Can higher octane fuel cause power loss?

[strother]

First, pardon my ignorance. I'm trying to learn as much as I can about cars since I bought my 82 911SC a couple of weeks ago, but I don't have much of a base.

Here's my issue. When I first got the 911, it was quick
with great throttle response. Now, it is downright sluggish. I mean, silly sluggish. I feel like I can't pull out into moving traffic because I can't match the traffic's speed.

Other than that, the engine sounds a little "breathy" and kinda muffled? There is also always a strong fuel smell. When the engine is cold, it idles high, around 1100 or so, and it surges a little at idle, with a regular cadence. Sometimes when it is cold, it will idle up near 2000.

Today, I took it out to drive it fast, and it didn't want to go above 5000 rpms in fourth gear.

This problem has coincided with the first time I filled up the tank. I've been using the highest octane available, 93 here, I think. My gas mileage has been awful. Something like 9 mpg or worse. If the car was tuned for a much lower octane, could that be the cause?

[strother]

Originally posted by SoCal911SC
No way that just a small boost in octane could cause all those problems.

I'm sure you're right. I'm trying to figure out what I've done in the past couple of weeks that would change the character of the engine. BTW, it's got around 45,000 miles on it. I haven't beat the hell out of it since I got it, either.

[wholberg]

I used to have a '78 911SC that had symptoms similar to what you describe, including the gas smell. Turned out it was a partially plugged catalytic converter. The simple fix was to remove the cat and "unblock" it with a heavy metal rod. The result was a test pipe that looked just like a factory cat. Waaay quicker after that.

[strother]

Interesting about the plugged cat. How long does it take to get plugged? Is it a slow accumulation process? I feel like my problem developed relatively abruptly.

[wholberg]

On my 78 it happened just like you described. One day it ran fine, the next it would not go over 50mph. A couple of days later, top speed was about 35 and it would not rev freely. I just had a hunch that it was an exhaust blockage, and started taking the exhaust off one piece at a time and seeing how it revved. In your case, I would start with the cat first.

[copper]

Octane ratings CAN be an issue.

Not sure which model of 82 SC you have, ROW, Japan, or USA, but if it is either US or Japanese, then you have a compression ratio of 9.3:1 as compared to the 9.8:1 in the ROW.

My 69 has a ratio of 8.6:1 and does not handle higher octane fuels well at all. Check the many threads on fuel octane and you will see many common opinions.

Higher octane fuels have a much higher combustion temperature. The lower compression engines are generally not able to generate the heat required to adequately burn these fuels efficiently and therefore you get misses, rough running, rougher when cold, unburned fuel smell, backfires (from combustion in exhaust system) and poor mileage.

Do the above symptoms sound similar? Does it run a little better once warmed up?

Run the fuel out and go back to 87 octane. If you're not running a high compression engine, or a supercharger, there's really no benefit to a high octane fuel, or octane boost treatments.

Jim

[strother]

I'll check that out. Is it possible that I'm just running too rich?

[beepbeep]

While higher octane fuel does not contain more energy per weight unit (it's merely more ping-resistant), your problem isn't fuel but something else.

Plugged cat, bad A/F ratio etc. It's not hi-octane fuel. Pouring hi-octane fuel in low-comp engine is just wasting money, but it won't hurt it.

[strother]

Originally posted by beepbeep
While higher octane fuel does not contain more energy per weight unit (it's merely more ping-resistant), your problem isn't fuel but something else.

Plugged cat, bad A/F ratio etc. It's not hi-octane fuel. Pouring hi-octane fuel in low-comp engine is just wasting money, but it won't hurt it.

Thanks for your answer, and your advice about wasting money on high octane fuel. I really am a naive consumer! I promise to read my owner's manual, but I might as well ask the question while I'm here: Can I use the lowest octane fuel sold at the station? I think it is 87 or 89 here. (My previous cars were neweer models and required premimum fuel -- I've developed a habit, but would love to kick it.)

[wholberg]

If you have the owner's manual, it gives minimum octane requirements for gasoline. If not, I am sure someone on the board will have that information.

[911Mac]

Sounds like you have something way more serious going on than just an octane issue. I have an 82 911SC, it runs fine on regular or premium. I can't really feel any difference between them. I got around 22 mpg with CIS, car is now EFI which has bumped the mileage up to 24-25. All numbers are for predominantly highway driving.

Because your problem began when you refueled the car, have you made sure you didn't get a bad load of gas, or didn't stir up a lot of dirt in the fuel tank that may have blocked the screen or fuel filter.

Not saying it couldn't be a plugged CAT, but it might be easier to verify the fuel system pressure is in spec and the mixture is set correctly before you go through the trouble of dropping the CAT.

Good luck getting your car running the way it should and welcome to the board.

-Mac

[magic930]

If fuel pressure is OK then:
Check for a vacuum leak. A vaccum leak will definately create the higher idle which then hunts because it goes lean-rich-lean-rich.
A vacuum leak will also prevent the timing retard and advance mechanism from working correctly and would cause the lack of power. Incorrect timing will also contribute to the poor gas mileage.

[Jim Garfield]

Originally posted by strother
I promise to read my owner's manual, but I might as well ask the question while I'm here: Can I use the lowest octane fuel sold at the station? I think it is 87 or 89 here.

Reading the manual is the best advice re: octane requirements. The manual is very comprehensive and informative, surprisingly it's actually a good read.

Your problem doesn't sound octane related though. I agree that it sounds more like a blocked cat or a fuel mixture problem.

[Doug Steinel]

The engine on my car is a stock '82 SC and it runs fine on 93 octane and gets 20 MPG. Matter of fact, I am afraid to burn the 87 octane so pay the premium.

Lot of possibilities causing your problem. Why don't you pull some of the plugs and see if they are fouled? If they are OK, move on to possible exhaust plug. If that checks out, it could be a CIS problem. Are you good at troubleshooting CIS? I would recommend Watson book.

[stlrj]

All SC's were designed to run on 87 octane US and I have to agree that your problem doesn't sound octane related, but mixture related.

From what you describe, I'll bet that your mixture is way too rich which would explain the gas smell, sluggish performance, high idle with surge.

Warm up regulator disconnected or not getting 12V?

That's where I would start.

-Joe
74 w/86 3.2

[wholberg]

Ah, the solution appears to be so simple (I hope you didn't have anything planned for this weekend)

[Doug Steinel]

...or next!

[Jack Olsen]

My (uneducated) money's on the catalytic converter. They can plug up surprisingly quickly.

[Por_sha911]

Definitely sound like a breathing issue or a problem with your ignition or fuel system. Have you "base-lined" the car (plugs, cap, rotor, air & fuel filter, clean the ISV...)? Its always better to diagnose a problem when you have eliminated as many variables as possible. Removing the cat is not a 30 minute job to a newbie so I'd start with stuff that you should do when you get a new car anyway (unless you have excellent records of these things being recently done).

[john walker's workshop]

typical symptom of a non-working frequency valve. feels like you're dragging an anchor. use the search function... frequency valve, or oxygen sensor relay. very common, and a easy fix.