Newbie 318 piston question

[odie 2882]

Hi everyone. I'm just curious as to the reason why more people don't use or recommend the kb399 domed pistons as a easy way to boost compression in their 318 builds. Lack of compression seems to be the biggest hurdle to fix when building a 318. Just curious I'm sure there is a reason. Thanks.

 

[rumblefish360]

Good question.

 

[George Jets]

That is an interesting question, worth looking into.

Like the high compression height of those, would have to be used with the open chambered heads. With those valve reliefs looks like cam selection won't be a problem as far as interference is concerned.

The harder to find factory mopar pistons have a compression height of 1.78 ch sitting .040 ths down in the hole which is pretty good for factory pistons.

Aftermarket stock replacements can be 1.755 ch.

These KB399 have a 1.81 ch sitting .010 above deck.

Was lucky enough just by chance to pick up a fresh set of 318 open chamber heads at a swap meet a few years back. Did not realize until looking at them lately here that someone had installed 360 valves in those 318 heads.

So yes, this could make an interesting combination too with the KB399 pistons.

Think stock 318 open chamber heads, a mild cam, and the KB399 pistions would wake up a basic 318 engine.

Although the factory 1.78 ch flat top pistons were able to produce 9.2:1 cr @ 230 hp, stock heads, that's with a 2 bbl carb.

Looking like the KB399 pistons with 68 cc open chamber heads are rated at 9.2:1 cr too.

KB399 have the higher compression height of 1.81 ch, but maybe they are loosing some chamber volume with those big valve reliefs. Don't Know ??

Worth looking into . . .

 

[rumblefish360]

You could use these with closed chambered heads. You just need more gasket thickness. Also, perhaps, ether opening the head chamber or piston shaping. All things in flux to fit it.

 

[PRH]

I think when most people think of the return on investment on actually “building” a 318...... vs say, a 360....... they come to their senses.

 

[teringer]

this is the way I have done all 318 I've done a few with a 360 head it's hard to tell someone it's not a 340

 

[Max1196]

George those compression ratio factors appear to be for the KB167 not the KB399.

the KB399 is more along the line of 12.2 to 1 with 57cc and 10.4 to 1 with 68cc heads!

 

[PRH]

KB periodically changes their piston designs..... without issuing a new part number.
That’s just a preface to saying, I’m looking at a 2016 catalog, so the info might not be 100% accurate today.

My book shows the KB167 to be a flat top with 5cc valve pockets.

The KB399 has a .200” tall dome that has an effective volume of -6cc.

So, there is an 11cc spread between the two pistons.

Both show a CH of 1.810.

 

[C130 Chief]

Back in the 80's and 90's the piston choices for a 318 were either stock replacement pattern flat-tops or 11:1 domed pistons. If you were running large port open chamber heads you needed the dome or you had to mill the heads quite a bit to get reasonable compression. A domed piston in a large open chamber isn't as efficient as a flat top with quench, but a lot of engines went together that way and ran fine. Valve clearance can be an issue with domed pistons.

 

[George Jets]

George those compression ratio factors appear to be for the KB167 not the KB399.
View attachment 1715565181
the KB399 is more along the line of 12.2 to 1 with 57cc and 10.4 to 1 with 68cc heads!

Max, that seems to be more in line with the 1.81 ch of the KB399.

Here is the article I posted above from CNC-Motorsports about the KB399, could be a misprint . . easy to get them mixed up when they are publishing info.

 

[George Jets]

I see there are a few companies showing the same spec info for the KB167 and the KB399.

Kind of would be nice to get a person's hands on one piston of each to see what the real differences are ???
Test fit them in a bore too, see the CH difference.

 

[PRH]

The KB catalog clearly shows the difference between those two pistons.

 

[273]

Most doing a 318 aren't rebuilding it, most want bolt on performance, or it's a low dollar build so they use stock type pistons.
But ya if you crank up the cr add some good heads and cam build it like it's a 340 no reason really not to beside there's 360 out there.

Here a 477 hp 318
Iron-Headed Mopar 318 Magnum Engine- Popular Hot Rodding Magazine

 

[YY1]

Is the KB 399 a factory replacement for the 67-68 pistons or 85-91 pistons from the 9.2:1 engines?

 

[273]

Is the KB 399 a factory replacement for the 67-68 pistons or 85-91 pistons from the 9.2:1 engines?

Edit*

 

[PRH]

Quench pad?

Uh, that’s just a regular old dome.
.200 high.

A quench pad would be on the opposite side.

 

[krazykuda]

It would be a good piston to use in a 318 and then put a set of 340/360 heads on it...

I ran a 318 with TRW forged 10.5 pistons and it would detonate and overheat within 20 minutes because the compression was too high with 318 heads... I found a set of 360 heads and swapped them on and ended up with 9.2 compression and no detonation or overheat problems any more....

 

[odie 2882]

It would be a good piston to use in a 318 and then put a set of 340/360 heads on it...

I ran a 318 with TRW forged 10.5 pistons and it would detonate and overheat within 20 minutes because the compression was too high with 318 heads... I found a set of 360 heads and swapped them on and ended up with 9.2 compression and no detonation or overheat problems any more....

 

[odie 2882]

Thanks for all the replies guys. Going with 360 heads and these pistons seems like a good combination to think about then.I have a 87 318 roller block in my 69 dart right now. I think I might be pulling it for a rebuild over the winter and just was trying to get a plan together. Good to know I'm on the right track.

 

[AJ/FormS]

Lack of compression is the #1 hurdle to building a strong street 318. With the right combination of parts, you can make a real nice ripper.
IMO, a closed chamber is the place to start.
The swept on a .030-over 318(323.338 cubes) is a meager 662 cc, so getting pressure is often a challenge. Closed chambers will make it both easier and better.
For instance, to get to 9.7 with 662 requires a total chamber volume of 76cc. If you start with 72cc openchambers, you'll never get there with flat-tops, without head-milling at additional cost.
But at zero-deck with .039 gaskets you could be looking at ~62.4cc, Magnum heads are a natural. They've got everything your teener needs.

 

[Garys72Duster]

Just picked up a 72 Dart, 318. The receipts show it has the KB 399 pistons at .030 OS. Head casting is the 974 (360) head. Machining shows the block was decked to zero deck clearance. Some porting was done, and the motor sounds really healthy. Almost ready to test drive it, after changing the carb, and fixing the NSS along with a bunch of other stuff. Compression test is showing 170 lbs, not bad for 5400 feet. I'll let you all know how it runs, pretty soon.

 

[Krooser]

CNC must sell a few of those slugs. The local race sanction has a bmod class that uses 318 as well as the street stocks.

 

[toolmanmike]

I'll let you all know how it runs, pretty soon.

Please do.

 

[ChargedDart75]

Used them with a .030" 318 with Indy 360X heads. Make sure to check the piston to head (both dome and quench pad) for proper clearance, and get the ring end gap right and you'll be good to go.

 

[AJ/FormS]

@Garys72Duster
Ima thinking, if it was me; I'd be looking to borrow a second Compression Tester.

"Really healthy", to me, sounds like it could be a 292* cam. >To make 170psi at 5400 with an Ica of 72* would require about a 12.6/1 Scr. That's a mighty mighty small chamber.
>With a 268 cam, the Ica could be 62*, and the Scr would need to be ~11.6. This is about 62.5cc total chamber volume.
So yeah, I'm interested too . Cuz if you can get that to take WOT at full-timing, with iron heads ...... I got a 318 here in need of pistons .......................