Hypereutectic Pistons & CR

[Brandon K.]

Anyone had any problems with hypereutectic pistons? I know of one person who had the top come off one, and needed his engine sleeved. Was wondering about the experiences of everyone else. I used them in my 67 sb, but havent started the engine yet.... trying to decide what to use for a 69 350/350.

Also, what is the highest compression ratio you can use (with point ignition) and still be able to use pump gas?

thanks, Brandon

[Mark #28455]

Re: Hypereutectic Pistons & CR

The biggest difference with the hypereutectic pistons is that they require a wider top ring end gap. If the engine builder uses the "usual" gap, it is likely the ring ends may touch during hard use of the engine and cause the top ring land to break off. This will look like a chunk of the piston top broke off.

Many budget oriented racers are using hypereutectic pistons with good results. There are dirt track cars around here regularly making well over 400 horsepower and turning over 7000 RPM. While I won't guarantee your success at trying that with your motor, I also can't see why your pistons wouldn't hold up to street use.

[Jack H.]

Re: Hypereutectic Pistons & CR

On what's the highest CR you can go and still use pump gas, well that's a loaded question. If you happen to live at altitude (I'm 1/2-mile ABOVE Denver), you can easily get away with 11:1. Also, there's a definition question here about what's 'pump gas' -- the octane content varies across the US based on location specific blend formula(s).

In most places, you can get 92-94 octane at the pump (research + motors method averaged). At/near sea level, you'll be 'pushing it' with an 11:1 CR engine and might have to retard your timing or use an octane boost additive. Most feel that something in the 10:1 range is 'comfortable' for today's range of premium available from the average pump.

[Joe L.]

Re: Hypereutectic Pistons & CR

Brandon------

Some engines that use hypereutectic pistons:

92-96 LT1
1996 LT4
1990-95 LT5 (ZR1)
ZZ2,ZZ3, ZZ4 crate engines
LS1
LS6 (Z06)
LS2
LS7 (2006 Z06)
8.1L Gen VII big block (truck)
8.1L Gen VII big block (all marine variants)
500hp/500cid Viper engine

[Brandon K.]

Re: Hypereutectic Pistons & CR

Thanks for your answers, gentlemen.

So should the rings that come with the pistons be correctly pre-gapped? If the top rings have such a large gap, should the lower ones be gapped smaller (file to fit) to compensate? I understand that pre-gapped rings usually have a larger gap than necessary.

thanks again, Brandon

[Mark #28455]

Re: Hypereutectic Pistons & CR

You will have to check the ring end gap and adjust as needed - use a ring filer. Most pre-gapped rings are NOT correct for hypereutectic pistons. That's likely why your friend's piston broke.

Don't assume anything about correct ring end gaps - follow the instructions that come with the pistons to the letter and you won't have problems.

[Craig S.]

Re: Hypereutectic Pistons & CR

Brandon - I agree with Mark, never assume the ring gaps are right if the bores are honed to the piston spec. I am not aware of pistons that come with rings, usually they are sold separately, and come in file to fit or pre gapped. I prefer file to fit......Craig

[John H.]

Re: Hypereutectic Pistons & CR

We buy the Viper hypereutectic pistons from Mahle, who supplies them already ringed, with our "cracked" rods and 200,000 psi SPS bolts installed; as the piston manufacturer, they ensure that the ring gaps meet their specifications. We just install the bearing inserts and assemble them to the engine.

Don't make the mistake of buying a standard pre-gapped ring set and installing them out-of-the-box on hypereutectic pistons; follow the piston manufacturer's instructions to the letter for ring gapping, particlularly for the top compression ring.

[Brandon K.]

Re: Hypereutectic Pistons & CR

And how much might I expect to pay for the luxury of not having to measure the gap? (I know Mahle pistons ain't cheap!)

Brandon

[Clem Z.]

mahle employees are not UAW members

so you are sure the fit is correct

[Chuck S.]

Hey, "We Buy..."

Has a present tense verb, John...Don't tell me you've weakened and gone back to work?! If so, yer givin' the "Wurfless Club" a bad name.

Wurfless Forever In West Texas

[John H.]

Re: Hey, "We Buy..."

Nope, haven't gone back to work, and don't intend to - after six years of being immersed in that car and its engine and in the race team, it's hard to refer to it in the past tense