1968 383 compression with silvolite 1271 pistons

[grizzyboat]

Hi everyone i am new to this site and have the following questions.
I have a 1968 D383 with
906 heads minus 0.10 measuring at 88cc
composite gaskets at 0.40
plus .030 silvolite 1271 flat top cast ch 1.848 pistons
deck ht standard
pistons 0.90 down the hole
standard melling spd-11 cam hydraulic
inlet manifold 2806301
carter E84401s carb
rod length 6.358
windage tray
this seems to come out at at 7.7-1 cr
and 113 psi comp
would this be a good combination for low end torque
Before i stripped the eng it had 150 psi compressions with standard bore and steel shim gaskets
not sure what the ch of the old pistons as the machine shop swaped them over for me and never returned the old piston.

These pistons would have been origional though.
I am sure i held the old piston up to the silolite piston with the wrist pins in line and the comp ht was higher on the silvolites thinking to myself that they have allowed for the composite gaskets.
with the old pistons at 150 psi this works out to 10.2-1
cant work out why if i use the silvolites and a steel shim 0.20 minus 0.10 on the heads 1 can only get 8.00-1
I dont want to go any further untill i find out whever i need to change these silvolite 1271 pistons.
I am aiming to get the best torque out of my jensen.
Also what torque convertor would be best.727 torqueflite
Will be running on unleaded pump gas with hardened exhaust valve seats.
Hope someone can help me on this please thanks Bob

 

[Mopars & Missiles]

Most obvious and quick look answer I see is the 0.090 in the hole. BAD, BAD, BAD, unless you are putting a blower on it.


You really need to have that block squared up and decked to 9.900" to come close to solving this problem. That will put your pistons right at zero deck. Pistons are also not a performance oriented type, but to each his own. Good luck!

 

[lewtot184]

those low compression 383's drive ok but are real power turds. played with a couple of them, very disappointed with performance.

 

[grizzyboat]

Hi again thank you for your reply.
Would it be best to replace the pistons with these and use shim gaskets.[h=1]Speed Pro/TRW Chrysler/Dodge/Plymouth 383 Forged Flat Top Pistons Set/8 +.030" which are 1.920 ch and shave another 0.20 from the heads.[/h]many thanks Bob

 

[Garys1969RR]

Yes those would put you at .022" below deck on the blueprinted deck height of 9.90". Use a closed chamber head of about 80 ccs, .020 steel shim gasket, this will give you decent compression as well as .040" of squish which helps a lot. The 67 and earlier motors had good squish/quench which is one reason they ran so well.

- - - Updated - - -

Your C/R with .022 deck height, .020 gasket, and 80cc heads comes to about 9.8 to 1. 75 cc heads chamber volume puts it at 10.3 to 1. This is more like the way they were built in 1967.

 

[Cranky]

But not with 906 heads. The 67 and earlier engines were still using closed chamber heads. With the 906's your quench side of the head will give you approximately another .080" of quench distance along with the .020 down pistons and .020 gaskets. That's not enough to help a whole lot and if your cam produces anywhere near decent PSI, you may have ping issues.

 

[Garys1969RR]

Yes that's right. The combustion chambers of the 906 heads with a large quench distance can actually add to ping problems. The turbulence created with a tight squish distance will help to reduce ping issues. I had more ping with 906 heads using a dome piston at 12 to 1 C/R, than I have with 915 heads at 13 to 1, And a quench distance of .037". Am currently using a blend of 91 octane premium, 100 octane race gas, and a little E 85, and no ping issues!

 

[grizzyboat]

Hi thanks for all the info, i have a chance to buy some 516 heads 73.5cc closed chamber, would these do the job with 1.74 ex valves and the 1.920 ch pistons.
Would i have problems running the standard cam or would there be a better one to use.
the fuels over here are 95 and 98
Many thanks Bob

 

[Garys1969RR]

Hi thanks for all the info, i have a chance to buy some 516 heads 73.5cc closed chamber, would these do the job with 1.74 ex valves and the 1.920 ch pistons.
Would i have problems running the standard cam or would there be a better one to use.
the fuels over here are 95 and 98
Many thanks Bob

You can determine deck height of piston by adding 1/2 the stroke, plus rod length of 6.358, plus C/H of 1.920. Then subtract that number from your deck height which is 9.90". You want your piston to be .017 to .022 below the deck surface at TDC. Then use the.020 head gasket, and 516 heads. This will give you .037 to .042 squish distance, and will help the engine run better.

- - - Updated - - -

And with 98 octane fuel, that cam should work well.

 

[IQ52]

Damn, a standard block height is 9.98. No way I'd be milling my block some .080". You'd have to mill the intake manifold .100" just to get it to fit.

I'm reading the suggestions over, doing some calculating and think he'd save himself some headaches and be better off if he'd just stick 3 or 4 pounds of C4 under the carburetor and touch it off.

 

[Garys1969RR]

Sorry, I had an incorrect standard deck height for a 383. It is as IQ 52 stated: 9.98" from crankshaft centerline to the top of the deck.

 

[IQ52]

With the 516/73.cc heads and the 1.92 CH pistons and a .02 thick gasket the static compression will be 10.8:1. If you use the SPD-11 cam with that combination the dynamic compression will be 9.5:1 and it will rattle itself to death on pump gas.

Keep everything the same, except use an .039 gasket and you get 10.3:1 and 9.0:1 dynamic.............detonation still.

516/73.5cc, the 1271 pistons and a .020 gasket and you got 9.0:1 and 8.0:1 dynamic and it probably will run on pump gas.

906/88cc, 1.920 CH and the .020 gasket and you're looking at 9.3:1 and 8.3:1 and if you get some bad fuel it's going to knock.

Change the above for a .039 gasket and you're at 8.9:1 and 7.9:1 and you're probably safe.

 

[Garys1969RR]

Just curious as to how much a 383 block deck can be milled safely? Is there a limit of say .100"? Can a 400 or 440 block be milled that much?

 

[IQ52]

Just curious as to how much a 383 block deck can be milled safely? Is there a limit of say .100"? Can a 400 or 440 block be milled that much?

I don't know the answer to that.

This is just my opinion and nobody has to give a damn about it. My problem with extensive block milling is that you bastardize the block so that nothing else fits without modifying those parts too. If you mill a stock block .100" and then mill the heads .060", then the top of the block is almost 1/4" high and you have to mill the intake surface/surfaces of something .197". Better to design or buy the correct piston.

 

[Cranky]

Just curious as to how much a 383 block deck can be milled safely? Is there a limit of say .100"? Can a 400 or 440 block be milled that much?

Depends on how thick the deck is. You can usually see how thick it is though the coolant passages. And I'm with IQ on doing that. A block should never be milled that much but taking off .005 to .010 to clean up is the norm. If you take off .100 (if you can), you even shorten your bolt holes.

 

[68gtx]

how about zero decking a 440 block, does this screw it up for intake manifolds and stuff as well?

 

[Meep-Meep]

7.7:1 CR and 113 PSI of cylinder pressure is an air cooled Volkswagen. In my opinion you want to end up with about 8.5:1 and 150-160 PSI. This will work with 87 R+M/2 assuming the mixture is correct and you have good fuel distribution (proper street intake like a dual plane) and a normal advance curve. Cam should be a mild grind suitable for 8.5:1 CR. Edelbrock makes some good daily driver cams.