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Discussion Starter · #1 ·
Ive got a completely fresh 350 long block back from the shop, its got brand new aluminum heads (2.02 , 1.60), lunati split duration cam with 1.6 rockers totaling around .530 lift, steel crank, all being backed by a 700r4 but here is the unsettling part... cast pistons. i sent this engine to the machine shop a long time ago and hadn't known as much about parts matching as i do now, that machine shop was lazy and i have found a much better place i take all my engine to now. so the question is what are my limitations in terms of rpm? should i get these replaced with forged?
 

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More for Less Racer
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As long as you don't run it into detonation, the cast pistons will live to a higher rpm than the stock rod bolts will.

The cast pistons will handle 6500 rpm as long as it isn't held constant at that rpm....at 6500 rpm the stock rod bolts are under serious stress.

Detonation will kill the pistons almost instantly above 4500 rpm, so compression ratio, quench clearance and fuel octane are all issues here.
 

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Forged piston will fail safe. In other words, a forged piston can have the center blown out of it and it will still hold the rod in place and save the motor.

If a cast piston fails, it will come apart and the loose rod will take out the engine.

I would use forged Pistons in a grocery getter. You can set up Speed Pro forgings at .0035" skirt clearance with 5/64" top and second rings and still maintain good oil control.
 

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Not to be rude; I'm curious.

Why would you put a 50yr old design forged piston with antique rings in a motor you're investing in under the pretense of 'durability'? Its a 350 for gosh sakes. I think my local quickie-mart can get me a good deal on a modern forging with a modern ring pack. If I was really concerned about 'durability' in a street engine to want a forged piston; I'd easily pick a metric ring pack forging. Quicker sealing, free horsepower and better fuel economy. Unless you're playing with 265s, 283s and 307s; theres NO reason to buy pistons with old school rings. Unless its a quick and dirty 'dingleball hone and go' with cast iron rings.

The OP would be fine with a hyper-eutectic piston, and following the piston mfg standards for assembly. As Eric said, I'd pop for ARP fasteners and a re-size job; if not a more modern rod altogether.
 

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Ive got a completely fresh 350 long block back from the shop, its got brand new aluminum heads (2.02 , 1.60), lunati split duration cam with 1.6 rockers totaling around .530 lift, steel crank, all being backed by a 700r4 but here is the unsettling part... cast pistons. i sent this engine to the machine shop a long time ago and hadn't known as much about parts matching as i do now, that machine shop was lazy and i have found a much better place i take all my engine to now. so the question is what are my limitations in terms of rpm? should i get these replaced with forged?

RPMs are more a limitation on rod reliability and ring seal than the piston. The piston itself is more at risk to detonation and preignition for sudden death. At that these events usually cause a ring failure that takes out the side of the piston. That is because detonation and preignition raise the temperature of the rings beyond the ability of the gap to keep the ends apart. They butt together to bend the ring till it busts the piston. Old mid alloy cast pistons usually hold together but burn out a side or loose the center, hyperutectic castings will take a much higher beating than old alloys but when they go they frequently just come apart and let he rod and pin bash the bore wall to death. Forged pistons push the failure limit higher and usually hold onto the pin and rod when they fail but they certainly aren't immune to failure.


Often at high RPM it is the inability of the rings to maintain seal that leads to detonation and preignition failures by allowing large amounts of oil into the combustion chamber which quickly reduces the octane rating of the fuel resulting in detonation. Or by forming carbon deposits that overheat causing preignition. In actuality both of these things are happening at the same time in this operating condition.


As for your pistons it depends a lot on how often the engine goes to high RPMS at and over 6000 and how long it stays there. In actuality if the rings are gapped in the expectation that the engine will run fast and hard for long periods or be subjected to heavy shots of nitrous or super charging of some sort it is possible for cast pistons especially hypers to put up with the power forces rather well.


So it comes down in many ways as to how the engine will get used. And of course what you haven't mentioned which is what are the rods, what is the crank, is the block a 2 or 4 bolt, how well is the rotating assembly balanced, and how good of a damper have you put on it.


Bogie
 

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I cant believe no one is asking how many bolts are on your mains! This is a huge limiting factor of RPM. Some say that with 2 bolt mains, 6000rpm is pushing it....
The 2 bolt vs 4 bolt main issue comes down to application. Daily use with some occasional tire burning up to 6500 is fine on any well prepped 2 bolt main block in the 300-400HP range. On the other hand, alot of strip use or street abuse is another story.

At least that's my opinion - humbly:thumbup:
 

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The 2 bolt vs 4 bolt main issue comes down to application. Daily use with some occasional tire burning up to 6500 is fine on any well prepped 2 bolt main block in the 300-400HP range. On the other hand, alot of strip use or street abuse is another story.

At least that's my opinion - humbly:thumbup:
God, im wondering how long my 383 2 bolter is going to last. I run that thing at RPM for hours at a time!
 

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I think the 2-bolt vs 4-bolt thing has a healthy dose of "more bolts must be better", not to mention bad balance jobs, suspect machining etc etc.

Good prep, attention to detail, a good solid tune that keeps you out of detonation, and a 2-bolt block 383 should live for a long time at 6500rpm.

I wouldn't even pay extra for a 4bolt block to use as a core (gasp!) For me? its either 2-bolt streeter stuff; or splayed 4bolt conversions....or screw all this old fashioned stuff and go with an LS platform :-D
 

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I didnt want to hijack the thread, but yeah, I make about 420hp on my 383 2 bolter and do road course work with the car. Typical sessions are 30 min long and I dont see much below 4000 rpm. I think i put 4 hours of course time on in one day. My setup is making hp all the way to 6000rpm, but i never take it past that. Only time i have ever seen any detonation was at low rpm when i had some tuning issues when i first got things running.
 

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If your car must have a catalytic converter to pass annual state inspection , you must use standard cast Pistons or cast hyper-eutectic Pistons.

Otherwise, it is forged Pistons hands down.
This is just simply not true. A forged piston will not cause you to fail emissions testing anymore than a cast, or, hyper piston.There are modern OEM engines on the road everyday that pass emmisions with forged pistons.If the clearance & ring gaps are properly set, the emmisions will be no worse on a forged than a cast/hyper.If they are not set properly, either will fail.Unless the test is performed on a cold engine at startup, would be about the only time, that MITE, hold up.
 

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fat tire
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I cant believe no one is asking how many bolts are on your mains! This is a huge limiting factor of RPM. Some say that with 2 bolt mains, 6000rpm is pushing it....
back in the mid 70s I ran a dirt car with a 327 .060 over 2 bolt main steel crank, 12.5-1 forged pistons with a 5:38 rear. it twisted 7200 Rs for two seasons. it was still running when the track closed
 

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Discussion Starter · #15 ·
would need to know a lot more about it. But when asked if cast or forged forged is the over whelming best bet.

Compression and rod type are also important. If the rods are stock small journal rods and have the stock bolts there is certainly an rpm limit. about 6K without arp bolts in stock rods and some small journal stuff may let go sooner than that. Sp pistons might not even be the issue. Some factory rods will let go a lot sooner than 6K rpm of sustained use.

fyi are they cast or hyper pistons most are not cast in the after market world. What is the quench as well.
Stock rods with arp bolts, flat top pistons, around 68 cc chamber heads
 

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The only SB Chevrolet engine that needs 4-bolt mains for high RPM is a 383 CI stroker. The rod angularity of the 3.75" stroke and 3.700" rods places a higher load on the main caps of those engines at high RPM.

All the stock two bolt SB Chevrolet engines can go to 7,000 RPM safely if upgraded with main cap studs, forged Pistons and aftermarket 3.700" steel rods.
 
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