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Highest Compression for 87 Octane?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by bigblock454, Sep 8, 2002.

  1. bigblock454

    bigblock454 Clack Clack Clack Premium Member

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    How much can I get away with? Is 9.0 to high?
     
  2. dyeager535

    dyeager535 1 ton status Premium Member

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    There are many variables that figure in. Of course compression ratio, but combustion chamber and piston design are two very large factors.

    The 305 was running 9.2:1 on 87 back in the early-mid 80's, with the crappy heads...

    You can probably come up with a good guesstimate of how high you can go, but finding a hard number is likely going to be pretty tough.

    I'd be willing to say that 9:1 should be pretty much attainable with any heads, as long as the pistons were standard flat or "dished" pistons designed for 9:1.

    Of course, you'll need to watch timing and what temp the engine runs, but thats pretty much a given with any motor.
     
  3. wrathORC

    wrathORC 1/2 ton status

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    Run 89 octane and don't worry about it. The extra nickel per gallon is worth it.

    87 octane on a 8.5:1 motor will ping like a dryer filled with quarters if it gets hot and is ran hard. Run 9:1 and 87 octane on a hot day and it's guaranteed to ping unless you run 4° of timing and have a weak timing curve. Of course, electronic spark control with a knock sensor will eliminate this problem by retarding the timing. This, of course, robs horsepower and toys with fuel economy. AKA gutless wonder high compression 305s found in trucks.

    In theory, if you keep the motor cool and drive with the secondaries wired shut you should be alright with 87 octane. Anyone trying to get performance out of 87 octane fuel should have their head examined. There's a reason why people call it 87 octane and not 13 heptane. Nobody likes heptane.
     
  4. xnoahx

    xnoahx 1/2 ton status

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    Yeah, dont worry about getting by on 87, its not much more to put in 89 or 91. After I put vortec heads on my truck ive used mainly 91. the few times I have gone cheaper ive noticed a difference in drivability and with the lower grades it would diesel more often. Then there was one time i accidently put about half a tank of diesel in my truck, that really was some hard drivin /forums/images/icons/shocked.gif
     
  5. dyeager535

    dyeager535 1 ton status Premium Member

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    Higher octane without more advanced timing and/or higher compression is a waste of money, and may HURT power. I know you covered the timing part, but that isn't true either. I didn't check timing on my 305 (again, 9.2:1) with a timing light, and advanced timing as far as it would go without pinging under load still starting fairly easy, and NEVER had a problem with pinging either. No knock sensor.

    Anyone that can't keep their engine cool enough to run 9:1 compression on 87 should have their head examined.

    That extra nickel per gallon is $1.50 a tank in my case, which is another whole gallon. If I'm not getting better economy (which would mean more power for the same amount of fuel, and my own testing has proven that 89 gives me nothing over 87, except a lighter wallet) then I'm throwing that money away.

    Theres a reason the government went after gas companies for trying to encourage people to use more expensive higher octane fuel, in vehicles that were not designed for it, and don't get any benefit from it. Gas companies were implying that higher octane was better for all, when in fact its not.

    Newer vehicles (and yes knock sensors help) run high compression on 87 because of efficient design. It will be harder to attain the same results with older combustion chamber design, but it doesn't mean that 9:1 means 89 octane in all cases.
     
  6. HarryH3

    HarryH3 1 ton status Author

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    Wow, mid-grade only costs a nickel extra there? At most stations in Colorado it will set you back an extra 15 cents per gallon. Hi-grade is 25 cents per gallon extra. The real pisser is that they sell 85 octane at this altitude as "regular", so they sell 87 as mid-grade and screw us royally for the "privilege" of buying what is regular in most places. /forums/images/icons/mad.gif In these parts you get:

    Regular 85 Octane
    Mid-Grade 87 Octane
    Premium 91 Octane

    But our "regular" is priced about the same as the 87 octane "regular" in the rest of the country.
     
  7. wrathORC

    wrathORC 1/2 ton status

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    Economy 86
    Regular 87
    Premium 89
    Super 91/92
    Ultra 93/94

    Economy is 8-10 cents cheaper than regular. However, you might as well run kerosene in your motor if you're going to do that. "Regular" smells really bad compared to the others. Premium is 5-7 cents a gallon more expensive than regular. Super is another ten cents on top of that and if a place offers ultra then it's yet another ten cents higher yet.

    I pay 1.66/gal for 89. Gas prices here haven't changed since the middle of March. Some people might call it price fixing but I call it anal rape. Where I used to live 89 octane now is 1.32/gal.

    Where you're at (if you're in the mountains) the pressure is less so heptane (which combusts really easy under pressure and temperature) doesn't combust as readily. I think it's just the gas companies being cheapskates.
     
  8. wrathORC

    wrathORC 1/2 ton status

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    All truck 305s were high compression (in other words, 58cc heads and 9.2:1 compression). All of them that I'm aware of ran a "recommended minimum" of 87 octane. I know at least in 84 they had knock sensors.

    On a cold winter day my 305 would spark knock if I put the pedal to the floor. It went away with 89 octane. The only reason I tried 87 octane is because I foolishly believe somebody here on CK5 that I wouldn't notice any difference. It wouldn't even get out of its own way (not that it could before with a 140 horse 305 moving 4400lbs) after a tank and a half with the 87. It took another two tanks before I got gas mileage and horsepower back.

    Every vehicle I have driven has seen an improvement in fuel economy over 87 when running 89. 1mpg when you're only getting 10-12 is a big difference. It pays for itself. The extra horsepower and the less wrist pin wear and tear is a bonus. I have never experienced an improvement in fuel economy with any octane greater than that with a low compression motor (below 10.5). Speaking stock engines, unless you're running a F-body then there is no reason to use 92 octane.

    Perhaps in places other than Michigan it is different and the two grades are closer to equal. But if you smell or taste 87 octane it smells and tastes completely different than 89 octane.

    If you were to take a newer motor (say, yank one out of a 98 truck) and throw a carb on there and a distributor without a knock sensor you're going to experience the same problem. You could cheat and use a set of heads that cooled better (ie aluminum) so then the heptane doesn't ignite as easily or a set of spark plugs that run cold but you can do the same with any motor.
     
  9. pr1aw

    pr1aw 1/2 ton status

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    Its what you particular engine will handle. How good is your cooling system, how bad are the springs in your distributor, there are alot of factors. In my Toy I was running close to 11 to 1 comp (195-205psi per cylinder) on 87 with no pining. If your engine will run on 87 without pinging I would run that. The lower the octane the more BTU(energy) you'll get out of the burn (power). Pinging is pre-detonation, lighting off of the fuel/air mixture before the spark happens. I would check your timing and then run a quality carbon remover in your next tank of gas.
     
  10. TIGHTWORK82 K5

    TIGHTWORK82 K5 1/2 ton status

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    how about 110 would it burn up the heads in a 305. or pistons?
     
  11. Pure Insanity

    Pure Insanity 1/2 ton status

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    Just the opposite. It wont run as well on 110.

    Octane rating is the resistance to burn. The higher octane will not burn as efficiently in something w/ lower comp. Thats why high comp. race cars run High octane racing fuel. If you put 87 oc. w/ a 13.5 to 1 motor it practically wouldnt need plugs. Exageration of course but you get the point.

    Whats diesel fuel, like 40-60 octane I think. I just seen it the other day but dont recall.

    Real world example. When I bought a new car back in 95 I ran nothing but 91 in it. (The owners manual suggested 87) After a few months I heard what I just said. I dumped 87 in it and it ran better! Not to mention saving 20 cents a gallon.

    PI has a 9.5 to 1 motor w/ Vortec heads. I run 87 in it. I wish I could say what the timing is set at but I have no clue. When I "set by ear" and use a Snapon light w/ advance its in the high 40s low 50s advance. Needless to say I just set by ear. Next time I pull the motor apart Ill get another timing cover and balancer so I know I can use them. All I can figure is I have a mismatched set.
     
  12. BowtieBlazer

    BowtieBlazer 1/2 ton status

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    I run 87 octane in all my 350's with no problem. I just buy quality gas. I use Chevron's or Shell stations and nothign else. I also use the busier stations to ensure I am getting fresher gas than I may get from the small town station that the tanker visits once a month rather than 3 times a week. Every now and then I will dump in a bottle of gas treatment. Nothing special about my 350 w/195 thermostat
     
  13. dyeager535

    dyeager535 1 ton status Premium Member

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  14. Blue85

    Blue85 Troll Premium Member

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    </font><blockquote><font class="small">In reply to:</font><hr />
    But if you smell or taste 87 octane it smells and tastes completely different than 89 octane.

    [/ QUOTE ]

    I was wondering if you were crazy saying that 8.5:1 is questionable on 87 octane, but now I see what has made you crazy: sniffing gas and injesting petroleum.

    If an engine is worn out of screwed up, it may require more octane, but that's too hard to classify. A newish 350 near factory compression will run fine on 87 octane unless you have the timing advanced beyond normal.
     

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