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hicomp
Dodge Dakota
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10/23/2008
09:13:03
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 Subject: Which Octane?
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Im building a motor right now from the ground up and i plan on running it higher compression than stock instead of boring it and stroking it. At which point do i need to switch from 87 octane, to 89, to 91, to 93, to race gas(or octane boosters if you trust them)
I dont have a target compression ratio yet, so im trying to figure out at which compression ratios you have to use which octane gas levels
thanks for the help and advice, i cant seem to find anything online
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.boB
Dodge Dakota
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10/23/2008
12:07:10
| RE: Which Octane?
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What's important is cylinder pressure. Static compression ratio plays a big role in that, but so do a number of other facters. I have a car with almost 11:1 compression, but it runs really well on 87 octane. For pump gas, you should aim for less than 195psi.
RBRracing.com has a tech page with formulas to calculate cylinder pressures. It's a HArley site, but the calculater works for V8's, too.
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hicomp
Dodge Dakota
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10/23/2008
14:39:13
| RE: Which Octane?
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well im planning on probably having a comp ratio of about 11.5-12 maybe a little bit higher, so i should be fine with pump gas im assuming, but ill check out that site, thanks bob
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daddio
Dodge Dakota
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10/23/2008
15:06:32
| RE: Which Octane?
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aluminum heads will be less susceptible to pinging than iron. if you're going that route.
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Kowalski
GenIII
10/23/2008
18:17:32
| RE: Which Octane?
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Good point, I think aluminum heads are often worth a full number in the ratio before needing higher octane. Other variables include the amount of ignition advance, the final drive ratio, and the weight of the vehicle. You really have to look at it as a complete package; rather than the octane requirement being dictated by any one given compression ratio.
Lead, follow, or get out of the way
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Mikey
Dodge Dakota
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10/23/2008
18:48:16
| RE: Which Octane?
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It is certainly better to start with an octane rating that is too than too low. You can always work downward by filling up with the next ower grade when the tank is half empty. The lowest useable octane rating will usually give the best performance.
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dakota49
Gen I
11/23/2008
21:04:46
| RE: Which Octane?
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Some engines with high compression can use a slightly lower octane because of the cam having long duration (meaning the valves are held open longer than stock, and both are open at the same time valves that is) so in some cases if you were to put an RV cam in an engine that ran fine with 87 octane, it can ping with an RV cam. That can happen because the intake valve opens sonner give the cly. more time to fill with air/fuel, but it really helps with low RPM torque. So I would say the cam would be the deciding factor though compression ratio is important as well.
Just generally speaking, iron heads try to stay below 10.5 to 1, alum. 11 to 1.
Good Luck
Dakota49
1988 3.9 4X4 5speed
dakota49
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