lower octane fuel=performance drop

[GasSavers_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?

[GasSavers_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