496 tuning advice needed
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496 tuning advice needed

  1. #1
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    Default 496 tuning advice needed

    I have a 496 BBC with a Holley HP 950, single plane intake, Dart Pro 1 335 aluminum heads, 10.5:1, nitrous/blower solid roller cam... 114LSA... 110CL...255/262 duration @ .050"... .652 intake/exhaust, MSD 85551 dizzy, MSD 6ALN and a Blaster 2 coil.

    I'm looking for some base points to start with for carburetor jetting both primary and secondary, a recommended spark plug... preferably an NGK and what gap running on 91 octane without any NOS? Also, what do you think the maximum safe timing should be?

    Thanks for any help!

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    Some guy obnoxious001's Avatar
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    You sort of left out some important facts, what type of car or boat is this going in, what type of use will it see. You don't plan to run NOS, or will change setup if you do?

    I would probably try a -7 NGK, .035" gap. "Normal" timing for a performance big block Chevy often works best at 34-36 total. Is your carb new? I would start with factory jets and make some test runs until you zero in jetting. Altitude, temperature and humidity all will make some difference.

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    Thanks for the reply. It is in a 19' Winger jet boat. I belive when the engine was originally dyno'd it made the best power at 37 degrees so I will try 36. The carb that's on it now has 88's in the primaries and 96's in the secondaries. Is that jet setting pretty standard?

    I was looking at the NGK BKR6E which is one plug hotter than the NGK R5671A-7 so I will try both.

    Would you change the spark plug gap if the fuel was changed from 91 to 110 or a 50/50 mix?

    My plans are to eventually run a 200 shot so I figure a -8 or -9 plug. Would you recommend the same .035 gap for that scenario? The boat has a separate fuel system for NOS so I will run 110 in that fuel cell for sure.

    Most of my time will be off the bottle so if I have a -8 or -9 in the engine will it be able to keep from fouling the plugs?

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    Senior Member Futs's Avatar
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    for plugs, I would run a 7 or 8 on motor. for NOS start with a 10 once you get the tune up safe, you can creep up on the timing and fuel pressure for the NOS I would also pull about 9 degrees to start with for the NOS. if its happy on pump gas, usually your not going to pick up anything by switching to race fuel but an empty wallet. Save that for when you spray it. 91 on motor and 110 as your enrichment will work good.

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    Quote Originally Posted by drc9805 View Post
    Thanks for the reply. It is in a 19' Winger jet boat. I belive when the engine was originally dyno'd it made the best power at 37 degrees so I will try 36. The carb that's on it now has 88's in the primaries and 96's in the secondaries. Is that jet setting pretty standard?

    I was looking at the NGK BKR6E which is one plug hotter than the NGK R5671A-7 so I will try both.

    Would you change the spark plug gap if the fuel was changed from 91 to 110 or a 50/50 mix?

    My plans are to eventually run a 200 shot so I figure a -8 or -9 plug. Would you recommend the same .035 gap for that scenario? The boat has a separate fuel system for NOS so I will run 110 in that fuel cell for sure.

    Most of my time will be off the bottle so if I have a -8 or -9 in the engine will it be able to keep from fouling the plugs?
    You can try the 8 and see what happens. I normally run them in recreational blown or turbo engines, with 9's getting put in stuff that will be run harder and/or longer. I put 7's in nearly every recreational NA engine that leaves here.

    I will state that I an not a NOS guy. I did learn a bit helping Mike Finnegan at Parker early this year, he runs a bunch more compression, and sprayed it pretty good to run the 6.500 index. I think he ran a 9 plug, but don't quote me on that, with a tighter .025" gap (I did gap some of his plugs so know that to be fact).

    Pay close attention to what you are told about pulling timing out with the NOS.

    If you are going to run this type of engine, you need to be able to read plugs and get your jetting right, particularly if that was not the carb on the dyno. Get some help from a friend if necessary(in person).

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    They have these fancy new sensors, just came out recently 30 years or so ago that tell you exactly what the air / fuel ratio is.

    https://www.ebay.com/i/270979863067?chn=ps&dispItem=1

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    Senior Member Futs's Avatar
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    Quote Originally Posted by Mark55 View Post
    They have these fancy new sensors, just came out recently 30 years or so ago that tell you exactly what the air / fuel ratio is.

    https://www.ebay.com/i/270979863067?chn=ps&dispItem=1
    don't trust those on the bottle. read the plugs, they don't lie !!!

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    steelcomp was here
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    Quote Originally Posted by Mark55 View Post
    They have these fancy new sensors, just came out recently 30 years or so ago that tell you exactly what the air / fuel ratio is.

    https://www.ebay.com/i/270979863067?chn=ps&dispItem=1
    That can get you in real trouble real fast, especially if you're running a N20 plate system. It'll tell you an average on one side of the motor but tells you nothing about individual cylinders.
    If you're too lazy to learn to read plugs, you need another hobby.
    If God is your co-pilot, change seats!
    Acts 2:38, the perfect answer to the perfect question.

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    The goal is definitely to learn how to read plugs correctly instead of hey... it has color... I'm good. I've been reading a lot about it and there's a lo more to it than I ever thought. I bought a flashlight/10X magnifying combo to get a better look.

    I found an NGK chart that listed the recommended plug for a BBC with Dart Pro1 heads and up to 11:1 to be the NGK BKR6E. That's why I had been looking at that plug but it seems most seem to think that's a little too hot.

    Thanks for the warnings about the O2 sensors especially with NOS.

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    Quote Originally Posted by drc9805 View Post
    The goal is definitely to learn how to read plugs correctly instead of hey... it has color... I'm good. I've been reading a lot about it and there's a lo more to it than I ever thought. I bought a flashlight/10X magnifying combo to get a better look.

    I found an NGK chart that listed the recommended plug for a BBC with Dart Pro1 heads and up to 11:1 to be the NGK BKR6E. That's why I had been looking at that plug but it seems most seem to think that's a little too hot.

    Thanks for the warnings about the O2 sensors especially with NOS.
    There is a lot to it, but there does not have to be as much to it.
    The guys that know their shit and write the articles are looking for every last possible hp/ et/ etc.
    You as mr pleasure boater do not need to be there, nor could you be...
    Temp changes, humidity, water grains, elevation... to variable from day to day... not to mention different lakes/ rivers, erc.
    SO... don't be intimidated.
    Wags

    PS... repeat... sensors o2 or egt can and will get you in trouble if you tune on them alone.
    Ask me how I know.
    Last edited by wagspe208; 08-18-2017 at 08:27 PM.

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