TJM

Is 87 octane OK in the L98 specifically an 1989 model?

I may be missed informed about the Knock sensor sensing detonation and retarding timing?

Thanks

TJM

CFI-EFI

Octane is only the measure of the gasoline to inhibit knock. If it doesn't knock, more octane provides nothing but added expense. If the knock sensor detects knock, the signal from it to the ESC should cause the timing to be retarded, eliminating further knocking and preventing damage. If this should occur at WOT, the most likely time, the retarding of the timing will momentarily reduce performance.

87 octane can easily be "OK" in your '87 depending on the characteristics of your specific engine and your wants and performance needs. Try it and see. Wait until you are low on fuel and pump in 5 to 10 gallons. If it should knock really badly, (unlikely) you will have plenty of room in your tank to dilute it with the higher priced stuff. I have driven and raced my 9.0:1 compression, 1984 for years using 85 octane exclusively, with no ill effects.

RACE ON!!!

JackDidley

When my 87 was stock, I tried 87 , Knock, knock. It has mods now and the chip is tuned for 91 minimum. The premium allows more spark agressive advance, it actually gets enough better MPG to more than offset the price per gallon.

seenalot

I have a non mod 1989 L98 and use 87 octane for normal driving. I'm not pushing it and performance is fine.

TJM

The question came up in another thread, sort of non-related to that thread, so I asked the separate question, here.

The other 'fella had performance issues, using 87 octane in his '87.

My '89 won't tolerate it with the chip I have, so it's a non issue for me.

Mine is just a Sunday driver, and a tank a month is not unusual, the cost difference being negligible.

Thanks,

TJM

anesthes

Not to mention more advance keeps the exhaust valve and port cool.

I was talking to a guy on thirdgen.org last week regarding some tuning I've been doing, and eventually we got on the topic of e85 with boost and the sorts. Well anyhow, the moral of the story is with slightly higher octane he can run more advance and that more advance translates too. aww, hell I'll just copy his statement here..

"I would like to try E85 too. I have a bone stock 1986 305ci TPI (9.5:1 compression) with 8-9 PSI of boost that I run 18* of advance with the water/alky injection on (50% water / 50% eth&meth mix).That is running 87 octane E10 gas. By using 93 octane E10 gas, I can add about 6* of timing and this gains 1 second in the quarter mile ET."


So what does this have to do with the OP's stock '89 Corvette? Well, the '89 has the 113 heads and a more aggressive advance table than say a lame earlier ECM with iron heads. In fact, the $6E mask on his '89 ECM will take out about 10* of advance when it encounters knock. So one would suggest, if you wanted to maintain performance you would put the fuel in the car was designed for.

By '90 the engineers at GM realized that people would often 'cheap out' and put lower octanes in the car, or that some regions just sold crappy gas. (midwest/banjo land I'd guess) so they added some code in to try and detect if 'premium octane' was being used, and adjusted the advance accordingly. Older masks rely on knock to happen first which is never a good thing.

-- Joe

Tom400CFI

How does this work? Is there an octane sensor in the gas tank? I think knock counts is the only way the ECM could make any assumption about fuel octane rating...

TJM

Reading the GM factory book, this is the only ECM input [ fuel related ] that would affect timing?


I'd have to agree.

TJM

anesthes

Depends on the year/bin, but it's all derived from the knock sensor. It's the functionality that differs.

I think the EARLY crap, like '84 and '85 it will simply retard with every knock count and that is that.

The later stuff, especially $8D (speed density) has low octane routines I mentioned eariler. It's based on knock counts over a threshold, if the threshold is met it sets the "low octane" flag, and limits the the advance across the board until the car is shut off and restarted.

So like I said earlier, the later stuff is better because it will go into that mode and keep the engine from destroying itself. The earlier stuff will just keep retarding/knocking/retarding/knocking. I don't know about you guys, but in any C4 I've ever been in I couldn't hear engine knock over everything else (exhaust drone, resonance, etc) yet saw the counts on the scanner. So I think it's safe to say those who are running low octane and are fine are pretty ignorant to what is *really* going on.

-- Joe

anesthes

Correct. I can show you guys the the tables + routines I'm talking about, the attack rate and decay rates for the older bins, etc. I cannot show you the earlier crossfire stuff because I don't have an ability to read those ECMs. I don't want to get into the whole CFI war again, but this is part of why I don't understand why folks modify CFI setups - you can't tune those ECMs. I guess there is a few things you can do with a hex editor, but nobody does.


-- Joe

TJM

Joe, your speaking to a virtual rock, here.

Can you dumb this down a smidgen?

TJM

anesthes

Well, the moral of the story is low octane could/should/will produce a knock. The ECM will retard the timing. Performance will be poor, exhaust temperature higher.

Older ECM's won't 'globally' retard, so you'll still get knock which obviously is harmful to the engine.

Newer ECM's globally pull it out, so you'll just have poor performance with less chance of damage.

Crossfire ECM's cannot be tuned, really.

-- Joe

TJM

The rock thanks you sir!

I'm guessing an '89 is limited to a burned chip? I have never been advised that it is tunable otherwise?

TJM

convas

As supplied by GM yes you need to burn a chip to tune it but there are alternatives like the Moates products which allow you do real time tuning using the stock ECM.

http://www.moates.net/index.php?cPath=50

anesthes

Yes.

86-88 use $32B software, 89 uses $6E, and 90-91 use $8D.

85 is a one off year. Very strange ECM, not sure what GM was thinking.
84 is left over technology, not really programmable.

You can't really 'tune' an EFI car other wise. Anything you can do mechanically will simply change things globally which is a hack at best.
If you look at a typical VE or MAF (fuel table) you will see that various places need more fuel with others, and it's not linear with RPM. Some guys think they can just change the fuel pressure and that will cover it. I cringe every time I read posts where it is suggested to adjust timing at the track by moving the distributor.

-- Joe

anesthes

I use an ostrich emulator from moates. It used to take forever to scan, burn, scan, burn. Now I take a cruise, pull over, make some changes, cruise some more, etc. The emulators are nice.

-- Joe

bczee

If your objective is to cut cost at the pump by going to the low grades of gas, try using a ODBI scanner (not a code reader) and take a drive or WOT and monitor the Knock count, If it is excessive, then you should step up to the mid grade and monitor it again.

Let us know what you find out.

69427

It is the mark of a quality ESC calibration when the knock is detected without the driver noticing what's happening.

69427

CFI ECMs are tunable. Been running the stock (with modified cal tables) ECM in my '84 turbo for years. I admit it's not as sophisticated as later ECMs (and I'm planning on swapping to a later ECM when I finish my other projects), but it's just a matter of looking at the code and making the required cal changes. Not a big deal. I've got several hundred track miles with this turbo 355 setup, and haven't melted anything.

69427

Regarding CFI ECMs, again, I disagree.