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Old 07-01-2010, 10:54 AM
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Which Octane To Run?

Looking at this crate motor: http://www.summitracing.com/parts/NAL-12530283/
Which octane would I have to run with it? It's 9.4:1 CR.

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Old 07-01-2010, 11:06 AM
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It will run on 87 octane, if ping is detected, slow the timing down 2 degrees until it`s gone.
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Old 07-01-2010, 11:32 AM
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Cool, thanks. Exactly what I wanted to hear.
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Old 07-01-2010, 11:36 AM
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Would the starter and bolts off an earlier Gen I block work with this?
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Old 07-01-2010, 11:44 AM
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Yes. Make sure they are genuine starter bolts with a shouldered head and a knurled section above the threads. Clean them spotless before using them. a 168 tooth 14 inch flexplate will use a staggard mount starter that will also bolt on.
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Old 07-01-2010, 11:45 AM
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Cool, thanks again.
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Old 07-01-2010, 06:06 PM
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Quote:
Originally Posted by Sinister
Looking at this crate motor: http://www.summitracing.com/parts/NAL-12530283/
Which octane would I have to run with it? It's 9.4:1 CR.
This is the L31 Vortec truck engine, yes with the Vortec heads everybody wants. It will run on 87 octane regular in a production truck of the period 1996-2002. But these are port fuel injected with a computer controlling the ignition timing that has a detonation sensor in the circuit so when pinging is "heard" the timing is automatically retarded to suppress it with out the driver being aware of that happening.

In a carbureted situation with a conventional ignition where there is no computer control of the timing with detonation sensing, the engine may require higher octane, or a richer mixture, or less timing advance to avoid detonation than is optimum for power and economy.

Bogie
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Old 07-01-2010, 07:13 PM
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It might be worth mentioning that with the increase in gas prices, the cost for added octane is a "bargain" if you look at the percentage extra that you pay NOW for premium (about 8%?) over regular, compared to the difference in the two grades 'back when' (about 20%).

How's that for spin doctoring a for-crap situation?
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Old 07-01-2010, 11:09 PM
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Not to hijack your thread but will 89 octane support 9.5 compression?
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Old 07-02-2010, 01:01 AM
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Quote:
Originally Posted by 383_Gladiator
Not to hijack your thread but will 89 octane support 9.5 compression?
its all in the cam and the tune.

Big cam= less octane
Retarded timing = less octane
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Old 07-02-2010, 09:17 AM
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That looks like a good deal on that long block. As mentioned the tune is the determining factor with making these mid compression engines tolerate lower octane. It is a real balancing act, and there are many things factoring in. As I designed my 383 I was hoping to end up with a motor that would be happy at 87, I ended up with 9.45 SCR and 8.58 DCR with a 221 @50 cam using the vortec heads. Looking at the info on your block it is just a guess but with that small of a cam you are going to have a quick intake closing point, running some calcs on the united compression site using say 30 degree's as and intake closing (+ the 15 the site needs) so with 45 this motor will be ruffly the same SCR and DCR as mine. I run a carb and would like to use 16-18 Deg as my base timing that is what my cam wants but up here the motor pings under part throttle load. When I move my timing down to 12-14 BTDC it is way better but then the idle drops and I need to open the base idle adjustment passed the transfer slots which takes away from the idle circuit exposure on my 1405 eddy causing other issues. The long block in this case will live with a lower base timing so who knows but it may just work fine with 87 octane, mine won't right know but I am determined to try everything to get it there. If you can muster it consider the fuel injection route for yours with the mentioned knock sensors etc.

Some times I wish I would have just went with a 10.5 scr flat top piston high horse 383SBC, rather than try and get a mid compression engine to live with the 87, running premium is the price I am now having to pay anyway, I could have built in some more power for the same price knowing this, not that I consider my mid compression 383 a bust, it hauls *** at WOT with no ping.
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Old 07-02-2010, 02:28 PM
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It is a good deal on the longblock, really can't build one for that cheap. With a warranty to boot. I really want to run 87 octane though, might have opt for the shortblock version and build it up from there.
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Old 07-02-2010, 03:41 PM
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why not run higher octane? I don't even put regular in my lawnmower. midgrade would be just fine, and you won't notice the increase. People worry over, "OH! gas just went up 5 cents!". if mid grade is 10 cents more expensive, it's a dollar for every ten gallons. it's not worth not filling with someting better. For an extra two dollars (based on 20 gallons) you can increase your timing just that little bit more for more power, and better fuel efficiency. Believe it or not, you will see better returns on midgrade for the dollar vs reg, but maybe not premium unless you tune for it.
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Old 07-02-2010, 03:50 PM
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This motor is going in a truck. Say I'm driving cross country, out in the middle of bumf@#k Texas, what if they are out of mid grade and premium? There are some small towns out on I-10 out there. I like a motor to be able to run 87 because I can run any grade of gas then.
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Old 07-02-2010, 04:15 PM
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Quote:
Originally Posted by Sinister
This motor is going in a truck. Say I'm driving cross country, out in the middle of bumf@#k Texas, what if they are out of mid grade and premium? There are some small towns out on I-10 out there. I like a motor to be able to run 87 because I can run any grade of gas then.
Carry a couple bottles of octane booster and/or a distributor wrench. The booster will get you by until you can get some decent gas again and if it's really bad pull the timing back a hair temporarily. They used to make a little timing tab that mounted on the distributor so you could make minor adjustments without a light. I haven't looked but I'd guess you can still get them.
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