Hi Everyone,

First off, let me say I know I am over-thinking this.

A few pistons in my 305 LU5 (Crossfire) are scuffed and need to be replaced. I will rebuilding the engine back to stock (for originality) with the exception of a very mild cam change. So no bolt-ons or big power adders, etc. (not that you could do that much with a CFI anyway). This means I'll be creating 165-175 HP max.

Given this is a completely stock rebuild, is there any benefit to going with hypereutectic pistons over regular cast?

This motor won't make enough power to hurt itself and I am not sure I will get any benefit from the slightly lower weight and lower thermal characteristics of the hypereutectics.

From what I've read so far, hypereutectics seem to be sensitive to ring gap and detonation, but I also know they are in widespread use so they can't be all that fragile and in my application they won't be under any real stress.

I'd appreciate opinions on this. As I said, I know I'm overthinking this and should probably just get regular cast slugs and move on, but I am curious if I would gain anything by going with the hypers.

 

If you are not re-balancing the crank, then whatever pistons you choose must be nearly equal in weight to what you are replacing.

 

I say go with hypers. They are stronger in compression and tension than a regular cast piston because of the added silicone. Hypers are still a cast piston though. But they have better sealing than regular cast and run tighter in the bore than a regular cast piston. And many aftermarket hypers can be bought with a "coated" skirt to help sealing even more.

If you want some budget hypers take a look at Silvolite #1482HC for 1982-1986 305's. Jeg's sells'm for $150/set. I'm looking at similar piston for my budget LT1 build. But there is the added cost of pressing them on as they are a pressed fit piston and require press fit rods to. I don't think you will find stock cast pistons cheaper than that.

As for sensitive to ring gap that's the first I've heard of that. And you say sensitive to detonation too doesn't make sense as they are stronger than a regular cast piston (due to better material strength) which would make a regular cast piston even more sensitive to detonation.

 

Don't get so would up in minimizing the "cost" of what you are buying TODAY, that you cost yourself MORE in the future. Building an engine affords plenty of opportunity for doing just that. Spend more than the absolute bare minimum possible, WISELY, in places that make a difference to your outcome, and you'll be MUCH happier in the finished product. There are few things in life that are worse than spending $2000 or whatever to build a motor, and have it make less power, burn more oil, cost more to maintain, and last a shorter time, than it would have if you had just left it alone.

Hypereutectics ARE different about their ring gap than either forged or regular cast metal ones. They tend to spec a bit greater gap than others, but it's not a big deal, just follow the instructions and they'll be fine. Be sure as you "adjust" them though, not to nick the sealing surfaces of them - not the "face" where they seal to the cylinders, nor the "sides" where they seal to the grooves in the piston - or throw up a burr or any of that.

Hypers do have an advantage in a stock-ish motor, in that they don't "leak" as much heat out of the cyl and into the crankcase as other types. In that regard they produce a bit more power and get a bit better gas mileage than other types, all else being equal. (which as we all know, it never is...) Detonation is more destructive to them BECAUSE they are "harder": they tend to shatter, like glass. But in a stock-ish motor, if you have ANYWHERE NEAR enough detonation to cause that, you have MUCH bigger problems anyway; I wouldn't worry about it.

More important to long-term happiness is, have the block bored and honed with a torque plate. If there is ANY ONE THING you can do that will pay off in a better-running, longer-lasting, less oil-burning, etc. engine, it would be that. Again, the small increase in cost will pay for itself over the not very long term in improved gas mileage and lower maintenance cost.

One problem with the cheeeeeeeeper hypers is, they add the .020" of "rebuilder" deck clearance, which KILLS compression and therefore also power. There are a few that don't, but they're not the cheeeeeeeepest ones; some of the Keith Black ones in particular might not compromise your build that way. The miniscule difference in price will probably pay for itself in improved gas mileage over the cheeeeeeep ones.

Watch out on the cam change, to stay COMPLETELY within the range of the stock tune. The ECM for CFI is basically from the 70s; very slow, very little memory, very crude. A popular upgrade for it, years ago, was to put a later-model TBI ECM in there, since after all, that's all CFI is in the first place. Somebody... can't recall who... even made an adapter harness to where it was just a plug-in, no idea if any of that is still available.

Otherwise, about the biggest cam you can go with that won't cause trouble, would be something along the lines of the Comp XE250 or their "dual energy" 255; or even just the cheeeeep Summit 204/214 one.

 

Given your power goals and intentions of keeping the CFI, I would knurl the piston skirts, and slap on a new set of rings - assuming the cylinders are in good shape (don't hone them if they are reasonable). Swap cam if you wish, and move on. Anything beyond that really isn't worth doing to a 305 at this time.

Otherwise it's a full rebuild - bore and plate hone, line hone, rebalance with new pistons, etc, etc - and as has been stated MANY times - not worth rebuilding a 305 - get a 350. Keep the old engine under the stairs if "numbers matching" ever becomes a concern in the future.

GD

 

Thanks to everyone for the feedback and comments. This is kind of stuff that makes thirdgen such a great resource.

@NoEmissions84TA: I completely forgot about the weight possible difference between the standard cast and the hypers relative to the crank. I'll check with my engine guy.

@cardo0: I read a few things online (FWIW) about the hypers being more sensitive to gap and that it can cause failure.

@softakingdom: Thanks for the detailed info. I am definitely not looking to cheap out on the slugs by going with standard cast vs hypers. My goal is reliability since I don't want to do this again any time soon. My question is really more about whether or not hypers (irrespective of cost) would really net me anything long term. The points you bring up about sealing, ductility and shattering are exactly what the types of things I've read about. Thank you also for the tip on 0.020" deck clearance. The CFI has 9.5 compression and I definitely don't want to give anything up. My engine guy went over the block and it needed no machining - just some light clean up. So right now everything is factory spec. The cam I have installed is a Comp XE256 which (as I recall when I was researching it) was the upgrade cam of choice for stock CFI motors. The CFI ECM is super slow and I will have to take a look at what it takes to upgrade to the later model ECMs. I am sure that would make a big difference.

@GD: The block is numbers matching and with the exception of the cam swap, I am keeping the 305 installed purely to retain the driving experience as it was when the car was new. For me, this is not about performance but it is about how the car left the factory and what someone buying a new-for-'82 Z28 experienced.

Thank again to everyone for your comments. I think I will go ahead with the hypers.

 

Woof. It wasn't a good era for sports car engines. OK if that's what you want but quite frankly if the GM engineers could have given it more power and stayed within emissions guidelines they certainly would have. Personally I would rather experience what the engineers envisioned it *could* be prior to the engine being strung up by it's genitals as a sacrifice to the EPA. I would say that most prospecting buyers of new '82 Z28's either had no idea what performance should feel like or walked off the lot and went looking for a used 60's model. Either that or they were just in it for the cool looks of the car - cause it certainly didn't have anything to back them up with. My 86 LB9 was and is a joke with 190 compression on all 8 (runs amazing, just no *****).... and that was arguably after they got some performance back in them with TPI.

Also - if you are changing the cam - how is that going to replicate new car buyer experience? Almost no one voids their warranty like that.

GD

 

@GD: yeah, you got me there on changing the cam with respect to originality and experience. So maybe I'm at 90% experience.

I had an '84 L69 back in the day and it was a blast driving that car around -- even though it also only had 190 HP. I'm sure if I were to get behind the wheel of one today I might feel differently, but I have a couple of other GM LS cars in my stable to feed the performance need, so the '82 is purely a nostalgia trip.

 

I think if I were in your shoes, I might consider changing the heads and upgrading to the Renegade Crossfire intake manifold. It would still be "in the spirit" of the factory CFI injection - just better. Remember, your intake has those 2/3 sized intake ports.





They claim 32 HP and 16 ft/lbs TQ increase from the intake change ALONE.

 

That's no lie. The ports on the factory CFI intake are truly horrible as the pic you posted shows.

I believe the 32 HP/16 TQ numbers you are quoting for the Renegade are dyno results from a Corvette 350, not the 305. Even so, I would imagine there is a good gain.

When I was researching the intake a while back there were definitely comments about fitment and foundry issues people had to contend with. Nothing serious, but like any aftermarket part, it needed a little tweaking. I'll have to do some research to see if anyone has done a Renegade install recently and with a 305 ...