Failure of stock cast 3.9/4.0 pistons at higher RPM? Or Forged options?

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turbodave
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Failure of stock cast 3.9/4.0 pistons at higher RPM? Or Forged options?

Post by turbodave » Mon Dec 30, 2019 2:28 pm

AE Hepolite
HRC1828
22059

The "books" suggest that 6K rpm is all these are good for, with maybe an occasion blast up to 6500 rpm.
Has anyone taken these higher on more than an occasional excursion and suffered (or not) a failure?
I'm trying to decide if I should stick with them and pick a cam that starts to peter out at 6200rpm but has a strong mid-range or go forged (second half of this post).
I've got solid adjustable rockers (and would therefor use a solid, not hydraulic cam) so the only limitation will be the factory pistons.

This will be used in an endurance racing engine (literally running for 12 hours over a weekend) so I do need something that's going to work well and be reliable.

Or, do I go for forged pistons?

Maybe the Chevy 305 engine or ford modular 3.700" ?
Will the factory 3.9 liners be reliable with the 3.736" bore of the Chevy 305 pistons?
Is the shorter 1.22" compression height of the ford piston compared to the 1.56" of the 305 better or worse?

What rods could be made to work with either? I've not prioced up a set of Omega's but I have a feeling its entirely possible they are way more than I'm wanting to pay when I can get a set of the above ones for $600...

Lots of questions, looking at options and suggestions.



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Rossco
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Re: Failure of stock cast 3.9/4.0 pistons at higher RPM? Or Forged options?

Post by Rossco » Mon Dec 30, 2019 3:27 pm

Hi Dave

Why not just settle with a rev limit of 6k, for the cost of forged pistons it would make any additional rpm very expensive.

Mc Tool
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Re: Failure of stock cast 3.9/4.0 pistons at higher RPM? Or Forged options?

Post by Mc Tool » Mon Jan 06, 2020 6:40 pm

You just need talked out of forged slugs. :D They are generally heavier than cast slugs , thus placing more stress on the rods and rod bolts ,specially if you want to increase rev limit. So you'd be up for at least, a set of rod bolts and some rod work ( shot peened & polished beams ) or even a set of Carrillo/chev SB rods.
I have never been endurance racing but I would have thought that with reliability and cost in mind it would make sense to get something workable with a modest rev limit . :)
I wish I was young again , Id be heaps smarter than the first time

bigaldart
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Re: Failure of stock cast 3.9/4.0 pistons at higher RPM? Or Forged options?

Post by bigaldart » Sat Jan 18, 2020 8:42 pm

We run 6^ Scat Chevy Rods, narrowed to suit, also need to narrow Chevy bearings to suit. Keith Black Ikon forged pistons built for a stock stroke, stock bore Chevy 305. Piston is 3 thou out of the bore and is flat topped. However we run a 40 thou thick head gasket. This gives around 11-1 compression. This set up has taken a lot of abuse and stood up well.

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Re: Failure of stock cast 3.9/4.0 pistons at higher RPM? Or Forged options?

Post by oneaves2005 » Thu Feb 13, 2020 2:53 pm

In the BCV8 championship, there is a class that uses standard pistons (and bottom end for that matter), there are around 8-9 cars revving to 6500+, no piston failures that I can remember in the last 6-7 years I have been involved. The cars have around 230bhp, with a mild camashaft and intake and exhaust mods only.

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