I've been preaching on the forum forever that you should run the lowest octane fuel that you can without getting any timing retard. Meanwhile I always put 93 in my car. Why? Well it's heavily modded and I just haven't had the chance to data log it and see if I would get any timing retard on lower octane fuels so I've just been playing it safe.

With gas prices the way they are I figured now is as good a time as any to start experiementing. When I got gas today I filled up the tank with 89 instead of 93. Now granted there was still some 93 left in the tank. I had about 2 gallons left so going off of my calculations I feel like I've got 89.5 octane in the car right now. Guess what? Well it ran just fine on the way home. I am not going to simply rely on my butt-meter dyno though. I've got to go some places tonight and I intend to data log the car and see if i get any knock counts, etc.

If I'm getting timing retard I'll drive easy until this tank is gone and then go back to 93. If I'm OK I'll try 87.

I know there will still be people who want "only the best" for their car and will pay $.20 more per gallon than they need to. I guess it's you're right to do so, and I've got a bridge for sale :)

I'm not sure what my results will be and I know this is beating a dead horse, but at least I'm gonna try to beat said horse with data, not with rumor and theory. I'll keep you guys posted.

 

Nathan, I agree with you on this subject. The problem with high octane fuel is it burns slower, and leaves more carbon behind. It also contains less btu's which is energy. You should only run as much octane as needed.

 

I just did the same with my mildly modified 86 (cam, headers, etc). I've always ran premium and swithced to 88 octane with no problems.

 

Isn't that like having a hot babe and putting an overcoat on her instead of a miniskirt? It does the same thing but it's just not right. :auto: :lol:

 

Cool, keep us informed. As a regular unleaded guy, I'm interested in the results.

 

:lurk: I always use premium in my bone stock '93, 'cause the manual says to. But after cleaning all the carbon out of my TB, I'd like to go to a lower octane, but only if its *safe*. BTW: [dumb questions] how do you check for knock counts?

 

While I agree with what you're saying, I think 87-89 octane is going to cost you some performance, as in additional knock counts.

Its recommended to use 91 Octane in my 89 and while it will run on less, it won't run as well as if I used the right octane.

Higher octane won't hurt its just not necessary to exceed your engine's requirements.

But the right octane is required for best performance. :cheers:
http://www.ftc.gov/bcp/conline/pubs/autos/octane.htm

 

I run 85 octane and 14* base timing. *I* don't feel 13's is too awful for a worn out, stock long blocked, Crossfire. Lower IS better, until it pings or throws knock counts.

RACE ON!!!

 

Nathan, if it does start "pingning" on you, maybe you can add some octane booster to get through the fuel that's in it now, rather than damage a valve. I know that you know your stuff about cars, but you might have forgotten about the octane booster. No offense intended.

 

It's not likely to knock until it is at WOT or near WOT, and then the knock sensor and EST will "kick in" to save the engine. No need to worry about valves or octane boost.

RACE ON!!!

 

Cool!! Thanks. I didn't know that. I'm not used to these fancy Corvette features. I'm still amazed that I have features on my '85 that are just becomming standard on regular cars.

 

Octane booster is absolutely WORTHLESS. An octane point is measured in tenths, not whole numerical values. So an 8 point octane increase would go from 87.0 to 87.8. Most people don't know that. Its much cheaper to buy higher octane fuel.

Nathan is on the right track though. Until you move into forced induction cars, which build a ton of heat and increase the chance of detonation, lower octane is better.

 

I dont see why anybody would continue to argue about it.

 

Ok, guys, some preliminary results are in. As it's been stated the computer will do it's job and keep my car safe so I'm not worried about that. As such I intentionally took the back roads to my destination and made sure to give it a little bit of hell.

The car felt great, there was no loss of power on the butt-master dyno. Upon reviewing the data logs though I can see that I did get a little bit of timing retard at WOT. I compared this data log to one from a long time ago. Basically I have got about the same amount of timing retard at WOT before, but it was extremely rare. Withe the lower octane fuel I appear to be getting said retard more consistently. Part throttle and cruise is fine, WOT seems to be the only time it's a problem at all.

Knock counts are actually pretty low when it does pull timing, I also never heard it pinging at all so I can't believe that it's anything significant. Granted a small amount of timing retard can cost you big time in the performance department. My initial thought is that for my car I should stick with premium for best performance. It also tells me though with my mods if I can run middle grade with very little timing retard, any stocker should be able to run it with NO problems.

I'm certainly going to keep data logging it for the next few days and see if anything changes. If gas prices keep going up the way they are I amy try 87. Even if it pulls a lot of timing with 87 at WOT I should be fine if I keep my foot out of it, which I need to do anyway just to get better mileage.

I'll continue to keep you guys updated.

 

If gas prices keep going up, I'll be riding a skateboard to work. :lol:

 

:iagree: it might be time for the ol' bicycle. :jester

 

I'm just trying to drive a little less, but at almost 22 mpg I can't complain about the car, and I romp her with regularity, but I have been on 87 for about the last 2 mos. since the last extensive discussion of this took place. We got the fourth hightest gas prices in the country and they are going up at least weekly. It's :crazy: and a lot more than 20 cents a gallon more here. 87 is right at 2.15 :eek:

Thanks for the data Nathan :thumbs:

 

:iagree:

Anytime I've run 87 or 89 in my cars I've seen a decrease in economy, I never got the enthusiasm up to do data logging on it, but I did hear pinging on accelleration and I was getting a 1-2 MPG less... I do drive a little more aggressively than most people though so that might account for a lot of it...

FWIW, I sometimes get a slight pinging on 91 (we don't have 93 here), so I know that lower grades won't cut it for me...

 

(in a small block chevy)
at the Same airflow, near WOT,
With optimized spark in each case,
with optimized A/F,
What is the power difference between 87 and 91?
Surely somebody has done this test.

:smash:

 

Nathan.... remember you'll always want a healthy margin. What happens when it's a hot day out (was 100 here last week on the coast) and you've got the A/C on.. and you need to punch it from a standstill? You're going to need more octane, that hotter running engine will be more likely to knock.


So, I'd add to you recommendation and say, "Run the lowest octane your car would require under the worst expected conditions you'd ever need to run WOT in."

I don't need the vette's superior cornering capability all the time either, but I enjoy having it for those select times when I need a cheap thrill, or the unexpected circumstances when it saves my hide.