spark plug for 383

[fdbdw]

74 350 block rebuilt to 383 stroker 10-1 compression,

I found a thread that stated with 10-1 comp and using accel shorty plugs to use Plug # 0414s and now I am using the ones from my old 350 motor wich are 576s, here are the differences and my questions are, will these be better for my 383, will they fit and what do these specs mean, thank you

0576s- ..460in reach/ heat range 6/ tappered

0414s- .750in reach/ heat range 4/ gasket

[cableguy0]

the cylinder head dictates the spark plug you need to use. what heads are you running on your 383?

[Wild83C10]

Yup just as cableguy said, it depends on the heads. Each manufacture will specify which plug to run. Either look at the instructions that came with the heads or go on the manufactures website and look up your heads.

[Rodms]

What heads are on it?You can buy 3 or 4 for stock chevy heads.

[fdbdw]

76cc 194 big valve heads

[Rodms]

You can use any plugs from the 13/16 to 5/8 depending on how hot you want them,If you go to the parts store and tell them to look up some 350 engines look up 60s vette motor 70s and then back to the r45 plugs in the 70s,I think he big plugs are 44s,I had better luck with them in my old double hump heads.Good luck,I'm going to use NGKs in the dart heads.Also I would think the further you come to valve he better and piston the better off you are

[Captainfab]

Are they the original heads from the '74 350? If so they will need a tapered seat plug.

Originally Posted by fdbdw

76cc 194 big valve heads

[fdbdw]

they are orginal heads, my main concern is, should I be using a hotter plug then the 0576s with the new motor, I do have to use shorty pugs due to my headers, I really appreciate all the help

[Marv D]

The higher the compression the COLDER the plug you would want to run. The heat range is a guide to how well the plug will disperse heat into the head. You want to run as cold of a plug as possible, that will stay clean and not fuel-foul with the poor fuel atomization / dispersion at idle.

If you know for a FACT (as in all components were measured and compression calculated) your at 10:1 Then I'd suggest you run 2 heat ranges colder than a stock 8.x:1 350 for whatever year the heads came from.

But a whole bunch of this is dependant on the cam you choose.

[fdbdw]

cam is #mc2203,
209/216 275/278 .435/.455 107/117
1500 to 4000 , thats interestin cause I thought the hight compression the hotter plug your would want, what heat range should I be using, thank you

[Marv D]

Assuming you truly have 10:1 compression (not just advertised 10:1 pistons in a unknown chamber / deck) .. that cam looks a little on the mild side to me. A wise old engine builder once taught me a simple rule for compression.

For 10:1 mechanical compression, Use a MAXIMUM of 250° @ 0.050"
For 11:1 use a MAXIMUM of 260° @ 0.050"
For 12:1 use 270°..............

Do note that is the MAXIMUM duration for that compression, or in other words it REQUIRES 10:1 compression to support 250° of cam.
If your at 10:1 and only 206° @ 0.050" I bet you building some man cylinder pressure and are having some SERIOUS pinging aren't you?

Backing up a little,,
Compression = heat. The combustion process = heat. Heat is good because heat = power. Thing is too much heat,, and the engine detonates the A/F mixture before the ignition has a chance to. OR, the edges of metal objects in the chamber become so hot they 'glow' and ignite the fuel as it enters. The spark plug is the 'screw-in' object in the chamber that offers the most potential for sharp edges. Threads, the ground strap, the edges of the center electrode, all of this is made of materials that will transfer their heat into the surroundings (the threads in the head that is cooled by the water jacket, and even the slight cooling that is seen by the incoming A/F mixture etc)

SO, if you build a high output, high compression motor you have all this chamber heat to get rid of. You need a COLD plug so the plug doesn't act like a glow plug at high rpm's.

IF you build a low compression motor with too much cam, then you would have issues with the intake valve closing too late, making low compression even lower (remember,, if compression = heat then lack of = less heat) I that case, with the poor mixture of a big cam at idle,,, you would have a plug that is so cold it can't keep clean and get's fouled by fuel at idle. Once loaded up it carbons up, over and over and all of a sudden the plug fails to fire at all.

Low compression, low output motor needs a HOT plug to keep it burning clean. A high compression high output motor you have to keep reducing the heat range to get a plug that can get rid of the heat to it's surroundings.


Here is a very good page on reading plugs to pick heat range.
http://cochise.uia.net/pkelley2/sparkplugreading.html



I don't know exactly what your combination is, but if that 383 was built with flat top 4.030" pistons with a true 1.425" CH (typically advertised as 10:1 having the normal 5-6cc valve reliefs), the block decked 0.010" just to clean it up, a builders 0.040"ish composit head gasket, and the stock 76cc GM heads from the 74 smogger era, you have 9.3:1 compression. (which is still a tad high for that cam IMO,, but that's an 'opinion')


In that case I would say run ONE HEAT RANGER COLDER THAN STOCK plug for the original motor, with a HEI ignition set teh gap to 0.040" instead of 0.045" and yo should be golden. Genreally that would be a Autolite R43T or Champion RJ12

As a BTW,, if you are not 100% POSITIVE of the components and measurements, here's an example of how WRONG you can be in assuming "10:1" pistons are 10:1

In the above, if you did not deck the block that 0.010",,, you only have 9.15:1 compression. If you use some of the KB builders pistons that put the piston as much as 0.040 down the hole,,,, you only have 8.5:1 compression
If those heads were the big 80cc chambers like the 441x head used on 350's and 400's,, you only have 8.2:1 compression.

Make SURE you know exactly what you have as the engine is built, (if you didn't build it ask the builder for a total parts list) and know exactly what the resulting compression is going to be before you assume you have 10:1. Everything from the fuel you run to the spark plug you choose is dependant on it.

[fdbdw]

Great info, thank you