Mtaibbi

I'm buying a 1990 C4 convertible (with the L98 engine) from a childhood pal who bought it new and has kept it in showroom condition all these years (for example, it's never been rained on). I'm deciding now whether to ship or drive it from its New York home to my primary residence in LA, and while my friend says he's always used premium gas I've read some threads asserting that regular is just fine. Is it?

grampi50

Why didn't you post this in the C4 section?

Vetteman Jack

Moved to C4 General.

vader86

A 1990 would need at least midgrade due to the compression.

My 86 only needed 87 octane, my 88 only needed 89 octane. You should run the lowest octane possible that doesn't knock. The detergent packages at most places will be exactly the same.

The way to be sure is connect a scanner and see how many knock counts you get.

Tom400CFI

Drive it.

You'll be making memories and starting a real relationship with a cool car.

PatternDayTrader

There's no way I would do anything besides drive it home, and by the time I got home, I would know quite a lot about the car. One of those things would be if I thought premium fuel was absolutely required, another would be its top speed, total fuel range at the posted speed limit plus 30%, fuel mileage at one hundred mph ect ... the important things.

Paul Workman

I've lost track of the exact date, but IIRC it was in the late 80 when the EPA mandated that ALL cars made in the USA be capable of running on 87 octane. Computers and (knock) sensors made it possible.

When knock is detected, the computer will pull timing until the knock counts fall below acceptable parameters. Even the LT5 with its stock 11.0:1 compression will run on 87 octane.

Performance WILL naturally suffer temporarily as result of the computer detuning in response to the higher knock counts. However(!), the computer will keep bumping (testing) the envelop and when premium fuel is reinstated to the tank, timing will be crept up until knock threshold is reached - thus restoring performance to the level of the computer's per-determined program limits.

This is why you will see "USE PREMIUM FUEL ONLY" written on the gas cap - It's NOT because the car won't run on regular, but performance detuning (via the computer) will automatically result otherwise.

So, what happens if a Vette is converted to a carb, when regular (87 octane) fuel is used after the tune was optimized for (say) 93 octane? KNOCK CITY, that's what! The timing is too advanced for the low octane fuel.

So, if ferrying a (C4) across country, assuming the computer etc. is intact, one can save some money by driving on 87 octane. And, mileage may even be a mile or so better. Just don't be so sure you could blow off a (pesky Mustang or whatever) you encounter on the expressway like you maybe used to (had you had a load of 93 octane instead of that 87 stuff)!

KenMathisHD

If your friend has always run it using premium, then I'd run AT LEAST midgrade if you have the potion. The difference will only be worth a few bucks at most, and unless there's none of either around there's really no reason to run it on 87.

Crossed Flags Fan

NY to LA = 3,000 miles. Assume MPG is 25 and difference in price of 87 octane vs 91 octane is .25 cents.
3,000/25 = 120 gallons
120 gallons X .25 = $30.00

PatternDayTrader

If you drive fast enough to make that trip without stopping, then you will probably average somewhere around 17mpg, but yeah, in the big scheme of things theres not a lot of difference when you think of it in terms of individual trips.

FAUEE

If you'e worried about the price difference in regular and premium, a Corvette is not the car for you. Everything is more expensive on them. You will not enjoy the car of you have a nickel and dime mindset.

JrRifleCoach

Burning premium in a low compression motor is wasting your money.

Yet the "nothings-to-good-for-my-baby" crowd will ignore the obvious.

856SPEED

Funny spin on it too!

FAUEE

I can't speak to the l98, but the lt1 requires midgrade. Where I live, that' like 5 cents cheaper than premium, and you don' have to worry about the mix being right (they just mix regular and premium to make mid grade).

Lots of modern-ish his CAN run 87. They just knock and pull timing. My friend has a silverado ss, and it knocks going up hills on 87. It runs, but not well when load is on it.

Renfield

Yep. Get a Prius and call it a day.

mickey5

Lets digress for a minute. If your going to drive it home find out if the injectors were ever changed. Mine failed the first time I took an extended trip after driving only several hundred miles a year the previous 10 years. The failure was sudden and undrivable. Beside the fact the chevy dealer didn't know what was wrong initially they don't have injectors on the shelf when they finally found the problem.

The original injectors will fail due to ethanol...its just a matter of when.

Taking along an extra set of injectors is cheap insurance. Paying a tech to change them isn't...figure 600-800$

ztheusa

You won't save ANY $$$$.

The knock sensors will just tell the computer to run richer/retard timing to keep the engine happy. Your MPG will go down accordingly. It might even cost MORE!

Tom400CFI

You're going to have to cite your source for that one.
I'm not saying what the OP should run, but the knock sensor ain't gonna make it run richer, and it ain't gonna cost more either. Come on.

ddahlgren

When you pull timing out the engine runs less efficient. That translate directly to worse mileage. The timing was taken out due to the cylinder pressure and cylinder pressure is what makes the car move. You still have to turn the same rpm at a lower efficiency. No source needed other than a calculator or a very small understanding of how the engine works.

Tom400CFI

Right....I know how engines work, and I also know that YOU already know that. You might want to review your own understanding of how the engine works, though.

When we're driving in such a manner as to be concerned w/fuel economy, (driving across country) cylinder pressure isn't typically a big problem w/regard to detonation; we're running with a very small throttle opening...aren't we? Yes we are. We're also running with some EGR. We're also running on the lowest end of the acceptable temp range (down against the stat). Under these conditions, detonation isn't typically a big problem, regardless of octane. So how are we going to get detonation, retarded timing, and lowered efficiency? We aren't very likely to.

In addition to all that...how do you explain this;

"The knock sensors will just tell the computer to run richer" (?)
B/c ^That was what I was REALLY interested in hearing about.