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Old 12-03-2010, 09:19 AM
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Plug gap help please SBC 400

I picked up this 400sbc casting 3951509 with haed casting 3998997.
My question is what the heck spark plug gap and part # on this thing.
So many diff opinions .045"-.060" The old plugs were AC with a "X" at
the end leaning towards .060" but with used engines who know if they were the right ones. Casting says 74-80. Engine seems slugish and could be plug,
not sure. This is test run cannot drive it cuz its in a project ruck so just running it. I have HEI in her
Help please? or thoughts? experience?
Thanks a ton

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Old 12-03-2010, 10:12 AM
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Gap the plugs at .035" If the plug u are using comes with a wide gap .060 and you gap it down to .035" you will need to shorten trim the ground strap so it does not hang over past the center pin. I recomend Champion RV8c plugs in that head on your 400 for Hi perf street driving (.035" gap) Timing should be 34 to 36deg at max advance. (92 octane)
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Old 12-03-2010, 10:22 AM
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SBC 400 plug# and gap "F-BIRD'88"

thanks for the info. But .035" ??? What is special about the head?
Why so much confusion over the plug # and gap even on google there
is controversy on the 400sbc??????
thanks again BIRD
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Old 12-03-2010, 10:40 AM
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Quote:
Originally Posted by Fastest65
thanks for the info. But .035" ??? What is special about the head?
Why so much confusion over the plug # and gap even on google there
is controversy on the 400sbc??????
thanks again BIRD

Just cause its on google does not mean it is good information.
There is nothing special about those smogger heads.
.035" is the standard-default plug gap for GM V8's. (and it works fine)
Champion RV8c or equal (AC R42T)work well on any of the motors that use a .460" reach spark plug for high perf street driving. (they are just slightly cooler that "stock") (AC-R44TS) Some smog era SBC's use a wide .060" plug gap for emissions purpose. You use .035".
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Old 12-03-2010, 10:47 AM
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400

slugginsh isnt usually a complaint with a 400.go over the time and tune.is there an aftermarket timing timing pointer on there?they are often way off.
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Old 12-03-2010, 11:16 AM
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Quote:
Originally Posted by Fastest65
I picked up this 400sbc casting 3951509 with haed casting 3998997.
My question is what the heck spark plug gap and part # on this thing.
So many diff opinions .045"-.060" The old plugs were AC with a "X" at
the end leaning towards .060" but with used engines who know if they were the right ones. Casting says 74-80. Engine seems slugish and could be plug,
not sure. This is test run cannot drive it cuz its in a project ruck so just running it. I have HEI in her
Help please? or thoughts? experience?
Thanks a ton
I'd suggest you sort out the timing curve and then get a solid tune on the carb.

With the truck sitting still, it will run the same w/the plugs gapped anywhere from 0.060" to 0.030" if the plugs, wires, cap, rotor, coil and module are all in good working order, unless the cam has a lot of overlap. What I'm getting at is the "sluggishness" isn't caused by the plug gap, unless it's far outside of the norm.

If you're using a used OEM distributor, be sure the advance plate is free to turn easily- they will have dry bushings after a long life, and can need cleaned and relubricated.

THIS is a description of an HEI rebuild.

Another thing you can experiment with once you're up and driving is what vacuum source you use for the vacuum advance. There are different adjustable vacuum cans and vacuum advance limiter plates that can be used to tailor the ignition to your engine.

The ACCEL #31035 is an adjustable vacuum advance can for GM HEI that allows adjustment to the amount and rate of advance. Comes w/instructions and tool.
Accel #31034/31035 Adjustable Vacuum Advance Can Instructions:#31034/31035 Instructions. Crane also sells an adjustable can- Crane #99600-1, 99600-1 INSTRUCTIONS..

Usually, around 10-12 of vacuum advance is about right for a performance engine's timing set-up. Most OEM vacuum advance cans will have more than this, though, so to limit the total amount of vacuum advance w/an HEI-type distributor, you will need to physically limit the vacuum advance can's travel w/a vacuum advance limiter plate like the Crane #99619-1, #99619 Instructions. Or you can easily make one.

DETERMINING TDC will allow you to be sure the timing tab and damper are correct for TDC.

MAKE A TIMING TAPE to see what the total timing is, w/o needing to use a dial back timing light.

Last edited by cobalt327; 12-03-2010 at 11:29 AM.
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Old 12-03-2010, 11:22 AM
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400 SBC Sluggish Plugs??? Timing??

Thanks 70060EPRO
This engine is reman so I assuming .030 over. with
an alleged 23-23.5K on it. It is was clean in the lifter
valley and clean under the valve covers, Been sitting
so I primed the oil p[ump with my crappy cordless and it
had trouble spinning it. Any way fired it first pop I was suprised
Fired it like 4 times now and let her run. Oil pressure is up around
55-60psi. Cannot tell right now on the timing gauge, but looks stock.
I just brought it up on #1 and ploped the distributer in. Just seems slugish
to me, revs great, idles kind of weird like no compression, but no blow by,
great oil pres revs good. This is why I was leaning towards plugs/gap or timing, NOT SURE actually??
Thanks again for help
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Old 12-03-2010, 01:39 PM
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In the old days when using points before high energy ignition was introduced it was .035 on the gap. With the use of H.E.I. and the use of MSD CD or equivalent ignition system you can increase the gap .04 to .05 gap.
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Old 12-03-2010, 02:10 PM
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400

most of the problems we see here like that are tuning issues.mainly a distributor curve thats meant to run a vacuum advance with the advance disconnected.id bet when you dial it all in itl wake up-----billy
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Old 12-03-2010, 02:19 PM
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70060EPRO thanks More 400sbc ?'s

Not sure what you meant?? you mean plug the vacuum hose
from carb base to vacuum advance on distributer and leave un
hooked? I was told set the timing unhooked then hooked it up
with a total timing of around 34 so adjust accordingly.
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Old 12-03-2010, 02:41 PM
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timing

no,you would un hook the vac can and plug the carb port temporarily to check initial timing.a timing tape on the balancer or a dial back timing light.you would first see how much mechanical advance you have.stock is usually 20 or 22.if you could find this out say its adding 22 mechanical then you want to set the idle initial timing at 12 or so add those two together and you have 34 total initial plus mechanical.this is safe and close to where you want to start on older v8.ok with a timing light with it set at 12 at idle hook the vac advance up and point the light there.if you dont have a gegreed balancer just look and see if the mark on the balancer moved about three quarters to an inch when you hooked the vac up.if it didnt you might be connected to ported.move it to manifold vac and look for movement again.if your not you have a bad vac can and just found the problem.if you do get vac advance when its hooked up your in business. this is about as close as your goin to get without a dial back timing light or timing tape.for a quick check to see if the timing is late,turn the distributor counter clockwise about a half inch and snug it down.gently test drive it and see if it responds better,if it does you know your headin in the right direction.---billy
if it knocks or pings get it back home and go back clockwise a quarter.
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Old 12-03-2010, 02:54 PM
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70060EPRO thanks ?'s ?'s sbc 400

before I threw the distributer in I put my mouth around the
vacuum diaphram and could not move it by drawing in. Almost
think it is N/G. To check the ports at the carb base..manifold vacuum is
full time and ported it only when you crack the throttle??? It does sounds like something is up with the timing, carb, plugs now that I hear you passing info.
thanks again....dam thing..
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Old 12-03-2010, 03:17 PM
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GM backed off that big gap (.050-.060) years ago and recommended .045 as plug life suffered. .035 really won't take advantage of the HEI ignition with such a small gap. If your engine is stock (cam and comp. ratio) you don't want to go any colder on the heat range than 1 step. On my 350 the factory plug recommendation was AC R45T's and I changed to R44T's with no problem.
R44t's have a recessed insulators and if they run a little dark after a while you might try R44TS that have an extended tip that run a little warmer and stay cleaner. It's up to you and trial and error to find what runs best.
I don't think anyone on here has put an engine in and not changed the tune over time unless they got extremely lucky! Besides, who ever stood back and said "good enough, I'm done with it"
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Old 12-03-2010, 03:25 PM
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400 SBC Sluggish Plugs??? Timing?? SQZBOX

Yup, I agree. dad used to say "good enough, isn't"
Someone on here said to run .035" in this thing knowing
I have HEI...didn't put too much stiock in that. I think I
have R44t or ts in there now at .045" so I must have a timing
issue(s) seems like it wants to run harder but is being held back.
Thing is I can't drive it, just run it..no rearends in her right now.
Thanks
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Old 12-03-2010, 03:58 PM
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Quote:
Originally Posted by Fastest65
I picked up this 400sbc casting 3951509 with haed casting 3998997.
My question is what the heck spark plug gap and part # on this thing.
So many diff opinions .045"-.060" The old plugs were AC with a "X" at
the end leaning towards .060" but with used engines who know if they were the right ones. Casting says 74-80. Engine seems slugish and could be plug,
not sure. This is test run cannot drive it cuz its in a project ruck so just running it. I have HEI in her
Help please? or thoughts? experience?
Thanks a ton
The power of the ignition determines the tolerable spark gap at the plug, the head has nothing to with that. Heat range would be a function of the head based on how much cooling it can deliver and what the compression pressures are. More compression or engine temperature assumes more temperature the plug has to manage which would lead to a colder plug to get rid of its operating temp faster and the converse for a lower pressure engine. A colder running engine or one that pumps a lot of oil through the combustion chamber because of bad rings or guide seals needs a hotter plug to keep from fouling the electrodes.

When HEI first came out it was thought that wider gaps needed to be used especially with the low compression, low density mixtures of the smog engines of the era where it was though that the wider gap meant a longer length of spark that stood a better chance of lighting these mixture. This proved to be untrue and gap recommendations went back to something in the range of .035 inch.

Bogie
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