Question about fuel: 87 Octane w/8:7-1? or 91?

[cadipacer]

Just noticed the tune up sticker on top of radiator for the 73 ELDO I'm parting out (sold body),, it says use 91 or better!??? I always thought the higher the compression the higher the octane!, but this seems to go against that theory, I've been using 87 with very good results,, timming set at 9* with a new timming chain & gear set.
Has anybody ever ran the same 87 octane and HAD a problem?
or does everone run 91 octane? thanks for any input.

[Terrible One]

My grandfather always ran 93 in the '76 before he passed away and willed it to me. I too thought about the low compression and filled up with 87. Did just fine for me, but I only filled it up with 87 once before the restoration started.

[abcdefg1675]

my '77 didnt run too good in 87 when I had it. Ran rough, make a ticking sound in the engine. It all went away when I burnt through the tank of 87 and put 91 in it. Gas wasnt too bad a few years ago when I was driving it.

Although I cant really say if it normally runs good on 87. Im sure I had enough problems with the car (carburetor/distrubutor and ignition, possibly vaccume leaks) to make it run bad on 87. Im not sure, though. I thought 91 wasnt any different from 87 aside that it was more expensive and had additives to it.

Once I get this megasquirt stuff figured out, and come back from the airforce, I will run 87 through it. 8.5:1 isnt extreme at all, but possibly the end gasses or carbon buildup can be enough to cause pinging under a heavy load?

[STDog]

If the engine is clean, and properly tuned, 87 is OK for low compression engines. If there's much carbon in the chamber, you get knocks, especially under a load. 89 should clear it up untill you clean the engine up.

My 10:1 472 wouldn't run on 92 untill after the rebuild. Too much built up in the chambers. it took ~95 (92+100LL avgas in a 2:1 mix) to run normal timming and not knock. Now, no problems with 91, and I could probably run 89, especially in the winter, but why risk it? $2.50 extra every 2 weeks (25 gal fo 2 weeks of driving, and 10cents a gallon more).

Although I cant really say if it normally runs good on 87. Im sure I had enough problems with the car (carburetor/distrubutor and ignition, possibly vaccume leaks) to make it run bad on 87. Im not sure, though. I thought 91 wasnt any different from 87 aside that it was more expensive and had additives to it.

There's a big dfference. You should read up on the octane rating methods. No in a low compression ( <= 8.5:1) you won't really know, but in higher compression engines it matters. Or boosed, which is effectively more compression by forcing more in the chamber.

Also, the number at the pump is an average (RON and MON) and the numbers vary by supplier. MON is the one that really matters to the engine under load. That's why you mighty run OK on 87 of one brand, but need 89 in another brand. It cheaper to increase RON.

[DOC]

The posted octane numbers in 1973 were RON (reaserch octane number). The goverment testing agency used MON (motor octane number). To eliminate confusion the gas companies were required to average the two and post that number. That is the reason the octane numbers are less than they used to be. If you look at the posting on the gas pump it will say RON plus MON divided 2.

DOC

[abcdefg1675]

I remember reading in my cadillac service manual (the big heavy dictionary sized book), the 1977 edition, that it should use 92 RON octaine. I think the RON number is different from what is at the pumps. Ill have to google this again.

Its been about a year since I read this book front to back, twice, but I knew it was around there.

Also, my 425 ran like it had a hot cam at idle. Real lopey exhaust note, engine would sometimes jerk the car when it missed.

And something Ive notced, my parents 1987 toyota corolla (4A-FE engine) pings to no end when its warmed up and gets lugged. Ive looked in it once, and it does have some serious carbon buildup. Ive heard a lot of talk about something called seafoam, for cleaning out carbon.

I also have a few automotive encyclopedias copyrighted in 1977, Ill be doing some reading on this. It has a good part in it explaining detonation, preignition, pinging, fuel octaines... I will post a few pictures about it if anyone wants me to. Im very bored... two months left till I leave for the airforce...

[cadipacer]

FROM ORGNAL POST,,, I want to thank everyone for their replys
this has helped,, I will shop around for higher octane at a name brand
stations,, and see If the Pacer perks up with a higher octane.

[DOC]

Cadipacer,
If you have any pictures of the Pacer, I would sure like to see that install.

DOC

[Terrible One]

He sent me some. I'm a big fan!