Supercharged 3800 Series II. , in two cars , 350 /350hp in vette
Premium gas., in these three, middle gas in 1940 chevy w /350 /300hp if I put reg in 40 chevy it knocks at higher rpms...

 

In the truck I run 87 and in the car I run 91

 

L36 no mods just a drop in K&N and I run 87 tried the 91 for a week got less MPG's on the tank !

 

Exactly, that's why I'm wanting to go up to 91 now. I was trying to keep my fuel expense down while commuting during what was my last year of college. Mostly highway cruising at that time, probably 85-90% of my driving.

 

If your car is stock, it will say on the gasoline door if you need premium. Otherwise, you're probably just wasting your money with higher octane. Here's what the 2004 Manual says on this:

Gasoline Octane

If your vehicle has the 3400 V6 engine (VIN Code E)
or the 3800 V6 engine (VIN Code K), use regular
unleaded gasoline with a posted octane of 87 or higher.
If the octane is less than 87, you may get a heavy
knocking noise when you drive. If this occurs, use a
gasoline rated at 87 octane or higher as soon as
possible. Otherwise, you might damage your engine.
A little pinging noise when you accelerate or drive uphill
is considered normal. This does not indicate a problem
exists or that a higher-octane fuel is necessary. If
you are using 87 octane or higher-octane fuel and you
hear heavy knocking, your engine needs service.

If your vehicle has the 3800 Supercharged V6 engine
(VIN Code 1), use only premium unleaded gasoline with a
posted octane of 91 or higher. In an emergency, you may
be able to use a lower octane—as low as 87 —if heavy
knocking does not occur. Refill your tank with premium
fuel as soon as possible. Otherwise, you might damage
your engine. If you are using 91 octane or higher-octane
fuel and you hear heavy knocking, your engine needs

service.

California Fuel

If your vehicle is certified to meet California Emission
Standards (see the underhood emission control label),
it is designed to operate on fuels that meet California
specifications. If this fuel is not available in states
adopting California emissions standards, your vehicle
will operate satisfactorily on fuels meeting federal
specifications, but emission control system performance
may be affected. The malfunction indicator lamp may
turn on (see

Malfunction Indicator Lamp on page 3-39)
and your vehicle may fail a smog-check test. If this
occurs, return to your authorized GM dealer for
diagnosis. If it is determined that the condition is caused
by the type of fuel used, repairs may not be covered

by your warranty.

 

2007 manual info:

Fuel

Use of the recommended fuel is an important part
of the proper maintenance of your vehicle. To
help keep your engine clean and maintain optimum
vehicle performance, GM recommends the use
of gasoline advertised as TOP TIER Detergent
Gasoline.
The 8th digit of your Vehicle Identification Number
(VIN) shows the code letter or number that
identifies your engine. You will find the VIN at
the top left of the instrument panel. See

Gasoline Octane

For all vehicles except those with the 5.3L V8
engine (VIN Code C), use regular unleaded
gasoline with a posted octane rating of 87 or higher.
If the octane rating is less than 87, you may notice
an audible knocking noise when you drive,
commonly referred to as spark knock. If this occurs,
use a gasoline rated at 87 octane or higher as
soon as possible. If you are using gasoline rated at
87 octane or higher and you hear heavy knocking,

your engine needs service.

If your vehicle has the 5.3L V8 engine (VIN
Code C), use premium unleaded gasoline with a
posted octane rating of 91 or higher. You may
also use regular unleaded gasoline rated at
87 octane or higher, but your vehicle’s acceleration
may be slightly reduced, and you may notice a
slight audible knocking noise, commonly referred
to as spark knock. If the octane is less than
87, you may notice a heavy knocking noise when
you drive. If this occurs, use a gasoline rated
at 87 octane or higher as soon as possible.
Otherwise, you might damage your engine.
If you are using gasoline rated at 87 octane or
higher and you hear heavy knocking, your engine

needs service.

California Fuel

If your vehicle is certified to meet California
Emissions Standards, it is designed to operate
on fuels that meet California specifications.
See the underhood emission control label.
If this fuel is not available in states adopting
California emissions standards, your vehicle will
operate satisfactorily on fuels meeting federal
specifications, but emission control system
performance may be affected. The malfunction
indicator lamp may turn on and your vehicle may
fail a smog-check test. See

Malfunction Indicator
Lamp on page 181. If this occurs, return to
your authorized GM dealer for diagnosis. If it is
determined that the condition is caused by the
type of fuel used, repairs may not be covered

by your warranty.

 

i use 87 in the good old 3100 think i should bump it for fuel economy gains or not worth it 40 is no longer a fill up here in mass

 

Just my opinion, but I would stick with 87 octane. Your car is tuned to run on 87 from the factory. Unless you modded the engine (K&N drop in filter doesn't count), the extra cost of the higher grade gas would out weigh the fuel economy savings.

 

a gallon here and there of it to clean out my uncle said he used to fill her with higher octane prolly why she runs like new at 157k

 

Higher octane doesn't clean, it has all the same stuff in it unless they specify on the pump.

All higher octane does it detonate LATER. If your car isn't set to detonate at that point, your car is no running efficiently.