Forged or cast 427 pistons...?

[Greg L.]

Okay, now I need to pick your brains again.

Are there any noise issues during warm up with todays forged pistons? I just had a look in my O/H manual and cast 427 pistons run at about .001-.002 but the forged ones run at I think it was .006-.008 so that would explain the noise untill the skirts expanded. Just wondering if todays forged pistons still run at those "loose" limits or if I would be okay noise wise going with forged as well as cast.

Thanks,

Greg Linton
#45455

[Joe L.]

Re: Forged or cast 427 pistons...?

Greg-----

Some of the forged pistons available today can be used at tighter wall clearances than forged pistons of old. However, today's cast pistons are a lot more improved than cast pistons of old.

Hypereutectic cast pistons are the way to go for any street application. They are now used for virtually all of GM's high performance engines, including the LS7 engine in 2007 Corvettes. I really doubt that GM used them for a cost savings of maybe $100 per set over forged when they are being used with titanium connecting rods costing several hundred dollars each.

The best advice I can give is this: if you are building an engine for street use, hypereuctectic cast are the way to go; if you are building an engine for racing, forged pistons are the way to go.

[Duke W.]

Re: Forged or cast 427 pistons...?

The OE forged piston specified clearances are about half of what you said. See the applicable service manual or AMA specs for the specific engine.

My opinion is that if you have a SHP engine that originally had forged pistons, you should use the OE replacements from Speed Pro, which I believe are available for all OE applications. Compression ratio can be managed to acceptable levels for current unleaded premium gasolines by careful measurement and management of deck clearance, chamber volume, and head gasket thickness.

The OE forged pistons didn't "cold slap" in most cases, but they probably did make a little more mechanical noise, which is part of the SHP cacophony, especially with mechanical lifter cams.

There are two common aluminum alloys for forged pistons - 2618 low silicon alloy and 4032, which has higher silicon content.

The OE forged pistons are 4032, and since they expand less than 2618 they don't need as much cold clearance; 2618 is the most durable and will take the most abuse (detonation, boost, etc.), but need greater cold clearance. These are the serious all out racing pistons.

Running clearance is also somewhat a function of piston geometry details, particularly the transition from the skirt to the crown. Always follow the OEM or piston manufacturer's recommendation for cold clearance.

Duke

[Mark #28455]

for street, use the hypereutectics

The new hypereutectic pistons are FANTASTIC for street and street/strip type racing. They are a high quality product that should last 100,000 miles with reasonable use and maintinence. See the KB-silvolite web site for more details. I have used both the KB and Speed Pro pistons and am very satisfied.

Mark

[Jim T.]

Re: Forged or cast 427 pistons...?

My 85 Corvette with original engine came out of the Bowling Green factory with forged pistons has no piston slap after 22 years of driving and 102,000 on the odometer.

[Clem Z.]

Re: Forged or cast 427 pistons...?

F/M forged piston L-2300 are put in at .0015 and L-2268 are put in at .0035 so there should be little if any noise when cold

[Barry Chappell]

Re: Forged or cast 427 pistons...?

I only have experience with SB forged. My engine came with TRW forged and from new was always noisy until warmed. Sounded like a diesel when cold.
I rebuilt the engine in '96 with .030 TRW 11:1 forged.
However, I fitted them to an exact .004 (tighter than GM spec) and the engine has not displayed the cold "rap" as it did when new.

My oil consumption now is less than when the engine was new. For my driving habits now, .004 is quite sufficient.

Barry

[Duke W.]

Re: for street, use the hypereutectics

There are a number of reports of broken Keith Black hypereutectic pistons, and some "engine builders" refuse to use them.

KB hypereutectic pistons have a very high mounted top ring, which runs hotter than the lower top ring location on OE and typical OE replacement pistons and requires greater than OE top ring gap. If ring gap is insufficient and ring butting occurs, the top ring land will break.

I believe that the vast majority of these failures are due to the "engine builder" or whoever assembled the engine NOT following the KB instructions for minimum top ring gap.

For the "Special 300 HP" engine in the works, I specified the KB 157, which has a 0.5" cc dome. This slightly domed piston was required to achieve the design compression ratio range of 10-10.5:1 that is required due to the very late closing inlet valve.

Also, a cast, snug fit piston is preferred to ensure that mechanical noise is indistinguishable from an OEM-built 300 HP engine since this is intended to be a "sleeper" - idle and overall operating characteristics indistinguishable from an OEM-built 327/300 (and pass a PV), but a lot more top end power and useable revs.

For replacing OE cast pistons, I think the KBs are an excellent choice, and the KB 156 (-7cc) or Sealed Power/Speed Pro H660CP (-5cc) cast hypereutectics are more suitable to achieving the target CR range of 9.5- 9.75:1 for a 327/300 restoration that will use the OE 929 cam.

However, I still prefer the OE forged replacement pistons from Speed Pro for any SHP/FI engine that originally had forged pistons.

Duke

[Greg L.]

Re: Forged or cast 427 pistons...?

Thanks guys. I'm not looking for for anything fancy...just something at least as good as the General used back in 69 AND will just drop in without any fancy rings or rods, etc. So I guess either the hypereutectic or forged will be fine as long as the forged runs at less than .004 so that it's quiet. I guess the hypereutectic will probably be cheaper than forged so that's probably what I'll look at.

[Clem Z.]

make sure you follow the top ring gap spec

because hyper pistons hold more heat and need a larger top ring end gap than forged pistons. make sure your engine builder understands this or you will have broken top ring when they end butt. some times it will even break the piston

[Greg L.]

Re: make sure you follow the top ring gap spec

Thanks for the tip Clem. Actually I'm the engine builder except for any boring or decking that it might need...just kidding on the decking part!!

I'm sure I'll have plenty more questions before the valve covers get bolted on...

[Joe L.]

Re: make sure you follow the top ring gap spec

Greg-----

The information regarding the ring end gap for the KB pistons is VERY prominently called out in literature supplied with the pistons. They make the point very clear.

I'm also quite sure that virtually all, if not absolutely all, of the problems with KB pistons is due to failure to observe these instructions.

The hypereutectic pistons sold today are FAR superior to the conventional cast pistons used originally on all L-36 and L-68 Corvette engines. They are roughly twice as strong and far stronger than they need to be.