Do I really need a forged carnk and forged pistons? - Team Camaro Tech
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post #1 of 12 (permalink) Old Nov 30th, 00, 09:01 PM Thread Starter
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I'm building a 461cid rated at roughly at 420hp and 500lbs/tq (according to dyno simulations). It's a street car with an attitude.

Can I get away using the stock cast crank, or do I really need a forged crank? Some stores say yes, while machine shops say not really. Who's right?

What's the advantages and disadvantages with Hypereutectic and Forged pistons? Will I need forges pistons as well?

I'd like to keep the rebuild cost down, but if I need to go forged, I will. It'll just take a little longer to save up for the parts.

[This message has been edited by Allen M (edited 12-01-2000).]
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post #2 of 12 (permalink) Old Dec 1st, 00, 03:59 AM
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Allen, all my hands-on machining experience was as a kid in the mid 70's We NEVER used cast parts for performance applications unless it was one of those British tractor motors.

Cast is OK if you stay at 6000 rpm redline. Hyper?whatever pistons are just cast pistons. I wouldn't use cast slugs, but I would use a cast crank. Again IMHO.

I think its more important to make sure the machine work is top notch, the rods have no angualrity, saddles are right, etc.

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post #3 of 12 (permalink) Old Dec 1st, 00, 05:26 AM
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You'll get lots of preferences on this one, so I'll start the thread with mine If budget isn't a big concern, I like to use forged cranks, just because they're basically bulletproof. However, being more realistic, nodular cast cranks are just fine for 99% of situations, unless you're going to spin it over 6000 rpm frequently or go the nitrous route. Forged pistons are great, except they require more piston-to-wall clearance and tend to be noisy at cold start as a result; I'd only use them under the same 6000+/nitrous situation mentioned above. For a street engine, I've found Keith Black hypereutectic pistons to be trouble-free; they don't slap when cold, are less prone to skirt scuffing, and save you some money as well. If you intend to go the 6000+ and nitrous route with a forged crank and pistons, don't skimp on the rods - use high-quality "H"-section STEEL rods (like Eagle ESP's) or similar offerings by Lunati or Crower, with ARP or SPS bolts. Rods are the most critical link in the reciprocating assembly, and they break much more frequently than cranks or pistons. In any event, for a street engine, buy the best production-type rods and bolts you can afford; much stronger rods than the production parts are available for $300-$350 per set instead of spending $500-$700 for the top-of-the-line "H"-section rods. Don't even think about aluminum rods for a street engine - they're race-only and have limited fatigue life. Summary - a cast crank, hypereutectic cast pistons, and upgraded rods and bolts (properly balanced, of course)will work just fine in 99% of street engine applications. Just my opinion and experience.

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post #4 of 12 (permalink) Old Dec 1st, 00, 06:23 AM
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I ran a stock car with a 355, cast crank ,and stock rods (good bolts) on a dirt track . rpm's from 2800 in the turns to 6800-7000 at the end of the straights never had a problem .the motor is still running in someone elses car now ,
maybe i'am lucky I don't , but that was always rolling not stoplight to stop light

just my thoughts

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post #5 of 12 (permalink) Old Dec 1st, 00, 10:32 AM
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I concur with JohnZ on this one.

Although hypereutectic pistons are a cast alloy, they are not equivalent to a "cast" aluminum piston. The hypereutectic alloy piston contains approximately 20% silicon, which not only results in a much stronger composition, it does not thermally expand as much as a forged piston does. This allows a much tighter piston-bore clearance, which, as John stated, reduces cold piston rocking and blow-by. I know of many people running these pistons, including myself, in street applications over 450hp.

The downside of the hypereutectic piston is that, because it is more brittle, MAY be more susceptible to detonation than a forged piston, although I have never heard of an instance of piston breakage soley as a function of the material.

With respect to forged vs. cast crankshafts, especially for a nominal, properly built STREET application, I believe a forged crank is overkill. Besides low cost, a cast crankshaft tends to transmit vibration less than a forged crankshaft; the less resonances the better.



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post #6 of 12 (permalink) Old Dec 1st, 00, 02:33 PM
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Well, here's another topic David Pozzi and I had the opportunity to discuss at SEMA.

Tom Leib, President of Scat Crankshafts, stated that his SCAT cast steel cranks are being used in 650 hp small block roundy-round engines with no failures, but have also failed in 400 hp engines. Seems that quality machine work and using the proper vibration damper is key. He said to use only an elastomer style damper for street use. Seems the majority of broken crankshaft snouts are using a fluid-type damper.

For regular use above 6500 RPM a forged crank is required. According to Tom, RPM is the killer, not power.

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post #7 of 12 (permalink) Old Dec 1st, 00, 02:45 PM
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If you're not gonna beat on it then use the cast stuff!!

BUT, if you're gonna beat on it, buy the forged stuff!!!

Forged cranks are going for around $5 to $600 with some stroked a quarter. And something like a ROSS piston should fall in there too.

As for rods and bolts, RPM is the killer here!!!

Hold your rpm's down to 55-5700 w/max of 6000rpm and stock 3/8 bolt rods are fine with aftermarket bolts. Otherwise, get a lot better for 7000+rpm's!!

And, later on if you decide to use a little squeeze(NOS) or add a lung(blower), OR overwrap(high rpm's), you're covered.

And, don't forget to balance the whole she-bang. IMHO---pdq67



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post #8 of 12 (permalink) Old Dec 2nd, 00, 08:17 AM
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I agree with the quality machine work being a large factor in failures as well as vibration. A cast crank should be fine. We have a set of el cheapo .060 over 350 pistions that came from the budget rebuild kit at the machine shop. They are now in their 3rd motor with no piston failures(obviously). We're cheap and re-use as many parts as we can. The pistons are now in a 10:1 motor that has over 500 low 13 (and one 12.95) 1/4 mile passes on it plus being driven everywhere including the track. Anyway, we've had good luck with cast pistons, even the cheapest ones. We may just be lucky, who knows. I thought it would be good to hear another thumbs up for cast pistons.
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post #9 of 12 (permalink) Old Dec 4th, 00, 07:20 AM
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I agree with JonZ on the pistons. Good stuff when used properly. Cast cranks are OK for hot street use and some abuse. It IS all in teh proper machine work. Try a cast STEEL crank from eagle (JEGS). Cheaper than forged, stronger than cast iron. I have abused one for some time now, no problem.
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post #10 of 12 (permalink) Old Dec 4th, 00, 08:20 AM
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I have to agree with Carl C. on this one. A friend of mine owns a machine shop and he uses SCAT cranks and rods and hasn't had any trouble in street engines and circle track cars. I have the SCAT rods in my engine.

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post #11 of 12 (permalink) Old Dec 4th, 00, 05:24 PM
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Not saying I wouldn't rather have a set of top Quality forgings,but I have been running the KeithBlacks.I have 50+ passes on them and lots of hard street miles.355ci. 10.5 lbs. boost best 1/[email protected] I've had one failure which I felt was a result of reckless tuning decisions.Detonation butted the rings and broke 2 pistons. Luckily the motor suffered no other damage.Now everythings been together for A while with no other problems.600-700hp and 7200rpm shifts has sold me on these $235 slugs. At this power level I don't thing any piston would tolerate tuning mistakes. SOLD
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post #12 of 12 (permalink) Old Dec 4th, 00, 05:34 PM
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Just want to add that I wouldn't even consider blowing 500-600 dollers on a set pistons for a street moter that makes .91 hp per ci. or fancy crank+rods under 6500 rpm's for that matter.
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