My 383HP Restoration Build

[CompSyn]

I posted this over in the nest. Thought I'd share here too...

The objective was to build 1969 383HP with 906 heads as close to factory specs as possible while utilizing available parts. This was to be an engine restoration for my numbers matching 69 383 Road Runner and not your more typical street/strip build with big cam, headers, stall converter, etc.

Going into it, I never thought the vulnerable Speed Pro L2315NF forged piston was that great of a candidate due to a shorter compression distance of 1.920' and since they are forged, heavier and noisier, right!? Well. check out my actual as-measured numbers below.

One thing to note. The engine block was square-decked which really only amounted to a clean-up pass as the decks were not found to be that far off. Kind of surprising. So squaring up the decks help close the gap of the shorter compression distance of the Speed Pro forged piston. Also, the Speed Pro pistons are 4032 forgings so they run tighter in the bore than 2618 forgings. Speed Pro specifies minimum clearance at 0.0015". Heavy? Not so. When weighing the stock cast piston versus Speed Pro forged piston side-by-side, they were found to be within 6 grams of each other. After shaving some grams for balance work, the difference becomes negligible at best. Best of all. I picked up the Speed Pro pistons from Summit Racing for a reasonable price of $398.99, purchased 4/2020.

I also wanted to help dispel the commonly held belief that the 383 HP, at least in 68 and 69 had a compression ratio of 10:1. Not so. That number was nothing more than marketing hype. Others have reported the same as-measured findings I'm posting below. These engines could not be 10:1 unless they had significant decking and milling work done to them.

Side-by-side, here are the actual as-measured numbers with final calculated compression ratio at the bottom:


Stock 1969 383 HP-------------------383 Rebuilt with Speed Pro pistons L2315NF

Bore: 4.25----------------------------------------------------------------------4.28
Stroke: 3.375-----------------------------------------------------------------3.375
Rod Length: 6.358-------------------------------------------------------------6.358
Gasket Bore: 4.354-----------------------------------------------------------4.441
Comp. Gasket: 0.020-------------------------------------------------------- 0.020
Block Deck Height: 9.978-----------------------------------------------------9.971
Top Ring Down: 0.360---------------------------------------------------------0.380
Head CC's: 87.5----------------------------------------------------------------86.7
Piston CC's: 0-----------------------------------------------------------------------0
Piston-to-Deck: 0.001---------------------------------------------------------0.006
Piston Compression Height: 1.932--------------------------------------------1.920
Piston/Pin Weight: 994.5 Grams-------------------------------------1000.2 Grams

Calculated Compression Ratio: 9.34:1-----------------------------------9.40:1

Note: These were the findings with my specific 383 engine. Your specific production 383 engine may differ.

 

[F4R/T]

Sounds about right on factory specs.. imo

 

[BSB67]

Yep. Nothing surprising. Thanks for posting.

 

[Nate S]

Thanks for posting this. Not many people measure up the factory stuff (or have it to measure). This is a good reference. Did this to a ‘64 Imperial 413 rated at 10:1 and got 9.6:1.

 

[dodge1972]

Thanks for the information. My 69 383 was built last summer. Still waiting to run it on the highway.

 

[Geoff 2]

The original TRW/Speed-Pro/Sealed power pistons were great pistons & underrated.