Looking to purchase a set of 340 (20 over bore) pistons for an A-Body. Any recommendations on material? Forged or Hypereutectic for street/strip use.

I'm not an expert... but if you are NOT REALLY race with it but just an occasional visit to the track I think Hypers are quite enough, 30-40% cheaper than forged

just my 2 cents

This is can of worms on this site. Some here love them and others hate them. When considering the overall cost of a motor, why cut corners just because something is cheap? I’ve known two people who have had problems with them coming apart. My engine shop does not use them. My two cents.

Here is my take, ask youself if you have ever broke a stock oem piston? Ive burned the lands out of a few stock motors on high end runs by things not being adjusted and things went lean.

I broke a skirt when young, like 17 because I cracked a skirt on install. And it ratteled but went over 60k miles of hard street action.

Both times were my fault.

I one time beat the living hell out of a bracket motor as when I bought it I was told it had forged pistons, but it turns out they were hypers and all was fine.

So now if its not a full time track car, high rpm (under 6800) I use hypers because there is no reason not to.

For full time track, high rpm, spray or t brake id use forged.

I understand some thoughts on forged only but if one dont need them the extra 3-4-500 saved for people on a budget can mean the difference of driving your car this summer or watching it collect dust.

In the right application, why not. Shops will do all the machine work for you in the world knowing you brought hypers in but they dont like to build with them because people do not use them as intended and then the shop gets the blame.

I've done the hyper and forged thing and have never had an issue with either. what I would do is decide what type of gas you want to run and then see what fits that need piston wise. I do believe the greatest draw back to hypers are the dummies that won't read and follow the instructions. truth is that if this is a street or street-strip either will work if properly installed.

I would prefer to use pump gas... I'm planning to have compression in the high 9s, but no higher than 10:1 compression. I'm told today's pump gas (e.g. 93 octane), we should be trying to achieve compression below 10:1. Thoughts?

I also heard mixing race fuel (110 octane) with pump gas (93) is not good for the engine. Any truth? Especially for those just street driving.

I would look for pistons that use a thinner piston ring set (1/16"?) than the old 5/64" rings, and the correct compression height and dish/dome to get the compression ratio you want.
A quick search is only showing forged pistons for the 340 with the thinner ring pack?

The Old Speed Pro H143CP30 pistons for the 440 were Hypereutectic with the 1/16" ring pack.

Every Hyper piston I've seen that was damaged was because the tune up was off or the rings were not gapped properly. Some were from rich conditions, others detonation.

Mixing gas isn't an issue and if building a street/strip engine, I'd build it to run on pump swill. What are you going to do if you get caught low on fuel somewhere?

Street strip?
HyperU's on a half dozen engines over approx. 20 years without any issues what so ever.

I've seen more than one failed cast piston in the oil pan in more than 100 pieces from detonation
Those cylinders where ruined from the steel writs pin gouging deep grooves into the cylinder walls before the motor stops rotating
Same thing on any hard steel part, like a air cleaner nut, dropped into the combustion chambers intentionally(haters at the races) or accidently, instant destruction
The OEM car makers use cast pistons due to cost, almost the Hi Po motors from the factory,GM, Ford, Mopar, have forged pistons
My choices on piston type used in all the motors I build are meant to be driven like hot rodders drive, forged for high abuse
If your not going to abuse, hot rod, the motor use Hypers, if not, don't use them

I have to go with EV2bird on this one. I run two cars, both with nitrous. One is a (cough, Mustang) with hypereutectic pistons and I've run as high as a 150 nitrous shot on them.... but I DO NOT recommend it.....forged pistons for nitrous, blowers or turbos. However, if you're running naturally aspirated, then hypereutectic all day long...or just cast pistons. I believe you can run a tighter piston/wall clearance (less damage during engine warm up, less piston blow by, etc) with hypereutectic pistons vs. forged, though I think it's mainly the old 2618 (?) forged pistons that have "piston slap" when the engine is cold. I do plan to run 2618 forged pistons in my new "mellow highway-cruiser" 440 I'm building ONLY because I plan to leave the nitrous on the car.

Are today’s engines equipped with forged or hypers?

“Race” engines should be equipped with forged slugs.
Forged slugs take the abuse of racing easy.
Street and street strip cars can use Hypers.
Hypers can survive a lot of power. I wouldn’t go into competition and travel the nation chasing the money with them.
Build for intent. Over build no matter what your doing.

For just a mild street cruiser hypers will be fine. I have a set of Speed Pro hypers in my one 360 that I raced and it has 7 seasons on it. Over 1500 passes. Shifted at 6000, trapped at 6300. Freshened it up a few times and all is well. My dad and wife both had a set of KB hypers pistons that one piston decided it'd rather be in a million pistons...so personally I'd stay away from KB hypers if it were me.

I just did a cursory look at Summit for a 440 and the Speed Pro forged are around $450 and the KB hypereutectic are around $450. So what would be the advantage of the KB's other than a tighter skirt clearance? But I would be the first to admit this is like the color green, you either love it or hate it.

Bullseye...

I believe the KB hypers are considerably lighter than the speed pro forged. For a street/strip 340 that's more street than strip I would use the hypers and not sweat it.

What do the LS guys know that we dont?

The LS9 is the only production LS engine with forged aluminum pistons; all the other use hypereutectic (cast) aluminum alloy pistons - varied mostly by diameter to accommodate various bore sizes. LS cast pistons shouldn't be used on applications greater than approximately 550 horsepower. Also, the LS7 piston's inner bracing requires the use of the matching LS7 connecting rod.

For a long life street motor with no power added or spray, I was told the Hypers will hold the ring seal and control oil longer than the forged due to having a tighter fit than the forged pistons require.

There's nothing wrong with Speed Pro pistons, but I "think" the KB's are lighter too....engine revs up faster, less bearing wear, higher rpm potential.

I can tell you this i ordered a set of kb400 pistons for my dads 383 rebuild and it has been the best decision i ever made in my 30 plus hot rodding years!
Why?
Well 383 has very little choice for a good compression piston ,
I wanted same as you 10.1 we got 10.2 with .046 in quench with 1009 felpro gasket.
Edelbrock heads,rpm intake and comp 275 hl cam , with cat roller rockers and a hp 750 holley and you have pretty much a solid 13 sec car!
that burns no oil and the oil stays way cleaner than my 493 with forged ross pistons.

what i am saying for a tough sounding cruise night car no one knows what the pistons are LOL !