10:1 isn't super high on a piston engine, but it is a high compression ratio. 9:1 or 9.5:1 isn't uncommon to find either.

 

Just to put it in perspective, the Mitsu Lancer Evo VIII (with a turbo) has an INSANE compression ratio of 19:1!!!

The Audi A4 I used to own (with the 1.8T engine -- a small turbo) had an 8:1 ratio, if I remember right.

10:1 isn't all that high.

 

Are you sure?!? I thought it ran 19 pounds of boost, not 19:1 compression. Most turbo motors have lower than normal compression, not higher. If that thing is running 19:1 and 19psi, I don't want to be around when one lets go!

 

No kidding - I thing 19:1 + 19psi would equal one heck of a grenade, useful in the armed forces...

I gotta think it's probably somewhere around 8:1?

 

So then if 10:1 isn't high, what's with everyone saying 10:1 on the RX-8 is high and should use premium?

My hyundai tiburon was a 2.0L 4 cyl with 140HP (when I bought it) 132 after (hint hint ) and it was 10.1:1 compression, and it ran on REGULAR!

I really think the RX-8 is just spec'd for *** saving, or something. Besides we now know it isn't even 247.

 

What about the use of gasoline with ethanol? An 89 octane is very common in the midwest.....

 

Hey again

I just read through and am surprised at some of the replies regarding warranty issues. Using 87 or 89 octane could not possibly void the warranty since it is mentioned as an option (albeit with reduced performance)

It comes down to this. If the engine is running better, the soot disappears. gas mileage goes up, and the missing stops then I beleive there will be no question as to what I will use at least. Right now I can only confirm 2 of these as improved - engine idle (run) and missing (gone) Iwill continue to post.

 

after reading this thread, I said what the heck and filled up with 87 on my way to work (on my way to work, my gas indicator light turned on). I noticed no difference in performance. About 5 hours later, I started up the car and it did seem to idle a tad bit more smoothly than using premium.

One big thing I noticed is my fuel efficiency dropped bigtime. After driving a mere 40 miles, I was already down a quarter of my tank.. YIKES.. I think this probably was due to me wasting lots of gas testing out the performance. But I'm not yet ruling out the 87 octane factor.

On my last tank I got 250 miles on premium gas before the gas light came on... I drove it smoothly but didn't pay any particular attention to keeping low RPM's (I already passed 600 mile break-in period). Anyway, I don't think I'll reach 200 on this tank before the gas indicator lights up...

Currently my tank is at two notches above half and I've driven only 80 miles since the refill this morning.

I will continue using 87 and keeping close logs of my mileage. Then I will switch back to 92 and do the same. Hopefully I'll have something substantial to report in a few weeks. I'll be filling up every other day with this car. Boy... You'd think this car was Irish :^P

 

LOL! It may be a lush, but it's a hell of lot fun at the party! :D

 

dude what are u smoking!? Its all in ur head that 87 is gunna help, if you are driving aggressively I would HIGHLY advise to stop because it causes engine damage. After you fill up and have a full tank of fuel do you feel faster? Its just psychological, for optimum performance and to care for your engine's health use PREMIUM for the sake of your rotary!

 

you're probably right..

but I seriously don't think using 87 will damage my engine in anyway..

unless you can prove otherwise, I'll just continue with my little experiment...

Not a big deal, IMO. Plus, I got 100k extended warranty on this puppy, so as to use it as a daily driver... My real baby will be a a 360 Modena I plan on purchasing in a few years...

 

The "19:1 compression ratio" (for a gasoline engine) written about several posts back seems VERY high. I thought diesel engines (obviously 'high compression' because of the need to ignite the fuel at the top of the compression stroke) were commonly around 24:1 compression ratio.

 

its not 19.1:1 its 10.1:1 i think

 

The evo runs under 19psi of boost. The engine has a 8.8:1 compression ratio. This is actually a pretty high compression ratio for a turbocharged car, but that engine is damn solid...

Turbo'd engines generally run with lower compression ratios than an n/a engine, typically 8:1. If you look at turbo/supercharger kits out there, it isn't unusuall to see lower compression pistons in the kit (or in a stage 2 kit or something where you turn things up a notch).

To put things into a bit of perspective, my current car's engine has a compression ratio of 9:1 and requires the use of 91+ octane fuel.

 

NA rotaries are notorius for loving low octaine gas.. I don't see why the Renesis would be any different. You have to remember that what you thought you knew about cars is prety much out the window then it comes to the rotary.

These posts are taken from the RX-7 forum where this has been talked about to death.

That being said my car runs like poo on 91 octane.. When I first got it I ran it on premium all the time and could not figure out why it felt so different than when I got it. One day I tried 87 and WOW it cleaned the idle right up and gave more power.

Quote: Remember the following is for the 13b engine! I don't know enough about the Renesis yet but it sounds like it's a similar story.

It has been dyno proven that a 13B runs better on 87 oc than on 91 or higher unless the timing has been messed with.

The lower octane resists premature combustion better.

In addition it burns slower across the entire combustion cycle, which produces more power in a NA rotary than higher octane burns (more single sudden bursts at initial firing of the leading plug).

If your car runs better (and you have not messed with the timing) on higher octane than a lower octane gas, check the brand of gas you are using. Chances are that it will have a higher ratio of cleaning agents which is what you are proceiving to be better running. Adding a jug of techron would probably do the same thing.

remember that a NA rotary really can't be comparied to a piston engine with a knock sensor, that really will produce maximum power when high octane gasolines are used. Just too different of a burn cycle.

You want to use LOWER octane gass. In fact, you want to use the LOWEST octane gas possible. The lower the octane, the hotter the flame front, and the cleaner the burn. Higher octane is for high-compression motors that require a slower flame front and more compressability.

The fuel property the octane ratings measure is the ability of the unburnt end gases to spontaneously ignite under the specified test conditions. Within the chemical structure of the fuel is the ability to withstand
pre-flame conditions without decomposing into species that will autoignite before the flame-front arrives. Different reaction mechanisms, occurring at various stages of the pre-flame compression stroke, are responsible for the undesirable, easily-autoignitable, end gases.

and:

If you are already using the proper octane fuel, you will not obtain more power from higher octane fuels. The engine will be already operating at
optimum settings, and a higher octane should have no effect on the management system. Your driveability and fuel economy will remain the same. The higher
octane fuel costs more, so you are just throwing money away. If you are already using a fuel with an octane rating slightly below the optimum, then using a higher octane fuel will cause the engine management system to move to the optimum settings, possibly resulting in both increased power and improved fuel economy. You may be able to change octanes between seasons ( reduce octane in winter ) to obtain the most cost-effective fuel without loss of
driveability.

and:

Once the octane and run-on requirements of the engine are satisfied, increased octane will have
no beneficial effect on the engine. Run-on is the tendency of an engine to continue running after the ignition has been switched off, and is discussed
in more detail in Section 8.2. The quality of gasoline, and the additive package used, would be more likely to affect the rate of engine wear, rather than the octane rating.

And:

If you use a fuel with an octane rating higher than what the engine can use, you are just wasting
money by paying for octane that you can not utilise. The additive packages are matched to the engines using the fuel, for example intake valve deposit
control additive concentrations may be increased in the premium octane grade. If your vehicle does not have a knock sensor, then using a fuel with an
octane rating significantly below the octane requirement of the engine means that the little men with hammers will gleefully pummel your engine to pieces.

 

Am I reading this right? This is the exact reverse of what I've been reading about octane on the net..

It is commonly said that HIGHER octane burns significantly slower than LOWER octane fuel.

Please clear this one up, somebody? Because I'm majorly confused now.

 

My dealer told me to use premium. In the past I've always considered this "bunk". Premium is only for doctors with BMWs and money to waste.

I'm now on my 5th tank. The best mileage I've gotten is 16/tank. The lowest MPG is just a bit over 14.

I love the performance but I'm very troubled over the MPG. The 8 should perform near the figures posted on the sticker. At this point I am finding the MPG nowhere near my expectations.

My tailpipes are quite sooty so I hope my post 1k tuneup will make for some impovement.

As for octane, I just filled up, for the first time, with regular unleaded. Regular unleaded is now going for $2.20/gallon here in Los Angeles. Thank you very fu__ing much George Bush!

During the past week I have noticed RPM surging during my early morning starts. This condition is something new. The surging has nothing to do with AC. This is not the "popping" that some have commented on. I have not encountered "popping".

I appreciate having the FORUM as a resource to discuss all the various issues about the 8.

Thanks,
TDS

 

TDS: my car "surges" if you mean this.

You start it, it jumps up to 2K rpm or so.

It then idles down to 800Rpm and then jumps back up to 2K it does this 2 - 3 times than settles down after a bit.

I'm running premium since day 1. Others have mentioned this on the forum as well.

 

The Renesis compression ratio is 10:1. All turbo rotary compression ratios since 1989 have been 9:1. In '87-'88 they were 8.5:1. N/a compression ratios after '89 were 9.7:1. Before this they were 9.4:1 until you get back to 1980 and earlier when they were 9.2:1 or 9.4:1 depending on which car it was. Hope that answers the compression ratio questions. Forced induction can be done on high compression engines but boost has to be limited to a lower level, timing can't be too agressive, and higher octane must be used.

Lower octane gas will not hurt your rotary engine. Your compression ratio isn't THAT high. If you ever add forced induction, nitrous, etc, kiss the 87 octane goodbye. 87 octane has worked for 40 years now and will easily continue to work for many more years. Early engines ran off even worse gas than we have now and they were more finicky and less refined than the Renesis.

FWIW: The president is not responsible for your high gas prices. I don't care if he's a Republican, Democrat, or otherwise. The president doesn't have anywhere NEAR the control of what everyone blames them for. It's the damn childish feuding between our 2 biggest scandalous political parties. Come to Texas to see a fantastic example of how not to get anything done. The U.S. has 10 times the amount of oil that the middle east ever had (It also costs more than 10 times more to produce here). Central Africa has even more. If we were to double our current consumption rate of oil (the world) we still wouldn't run out for over 250 years! This also makes some very foolish assumptions. 1) Our consumption rate will never get lower (or higher) 2) We will never find any more oil anywhere else in the world 3) We will always be dependent on oil due to a lack of newer technology. Blame OPEC and the middle east, not YOUR own country. Gotta vote for a proper governor before you can worry about how a president does his job anyways.

 

Sorry yeah you read it right BUT he wrote it wrong.. I did a quick cut and paste of a few different posts and this was was WRONG!!!

Okay the lower the octaine the EASIER and FASTER it burns.. Higher octaine burns slower and resists detonation... something you really don't worry about on an NA rotary. The na rotary wants gas to burn as quckly as possible.. This part (from a different poster) was correct.

"You want to use LOWER octane gass. In fact, you want to use the LOWEST octane gas possible. The lower the octane, the hotter the flame front, and the cleaner the burn."

Wanna try to avoid those carbon deposits.

Off the topic.. Do you Renesis guys know about carbon buildup in rotaries and how to avoid it? stolen from atkins rotary so I didn't have to retype it.

1) Never shut the car off before the idle drops off of the enrichment system (choke)

2) Do Tune Ups every 10-12,000 miles and only use NGK Spark Plugs - Change the air & Fuel filters at this time.

3) Unless driving a turbo - use the lowest to mid grade of fuel at a good service station. Here in the Puyallup / Tacoma / Seattle area of Washington state that means Texaco and Chevron. Both of these companies dedicate their expertise in using the min. amount of oxygenate additives and add other chemicals (like Textron) to help the additives burn. If using Supreme or hi octane in a non turbo, the higher octane makes it more difficult to burn all of the fuel allowing for excessive carbon. Turbo or supercharged cars need the slower burning fuels to prevent pinging or knocking which is very hard on the apex seals.

4) Use the BG 44k fuel injection cleaner every 4-6 months (more if all city driving or just short distances- less if mostly freeway miles)

 

Thanks for tyhe info on the 87 Octane gas. I assume you're going to run a few tankfuls through before determining if it's good for you in the long run. Are you planning on nting the gas mileage as well? it would be interesting to see what impact the change in Octane mught have on fuel mileage.
Again thanks. i look forward to reading your future posts.

 

I've been using the 93 octane for most of the 2500 miles that I have on the car. I don't have any idle problems BUT the one time that I let my dealer fill up my car, I notice (within minutes) that I had a funny BBRRRRRAAAPPP noise (rotary version of ping?) when I would push the engine to more than 6000 RPM. Once I ran that gas out and filled up with the 93 version, the noise went away. I'm going to try and use the next level down which is 91 octane. I'm hoping that this middle of the road approach will do the trick!

DeBug

 

okay, here is my first data to report. Not enough time has lapsed yet but some conclusions can be drawn,

My commute is the primary use of the car. It is a 17 mile one way jog through a 4-6 lane road/city driving with several lights - like many towns in FL. The first month - running premium gas I averaged 15.89 mpg and of course I drove fairly easy the first 600 miles. The car sputtered and missed at initial startups and at red lights and there was a definite vibration at idle. The soot on my rear trim area was similar to a diesel car.

I switched gas to 87 octane as per the manual's allowance for such. It took a half tank to notice a difference in the smoothness of the engine. The checked gas mileage at that point - a halftank was 14.5 MPG - YIKES! I filled the tank with 87 and drove it this week. I got gas last night and got 16.29 MPG. Now, considering this is city driving and the fact that I am now using RPM's up to around 7500 - driving it harder than before. I can safely say that I have gained around 1mpg. What is interesting is that the mileage at first shot WAY DOWN.

The engine is running so much better. If the idle were completely smooth (like you hear the engine but don't feel it) I would think I was driving a Huskavarna or a Singer sewing machine. IT is almost as if I am driving amongst slugs - SUVs, work vehicles, big sedans - they are all in the way! oh, and the greatest thing is the little purring sound when you pull it in the garage!!!!! Nice pet car. Nice pet car.

 

I have similar experiences. I filled up near empty last week at 89 octane, than I drained that tank down to nill almost and fill up again with 89. I'm running purely 89 and the car doesn't sputer on morning starts anymore!

Gas mileage is a little better too it seems! Woohoo, I'm sticking with 87/89.

 

Thanks fir the input. I hope you'll continue to post after you've run a couple of more tankfulls through. I'm about to switch.