New member with 383 question

[RB61]

I am re assembling a rebuilt block and find that the flat top pistons are approximately .040" down from the deck. I have read that blueprint specs are closer to .015" but that most blocks came from the factory closer to what mine reads. I have 73.5cc heads.

I would like to get some feedback as to what my compression ratio is.

What are some non drastic options to raise the compression ratio if I decide I want to?

Thanks- great forum

Ray

 

[Car Nut]

aboard!

 

[mopar 3 B]

Others here really worry a lot about this. My solution would be find a camshaft that will work with your complete package and forget it.

 

[Photon440]

Hi RB61, welcome:wav:

 

[Xp29h]

Welcome!

I know my pistons are well below the deck as well. I don't recall the exact distance. Lower comp too, 8:1! I have tried different cams too. I'm currently using a melling cam. I'll post the pic with info. I will say this, the runs very well on top end. Above 2500 rpm great and strong. I can't seem to get the darn thing to go good with lower rpm.

I should mention I don't think the p.o. Didn't clearance the Pistons and rings when rebuilt and therefore it's way too tight creating ring issues and possible ring land issues as well. All of this would cause my low rpm sluggishness. If this wasn't the case, I think my cam would be working well.

Lower comp motor set up right should run well.

Good luck

 

[The Horvaths]

Xp29h, did you degree your cam? It sounds like it could stand to be advanced a couple degrees to get better low-end response.

As for piston height, if your building it to drive, it's fine. Thinner head gaskets will pep it up a wee bit.

 

[Xp29h]

No I did not. At the time, there was a ton going on trying to figure out what the PO had done, so I wanted straight up so I could start figuring things out.. But would be something for RB61 to consider maybe!

 

[mr. b]

welcome to FBBO great to have you

 

[IQ52]

Calculate compression ratio easily.

https://www.uempistons.com/index.php?main_page=calculators&type=comp

 

[Cranky]

I am re assembling a rebuilt block and find that the flat top pistons are approximately .040" down from the deck. I have read that blueprint specs are closer to .015" but that most blocks came from the factory closer to what mine reads. I have 73.5cc heads.

I would like to get some feedback as to what my compression ratio is.

What are some non drastic options to raise the compression ratio if I decide I want to?

Thanks- great forum

Ray

It also depends on what year the 383 is and if it still has factory pistons or not but I don't think I've seen a factory piston that far off from blueprint specs before. Not saying it's not possible tho and like someone mentioned, .040 in the hole isn't as bad as some think. What heads are you using and have they been milled? Lots of calculators out there and IQ posted a good one.

 

[Meep-Meep]

I can tell you what my 383 was when I cc'd everything. Flat top pistons .005" below deck, block decked to blueprint spec, composition gaskets, 915 closed chamber heads. All that ended up at 9.5:1 CR and with the MoPar .528" cam it ran 12.65@107 in a 68 RR.

It seems to me you don't have enough compression to run that cam.

 

[The Horvaths]

It seems to me you don't have enough compression to run that cam.

Good point. The devil's in the details.
That goes for degreeing a cam as well. "ALWAYS degree the cam" is my mantra. Your crank, cam, and timing set all have tolerances. These can add up or subtract in either direction. The end result is that you don't know what you actually have until you measure it. As long as you're in there, you may wish to verify valve-to-piston clearance as well. Trust me on that one!

 

[jason lewis]

A couple of options if you are still looking.If you run a composition head gasket then you are doubling the the deck clearance,which will effectively kill the quench.If you run a shim gasket which are about .020 then you would be at the max of what quench will effectively help.Personally I would have the block decked,then run the composition gasket.You can degree the cam in a little tighter and pick up some comp,and a little rowdier sounding .And it will bring your rpm range down a bit.And as others have said,check your piston to valve clearance.Changing valves thats eliminate any cup to them will raise the comp a little to.I'm a compression guy,so I'm biased.

 

[RB61]

""Changing valves thats eliminate any cup to them will raise the comp a little to""

Thanks for the tip. I don't know what this means, but I can now look into it.

 

[lewtot184]

Welcome!

I know my pistons are well below the deck as well. I don't recall the exact distance. Lower comp too, 8:1! I have tried different cams too. I'm currently using a melling cam. I'll post the pic with info. I will say this, the runs very well on top end. Above 2500 rpm great and strong. I can't seem to get the darn thing to go good with lower rpm.

I should mention I don't think the p.o. Didn't clearance the Pistons and rings when rebuilt and therefore it's way too tight creating ring issues and possible ring land issues as well. All of this would cause my low rpm sluggishness. If this wasn't the case, I think my cam would be working well.

Lower comp motor set up right should run well.

Good luck

the .050" duration, lift and LSA are nearly identical to the 284-.484 mopar cam. the Melling seat numbers probably include the ramps. more cam than i'd run with an 8:1 383.

- - - Updated - - -

I am re assembling a rebuilt block and find that the flat top pistons are approximately .040" down from the deck. I have read that blueprint specs are closer to .015" but that most blocks came from the factory closer to what mine reads. I have 73.5cc heads.

I would like to get some feedback as to what my compression ratio is.

What are some non drastic options to raise the compression ratio if I decide I want to?

Thanks- great forum

Ray

i'm getting about 10:1 with a steel shim gasket and .030 over.