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Replacing early style plugs on 69 427 390hp heads with suggested R45XLS plugs. Is it just me or does it not appear plug will thread deeper into chamber then old style plug ? Clearance issue ? BTW flat seat on head surface.
Replacing early style plugs on 69 427 390hp heads with suggested R45XLS plugs. Is it just me or does it not appear plug will thread deeper into chamber then old style plug ? Clearance issue ? BTW flat seat on head surface.
Note: deleted and replaced my previous reply as it was pretty messed up copy/paste with errors.
Tapered seat heads/plugs began began appearing in '70; AC Delco plugs with a T in the suffix indicate tapered seat design. The R45XLS would be an appropriate plug for your '69 heads. 43N was the original recommendation from GM for your engine but would be pretty cold plug for today's gasoline.
I have the NGK BPR4ES in my '67 original, stock configuration 390 which is equivalent to the R45XLS/R46XLS and it runs strong and clean.
2025 C3 ('68-'73) of the Year Finalist - Unmodified
2024 C3 of the Year Finalist - Unmodified
2023 C3 of the Year Finalist - Unmodified
Originally Posted by ddt
Replacing early style plugs on 69 427 390hp heads with suggested R45XLS plugs. Is it just me or does it not appear plug will thread deeper into chamber then old style plug ? Clearance issue ? BTW flat seat on head surface.
I'm using R43XLS # 5613522 in mine without any issues !
Have an old unilite with Mallory coil,no vac advance. Switching to HEI and opening gaps to 40-45 ? Then moving on to rich running quadrajet..never ends
427/400 - 100% stock with points / don't see why those wouldn't work on a 390 hp ?
Originally Posted by MelWff
You have a higher compression engine and possibly a different camshaft
I don't think so. The 400hp was the same engine as the 390, just a different intake/carb setup. If you look at build sheets, the order specifies the L36 (390) and adds L68 (400) to that.
Here is a '69 build sheet that shows the L36 as the 2nd line from the top and the L68 5 lines from the bottom.
Last edited by barkingrats; Oct 8, 2023 at 02:50 PM.
Reason: added pics
I don't think so. The 400hp was the same engine as the 390, just a different intake/carb setup. If you look at build sheets, the order specifies the L36 (390) and adds L68 (400) to that.
Here is a '69 build sheet that shows the L36 as the 2nd line from the top and the L68 5 lines from the bottom.
I hate to keep pushing back, but that's not what the specs say for the '67-'69 427s.
The 390/400s (L36/L68) were 10.25:1 while the 435s (L71/L89) were 11.00:1. Below is the AMA doc for '69, both '67 and '68 show the same info. Just like the L89 was an add-on option to the L71 engine assembly, L68 was an add-on to the L36 engine assembly.
I hate to keep pushing back, but that's not what the specs say for the '67-'69 427s.
The 390/400s (L36/L68) were 10.25:1 while the 435s (L71/L89) were 11.00:1. Below is the AMA doc for '69, both '67 and '68 show the same info. Just like the L89 was an add-on option to the L71 engine assembly, L68 was an add-on to the L36 engine assembly.
plugs are determined by;
taper or flat gasket
reach, how far the plug tip goes into head.
this info is based on head number id.
heat range, new gas requires 45 or 46 heat not 43
plug gap, determined by coil output.
points, stock coil supports .035
there are "super" coils that will make points support .045
I do that now on a 70 350.
hei will support .045
Spark plug
