Allright, I'm getting a real good deal on machine work so my 390 build is going to at least get started. Probably be a while before it's done, but at least I'll have the block ready to roll.

Anyway, I'm going to start by rough boring the block to find out what the minimum overbore is and then I'm going to pick up pistons to get exact measurements for the finish hone.

This brings me to my problem. Since all this is going to be is a sub-5500 truck motor I was looking at cast and hypereutectic pistons as an alternative to forged. That's when I noticed the compression heights of the pistons.

Idealized (rod length+compression height+1/2 stroke) the off the shelf Federal Mogul cast pistons will put the head .132" below deck (idealized at 10.17) while the SpeedPro hypereutectics are better but still not good at .033".

Is there any non-forged piston out there (taking budget into account) that has the car 390's compression height of 1.776 (+/-)?

Question 2 (or a continuation):

I noticed KB's hypereutectic pistons have a 1.78 pin height, but the speedpro forged pistons are only about $40 more. Here is name/supposed higher quality better or is mass produced, but forged better?

Again, 5500 RPM and lower, 400hp target, no power adders.

Justin

EDIT: and now i notice the KB's have a 20cc dish which will kill my compression unless I zero deck the block, but it'll still only be about 9.3.

It sounds like I'm talking myself into the forged pistons just for the sake of being able to get decent compression.

 

I'am all confused here?? The head .132" below the deck??

And IF your talking about quench area.....I would go with the speed pro's..just for peace of mind....and at .033" ...you can work with that with head gaskets, and your chamber size.....and sinking the valves a little lower in the seat to make the chamber a little bigger... and there's many other was to get the right CR your looking for...

Have too get back to MY House Heat and A/C.....

RJ

 

Head of the piston, guess i should have said face but i was in a hurry and slipped up.

yeah, off the shelf federal mogul cast pistons would put the face of the piston .132" down the hole at TDC and with 68 cc chambers would yield a compression ratio of 8.6:1 IF they were flat tops, which they aren't (but i can't find how big the dish is)

Speedpro hypereutectics put the piston .033 is the hole, but once you take into account the dish and head gasket thickness it only winds up being about 9:1 with 68 CC chambers.

KB hypereutectic are .012 in the hole, but have a 20 CC dish so it'd only be about 9:1, but with better burn characteristics than the speedpro.

speedpro forged would be .016 in the hole and with the smaller dish would give about 9.8:1

It seems like of the common (read affordable) 390 piston choices, the only one that is an actual replacement for a car piston would be the speedpros. Next would be the KB's, but with any sort of cam you'd just be bleeding off way too much compression.

So i guess I answered my own question. It really helped to lay it all out like this.

Justin

 

Well Justin, JMO...For the $40 lol..I'am a defenitly Forged piston guy for the money.... My last 2 builds were zero decked with the pistons..and I used the blue felpro .041" gaskets with 72 cc chamber heads....and with a little bowl work in the chambers brought the CC's up some....

But you'll also find if Ford advertise's 72 to 74 cc heads...you can bank there closer to 74cc's....I see them all the time....with 3 chambers running right at 74cc and 1 at 72 +cc's..

And as far as the cam goes, going for an aftermarket Cam... Thats why you choose a cam to you CR and the type of driving your going to do....Some will bleed off quite a bit...Depending on timing set up..and others wont...

Keep us posted!!

RJ

 

Yeah, i'm trying to run about 9.5:1 since with the level of cam I'm planning on it won't bleed off alot, but enough that anything less would just be a waste.

But all at once since i'm getting the block cleaned, maged, bored, honed, and cam bearings installed for nothing more than my time it opens up a couple hundred extra dollars in my budget. So now there's really no reason to go anything less than forged, even if there was a viable non-forged option.

Justin

 

Well Justin, It sounds like to me your heading in the right direction, Buddy!!

RJ

 

Get the Speed-Pro forged while you still can. I am trying to muster enough cash to inventory a bunch at my warehouse - they closed the factory in Missouri and are moving production to India. Odds are that once the 390 pistons are sold out it's gonna be a loooong time before we have a cheap forged option again - if ever.

 

OK I know these (choose your own word) can do a semi pathetic job of comp tech phone help.... But forging pistons ? {Apu voice} "Now listen sir this attitude is not going to get you these pistons any sooner"....."how will you be paying for this?"

 

Bob.... Lmao ...I had to read your post, 3 times to finely get it !! Duh !! LOL

RJ

 

 

Silvolite Hypereutectic flat top (part number 1131) has a 1.76 pin height. Go to United Engine and machine to look them up http://kb-silvolite.com. These sell for about $90 a set.

 

went there myself and bookmarked it for when $$ is in hand.. dont see any 4 digit KB numbers under pistons Hypereutectic..
B: What the hell does that word mean.(yEAH i'M THAT FAR OUTA THE LOOP) Cast I get, Forged,, Hyper what ? My dictionary doesnt have the word in it...
My reciept for 352 cast pistons however shine plenty light on the WTF was I thinking? scenario when the overbore was needed...

 

Allright, here comes metallurgy lesson 101.

Eutectic refers to an alloy mixture where the components are alloyed together in the proportion that gives them the lowest melting point while still not having the characteristics of either base material.

If you increase the proportion of the either substance, the characteristics of that base material will begin to show through and you are now over the eutectic point or "hypereutectic".

In aluminum you're typically talking about aluminum and silicon. The eutectic point of these materials is about 12% silicon to 88% aluminum.

Hypereutectic pistons increase the silicon content to about 16-18% to get some of the characteristics of silicon to show through. Silicon doesn't expand as much as aluminum when heated, so a hypereutectic piston can have a tighter cold fit in the bore than a standard cast piston and especially a forged piston. The silicon content also helps disperse heat more quickly so the aluminum in the piston itself doesn't get as hot and expands even less.

The side benefit to this is the hypereutectic piston is stronger than your typical cast piston in high-rev applications BUT is MUCH more prone to crack when exposed to detonation since the silicon content embrittles the aluminum.

So i guess paying attention in class does pay off every now and then


Justin

 

And i looked at those pistons. the 1131h is the hypereutectic one. Checked and they're up to $140 a set from summit.

Soooo. . . it's looking like 1131h's in +.030 with the Mr gasket .020 steel gasket would put me right about 9.7-9.8:1 if my block hasn't been decked. If it has, the felpro permatorque (.041) would keep me at about 9.5-9.6.

So that's about where i wanted to be. guess now I just need to get the block punched out so I can see how deep in the holes these things'll really be. Can't wait for december to come around. . . .

Justin

 

Well Bottom Line is some get really lucky with cast and Hyper's.... But you cant beat forged if you can swing it....

Ford even put Forged pistons in there motors that made Hi HP.....If your looking to build 400hp today...what are you going to want tomarrow?? enough is never enough! and if you go the Hyper route..you may have wished you hadnt...

JMO.....Cast and Hyper's I feel are fine for a lower HP build.....But what do I know....

I just go with what I know that works and stay's together? I hope...LOL

RJ