lower octane fuel=performance drop

[lunarhighway]

in belgium we have a choise of 95 and 98 RON octane fuel

wich according to wikipedia would be the equivalent of 90-91 and 94 in the us
there's a 10 cent difference/liter in price

98 is recomended for my engine, wich is carburated so it isn't going to adjust for anything like modern engines do. but since i recently read in the manual it could also run 95 (with the notice "if nocking occurs see your dealer for engine adjustments..." talk about engine management) i decided i'd give it a try, to see if i haden't been wasteing my money on perhaps a small performance gain. 98 is the most expensive fuel you can get here (around 8$/gallon )

I filled up yesterday with 95 and i suppose theres quite some fuel in the lines as it wasn't untill i came back from work i noticed something changeing... the gaspedal seemd to become spongy... and suddenly it was all the way to the floor with the car barely accelerating like it would otherwise do with less than half the travle. it didn't run exacly bad... it seemed to run smoother but the power and response just wasn't there.

i think i'll refill early this tank with 98 to straighten things out and see what the FE hit is, but it seems from an economic and performance point of view high octane fuel is the best choise for my car.

well...now i know what i'm paying those extra 10 cents for... maybe a 50/50 mixture might work out better... i could find out by filling up with 98 halfway trough the tank.

[Gary Palmer]

I don't think what you described is something due to the gas. The gas might cause the car to ping a little bit, in which case you could probably back the timing off a couple of degrees, but it shouldn't have the performance impact your describing.

I will be interested to see if your performance does come back.

[RoadWarrior]

My parent's old Kadett (Actually a Vauxhall Astra) with the 1.3 motor ran fine on 96 RON unleaded. Also confused as to why you're using a 1.6 carb, generally makes motors a bit "difficult" and usually thirstier when you upsize the carb without having it rejetted for that application.

[guest001]

Originally Posted by Gary Palmer

I don't think what you described is something due to the gas. The gas might cause the car to ping a little bit, in which case you could probably back the timing off a couple of degrees, but it shouldn't have the performance impact your describing.

I will be interested to see if your performance does come back.

I agree. It shouldn't make that big of a difference in power. um, I don't see your car even needing 90-91. hell, I run 87 on a 9.1 compression, maybe a little more cause I have the japan engine. you need a higher octane when you increase compression, which increses heat. what the hells the compression ratio of your car anyway.

all an octane rating is it ability to resist detonation. your car's not going to run better or worse. running premium in a car that doesn't need it isn't going to do ****.

[VetteOwner]

same odd thing with my chevette, says it can run E10 as long as its 87 octane or higher. which ours is. if i fill up with 90 octane(midgrade) it instantly gets 5-10 more mpg!!! it drives the same, might be a bit peppier but barely noticeable.

[lunarhighway]

well it was only my first impression, i'll drive it again today, but there definately was something going on there... i can't think of anything that would have otherwise caused such a behavior, but A-B-A testing should be

the 1.6 carb was just a replacement....the old one had a tiny gasket on one of the regulator screws that was leaky, and the thing just wouldn't tune, this one is the same, exept for the jets maybe, but since it ran much better and got better FE with the new carb i left it as it was.... maybe swapping the jets in from the original carb would do some good

[SD26]

Originally Posted by lunarhighway

in belgium we have a choise of 95 and 98 RON octane fuel

wich according to wikipedia would be the equivalent of 90-91 and 94 in the us
there's a 10 cent difference/liter in price

98 is recomended for my engine, wich is carburated so it isn't going to adjust for anything like modern engines do. but since i recently read in the manual it could also run 95 (with the notice "if nocking occurs see your dealer for engine adjustments..." talk about engine management) i decided i'd give it a try, to see if i haden't been wasteing my money on perhaps a small performance gain. 98 is the most expensive fuel you can get here (around 8$/gallon )

I filled up yesterday with 95 and i suppose theres quite some fuel in the lines as it wasn't untill i came back from work i noticed something changeing... the gaspedal seemd to become spongy... and suddenly it was all the way to the floor with the car barely accelerating like it would otherwise do with less than half the travle. it didn't run exacly bad... it seemed to run smoother but the power and response just wasn't there.

i think i'll refill early this tank with 98 to straighten things out and see what the FE hit is, but it seems from an economic and performance point of view high octane fuel is the best choise for my car.

well...now i know what i'm paying those extra 10 cents for... maybe a 50/50 mixture might work out better... i could find out by filling up with 98 halfway trough the tank.

I'll shoot in the dark as I don't have anything really to see.

But, perhaps, you've got a carbon build up in the combustion chamber. Maybe it's related to the carb, maybe it's related to something else, I don't know.

But as carbon builds up, and builds up in unique places within the combustion chamber, it can change the flame pattern. That increases the "need" for the engine to have a higher octane fuel.

Maybe it just needs to be decarbonized to return the combustion chamber to normal.

[guest001]

though, detonation from carbon build up has to be rare, cause I've never heard it or seen it. the question is what extreme conditions (mechanicly, a/f mixture, pcv, etc) have to occur to build significant carbon build up. or is it gas related. the 10% ethenal in some fuels

[lunarhighway]

maybe.... i've driven the car to work today... it's drivable, but it still needs more throttle to get going like it used to. this morning with a cold engine it was really bad tempered (but my automatic choke is having issues sometimes so that might not be related... unless the 98 would cause the temperature to rise faster wich would in turn make the choke come off sooner)

might running some acetone in the fuel work for the carbon if it would be there.

i've been looking for info on my engine but i can't find anything detailed. all i know is that it's an S version the S versions of opel engines had about +15HP over the normal version with the same displacement, and they where more optimised for high octane, but i don't know what exactly is different... maybe the timeing or something. or maybe the carb wich is a twin barrel. (i think ... pierburg 2E3)

[SD26]

Originally Posted by guest001

though, detonation from carbon build up has to be rare, cause I've never heard it or seen it. the question is what extreme conditions (mechanicly, a/f mixture, pcv, etc) have to occur to build significant carbon build up. or is it gas related. the 10% ethenal in some fuels

When I was in the lawn and garden industry, we saw it a lot.

Similarly, we see it in racing engines with fuels that leave a lot of carbon.

Really has something to do with combustion chamber shapes, fuel additives, lots of variables.

[RoadWarrior]

Ahhh, the S version is high compression.

[VetteOwner]

yea ive done A-B-A-B-A testign before lol it routinely got 5-10 more mpg than previous!

ive had the head off my chevette and theres not much carbon build up at all, i scraped what i could off the piston heads and the head but there wasnt much at all to scrape off...ive rebuilt the carb and used 3 cans of carb cleaner in it blasting out all the passages and the outside

i dont know whats going on...

on btw both fuels ive tried are midgrade+10% ethonal and regular+10% ethonal. so its not the ethonal at all....

[lunarhighway]

well after a fruitless search on the internet for my engines data, i looked in the manual and it's all there! doh!

the normal 1.3 has a compression ratio of 8.2 with 94Nm of torque
there's also a version with 9.0
the 1.3S has a ratio of 9.2 with 101Nm of torque

the S also has slightly better FE figures listed.

9.2 doesn't sound all that high though... what's an average compression ratio?

[RoadWarrior]

no 9.2 isn't all that high.

Hard to compare fuels between here and Europe though, Europe generally uses the RON rating and north america uses the RON+MON/2. RON is a more theoretical octane rating and MON is a rating based on testing.

Actually though with the bigger carb on that motor, you could try running on a high ethanol content fuel, E85 if available.

[lunarhighway]

update:

i've been driveing trough half a tank now.
the car feels less sluggisch than before, but still it's not the same as before.
however things are a bit more complicated.
i get the impression the car has a bit more low rev torque, only when it's up to speed the pedal get's spongy and if you're trying to accelerate uphill you'll end up flooring it.

is there a rational explanation for this? might this have to do with the ignition timeing?

also, but perhaps unrelated, i noticed when the engine was idleing cold, i heard something that sounded like one cilinder was a bit offbeat, it goes away when the engine's warmed up though.... maybe i should check the spark plugs and the cables

[VetteOwner]

is your car partially computer controlled? if it is sometimes they will self adjust and those older computers arent the best at doing this...

[dkjones96]

I'm curious about that too. Is the spark advance on that engine electronic?

I've seen this in our lab testing at work. An engine that requires premium(91) was brought in with regular(86) in the tank and you can see on the dyno graphs where the spark advance is pulling timing. It makes for a nasty looking dyno result. The lower octane won't make that big of a difference at lower throttle or low rpm and will actually benefit under those conditions but you will lose performance under heavy load so if you normally have to run that engine hard you'll feel it.

You did say however that you have to floor the car to do what you used to be able to accomplish at half throttle. But it still could be a case of bad timing. And 9.2 isn't that much at all if it's an Aluminum head engine. If it's an Iron head engine that is high.

[lunarhighway]

no not really. that's why i figured my car would be a good platform to test the effect of different octane fuel, but the difference surprised me. most trials i did with acetone or aeromods where along the line of "i think perhaps somethings going on but it might be me just wanting to feel something"... here i just felt my foot sink trough the pedal suddenly, and the car still wouldn't do what it used to...

the only "advanced" stuff are some vacuum lines and a temperature controlled valve that regulates the intake temperature.... different throttle settings and engine warmup time might upset these things perhaps a little to the car drives different, but i can't imagine these things makeing all that much difference


i think i'll try running most of this tank and than refill with higher octane again to see what my FE did and if the car will change it's behaviour again.

[VetteOwner]

hm if your mpg has taken a sharp nosedive im betting that temp valve in the snorkel that regulates intake temp is gone bad and keep the flap up (closed) so its constantyl usign the air froma round the exhaust manifold as its onyl scource for air. hot air= less dense= lower performace. get the car up ncie and warm, leave it running pop the hood and if you can look inside or feel inside th esnorkel where that dampner flap is it should be all the way down (wide open for cooler outside air).

the chevette did this and it got crap milleage and seemed to take extra oomph to get going. removed that sensor, plugged the vac line and runs like a champ.

[n0rt0npr0]

dkjones96,

Extremely enlightening post. This totally explains the differences of opinion over octanes. And calls out the heavy footers (lol, myself, dam*it!)
Thank you for offering up your experience.
~Will