Musada

Hey guys, I'm looking into buy my first Rx8 within the next few months and just wanted a bit of advice on what to look for when purchasing. Not really trying to spend over 9k for one.
I've owned 3 rx7s, so I'm not a total newb to Rotary engines. Also how much is it to get the Renesis engine rebuilt, and I noticed somewhere that the 8 uses premium gas...that true?

RIWWP

iTrader: (2)

In the subforum that you posted this thread in, at the top, there is a sticky named New and Potential Owners Start Here (https://www.rx8club.com/new-member-forum-197/new-potential-owners-start-here-202454/). Read the first 9 posts.

The only question you noted that isn't listed there is the premium gas question, and yes, the 8 needs 91+ octane. If you can run lower safely then it means that you are running with lower compression in your engine. The RX-7s could run 87 octane because they had lower compression rotors. Make sense?

Musada

thanx for the post, but i didnt see anything about how much a rebuild would cost for the engine

Emery_

iTrader: (8)

RIWWP could you clarify to me what you're saying here. You're saying that the only way an RX8 can run 87 octane safely is if the engine is running with low compression? Another question, would running with lower octane affect compression on the engine or maybe over time be a cause to lowering compression in an engine?

I always run premium, but I'm just curious on this situation.

RIWWP

iTrader: (2)

That wasn't one of the questions in your original post

Costs can range from about $2,500 for a bare bones minimum to >$5,000 for high quality with porting work, etc... OR a severely trashed motor that requires new everything.

Do a search for the Octane rating thread on here for an in-depth explanation of octane, but yes, running too low of octane can indirectly affect compression.

- Compression builds heat, higher compression = more heat
- Octane is the resistance to gasoline igniting due to this heat buildup (nothing to do with how much power the gas produces)
- Higher compression engines require higher octane ratings to prevent detonation.
- Running too low of octane in an engine that requires higher can lead to engine damage (true of all gas engines, not just rotaries).
- In a rotary, detonation from too low octane can bend or break the apex seals, which will destroy the engine's ability to hold compression.
- RX-7s could run 87 octane because they were more like 9:1 compression ratio (I think, not sure).
- RX-8s have a 10.1:1 compression ratio, and it requires higher octane. A perfectly healthy Renesis can not run on 87 safely. However, a Renesis with failing compression means that the engine can't compress the charge as much, so less heat, and you can run lower octane. It's a net loss in power and mileage though, so isn't something to be desired at all.
- Note, that you can have low compression in 1 rotor and not the other, and you can even have low compression in 2 faces on a single rotor and no compression loss on the 3rd. In cases like this, running too low of an octane can cause detonation problems for the health rotor and/or healthy rotor faces. It's the highest compression face that pretty much dictates the lowest octane you can run.

Emery_

iTrader: (8)

Thanks for the response RIWWP it makes sense. I did try searching, most of the threads I found were mainly people arguing about their personal experiences running regular and higher octane. But yeah it makes sense if the engine has high compression and high compression = more heat then the use of a higher octane gas is needed to prevent detonation. Good to be more educated.

Old Rotor

Using premium fuel is like insurance protection. When you get stuck in traffic the engine can easily go up 20-30F(in stock form). Now the engine will pre-ignite unless you have premium.

monchie

I use 87 octane Chevron with techron, and no major issues at all.

RIWWP

iTrader: (2)

Looks like Monchie's engine is failing then.

200.mph

iTrader: (9)

just like his posts and his mods

Wingznut

iTrader: (1)

That really doesn't mean anything. That's like saying that you never brush your teeth, but since you have "no major issues at all", it's all good.

You could be causing damage little by little, and by the time you notice your "major issues", it'll be too late.

All for what... To save $2 per fill?

monchie

Yes it's failing, how about yours...lol


Thanks for the compliment.


On this economy, it sucks, but you have to do what you have to do, I know.

RIWWP

iTrader: (2)

Nope! 7,000 miles is too short of a time for a reman to fail unless it's a bad installation. Give me at least another 3,000 miles before I get worried.

Wolfe

iTrader: (1)

A Mazda dealership told me it would be 4500 to replace the engine. (luckily my new engine has 15k miles on it). Call a Mazda dealership when you find a car and have them run the VIN number. useful information.

Emery_

iTrader: (8)

^ Yeah to anyone interested in buying an RX8 it's a must to find out about the engine, and get a compression test. When I bought my 8 I was a bit desperate to jump into one, and I assumed because it had an engine replaced at 34k miles and the car being 82k when I bought it, that the engine must be good since it was replaced in 08. I was very wrong, and now my engine has low compression. Hopefully I will be getting a new engine from mazda soon! I won't be making the same mistake again...