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OEM connecting rods

 
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Old 04-10-2018, 02:05 AM
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Default OEM connecting rods

any of you know how much power can they endure the original connecting rods of the 4.2 supercharger engine?
 
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Old 04-11-2018, 12:12 PM
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Originally Posted by deuce2000 View Post
any of you know how much power can they endure the original connecting rods of the 4.2 supercharger engine?

I don't know the number off hand but the powder metal sinter forged rods don't have much margin.


I would change them out above about 450 Bhp for forged affairs.


I picked that number because that's what we did LIMITED testing on for the XK-180 concept car years ago.
 
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Old 04-11-2018, 12:36 PM
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Our rods are cracked powdered metal rods. (Likely ASTM 4260 material) If you search the Chevy and Ford forums as well as engine build articles, you will find that here is a consensus that you shouldn't run this type of rod much over 500 hp.
Some have written that with ARP, or equivalent rod bolts, they have had success up to 600 hp. Some new Chevy powdered rods are being "engineered" to support 600-700 HP.

The biggest factor in "PPF" (precision Powder forged) rods is not that the material is weak, but that the rods are CAD designed specifically for the engine's performance specification. The rod mass is minimized for cost and maximized for engine life at a specified power rating. (Perhaps "the Count" knows what max HP the rods were designed to.)
 
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Old 04-12-2018, 01:22 AM
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the competition car of the Ferlito team uses the original connecting rods and has never had problems. it is probably a risk to greatly enhance the power of the engine leaving this rods but if I'm not mistaken even those who have fitted the famous Twin Screw kit left the original connecting rods without anyone having had any problems
 
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Old 04-12-2018, 03:07 AM
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Avos has driven years after years and huge amount of kilometers with stock 4.2 rods....the limit has not been reached.
 
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Old 04-12-2018, 01:40 PM
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Anecdotal evidence with no statistical Weibull back up data is all very nice and praising others kits in that it gives a warm fuzzy feeling, I'm sure.
It all comes down to whether or not you feel comfortable eating into Jaguars safety margins and what your appetite for risk is.


I'm very much a fan of over engineering my stuff
 
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Old 04-12-2018, 06:35 PM
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Originally Posted by Count Iblis View Post
Anecdotal evidence with no statistical Weibull back up data is all very nice and praising others kits in that it gives a warm fuzzy feeling, I'm sure.
It all comes down to whether or not you feel comfortable eating into Jaguars safety margins and what your appetite for risk is.


I'm very much a fan of over engineering my stuff
AKA - Fatigue limit. The highest stress that a material can withstand for an infinite number of cycles without breaking.

Assume the stock powdered metal connecting rods were designed for infinite life up to 450 HP. (Cross sectional area/shape of the arm at the rated yield strength of the powdered metal alloy used. + considerations for heat treating and with stress-risers relieved, and max engine rpm)

Once you go past the rated hp/rpm threshold, it doesn't mean that the rods will instantly fail, but now they will have a finite life. The more times the rod sees higher stresses, the sooner it will fail.

In the "race car scenario" The car may be raced a few seasons before they rebuild the engine. Perhaps they put a couple thousand miles on the stock rods. Say at 500+ HP, the race engine can last 10,000 miles of racing. They know that they have gone past the fatigue limit for the rods, but don't care because they will throw them away after a few seasons.

In Avos' engine build, I am sure that his rods have a finite life. I doubt that he drives his car continuously under high load situations. I am sure he judiciously uses the car's capabilities. Dyno runs and occasional street blasts, The rest of the time the engine is running at or below the rod fatigue limit.

Therefore, the rods you run should be designed so that they never exceed their fatigue limit. Otherwise you have a ticking time bomb.

This leads to one of the problems I ran into building my "super build" attempting to achieve a safe 800 HP at 7000 rpm. For the rods, I need a larger cross sectional area and more total mass to stay below the fatigue limit. Calculated by the rod manufacturer. The added rod weight made it so that I couldn't balance the engine. [Standard rotational mass + 1/2 the reciprocating mass.] I shelved this build approach until I get time to try a few other design approaches.
 
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Old 04-13-2018, 08:55 AM
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Originally Posted by Tijoe View Post
AKA - Fatigue limit. The highest stress that a material can withstand for an infinite number of cycles without breaking.

Assume the stock powdered metal connecting rods were designed for infinite life up to 450 HP. (Cross sectional area/shape of the arm at the rated yield strength of the powdered metal alloy used. + considerations for heat treating and with stress-risers relieved, and max engine rpm)

Once you go past the rated hp/rpm threshold, it doesn't mean that the rods will instantly fail, but now they will have a finite life. The more times the rod sees higher stresses, the sooner it will fail.

In the "race car scenario" The car may be raced a few seasons before they rebuild the engine. Perhaps they put a couple thousand miles on the stock rods. Say at 500+ HP, the race engine can last 10,000 miles of racing. They know that they have gone past the fatigue limit for the rods, but don't care because they will throw them away after a few seasons.

In Avos' engine build, I am sure that his rods have a finite life. I doubt that he drives his car continuously under high load situations. I am sure he judiciously uses the car's capabilities. Dyno runs and occasional street blasts, The rest of the time the engine is running at or below the rod fatigue limit.

Therefore, the rods you run should be designed so that they never exceed their fatigue limit. Otherwise you have a ticking time bomb.

This leads to one of the problems I ran into building my "super build" attempting to achieve a safe 800 HP at 7000 rpm. For the rods, I need a larger cross sectional area and more total mass to stay below the fatigue limit. Calculated by the rod manufacturer. The added rod weight made it so that I couldn't balance the engine. [Standard rotational mass + 1/2 the reciprocating mass.] I shelved this build approach until I get time to try a few other design approaches.
Don't forget that material has an impact:


I don't know the material properties of the powder metal con rods off hand- but if you go to forgings, will you be increasing the cross section significantly?


Even from 20 years ago piston designs have come a long way, and they've become lighter, however, I'm also assuming that you've gone to 92mm bore so probably NET weight gain I imagine.


Do you have the crankshaft forged already?
 
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