383 piston suggestion, help

SgtLee511

SgtLee511

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Guys being a 340 owner, i'm in a new world building a 69 383 rr, my engine is at the machine shop and was magnafluxed, its good. So it had a whole in one of the cylinders, so the shop is going to sleve that cylinder. The shop needs to know what pistons i'm going to use. Lol i hadn't even thought that far ahead, but here i am. So the bird is going to be a resto job, but i want to increase my performance a bit. I plan on using the 906's. Can you guys shoot me a couple of options for camshaft and piston combinations. I dont know at what point i would need the valve reliefs. Id like to keep the compression between 9.5 and 10.0.
 
If your after stock sealed power is about the same weight as the originals. google summit racing compression calculator and fill in the blanks. All the info is on their site as far as gasket thickness, bore of gasket volume of pistons. shop the site and wright down the specifics and plug them in. You will also need the bore and stroke
 
Thanks , called my machinist today, going with a seal power rebuild kit, made the decision to stay stock, mostly......keeping at as near to original as i can. Thanks for imput.
 
Zero deck flat top pistons with valve reliefs is the way to go. Stock rebuilder stuff always comes out with much lower compression than what the factory had originally. Result is poor performance. Today everyone uses the FelPro blue head gaskets that are .043 thick, where as the stock steel shim (which is hard to find) is .015 and the result is near a half point of compression. Then the use of aftermarket valves which are usually low performance replacements where the underside is concave and the seat gets cut and the valve gets sunk into the head a little further equates to an increase in CC's or volume of the combustion chamber. Then you mix in the giant four eye or eye brow valve reliefs on the piston equates to a loss of compression height. Then stock rebuilder pistons are usually made with less deck height due to blocks getting decked and lowering the deck height and the manufacturer of the piston makes up for this with lowering the piston in the bore since the piston needs to clear the head. All this ads up to a decrease in compression of about one full point. So your 9.7 to 1 stock 383 hipo becomes a 8.7 to 1 low performance. This will cause a 10% decrease in hp and torque. So my point is to spend a little more and buy your parts individually and get the better stuff (bearings, rings, pistons, etc) and your engine will be a shining star not an engine that is lack luster.
 
Wildly Optimistic

Here is a quote from JohnRR from the moparts forum:

"To clear up a few things, that have been posted a hundred times at least, the stock 68-69 383 HP piston has a CH of 1.932, on a factory blueprint deck of 9.980 it sits .0025-.003 in the hole. If you build a 69 383HP to NHRA BLUEPRINT you get close to 11.0 compression, the NHRA spec has the piston .021 ABOVE the deck and it uses a 79.5 CC head chamber volume, NEITHER is factory actual build spec. I have an untouched 56K mile 69 383HP in my 69 GTS Dart and this is how it , and another 69 383HP I have on an engine stand, are built as were others I have disassembled with original pistons and uncut decks." -JohnRR
Click to expand...

John goes on to say that a factory a 68/69 383HP would have been "9.2:1 at best". So one could say that the advertised 10:1 compression ratio was wildly optimistic.

A Sealed Power forged 383 piston has a Compression Distance (CH) of 1.920"

So with the the Sealed Power pistons, the blue head gaskets and the typical measured 90-92 CC volume of stock 906 heads, one could expect a compression ratio of 8.75:1.

Add a bigger than stock camshaft, especially a Purple Shaft, and now the 383 feels somewhat anemic.

This post isn't intended to be a knock on 383s. These big blocks can be built into the potent powerhouses we remember as long as we pay close attention to the critical numbers.
 
CompSyn said:
Here is a quote from JohnRR from the moparts forum:



John goes on to say that a factory a 68/69 383HP would have been "9.2:1 at best". So one could say that the advertised 10:1 compression ratio was wildly optimistic.

A Sealed Power forged 383 piston has a Compression Distance (CH) of 1.920"

So with the the Sealed Power pistons, the blue head gaskets and the typical measured 90-92 CC volume of stock 906 heads, one could expect a compression ratio of 8.75:1.

Add a bigger than stock camshaft, especially a Purple Shaft, and now the 383 feels somewhat anemic.

This post isn't intended to be a knock on 383s. These big blocks can be built into the potent powerhouses we remember as long as we pay close attention to the critical numbers.
Click to expand...

I'll agree with that, but I would like to add that everyone forgets about the valve reliefs. The typical reliefs in a race piston are 3 cc and the low performance sealed power reliefs are 5-6 cc. That 5-6 cc will drop the ratio even further. The reason NHRA allows the .021 deck height is due to the thick modern head gaskets and the need for proper quench. Also the NHRA allows up to .019" of offset grinding of the crankshaft which will result in an increase of deck height of .019".
 
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