Understanding terms like Compression height, etc...

[Kern Dog]

I have a 383 that I just tore down and will be taking it in to the machine shop soon. I was hoping to just be able to hone it and reuse the original pistons but there was a ridge on every cylinder. I'm aware that I might have to bore it and use new pistons. I was impressed though that The rod and main bearings are all standard and dated 10-69, the block casting shows 8-27-69.

Ordinarily I'd just skip right past this and use a 440 but this is the original engine for my 1970 Charger project.
If I am faced with replacing the pistons, I'll do it but I am learning that these 383s with the short stroke they have are hard to build compression.
I've read terms like compression height and dome height. Summit lists several with a compression height of 1.908. This would be helpful if I knew what a stock height was. My pistons sat .076 in the hole and the calculated compression was 8.12 with stock parts. At zero deck, compression goes up to just a tad over 9.0 to 1.
Does anyone know of an aftermarket piston that results in zero deck or at least close to it?

 

[Mebsuta]

For original 68-69 10-1 flat top piston, I measure compression height of about 1.934 to 1.935.

Distance from top of wrist-pin hole to top of piston + half the diameter of the wrist-pin hole = compression height of piston.

That is probably subject to a lot of argument. I don't know anything about pistons other than the factory flat tops. Probably the closest you can find in production is the Speed Pro with 1.920 compression height.

 

[wyrmrider]

not that I know of
the 383 road runner motors were above deck with open chamber heads
what heads kd? keeping matching number parts
I built a stealth 383/426 crank and with 440 ly rods and quench dome pistons so the customer could keep his "stock looking-numbers matching heads" 9.5:1
this was a 4 speed dana rear superbird
car had to sound stock so we came up wth a shorter than stock cam on the seat 218 @.050 .525 lift iron adjustable rockers from rocker arm rebuilders when they were in socal bushed with\ their hard chrome shafts
changing heads?
stroke it

 

[1 Wild R/T]

There have been a few shops that tried to get a piston made that would actually make a true 9.5+ compression ratio but none could get it done in a way that they could even break even... There are to many variables, cylinder heads, bore sizes, valve clearance...

Back in the day Chrysler made over three million 383's but these days there isn't enough of a market to make decent pistons viable...

 

[Kern Dog]

I am trying to keep costs down for now...Ha ha, yeah, who isn't ??
I'm going to try porting heads for the first time. I have several sets of '906s, '452s and a pair of '346s.

I saw these pistons listed in another thread:
https://www.summitracing.com/parts/slp-l2315nf30/overview/make/chrysler
I'm learning that compression height is a lot more critical on the short stroke engine. Too much and the piston sits out of the hole, too little and the engine is a 7.8 to 1 slug!

 

[1 Wild R/T]

I am trying to keep costs down for now...Ha ha, yeah, who isn't ??
I'm going to try porting heads for the first time. I have several sets of '906s, '452s and a pair of '346s.

I saw these pistons listed in another thread:
https://www.summitracing.com/parts/slp-l2315nf30/overview/make/chrysler
I'm learning that compression height is a lot more critical on the short stroke engine. Too much and the piston sits out of the hole, too little and the engine is a 7.8 to 1 slug!

As mentioned that is probably the tallest C/H your gonna find and along with that you need to be careful with cam selection cause there's no valve reliefs....

Some folks feel the KB400 with it's .200 dome and valve reliefs can work better... I'm not real impressed with either option...

 

[Kern Dog]

I was planning on a cam similar to the MP 280/474. I had that cam in a 9 to 1 440 and it ran pretty strong.

 

[beanhead]

I don't know all what's out there for 383s, but with an un-milled (book spec) factory deck height of 9.98 you would sit .0265 in the hole with those 1.908 pistons:
3.375 stroke ÷ 2 = 1.6875
1.6875 + 6.358 rod length = 8.0455
9.98 factory deck - 8.0455 = 1.9355, the available space (comp. height) between the wrist pin and deck surface.
1.9355 - 1.908 comp height piston = .0265 in the hole.
So in theory a 1.9355 comp. height piston would, IF the decks were spot on, sit at zero deck...a big 'IF'!
Now consider that you may want to do some deck milling anyway to get them squared up as the factory decks seem be all over the place (some more than others.) Then it's maybe just a few more thousandths off to get the 1.908 pistons to zero...that's not much cutting if you're going there anyway to get things squared up and nice.

 

[beanhead]

Dam I'm slow....when I started typing there were no replies yet! Ha! Hey if you're gonna try your hand at porting, holler if (when!) you screw something up, I've got a pair of unfuckered 452s that you can have if need be.

 

[1 Wild R/T]

I was planning on a cam similar to the MP 280/474. I had that cam in a 9 to 1 440 and it ran pretty strong.

A 440 isn't a 383.... My suggestion, Crower 271 HDP...

 

[vance.dykes]

Kern call a cam maker i just did this for my 451 for the pro-flow4 and was surprised with all 4 company i called were very close . so i liked that crane wanted to under stand what i wanted not what was on the shelf.

 

[vance.dykes]

Dam I'm slow....when I started typing there were no replies yet! Ha! Hey if you're gonna try your hand at porting, holler if (when!) you screw something up, I've got a pair of unfuckered 452s that you can have if need be.

with that much math it would taken me all day to wright that.

 

[moparedtn]

Just one thing that struck me funny reading (and learning) all these posts -
the RoadRunner/Super Bee 383's had the 440magnum heads, right? Those are the
906 heads, which are closed chamber, yes?

 

[vance.dykes]

Just one thing that struck me funny reading (and learning) all these posts -
the RoadRunner/Super Bee 383's had the 440magnum heads, right? Those are the
906 heads, which are closed chamber, yes?

no open. 915 was closed.

 

[moparedtn]

no open. 915 was closed.

Yep, you're right. I got it ***-backwards.

 

[beanhead]

with that much math it would taken me all day to wright that.

I gotta stop taking ice cream breaks in the middle of replying to posts!

 

[BSB67]

I have a 383 that I just tore down and will be taking it in to the machine shop soon. I was hoping to just be able to hone it and reuse the original pistons but there was a ridge on every cylinder. I'm aware that I might have to bore it and use new pistons. I was impressed though that The rod and main bearings are all standard and dated 10-69, the block casting shows 8-27-69.

View attachment 948582

Ordinarily I'd just skip right past this and use a 440 but this is the original engine for my 1970 Charger project.
If I am faced with replacing the pistons, I'll do it but I am learning that these 383s with the short stroke they have are hard to build compression.
I've read terms like compression height and dome height. Summit lists several with a compression height of 1.908. This would be helpful if I knew what a stock height was. My pistons sat .076 in the hole and the calculated compression was 8.12 with stock parts. At zero deck, compression goes up to just a tad over 9.0 to 1.
Does anyone know of an aftermarket piston that results in zero deck or at least close to it?

If you want the best value, use the 2315. Watch your piston to valve clearance.

A better piston is the Icon IC687. But better has a price.

Diamond makes a piston too, but the IC687 will give a better compression ratio.

 

[451Mopar]

The Speed-Pro 2315 pistons are decent if using a mild cam.
Pretty close to zero deck. My 383 was cut so the pistons are at zero deck (or really close to it, need to measure it.)
The lack of valve reliefs limits your cam choices. I think a 230 @ 0.050" cam may fit, but need to check piston to valve clearances.
The Edelbrock E-Street 75cc chamber heads would be a good upgrade for compression, flow, and less weight.

Just looked at the Icon IC687 mentioned by BSB67, and really like the specs of that piston, with the thinner 1/16" ring pack, small dome and such.
Summit shows they cost $547
https://www.summitracing.com/search...rdMatch&SortOrder=Ascending&keyword=IC687.030

 

[Kern Dog]

I am somewhat resistant to dumping too much money into a 383 when 440s are still easy enough to find. I have 2 in my backyardshop right now. One is a virgin bore, the other is an 846 mile rebuild that threw a rod but only needs a piston and two rods to be functional again. I go back and forth on this though....this is the original engine and the novelty of that is appealing. For the price of those pistons, I could have the 846 mile engine together.
One option that I hold is the idea of a future 4.25 crank stroker build. I could later switch to the 383 block for the sake of originality while hiding an additional 112 cubic inches!

 

[451Mopar]

The 496" 383 is nice if you have the funds.
The price difference between the 2315 piston and IC687 is less than $250, but the IC687 also solves some issues, like having valve reliefs, and higher compression ratio. On the other had, if using a mild cam < 230@0.050" duration the 2315 will get the job done.

I guess I keep thinking of my 383 build with the 2315 pistons, but forgot that the block was cut for zero deck height, and the 906 heads were also cut 0.060" for compression. Because of the milling, and being lower cost build, I used some shorter pushrods that were for a Ford engine. I forgot the specific engine, I built that engine in the late 1980's. It actually used the Crower 271HDP cam. I mention pistons with valve reliefs, because I over-reved the engine and bent a few intake valves. The 2315 pistons have nice witness marks on them from the valve hitting the piston. I had the recommended Crower single spring with damper, but they also recommended (as an upgrade) the dual springs with that cam. Really the single springs worked fine until I accidently down shifted from drive to first and fit about 8,000 RPM. That was the last time I used the low gear at any speed valve body modification in the trans shift kit.