8.6, 9, or 9.5 CR for initial NA eventual Turbo build? - Page 2 - Miata Turbo Forum - Boost cars, acquire cats.

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Old 08-28-2014, 07:01 PM   #21
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To put it in perspective, the difference on a dyno on a 7.6 liter gasoline v8 between 8.5 and 9.5 compression is about 20ft lbs of torque on a ~500ft lbs engine. If you rationalize that down using napkinscratch guestimating it won't be but about 4.75 ft lbs on a 1.8 liter. But the extra couple of lbs of boost you could run without detonation will be worth far more power than that. YMMV.
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Old 08-28-2014, 07:07 PM   #22
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Quote:
Originally Posted by Chiburbian View Post
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.
There is also this crazy thing called right foot modulation, simply stay out of boost when you are on crappy 91 octane and you should be fine.

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To put it in perspective, the difference on a dyno on a 7.6 liter gasoline v8 between 8.5 and 9.5 compression is about 20ft lbs of torque on a ~500ft lbs engine. If you rationalize that down using napkinscratch guestimating it won't be but about 4.75 ft lbs on a 1.8 liter. But the extra couple of lbs of boost you could run without detonation will be worth far more power than that. YMMV.
That is how I always see it, with 8.6:1 pistons you can find MBT on pump gas and if you are really missing that tiny bit of power just up the boost another psi or so.
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Old 08-28-2014, 07:14 PM   #23
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Quote:
Originally Posted by sixshooter View Post
To put it in perspective, the difference on a dyno on a 7.6 liter gasoline v8 between 8.5 and 9.5 compression is about 20ft lbs of torque on a ~500ft lbs engine. If you rationalize that down using napkinscratch guestimating it won't be but about 4.75 ft lbs on a 1.8 liter. But the extra couple of lbs of boost you could run without detonation will be worth far more power than that. YMMV.
Thanks for that sixshooter! Numbers make my decision easier than 'N/A miatas suck no matter what'.

8.6 it is. I just wish we had e85 here.
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Old 08-28-2014, 07:32 PM   #24
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There's more to it than simple toruqe peak gains, but I'm not gonna open that can of worms here.

If in doubt, go with 8.6 or 9.1
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Old 08-28-2014, 08:00 PM   #25
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Supertechs are listed as 8.6:1 or 11:1. Are there other recommended pistons? (sorry for the spoonfeed request).

NEVERMIND - Delete. Found 9:1
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Old 09-01-2014, 12:47 AM   #26
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Fyi... My little brother on his jdmAaa stock bp 94 block holds 15psi on e85 self tune for the past year and no issues. Same setup on 91 at 12psi posted 240whp (mustang dyno)with boost dropping to 10psi in the last 500rpm.

There is nothing special about the jdm bp motors then maybe the fact that it has 50k miles or less. But 15psi is holding just fine. This is also his dd miata. 6 days of the week it sees boost constantly.
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