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Discussion Starter · #1 · (Edited)
What is the difference between these plugs? Why is one core further in/out than the other? Which is recomendable for fuelie heads….if it even matters.

I was previously running Autolite 85 and the shop that replaced my pan gasket took it upon himself to change the plugs to Autolite 303 (.035 gap). He said this is a better plug for fuelie heads and because of my cam specs. Also I’m running an Eddie 600.

The car runs a bit better during acceleration but has a stumble at idle.

The pics are of an Autolite 303 and Delco R45S (Equivalent to Autolite 85)

The car ran fine with the 85s but when the car was at the shop they must have flooded it and when they checked the plugs they made the change. I never had an issue with it flooding.

thanks

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@SMO'KUM63
may I first ask what fuel pressure you're running with that Eddie? that's a 1406 if I'm not mistaken and they can't handle more than 5.5-6psi max so anything more than that will cause a massive rainforest inside the carb if it isn't brand spankin' new outta the box... shortly after that, it will start leaking past the seats and flood your motor like a tsunami. It will run rich!!! Been there many times with others trying to dial in the carb to find out they have 9-14 psi at idle... Anyhow, need to make sure your fuel system is set correctly before looking at the plugs however the plugs should be a point of concern as well. what do you have in your motor? compression? cam? timing? headers? ignition system and spark box? lossa stuff to consider first to figure out what heat range you really need or if the plugs you used before are best or if there's a colder/hotter plug to use to make your engine run better.
 

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Discussion Starter · #5 ·
Gents, these are my specs

60s SBC 327 maybe 9.5-10:1
Was new 1406 no reg
Hyd roller .510/.510 221/235 108 sep
Long tube headers
MSD small cap magnetic with 6AL

I’m gunna try the Delcos. They’re the equivalent to the 85s that were on when it ran fine.
 

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Gents, these are my specs

60s SBC 327 maybe 9.5-10:1
Was new 1406 no reg
Hyd roller .510/.510 221/235 108 sep
Long tube headers
MSD small cap magnetic with 6AL

I’m gunna try the Delcos. They’re the equivalent to the 85s that were on when it ran fine.
Should get enough spark with that 6AL so I'm with you and go back as you're saying to use what worked before. Not sure what the guy was thinking by changing to a different & hotter plug. I would still get a regulator for the carb. Cheap enough and will make a difference in tuning for sure but use what worked for you, not what someone else thinks would work better without really trying it out or even asking your permission... Kinda jenky...
 

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Discussion Starter · #7 ·
ok, thanks Stroker. So that 303 plug is a hotter plug?
 

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should be, typically, the higher the porcelain in the plug and the higher the #, the hotter the plug range. Check this site out and it will shed some light on the ranges you can experiment with in whatever brand you choose.


Good explanation and some really helpful info. Just need to know what your engine wants. I usually start cold and work my way up so I don't start with a detonation issue.
 

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Discussion Starter · #9 ·
So according to that link Autolite plugs identify heat range by last number. The current 303s are colder than my previous 85s. Guess my engine prefers a hotter plug. Thanks again Stroker
 

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I'm honestly not sure how they all correlate between a two- digit and a 3-digit for heat ranges between series. It makes sense and that's what the site states but I am unclear whether the heat range is the same from one plug model number to another with the same last digit... Be a good research project to be certain... Hope I helped more than confused :p
 

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Discussion Starter · #11 ·
So according to the link, Autolite plugs identify the heat range by the last digit on the p/n. The current 303 plugs are colder than the previous 85s. Guess my engine prefers a hotter plug.

The engine never stumbled before the plug change, so hopefully putting back the hotter plugs fixes that issue. I don’t think I need a fuel reg tho. Im running a mechanical Edelbrock FP that’s Max 6 psi 110 gph.
 

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Im running a mechanical Edelbrock FP that’s Max 6 psi 110 gph
perfect! Made just for that carb then :) Yeah, go with what you had if it worked. FRIGGIN Some people!! SHEESH! They need to leave your engine alone and just fix what you had taken it in for and only that.
 

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Discussion Starter · #13 ·
So according to the link, Autolite plugs identify the heat range by the last digit on the p/n. The current 303 plugs are colder than the previous 85s. Guess my engine prefers a hotter plug.

The engine never stumbled before the plug change, so hopefully putting back the hotter plugs fixes that issue. I don’t think I need a fuel reg tho. Im running a mechanical Edelbrock FP that’s Max 6 psi 110 gpf.
 

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Discussion Starter · #14 ·
So according to the link, Autolite plugs identify the heat range by the last digit on the p/n. The current 303 plugs are colder than the previous 85s. Guess my engine prefers a hotter plug.

The engine never stumbled before the plug change, so hopefully putting back the hotter plugs fixes that issue. I don’t think I need a fuel reg tho. Im running a mechanical Edelbrock FP that’s Max 6 psi 110 gpf.
 

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I would run the plug style that extends the gap out more. The R45S is a little hotter plug than what came from the factory and to me seems like a good plug to run in a street driven car. Definitely a mistake to change to that 303 plug. I'd go back the R45S or equivalent and run it around for 1/2 hour or so, then send us a pic of the plugs. There are several folks here who are really good at reading plugs.
 

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Discussion Starter · #17 ·
Thanks drm (y) I’ll do that this weekend.

I have to do my homework to better understand plug heat ranges and their purposes. I forgot where I heard that the hotter the plug it can be more dangerous to your engine.
 

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the hotter the plug it can be more dangerous to your engine.
the plug itself does not make more heat, it retains more heat in the combustion chamber based on insulator and electrode placement/stick-out so it can cause detonation by providing an ignition (pre-ignition) source to the entering mixture prior to TDC or °+/- TDC when both valves are closed and the cam is on the base circle, depending on the cam timing and ignition (°advanced) of course. It's a trial and error thing really, depending on your combo but as I stated earlier, start cold and work your way up to a hotter plug. If the car ran fine before start there, make a good hard pass on new plugs then shut off the motor immediately after and pull a plug from each bank. Do not let the engine idle at all. Post pics and we can help read them and give you some info on what to look for as far as lean/rich or in the hunt coloring. :)
 

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so, what else was done to motor at shop? if it did flood out maybe they made some other adjustments to carb, timing, etc? new plugs alone should not cause that much problem with way your motor runs
 

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Discussion Starter · #20 ·
I forgot to mention they also removed the intake to fix some oil leaks so that involved messing with the timing again and maybe carb adjustment.

But, I just got off the phone with MSD tech support and they said that the plugs should be gapped at .045. They are currently at .035.

I’m still going to run the tests that drm and Stroker advised andchange the gap. Hopefully that will get me back to where it was.

thanos again gents
 
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