Compression ratio vs octane
This summer I'm planning on buying new pistons and upping the compression ratio, probably to 10:1 or so. What octane are most of you guys running with compression ratios like that? I'm N/A and don't plan on ever going forced induction. I've searched but can't find a definitive answer.
Thanks, Parker
Rust-Oleum White 97
I would be surprised if there is a definitive answer. My answer is 91 octane. I live outside the Metromess, so I can buy E0, alcohol free gasoline. 93 octane is not available here in E0. The compression ratio of my engine is 10:1 and the timing is set to 10° BTDC. I am using the stock '94 ECU.
Last edited by cone-cerned; 04-14-2016 at 08:30 PM.
On the track, I am fearless.
If you were as slow as me, you wouldn't be afraid either.
1994 M Edition
CSP 67
I'm set at 12 BTDC, I don't think we have 91 in Plano, I'm pretty sure it's only 87, 89, and 93. I still run the stock ecu as well. Basically, I'm asking if 93 is okay with 10:1
Rust-Oleum White 97
A lot of newer engines have 10:1 and run just fine on 87. If your ecu and sensors are in good shape 93 is probably overkill.
AnthonyS
former owner of 4 NA Miatas
94 Laguna Blue awaiting Ford 5.0 transplant
Also have a 91 ZR-1 and 00 996 (911).
Most new engines have knock sensors, but most na/NB don't, that I am aware of anyway. I've noticed NAs will start to ping in hot weather. For me, a few bucks each tank is worth knowing the engine is better protected, if not keeping the timing advanced depending on the ECU.
Polished Turd Racing
Mick wrote: "I think Jerrett is the best autocrosser I have ever seen naked."
There was a grassroots article recently that tested fuels in their CSP miata and found no benefit to the E0 fuels.
The NB VVT motor has a knock sensor. I know cause I am using it. It took a little tuning to get it to filter correctly but it works great now. wouldn't be hard to grab one from a salvage yard and utilize it. If you are changing compression I would hope you are planning on running a standalone, and the good ones all have a knock sensor input.
I read the article, too. One car, with a Megasquirt, being tuned for each fuel by Megasquirt's Russ Patrick. And tested on Megasquirt's dyno. Then results were posted for the finished tune without any correction for temperatures of different runs. Their test has almost nothing in common with my situation. I'll stick with E0 until I see a test with stock ECU (no tuning) for each of the fuels.
A Megasquirt is on my shopping list, but it will be a while before I can get one.
On the track, I am fearless.
If you were as slow as me, you wouldn't be afraid either.
1994 M Edition
CSP 67
I wasn't really referring to the power. They found the same timing curve was best for both the 93 E10 and the 93 E0, indicating that they both have the same knock resistance. Which is to be expected really because that's how the octane rating is measured, by knock resistance in a reference motor.
What gets interesting is that reference motor is fueled through the intake valves. For modern direct injection motors, the octane rating of ethanol fuels is through the roof, somewhere around 170 for E85, but since it's not tested like that you dont see the benefit on the label.
Alright, thanks guys. I'll probably just run 89 to keep it safe.
Rust-Oleum White 97
Conversation took tangents, didn't it?
My track car is a '95 with its original engine. It's been fitted with NB2 pistons and a '99 head giving a 10:1 CR. I always fill with 93 (except at Hallett where only 91 is available) and haven't had detonation issues. Data point.
Will the NB2 pistons work with an NA8 head? I can't see any reason why they wouldn'tOriginally Posted by hornetball
Rust-Oleum White 97
I believe so, but you'd be doing a lot of work, while continuing to use an inferior head. Seems counterproductive.
Polished Turd Racing
Mick wrote: "I think Jerrett is the best autocrosser I have ever seen naked."
I'm a high school student on a tight budget, I can't afford a $300 head on top of the pistons and possibly machine work.
Rust-Oleum White 97
What autocross class will you run after the rebuild?
1995 M Edition
Unless you're engine is bad, I wouldn't even consider messing with pistons. If $300 is a deal-breaker, you should stop before you even start.
Polished Turd Racing
Mick wrote: "I think Jerrett is the best autocrosser I have ever seen naked."
Quit trying to quash his dreams... Meanie....
Haha thanks, and to answer him, yes, my rings and valve seals are bad. I'm burning a decent amount of oil. However, I have decided to go cheap and just get stock replacement rings for now and buy another exhaust cam to exhintake it when I replace the valve seals too.
Thanks all
Rust-Oleum White 97
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