Is my low compression due to low CR pistons?

[Yellow Submarine]

I have posted on this problem previously but from a different perspective. I have a 383 that was rebuilt in 94' according to the paperwork I received with the car. The car was not driven until I purchased it. The trip meter now shows a little over 100 miles. After working through some minor issues it now runs and idles well.
The concern is the compression tests I did show 120 psi +\-3psi. This is cold, no oil squirted in the bore, all plugs pulled, carb open and allowed to crank 3-4 revs till it did not go up any more. I also checked the gauge towards a known motor So I believe it is reasonably accurate. It has been suggested that this may be due to the cam being installed one tooth off or being ground wrong. The cam is of unknown specs but the engine rebuilder had it reground presumably using the origional as a blank. The cam seems very mild. Nothing else on the engine in aftermarket. Heads are 906.
Today I came across some online info saying many rebuilds of 383s end up with low compression (8:1-8.5:1) pistons. According to an estimation table (I know there's many factors) the compression of 8:1 corresponds to 117 psi. Given that, is it worth the effort of removing the engine timing cover to check the cam timing marks?

John

[1 Wild R/T]

I believe you have found your problem.... Easiest fix?  Cylinder heads with small chambers... I'm guessing you have 906's now?  About 90-92cc chambers...  75cc chambers should put you around 9.5-1...

[Chryco Psycho]

If it starts runs & Idles well the cam cannot be far off . compression may be your issue  120 psi is low

[Yellow Submarine]

Too bad the guy went through the expense of a complete rebuild only to have low cr pistons
installed. This is not the original block either. Thinking of just changing the intake and putting a bigger carb on it for now. Later, Maybe try to find an affordable 440 short block with higher cr and swap the fresh 906 heads to it
Or just get a stroked RB.
At any rate thanks for the help and input.
John

[1 Wild R/T]

The problem is in 94 there were no decent pistons available... The old TRW forged dome was 11.5-1 otherwise all you could get was 8-1.... I built the 383 in my Challenger in 95, I postponed the build waiting for a new piston that was supposed to be coming out for the 383 but after 7 months I needed to build it to stay on track... The promised new piston showed up around 2003....  I took a set of 915 heads & cut them .050 to get the chambers down to 76cc...

[Yellow Submarine]

RT , was the effort with the 915 heads worth it? I don't know what the flow is compared to the 906s. Trade off flow vs cr, which is best?

[1 Wild R/T]

915's & 906's flow about the same, 516's flow less... There are plenty of guys who have used 516's & run faster do to the compression gain... These days I'd probably order these....

http://www.summitracing.com/parts/tfs-61617801-c00/overview/make/dodge

78 cc will put you around 9.25-1, better flow than either the 906 or 915 & probably 75 lbs off the nose of the car....

[Yellow Submarine]

 Since the rebuild is fresh, how about replacing the pistons with higher compression ones if I can find at a reasonable price. It would be more labor but less expense than new heads particularly than Trick-Flow.

[dodj]

Or it is set up nicely for supercharging....

[1 Wild R/T]

Since the rebuild is fresh, how about replacing the pistons with higher compression ones if I can find at a reasonable price. It would be more labor but less expense than new heads particularly than Trick-Flow.

Problem is you'll only gain about a half a point....  There still are no great piston choices unless you order a custom set....  Do what I did get some 915's or 516's & cut them down to 72-75cc use steel shim gaskets & realize it's your best option...

[moper]

The problem is even the good pistons will be down in the bore unless the block was blueprinted when it was machined. 120psi is not that low all things considered. I've seen well worn others pumping 115psi that ran just fine.
I'd say replace the camshaft especially if it's an MP version of the factory 4bbl cam. I wouldn't bother tearing it all apart to replace pistons, requiring rebalancing, etc. just for that extra .5 point. Stick a little better cam in it and if it doesn't have them, run the thinnest head gaskets you can find and have fun.

[Yellow Submarine]

Thank you all for you insight and information. For now I will leave it as is. When I dissasemble the car for paint, I will probably change the cam to one that maximizes cylinder filling while minimizing agrivation of the low cr, if that's possible. As I said before, I have no idea of the current specs other than it is mild.

Thanks
John

[734406pk]

Check into a Hugh's Engine "Whiplash" series cam. They work well well with low CR engines and sound absolutely "Old school nasty"!

[Yellow Submarine]

Thanks for that 73. Ill look into that when the time comes

[ted]

915's & 906's flow about the same, 516's flow less... There are plenty of guys who have used 516's & run faster do to the compression gain... These days I'd probably order these....

http://www.summitracing.com/parts/tfs-61617801-c00/overview/make/dodge

78 cc will put you around 9.25-1, better flow than either the 906 or 915 & probably 75 lbs off the nose of the car....

2 grand in heads?