What piston for my 383 build?

[Nate S]

For starters, do you know how to calculate compression ratio? I’ll help if you don’t. For 906 heads you can assume chamber volume is 87cc. They may be a bit more or less but if they are virgin unmilled they should be around there. There are several 383 pistons available so you should be able to pick what you want. Head gaskets at .020” or .039 are a good way to adjust a bit.

With that cam it’s pretty similar to a stock magnum cam, wee bit bigger. I just built a 426W. Calculated everything properly and I’m at 9.25 : 1 with open chamber heads. I’m using a stock magnum cam ([email protected] intake). I’m finding that with proper 38 degree max ignition timing I need 93 octane, with 89 it pings a little on warm days. If you want to use pump gas this seems like a good place to be.

If you want a true 10:1 or so you’ll need a notably bigger cam to get away with it on pump gas. Also good to remember that the factory ratios were exaggerated. 10.1:1 was often about 9.75.

Rebuilder’s choice pistons for stock use are pretty good. Tend to come in exactly like originals. Watch out for dished top etc. often the volumes aren’t published and you’re guessing. Lots of higher end pistons too, just make sure you’re paying for what you need.

 

[MoparMitch]

Kb 400.......

 

[BSB67]

Kb 400.......

Sure, but the KB400 would net a lower compression than the other two pistons mentioned, and I think the OP is wanting to increase compression ratio.

 

[CompSyn]

Okey, new question!

What type of pistons should i use for my -68 383 with 906 heads, XE268H compcam, will the
Speed-Pro Forged Pistons L-2315NF 30 work? Or the KB Pistons?

This should not be a crazy HP motor, just enough to work good.

I'm doing a very similar build as I type this. I went with the Speed-Pro forged pistons. No valve reliefs with these helps with already problematic lower compression of the 383. And as you say, NO planned bigger cam down the road, no problem with lack of VRs. For $399 at Summit, they won't break the bank. With careful attention to deck height and head CC's, and using the 0.020" steel shim gasket, you'll be able to match the factory 383HP compression ratio of around 9.0:1-to-9.2:1, yep! that's where they were... So with today's crappy pump gas, that's not a bad place to be compression wise.

Also, if you're running exhaust manifolds and have not bought your XE268 camshaft yet, I'd suggest you give Bob Karakashian a call...

 

[MoparMitch]

Sure, but the KB400 would net a lower compression than the other two pistons mentioned, and I think the OP is wanting to increase compression ratio.

I got 9.50 with my build.....Twice.....

 

[BSB67]

I got 9.50 with my build.....Twice.....

Sure, you just need to cut more.

 

[MoparMitch]

Sure, you just need to cut more.

Ok

 

[451Mopar]

With the iron heads, pump gas and mild cam in an open chamber design, you might want to stay around 9.0:1 so the engine does not ping.
The good part of the Speed-Pro piston is the taller compression height and no valve reliefs will make the compression ratio higher (if no block/head milling is done.) They are also forged, but that has pluses and minuses. It is stronger, but requires more piston to cylinder wall clearance.

The pluses of the KB piston would be quieter operation (usually not a concern if running headers anyhow), and having valve reliefs if you wanted to run a larger cam over about 280+ duration. In the case you want the bigger cam, then you would want more compression anyway which may make the KB400 popup piston a better choice.

For me, I would just use the speed-pro pistons, and the cam you mentioned. Just make sure to use the recommended valve spring pressures.

If you decide later you want more power, drop on some better flowing (assuming your heads are stock) and smaller chamber heads like the Edelbrock E-Street heads.

 

[33 IMP]

What quality is your pump premium in Sweden? Our pump gas premium here is rated - research octane, plus motor octane, divided by two. Depending on which state we live in, our premium varies from 91, with 10% alcohol (garbage) to 93, alcohol free.
Whether you enjoy your engine will depend on if it can run comfortably on your pump gas.
If you just want a nice running engine, I would be conservative with the compression ratio.
Here in california , with 91 and 10% , I wouldn't build one over 9-1 with iron heads and a mild cam.

 

[CompSyn]

With the iron heads, pump gas and mild cam in an open chamber design, you might want to stay around 9.0:1 so the engine does not ping.
The good part of the Speed-Pro piston is the taller compression height and no valve reliefs will make the compression ratio higher (if no block/head milling is done.) They are also forged, but that has pluses and minuses. It is stronger, but requires more piston to cylinder wall clearance.

The pluses of the KB piston would be quieter operation (usually not a concern if running headers anyhow), and having valve reliefs if you wanted to run a larger cam over about 280+ duration. In the case you want the bigger cam, then you would want more compression anyway which may make the KB400 popup piston a better choice.

Not necessarily. Look at the alloys. The Speed-Pro pistons aren’t 2618 forgings and as such not as loud. Plus the Speed-Pros have a skirt coating if that helps any? Also, there’s the flat top versus dome piston and the flame travel debate...

On a side note. We weighed the piston pin combos; new Speed-Pro versus original cast. They were almost identical.

Thinking I’ll do a write-up on my 383 restoration build as a reference for anyone interested once I have all the numbers in.

 

[Kern Dog]

Hey guys,
I'm going to rebuild the original 383 from my project car. I've been running the numbers in the Summit racing compression ratio calculator and have come up with all sorts of combinations to get the ratio into a favorable range.
My first goal was to reuse the original pistons but with them sitting .076 below deck, I'm looking to those Speed Pro pistons.
The Speed Pro specs list a 1.920 compression height. Those KB400s show 1.908. That .012 means more compression all else being equal but does anyone know what that means in terms of TDC measurement below deck? I'm trying to get an accurate number for the compression ratio.

 

[451Mopar]

My 383 has the speed-pro pistons, but the engine was built around 1988, so no piston skirt coating.
The block was milled so they are pretty much zero deck. I haven't measured if they are exactly zero deck, but they sure look close.
The theoretical numbers show the 2315 piston would be 0.012" below deck if the block deck height is 9.980, with 3.380" stroke, and 6.358" length rods.
With an 84cc head, compression is 9.23:1 with a 0.040" thick (10cc) head gasket.
With the 0.020" head gasket (5cc), compression is 9.68:1
If the block is cut for zero deck, 0.040" gasket = 9.48:1 compression, and 0.020" gasket = 9.95:1 compression.

The thing that is going to throw these numbers off is the cylinder head volume is likely closer to 90cc unless it has been cut also.
With all the milling work, you will need shorter pushrods, and need to make sure the intake port sides of the heads cut so the intake fits.
As you see, a change of about 5cc in volume (head or gasket) changes compression ratio about 1/2 a point.

 

[Kern Dog]

The theoretical numbers show the 2315 piston would be 0.012" below deck if the block deck height is 9.980, with 3.380" stroke, and 6.358" length rods.
With an 84cc head, compression is 9.23:1 with a 0.040" thick (10cc) head gasket.
With the 0.020" head gasket (5cc), compression is 9.68:1
If the block is cut for zero deck, 0.040" gasket = 9.48:1 compression, and 0.020" gasket = 9.95:1 compression.

The thing that is going to throw these numbers off is the cylinder head volume is likely closer to 90cc unless it has been cut also.
With all the milling work, you will need shorter pushrods, and need to make sure the intake port sides of the heads cut so the intake fits.
As you see, a change of about 5cc in volume (head or gasket) changes compression ratio about 1/2 a point.

I appreciate the response. How do you determine the below deck height?
Do you take the rod length, the piston compression height and the stroke and add them together and compare to the published deck height?

 

[BSB67]

The pluses of the KB piston would be quieter operation (usually not a concern if running headers anyhow), and having valve reliefs if you wanted to run a larger cam over about 280+ duration. In the case you want the bigger cam, then you would want more compression anyway which may make the KB400 popup piston a better choice.
.

Again, why would you pick a KB400 over an Icon 687?

 

[Kern Dog]

My 383 is standard bore, the bearings are dated August 1969 so it is fair to say that the deck is standard too. IF the .012 below deck height is correct, this changes things! Any idea of the compression ratio differences between those KB400s and these Speed Pro slugs?

 

[BSB67]

My 383 is standard bore, the bearings are dated August 1969 so it is fair to say that the deck is standard too. IF the .012 below deck height is correct, this changes things! Any idea of the compression ratio differences between those KB400s and these Speed Pro slugs?

The compression 2315 will be about 0.25 higher than the KB400. The IC687 will another 0.20 in higher than the 2315.

 

[Kern Dog]

I'm trying to learn and understand this stuff. The published compression heights are 1.920 for the Speed Pro and 1.908 for the KBs. I thought that meant a .012 difference.

 

[451Mopar]

I appreciate the response. How do you determine the below deck height?
Do you take the rod length, the piston compression height and the stroke and add them together and compare to the published deck height?

1/2 of the stroke + rod length + piston compression height. so 1.690"+6.358"+1.920" = 9.968" (at TDC), Block hight spec = 9.980", so 9.968" - 9.980"= -0.012"

 

[1 Wild R/T]

1/2 of the stroke + rod length + piston compression height. so 1.690"+6.358"+1.920" = 9.968" (at TDC), Block hight spec = 9.980", so 9.968" - 9.980"= -0.012"

And all the math is based on this number which is the spec but usually not an accurate number.... Kinda like the cylinders heads are supposed to be 81cc but actually are 88-92

 

[451Mopar]

Again, why would you pick a KB400 over an Icon 687?

I wouldn't, I would pay the extra cost for the ICON piston, and build the engine a bit on the larger cam size knowing I will be using a high stall converter, but this is not my build and the original poster seems to want more of a daily driver that is simple and not too expensive?
What I was getting at is with the small cam you don't need the piston valve reliefs of the KB-162, and the speed-pro L2315 gives a slight bump in compression ratio over the lower deck height KB-162 flat top piston (if the block is not cut to zero deck the pistons.) And that if planning to use a larger cam that might need the valve reliefs like the KB-162 has, you would want to bump up the compression with the larger cam anyway, so the dome piston(s) (KB-400 or ICON-687) makes sense.
I think the discussion sort of got away from the ICON piston, likely because of cost and with the mild cam the less expensive pistons will get the job done.

Really, your talking a really simple and mild build, no reason to overthink all this stuff. Having the right ignition advance curve, and torque converter will have more of a performance impact than 1/4 point of compression which is close to the difference between the L2315 and the KB-162. Again, at the same block height.