I hope the tech section is the right place for this question. I am pretty sure GM and the owners manual says "premium fuel required". With the prices of fuel lately, just wondering if any of you 3800SC owners run midgrade (89 octane) or regular (87 octane) ?

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Tim
Red 88 Formula Auto 2.8 100K+ Miles

I don't care how much money you have, free stuff is always a good thing.

Hi Tim, I run only 93 octane in mine, I know it cost more and others may say its not needed. I figured I have enough in-vested ithe car anyway, so why not... I`m not going to put a ? behind that statement, because I know whats coming next.... Hang on.................!!

The difference between reg and hi test when filling up the Fiero is about 50 cents - why take the chance?

The manual on my 2005 GTP says to use premium. It goes on to say that no harm will result from using the lower grades but performance will be reduced. It seems to me that this guidance allows for octane matching. If you are just driving to corner store and back use the regular. On the weekend when you head to the track pour in a tank of the good stuff for added fun.

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I clearly see noticeable performance and MPG if I fill up with 87 octane over the 91 or 93 octane in my 2000 Buick Regal w/ a 3800SC

The 3800 SC can live with 87 octane but like Phil said for safety, fill up with 93 octane so you can have peace of mind on a WOT run. Its best to try several differant higher end Gas Stations(Shell, Cheveron, Ect.) and run there 93 or higher octane and see which one the motor likes the best, then stick with it. I found Shell 94 octane gives me better gas milage and performance when I run it, so thats what I stick with on every fillup. The 94 octane is a little higher priced but the few dollars or so diffferance is worth it in the end.

I've run 93, 91 and 89 octane in my 3800SC Buick. Since I don't run at WOT, Ive not seen any performance or milage differences. I've not run 87 octane. The price differences in Florida is $0.10/gallon between the various grades of gas. So 93 octane at $2.39/gal vs, 89 octane at $2.29/gal, 87 octane at $2.19 /gal works out to a couple of bucks per tankful. There are a few places that sell 91 octane which is around #2.32/gal. The computer will retard the timing to compensate for the lower octane grade.

the 2004 GTP will compensate for the 87 octane, the older gens (97-03) won't.

I run 93/94 no matter what, I'm not going to burn up a piston. I have thought about it but saving 10-20 cents isn't worth the hassle of worrying. You figure you fill up completely you're saving maybe 2.00-2.50?

I mean 87 might work in the winter, but with increased temps you're just asking for trouble. I know my friend in his gtp was using regular octane and moderating it, said once we hit the 80's he saw the KR coming on.

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3800 SC/IC Fiero fixed and weeeeeen'in again!

[This message has been edited by LoW_KeY (edited 07-09-2005).]

If you want to go strictly by the book....the 97 & 98 Pontiac Bonneville manuals for 3800SC engines say 87 octane. That being said......the choice is yours.

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88 Fiero GT
85 Fiero GT
05 Pontiac G6
90 Buick Grand Sport 4 Seater
What’s Behind You Is Of Little Consequence

All the higher octane does is reduce spark knock/ detonation / pinging in higher compression engines. If it dont do any of those, your just throwing money away. I use 89 in both vettes, the Magnum Hemi, and did in the turboed Ferraro. If i knew I was going to be 'hot dogging' in advance id use premium. All those manuals for mine specified 89 octane and I havent had any problems. In fact every engine of mine Ive opened up is clean as new inside.

A lot depends on the modifications if any your have done to the SC3800. A pure stock 3800 will operate fine on reg gas but as soon as you start dropping pulley sizes and adding a cam or high ratio rockers you had better start using hi test to take advantage of your mods. I run hi test in my modded SC3800 and still get 28-30 mpg highway

Thanks for all the replies. I just picked up a 99 Grand Prix GTP. (Can you envision a possible future Fiero swap) ? With premium currently at around $2.60 in these parts - I wanted to get some PFF feedback on this subject.

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Tim
Red 88 Formula Auto 2.8 100K+ Miles

I don't care how much money you have, free stuff is always a good thing.

If you want a stock 3800SC to last more then 50K miles run 91+ octane. A STOCK GTP will sometimes get 1-2 points of KR on premium, on regular that will be 5-6 points KR which can damage a piston. The pcm only pulls timing AFTER its heard knock.

If you want to run less then 91 octane unplug the wire going to your boost bypass valve this will allow the SC only to make half boost (about 3 PSI with a stock pulley). This doesn't not guarentee that you can run less then 91 octane gas, so buy a scanner. Start with just prem gas in the tank and scan on a hot day with the engine warmed up. Find a empty road and take if from stop/1-2 mph (no need to spend the tire much) well into 2nd gear. If theres KR then don't run any less then prem until you've done some KR fighting mods and have scanned again.

No matter what other people run and have ran for years there is NO guarentee that a few points of KR wont brake a piston and ruin your day.

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Low 14 sec 99 Pontiac GTP, 3.2L V6 4 sp 86 Fiero GT

Not on a 3800, but I have a 2.0 supercharged ecotec in my saturn, and the book RECOMMENDS 91 octane... same type of deal, says that performance will be lost yada yada.

I've run it with 87 before. I didn't really notice a loss in performance, but I wasn't comparing numbers, only seat of pants.

HOWEVER!!! It's a 20 cent per gallon increase around here for 93 vs. 87. This is my daily driver, and I figure I fill up once a week. each fillup it's about 2 bucks more to use premium. So... I figure $100 a year is some pretty cheap preventitive insurance.

So... in short. Run premium. It really doesn't cost that much more. It always looks horrible, but no matter what the prices jump to it's always a 20 cent difference.

One of my co-workers has a 97 GTP or a Regal (I forget which) with a 3800SC. He runs regular unleaded in it and gets crummy mileage and spark rattle.
That would pretty much do it for me.

If you have a way to monitor knock retard, give it a try. That way, if you notice the KR going up, go back to premium. You WILL notice it go up under hard throttle, but for commuting it might be ok.

Personally, I agree with Phil, why take the chance. Here Premium is about $0.20/gal more, so that's $2.00 extra per tank. That's not a lot to insure your engine.

On a 4.9 caddy my grandfather has he runs cheap grade and you can hear the thing ping and rattle. my dad and i drove it a week when we were doing some repair work/tune up we ran high grade and the pinging went away and it seemed to run a lot smooter. It's no a 3800SC. if you are recommened premium use it or midgrade dont go to regular.

If you realllly wanted to save the money and aren't worried about performance, you can pull the supercharger belt off and that'll leave u with a NA 3800 with 8.5:1 (I'm pretty sure) compression, and that'd be perfectly safe to run 87 octane in. On the other hand, the way the supercharger and intake are designed, those rotors are a major obstruction for WOT runs without the SC helping push along.

You could always modify the boost control solenoid to always keep the bypass valve open and reduce the output of the SC.

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- Kyle

Fiero Quarter Mile Database
Fiero Image Gallery

quote Originally posted by Strange Brew: If you want to go strictly by the book....the 97 & 98 Pontiac Bonneville manuals for 3800SC engines say 87 octane. That being said......the choice is yours.

I went out and got my owners manual for my 97 Bonneville SSEi and on page 6-3 it says engine vin code K n/a 3800 use 87 octane, for vin code I ,3800SC use 91 or higher octane

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85 GT 3.4
Auto 3.08
14.9 @ 90 1.9 60"

quote Originally posted by nitro: If you realllly wanted to save the money and aren't worried about performance, you can pull the supercharger belt off and that'll leave u with a NA 3800 with 8.5:1 (I'm pretty sure) compression, and that'd be perfectly safe to run 87 octane in.

Oh man the car will feel like a terd! lol my moms quad 4 grand am was probably just as fast when I didn't have a belt for a few days

Is it safe to detune the L67 to use 87 Oct? I have headers, no cat, no EGR, no codes, open exhaust, 104 Autolite plugs, XP-Cam and a DUB Pulley set from 3", 3.1" to 3.25", and a DHP that reccommends/requires? 93 Oct with an XP Cam flash. Last scan on Premium, I had 1.9 KR at WOT with the 3.25". If I buy and swap a 3.4 for daily driving, would that allow enough slack to use the lower grade stuff for low/no boost driving? Then I'll get the race gas for track time.

Of course I would use a scanner to make sure KR isn't an issue, but since I have a modded PCM... Would I have auto timing advance issues? I think it only advances shift points and timing when the sport shift switch is activated.

PS - I do find the extra $.20 justified, but there will be parts of my job that will require an hour of cruising speed on the highway. No sense wasting available performance with high octaine for Grandma driving, huh?

Thanks,
Greg

quote Originally posted by FieroGTguy: Is it safe to detune the L67 to use 87 Oct? I have headers, no cat, no EGR, no codes, open exhaust, 104 Autolite plugs, XP-Cam and a DUB Pulley set from 3", 3.1" to 3.25", and a DHP that reccommends/requires? 93 Oct with an XP Cam flash. Last scan on Premium, I had 1.9 KR at WOT with the 3.25". If I buy and swap a 3.4 for daily driving, would that allow enough slack to use the lower grade stuff for low/no boost driving? Then I'll get the race gas for track time.

I would guess with the 3.4 that you should at least be able to use 89 with little/no KR

quote Originally posted by FieroGTguy: Of course I would use a scanner to make sure KR isn't an issue, but since I have a modded PCM... Would I have auto timing advance issues? I think it only advances shift points and timing when the sport shift switch is activated. PS - I do find the extra $.20 justified, but there will be parts of my job that will require an hour of cruising speed on the highway. No sense wasting available performance with high octaine for Grandma driving, huh?

What the maximum timing advance that your modded PCM will allow? The max on DHP was 16 degrees last time I heard. I have a DHP in mine (+ other mods but no headers/cam) and I can run a 3.4 with full boost on 89 and 87 with half boost. With full boost on 87 I was good in first but getting 5-6 KR in second though its possible it could be false knock since I have a worn lower engine mount. This was also with tunning using a Mini-afc 2.0 (richen alittle to drop kr) and 70-80 degree weather. Now that it's gotten up to 90+ i've been running 89 with half boost until I can scan it.

I think it only raises the shift points, rev limiter, and allow skip shift when PS is on, everything else is full time.

BTW It seems like I get slightly better mileage on 87 then 92, but it could all be in my head

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Low 14 sec 99 Pontiac GTP, 3.2L V6 4 sp 86 Fiero GT