Interesting Octane observation...

[WS6 VERT]

They say you shouldn't use a higher octane rating than needed, and I think I found out why.

Ever since I put my "hi-po" motor in my car, I automaticly assumed I was going to need 93 Premium gas. Well, recently, my car has been hard to start, almose like when millican's fuel pump took a crap. I would not start, even after three attempts of extended cranking. So next time it needed gas, I put 89 Octane in it, and now it fires right up again.

I just figured that would be interesting...

[RacerX]

Sounds like your car isnt high performance enough.... :D

[RobP]

Back a thousand moons ago when the ZZ4TPI was running right I loaded it up with 89 octane. Ouch, that 10.5:1 compression ration ZZ4 did not like it. The first time I ever saw timing pulled by the computer.
My Impala has been pinging more and more with the summer heat. I put 5 gal of 89 in and the other 13 gal of 87. She has all but stopped pinging. Of course people at the pumps think I'm crazy!

[SportCoupe]

I think you're crazy too. A 18 gallon tank?!
Your nuts. smile.gif

[LL6]

BT might it not have been water/bad gas that was causing the hard start?

[SportCoupe]

By using the 89 instead of the 92 have you hooked up a scan tool and checked for knock counts/retard?

[WS6 VERT]

I havn't checked for pinging w/ a scantool yet. Even though I don't see why I would. Remember, I'm only running aprox 9.2:1 CR, and I don't have any wild timing.

It may have been a bad batch of gas, but remember, octane rating is directly proportional to the volitility of gas. The higher the octane, the harder it is to burn, and I have heard stories of "hard starts" on cars that had premium gas that were designed for regular. I guess I'd have to keep playing around to know for sure...

[Zelda84]

So while we are on the subject...the 350 in my 84 is running like a bear! Its an animal compaired to the 305.

So here is my fuel question. My 305HO required high test gas of 93 oct. Now the 350 engine according to the specs I found was an 8.3:1 engine new. It was supposed to run on regular. I have been putting in the high octane gas figuring with the bigger cam and better heads I got my compression up to about 9:1. Do you guys think I should maybe run mine at 89 octane instead? Or stay with the 93?
With the gas prices here of $1.44 for 87 $1.48 for 89 and $1.55 for 93. graemlins/sick.gif :eek:
I dont want to be wasting my money by filling up with 93 if the engine wont perform any better with it. Remember I'm running carb vrs your EFI.

[ May 30, 2002, 08:33: Message edited by: Zelda84 ]

[Pantera61]

If your mods are coming up to the stock 350's Timing/Cam/etc. then the octane rating on the stock on was 91. I use 93, considering no one makes 91 except for 7-11. I sometimes see unburned fuel out of the exhaust when I start it. Just an idea, I would try and scan it to see what it is saying.

[ragtop88]

I thought you went with the 76cc chamber heads? If thats the case you probably lowered your compression ratio as the stock Vortec heads are 64cc chambers. Cam timing will affect the dynamic compression ratio but typically the tighter LSA and more duration will actually bleed off cylinder pressure at low RPMs. If you are using the 76cc heads I'd bet you could run 87 octane.

[starbuck]

my car runs like pooie on 89...it tends to sputter a little off idle.

[RobP]

I've also kinda checked MPG on different octane fuels. I've always gotten better mpg the higher the octane. But the question is does the added MPG outweigh the added cost?

[86IROC]

Bernard, the ill - fated 355 TPI motor that was in my car a few years back had pistons that were supposed to be 9.8:1 w/ 64cc heads. I had Performer RPM 70cc heads, and the machine shop obviously didn't deck the block much. The result....car ran way stronger on 89 octane. I think that motor was maybe 8.8:1 CR tops. With aluminum heads you generally use the rule of thumb - increase the CR by one point over stock...roughly, give or take. That 355 I had was screaming for forced induction with that low CR. Too bad it only lasted 400 miles and ran a dismal 13.7 @ 100.01 on a 1.95 60 ft. :mad:

Anyways, my long-winded post just implies that I think w/ the 9.2:1 CR you have, you just don't create enough heat in the chambers to burn higher octane fuel. I think it's time to rob an Eaton blower off one of the cars from work. :D

[Pantera61]

ragtop88<--
Who where you writing that too? A long time ago I was planning on changing to vortec heads and stuff, but have changed to am LT1 with LT4 conversion. I guess that you are talking to bernard.

[MrSanRamon]

RobP & DenaliTech...

I read somewhere, a long time ago, that the specific energy content of gasoline was higher in regular than in ethyl.

I may see if I can climb up in the attic where that textbook is being stored. It's old, published in the 60's when gasoline was a completely different blend though, so it may not be worth the effort.

Currently, I believe that the oxygenates contained in gasoline cause the same issue as lead did in the 60's. Since oxygenates contain oxygen they do not provide energy content in the fuel, thus oxygenates will significantly lower the fuels' energy contents. Oxygenates are added to provide octane, not energy.

So, ones mileage should actually be better with the lowest octane fuel that will run in it. If one gets better mileage with a higher octane blend, then the engine may need that octane level to burn completely (with out minor detonation). But, it’s difficult to actually compare the mileage of one tank of gas to another.

As for starting, the lower volatility of the higher-octane blends could directly cause starting problems...harder to ignite. Or, they may contain compounds that, in lower CR engines, are not completely burned off during the combustion process and slowly start coating the combustion chamber and plugs...just a supposition on my part though.

Bob

[ragtop88]

Pantera, actually my comments were to Zelda....sorry for the confusion