suggest running a larger than normal turbo to help keep the back pressure down when on 91 octane to help prevent detonation.

Thanks for the insight, I'm thinking I should error on the side of caution and go with a more conservative compression ratio....given the fuel type I have access to and not trying to chase huge horsepower.

You mentioned a program to adjust the tune for E85. This is not needed with an MS3Pro and most other modern ECUs. You can get a FlexFuel sensor and have the ECU adjust fuel and ignition advance based on the content of ethanol on your fuel realtime.

I think 10.5:1 is great, but only if you have e85. If going pump I'd absolutely go lower comp.

Totally agreed, I think 9:1 is going to be a safer route as I can't reliably get E85 everywhere I go. Plus, I'm starting with a 2560R and want to push it pretty hard as it's already rather small.

Thanks for the insight folks, appreciated.

I realize this thread is, or has, come to a conclusion but I just saw it and wanted to add a few comments and observations having been down a similar kind of path. It’s been said many times before: “ buy once, cry once” and “cheaper isn’t necessarily less expensive.” There’s a gazillion posts and threads talking about the increase in cost and complication going above 250’ish. So I’m going off your goal statement of 350HP. If you go the smaller route and then move up to 350, you’re going to replace injectors, turbo, intercooler, oil cooler, and possibly (or likely) the intake, throttle body, manifold, exhaust to name a few. A few have resale value but most go for penny’s on the dollar. The rebuild time and hassle isn’t insignificant. There’s more people that speculate about 350-400HP build needs and complications than have actually done so (guys, I’m really not throwing stones).

Just offering an opinion from someone that has purchased 3 of nearly every one of these parts and wish I had followed the advice to set a plan and build to it. There’s a lot of cool learning and experience that comes from analyzing and fixing each weakness discovered while trying to achieve a reliable high HP turbo Miata but that experience can get expensive. Massive amount of knowledge and experience on this forum. Your humble and respectful post is what caught my attention along with the topic. Best wishes on your endeavor and Happy Holidays!

Last post is great advice.
You would have to buy a pretty big (and laggy turbo) to hit 300whp on 91. Like 2867 .86 big or even bigger.
Meth injection isn't all that complicated with a single nozzle in the charge pipes and controlled by your ECU. Use your washer fluid reservoir.

Do it once, do it right. Swapping a turbo is fine, but otherwise build it for your end goal right away

I ran a 2560r for a while on 93 pump. Anything over 18psi the turbo wouldn't make more power. I also run 8.6:1cr and that allows me to add quite a decent amount of ign timing even on pump gas. I think it's smart for you to go with 9:1 you'll be able to make a lot more power on pump than with 10.5:1. I made 296whp on 18psi with a older flyin Miata manifold and a 2560r on pump gas.

Thank you all for the insight, and great to know what is acheivable with a 2560r. I'm slowly figuring out the right build recipe and appreciate the help. I assume 300 or so hp makes it pretty quick : ) I've never had the pleasure of driving one at that power level.

300whp in a na/nb chassis is fast. It's probably similar power to weight ratio of like a 500hp mustang GT or something. With a 2560r the turbo will be on boost very early and it makes for a really fun car to drive. You'll love it.