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Old 08-19-2013, 04:00 PM
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what fuel should I use?

I have a 383 sbc with two relief flat tops, piston to deck is .010 with 64cc aluminum heads and I think the head gasket is .045 the cam IDs 480 lift 230 duration@.050 292 advertised with 110 lsa. My question is if I need to run a higher octane fuel, around here 91 is the highest at the pump, I have been meeting with the tune a little and at cruise speed and lightly acceleration I can hear a knock but don't notice at wot

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Old 08-19-2013, 05:30 PM
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Figuring max 12 cc for valve reliefs, I come up with 9.9:1 c.r., fine for 91 pump gas.

What's your base and total timing, and when is it all in? Heavy/light car, gear ratio, etc.
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Old 08-19-2013, 05:40 PM
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I guess I thought with two valve relief they were around 3 cc,the car weighs 2400 3.70 gear right now the total timing is 34 initial is 16 all in at 3300 I have been messing with the mechanical had it in by 2800 before
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Old 08-19-2013, 09:08 PM
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All in by 2800 is often too agressive.

A slower more stable curve maxing around 3500 is often better.
A shorter but slower curve allows more base timing. say 20-22- and 32-34 (a 12 ish deg curve)
Vacuum advance rate under cruise has to be adjusted.
limit to a max of 10deg at highest vacuum cruise.
adjust the rate as required.
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Old 08-19-2013, 09:16 PM
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To hear if what you are hearing is actually detonation....or... and help you tune it
build this set of "DIY engine ears" .. You get the amplifier listener #33-1096, and the alligator clip and audio cable w phone jack, from Radio Shack
The "hack" takes about an hour to do.

AutoSpeed - DIY Detonation Detection - Part 2

Last edited by F-BIRD'88; 08-19-2013 at 09:24 PM.
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Old 08-20-2013, 04:53 AM
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I slowed down the timing curve and put the heavy springs in and the black bushings and took a little out if it that's where I ended up with 34 total and seem s to be all in around 33-3400 rpm if I unhook the vacuum advance I can just barley hear it with it hooked up its quite a bit louder.
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Old 08-20-2013, 05:08 AM
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get a bigger advance limit bushing. allows more inital, less totoal.
Then get a handle on the amount and rate of vacuum advance
get an adjustable one.
This is why I always warn about building 383's with flat top pistons combined with 64cc heads.

Chamber volume should be 70+cc for this flat top 383.

Go to a different gas station for gas. They are not all the same.
And many dump 87 octane in the high octane tank, knowing that all modern cars
have EFI and knock management. So they don't knock. No complaints, more profit.
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Old 08-20-2013, 05:14 AM
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will any adjustable vacuum advance work in my msd? there web site doesnt give allot of info on that, and you think the 91 octane will be ok? there is not allot of places to get it around here but I will try other stations. Thanks
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