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Discussion Starter #1
hey what's up guys i'm new here.just bought a 91 crx si with a dohc zc. so i just recently found out that these engines are recommended to take 91 octane. ive been putting 87 on mine just wanted to know if i'm doing Any harm to the engine.just really paranoid because i just replaced the dizzy for it yesterday due to an internal oil leak.was that the cause of the wrong octane? should i drain my gas tank and replace it with 91? i still have like a lot of gas in there.
 

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They call me Awesome!
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No running 87 won't hurt your motor, especially if it's just this 1 tank. I'd consider swapping to 89, just to see if you can get better mpg.

And no that didn't cause your oil leak...
 

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Discussion Starter #3
No running 87 won't hurt your motor, especially if it's just this 1 tank. I'd consider swapping to 89, just to see if you can get better mpg.

And no that didn't cause your oil leak...
argh the thing is the previous owner who had the car for a month had been using 87 on it as well. ive had the car for 2 Weeks now. it ran shitty before and it runs a lot better now after i replaced the dizzy. it does struggle to start up on cold weather i have to step on the gas pedal to start it only when its cold though.
 

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They call me Awesome!
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Still probably not the gas. Most likely low on compression, it is an old motor...

Running 87 in it just means you may be a little low on power and slightly shy on mpg, other than that it's not really gonna hurt you.
 

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Re: zc octane question

All I have ever ran is 91 in my cars. I don't run it because that's what's recommended I normally run it because I feel like the engines seem to like the 91 better and I have always gotten better fuel economy from it. But if the beaver says your ok to run 87 in the dohc zc listen to him he knows his shit. He helps me out with my random ass dohc zc problems. Like I said I have always ran 91 you might try it and try running a can of seafoam through the engine to help clean up the injectors and the lines and throw on a new fuel filter and see if that helps you out any good luck
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Re: zc octane question

compression test then go from there. if old owner upthe compression then 91 octane should be used.
but factory should be 87 as others said
Factory actually recommends 91 or higher for the ZC's

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ej8
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Factory actually recommends 91 or higher for the ZC's

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Yea then i would use that.
Use i had my gsr i only use 91. 87 made it act a lil funny.
Its the high comp. I just dont know to much about the zc, but if there comp is higher the higher octane will help with knock.
 

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93 Legend L Coupe.
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*sighs*

On a factory JDM setup, with the engine stock, the ECU stock (PM7 or P29) and their overly aggressive timing maps (the EDM PM7 maps are much better, actually), the requirement was to run 98RON straight gasoline.

Here in the US you have nearly always about 10% ethanol in the fuel, much less aggressive timing maps in a PM7 or PG7, and if you leave the dang timing stock, 87 is FINE.

If you advance the timing, raise compression, run cams or pretty much do anything to increase power, you need a tune and should run 91. In fact, a bog stock DOHC ZC with a good tune will outperform any bog stock SOHC, period, even with a good tune. Once you start to open things up, there is no contest at all.

Running even a stock JDM DOHC ZC with the stock PM7 or P29 isn't going to get you a very smooth running engine IMO and experience. The timing maps are too hurky-jerky and fuel maps are WAY too rich above peak torque (commonly dipping into the 11s), which greatly decreases power output above 5.5K, and considering that the stock rev limit is 7.8K, it really just sucks. A good cleaned up basemap (like I've figured out) will run the engine better than any stock ECU, and can probably even run OK on regular, but I would recommend at least midgrade.

Look at the timing maps for yourself, then make some comparisons, then decide for yourself.
 

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Discussion Starter #12
just checked my ecu and its a PM7. idk if the previous owner raised compression or advance the timing so im switching to 91 once i finish the tank just to play it safe
 

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Depends 91 RON is 87 R+M/2. I think Japan uses RON so regular gas here is fine. However, With older motors they can sometimes benefit from increases octane due to wear, burning a bit more oil (which essentially lowers octane), etc.
 
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