Author Topic: What the difference between pre 1968 383's and after  (Read 513 times)

Offline DrEamer

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What the difference between pre 1968 383's and after
« on: September 13, 2017 - 01:43:42 am »
I am thinking about building a 383 for my Challenger. I noticed that certain head manufactures note that their heads are not meant for pre 1968 engines. My limited research seems to indicate that the 68 and newer shared heads with the 440. Is that true, or am I missing something else? My idea is to pick up a builder engine, but I don't want to limit myself with an engine that has very little aftermarket support.
Well, I had to start somewhere.




Offline Flatdad

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Re: What the difference between pre 1968 383's and after
« Reply #1 on: September 13, 2017 - 09:27:17 am »
From my limited knowledge: Pre-1968 big blocks had closed chamber heads, so the pistons would have to be down in the hole more to make the same compression as an open chamber head.

I have heard (but not seen first hand) that factory rated compression ratios were optimistic and actual compression was as much as a full point less than rated CR sometimes.

From what I understand, piston availability is pretty limited for 383's compared to 400's and especially 440's.

Good Luck
'70 Road Runner-B7 383 Auto, now with more cubes
'97 Ram 1500 Sport 4x4-Bright Red 318 magnum
'67 Belvedere II-Dark Blue 318 Auto, now owned by my brother
'73 Challenger Rallye-B5 318 Auto, had a 360 in it, SOLD :(
'42 Harley Davidson 42WLA-U.S. army green 45ci flathead
'71 Challenger R/T-B5 383 Auto, now 400 powered and rusty!
'86 W100-Yellow 318-2bbl 4x4 4spd, off road toy

Offline Beekeeper

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Re: What the difference between pre 1968 383's and after
« Reply #2 on: September 13, 2017 - 09:42:15 am »
I've heard that too about the differences. I've built 383s before and been happy. A good machine shop should be able to accomplish what you want. I'm a fan of letting the machine shop order your parts too so that it all comes together well. Any mistakes and it's on the builder. They can do lots of things to get the compression where you want it but a little bit lower like 9:1 is not necessarily bad. The key is using a cam that's designed to work with the compression ratio. My knowledge of cam selection is so basic that again, I allow my machine shop buddy to help me make the right selections for what I want. I just tell him how much power I want, where in the power band I want it, what sort of idle speed and quality I want, how much vacuum at idle I need, etc and he has never let me down.

Too bad you're not on the west coast...I have a nice 400 taking up space. The guy I picked it up from already milled the heads and intake to bring the compression back up to a decent level since the 400s were notoriously low.


Offline cudabob496

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Re: What the difference between pre 1968 383's and after
« Reply #3 on: September 13, 2017 - 11:07:23 pm »
yep, the more cam duration you select, the higher compression ratio you need,
so that cam won't bleed cylinder pressure.
72 Cuda, owned 25 years. 496, with ported Stage VI heads, .625 in solid roller, 254/258 at .050, 3500 stall, 3.91 rear. 850 Holley DP, Reverse manual valve body.

1999 Trans Am, LS1, heads, cam, headers, stall, etc! Love to surprise the rice rockets with this one. They seem so confident, then it's "what the heck just happened?"

Offline DrEamer

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Re: What the difference between pre 1968 383's and after
« Reply #4 on: September 14, 2017 - 11:47:39 pm »


Too bad you're not on the west coast...I have a nice 400 taking up space. The guy I picked it up from already milled the heads and intake to bring the compression back up to a decent level since the 400s were notoriously low.

I actually live on the west coast, but am in Oregon. If I remember one of your post, you are in southern California, which is still a little far. That said I attended a local car show/cruise in tonight. I ran into one of the local known hot rodders who has a hemi pro-street Challenger. After a bit of discussion on various engines, he said that he had a virgin 400 block with a 440 crank that had already been prepped to go into it. He also has a set of low mileage 440 rods to go with the rest of it. He gave me a price which seemed more then fair. It still needs the pistons, but to have a good start on a 451 engine, might make it worth it. :) 
Well, I had to start somewhere.

Offline Beekeeper

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Re: What the difference between pre 1968 383's and after
« Reply #5 on: September 15, 2017 - 11:12:10 pm »
Sweet! Sounds like you're well on your way to a great project.

Offline cjm

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Re: What the difference between pre 1968 383's and after
« Reply #6 on: September 20, 2017 - 12:27:34 pm »
The pre 68 is a completely different block. Although the cubic inches are the same, the bore & stroke are different. Also, the physical positioning of the cylinders are different. I believe that the pre 68's are higher.

Offline moper

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Re: What the difference between pre 1968 383's and after
« Reply #7 on: September 25, 2017 - 08:32:36 pm »
Not sure why the head manufacturers would note any difference. There is no difference that really matters.
There was a late '50s RB 383 rather than B height. Prior to '65 I think the valve covers had 4 bolts and I think '63 had bolt in rocker supports. I think '65 and earlier have a smaller crankshaft hub. Pre '68 use lifters that are smaller diameter radius on the lifter end of the pushrod and the heads are closed chamber until '67.  Some '67-'69 are A body blocks and have a missing ear on the Driver's side, and a tapped boss on the front left of the block. '68 use 906 type open chamber heads and the later radii on the lifters' pushrod seats. All that being said - the main bearing sizes, bore, stroke, rod length, etc are the same. From '68 up pretty much everything interchanges.

Online 1 Wild R/T

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Re: What the difference between pre 1968 383's and after
« Reply #8 on: September 27, 2017 - 03:58:44 am »
The pre 68 is a completely different block. Although the cubic inches are the same, the bore & stroke are different. Also, the physical positioning of the cylinders are different. I believe that the pre 68's are higher.

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Offline cjm

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Re: What the difference between pre 1968 383's and after
« Reply #9 on: October 19, 2017 - 03:22:58 pm »
Tell us more....

The 383 RB Bore 4.03" - Stroke 3.75" , 383 B Bore 4.25 - Stroke 3.375...

Offline 69bcuda

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Re: What the difference between pre 1968 383's and after
« Reply #10 on: October 19, 2017 - 07:32:25 pm »
The 383 RB Bore 4.03" - Stroke 3.75" , 383 B Bore 4.25 - Stroke 3.375...

..........
https://www.allpar.com/history/inside/plymouth-7a.html