4.0 Connecting Rods

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4.0 Connecting Rods

Postby PolloLoco » March 19th, 2009, 12:36 pm

Split from another thread because I was starting to hijack it.

dwg86 wrote:
PolloLoco wrote:So I'm trying to find a little more info on using the 4.0 connecting rods. They are longer so custom pistons are needed. I was already planning on boring 0.060" so I'd need new pistons anyway. Any benefit to either the 4.0 or 4.2 connecting rods when going 0.060" over?


Keith Black makes a forged stroker piston using the 4.0 rod(kb944 and kb945), so you don't have to have a piston made. The 4.0 is a longer rod, so it would be better to use. A longer rod puts less piston side load on the cylinder and longer dwell at TDC.


Doing my homework before I remember noting that the 258 crank with 242 connecting rods and stock pistons would create a negative deck clearance, meaning that the piston would be above the deck at TDC. Is this why budget builds use the 258 connecting rod too?

And answering my own question and expanding on dwg86's answer from http://www.angelfire.com/my/fan/faqs.html:
Q. Which connecting rods are better and why?
A. The longer 4.0 rod reduces the rod to cylinder wall angle theoretically resulting in slower cylinder wall wear. The longer rod increases piston dwell time at top dead center (TDC) providing free compression, maximum power from less ignition timing advance, and a reduced risk of detonation. The longer piston dwell time at bottom dead center (BDC) also allows more time for cylinder filling at the end of the intake stroke. The Eagle forged rods are the best of all and will stand up to the abuse of forced induction at high (>8.0psi) levels of boost without failure.


I do plan on boring 0.060" over so I might as well go with KB piston and use my stock 242 connecting rods for the added benefits. Or should I get new connecting rods too?
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Re: 4.0 Connecting Rods

Postby dwg86 » March 19th, 2009, 1:02 pm

Have yours reconditioned with arp rod bolts. ARP doesn't list rod bolts for the 4.0, use arp rod bolts fro the 258. They are the same.
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Re: 4.0 Connecting Rods

Postby Exos » March 20th, 2009, 4:35 pm

Agreed. I just received my ARP rod bolts today.... :banana:

Man they are expensive though..
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