Author Topic: Cobra Jet Conrod strenght  (Read 1549 times)

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Offline IanR-Cj-Aus

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Cobra Jet Conrod strenght
« on: June 07, 2012, 11:50:32 pm »
Can anyone tell me the Hp rating of the Cj Conrod. Yes I'll be putting ARP bolts in them if they are suitable for what I'm after.

Offline 428 CJ hardtops

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Re: Cobra Jet Conrod strenght
« Reply #1 on: June 08, 2012, 10:48:13 pm »
If you know they are good rods 500 hp with the ARP bolts should be fine much more than that and I would go aftermarket rod.CJs are not a big rpm engine 5800 shift point on my car going through the traps at 6200 .Most of them wont want or need quite that much
had many 428 CJ and SCJ cars over the last 30 years.I currently own a 68 1/2 coupe and a 69 R code SCJ Coupe and a 70 R code SCJ Cougar Eliminator .I am looking for a 70 428 coupe if anybody knows of one for sale.

Offline IanR-Cj-Aus

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Re: Cobra Jet Conrod strenght
« Reply #2 on: June 08, 2012, 11:23:05 pm »
Well that answers my question quite well. Thank you

Offline IanR-Cj-Aus

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Re: Cobra Jet Conrod strenght
« Reply #3 on: June 20, 2012, 06:30:41 am »
How about the crank? Are they good for the 500-550 hp mark?

Offline redscj

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Re: Cobra Jet Conrod strenght
« Reply #4 on: June 20, 2012, 04:40:35 pm »
     Lots of the issue is how much stress it has "seen" previously. Stress is accumulative. So a 100K miles at 1 horsepower level might be no issue. A brief exposure at a higher level changes the picture. Once the rods have seen x number of cycles at a horsepower there is no way to return that stress life back to the rod. So if you start with rods that have been beat on. You may have a failure earlier & at a lower horsepower than the next guy. Even with the best machine work.
     Sorry but it is really complicated. This is why rods are peened. Peening helps prolong whatever life is left in the rod. Basically by putting the surface of the rod in a state of compression the rod is able to withstand more cycles before failure. But if the rod is near failure already then peening probably won't help you very much.
     Think of bending a wire until it cracks. It will crack on the outside of the bend. By compressing the metal it needs to bend farther before it will crack. Thus the benefit to the rod.
     This same principle applies to the rod bolts. That is why a lot of people advise to start with new bolts. Bolts are relatively cheap, easy to replace & usually fail more often than the rod itself. Plus the new bolts are made to a very high standard.
     
Grant
69 Mach 1 Dearborn SCJ 4.30 4spd 6/17/69 Candy Apple Red
69 Sportsroof Metuchen SCJ 4.30 C6 5/28/69 Acapulco Blue

Offline crossboss

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Re: Cobra Jet Conrod strenght
« Reply #5 on: July 04, 2012, 02:23:58 pm »
I have heard rumors that the Stock Elim guys use the 390/std CJ rods because of the lighter weight and they are more common. Im pretty sure they dont use the heavy SCJ capscrew rods. Please correct me if I am wrong, thanks.
<My old C.J. heap  aka  S-B Racing!
Current lifelong project: 1969 Mustang Fastback/FOX chassis, Powered by a modern Can-Am 494 (Boss 429), Kaase headed, Autolite 1425 cfm 'B' Inline carb, 6 speed, 4 wheel discs, ala Trans-Am style--Whew!

Offline redscj

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Re: Cobra Jet Conrod strenght
« Reply #6 on: July 04, 2012, 03:53:19 pm »
     I've heard the same thing. Using top quality aftermarket bolts. The lighter weight has a small advantage. The stock guys are looking for every horse power fraction that they can get.
Grant
69 Mach 1 Dearborn SCJ 4.30 4spd 6/17/69 Candy Apple Red
69 Sportsroof Metuchen SCJ 4.30 C6 5/28/69 Acapulco Blue

Offline 70cj428

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Re: Cobra Jet Conrod strenght
« Reply #7 on: July 07, 2012, 09:44:11 am »
Quote
I have heard rumors that the Stock Elim guys use the 390/std CJ rods

In the early/mid 1980's alot of CJ Stock Eliminator racers used to use the stock CJ rod with new ford bolts  as no one made good aftermarket bolts for the CJ rods till the early 1990's , used to piss me off as ford sold the bolts ( and nuts) in boxes of 5. Other tricks were to size the big end with a piece of tissue between the cap and the rod, making the big end slightly oval with more clearance at the sides. the theory was that if the big end distorted at speed, the bearing wouldn't loose oil clearance at the parting line. Nobody used the SCJ rods as they were too heavy and had issues with the stock bolts also... Nowadays in stock, aftermarket rods are allowed, and I'd be surprised if anyone uses a ford rod anymore. As mentioned above, fatigue life of the rod is finite, and decreases dramatically as stress (rpm/HP) increases, so using used stock rods in a performance application is a real gamble unless the history of the rods are known from day one.  Given the value of your block/crank, and the cost of new aftermarket rods vs reconditioning your old rods with new ARP bolts, It doesn't make alot of sense to use stock rods anymore in anything that will see some hard use / track time.        Just my humble opinion, John
70 "R" code Mach1, Wimbleton White / Vermillion, Auto, AC,
70 "Q" code Mach1, Grabber Green / White, Auto,AC
70 "M" code Mach1, Med. Gold / Ginger, Auto, AC
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Offline 428 CJ hardtops

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Re: Cobra Jet Conrod strenght
« Reply #8 on: July 09, 2012, 01:55:03 pm »
I sure had my 428 CJ show what a rod or rod bolt failure can do last fall.New engine gets aftermarket rods and bolts
had many 428 CJ and SCJ cars over the last 30 years.I currently own a 68 1/2 coupe and a 69 R code SCJ Coupe and a 70 R code SCJ Cougar Eliminator .I am looking for a 70 428 coupe if anybody knows of one for sale.

Offline My427stang

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Re: Cobra Jet Conrod strenght
« Reply #9 on: July 09, 2012, 09:25:13 pm »
The rods are pretty good, its the stock style pistons that are heavy.  Reduce the weight of the pistons = increase the strength of the rods
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71 F-100 4x4 shortbox, 445 FE, 4 speed
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Offline eatmoparscrapchevys

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Re: Cobra Jet Conrod strenght
« Reply #10 on: July 13, 2012, 05:23:00 pm »
I used stock CJ rods with the arp bolts and aftermarket pistons and split the #7 rod in half and ruined a really nice block.  I was making 550HP.    Will be going with aftermarket storker kit this time,  any suggestions on a kit that can handle my goal of 600 plus HP??
69 sportsroof R-code not a mach 1
96 mystic cobra
91 5.0 GT