Max HP that a 4.0L rod can support?

[SilverXJ]

Any one know a rough hp figure that the 4.0L can safely handle?

[CobraMarty]

I have read most every post here and other sites and the guys who were racing these engines were using stock rods and up to 450-500HP and 7000 plus rpm and never had talked about a rod problem. They had other issues cam harmonics at higher rpm, that is kinda wierd. 505 has a 600+hp turbo but I don't know what rods. Good oiling and build and reasonable rpm and no detonation and they sound like they can last.

[5-90]

I have read most every post here and other sites and the guys who were racing these engines were using stock rods and up to 450-500HP and 7000 plus rpm and never had talked about a rod problem. They had other issues cam harmonics at higher rpm, that is kinda wierd. 505 has a 600+hp turbo but I don't know what rods. Good oiling and build and reasonable rpm and no detonation and they sound like they can last.

There is an issue with cam harmonics in the range of 6000-7000rpm, as Barney Navarro found out in the 1960's (he raced a destroked twin turbo AMC 6-199 at Indy for a few years.)

Below or above that range, the engine will live forever.

It may have something to do with the seven main bearings - I don't know if anyone experimentally opened up some bores to make it a five- or four-main block, but that may prove instructive, if someone has the time... (Eliminating main bearings will change the harmonics from firing impulses, by changing how the crankshaft is supported.)

However, no seven-main configuration would survive in the 6000-7000rpm band - I think the Navarro team went through a few engines to figure that out...

[SilverXJ]

I believe the harmonic is closer to 5500 RPM. The problem was the driver wanted to hold the engine there under power. Snapped the timing chain. Twice. Hesco has the info about that.

Also, digging around on Hesco yielded the stock rods support 400 HP.

[casual]

I was going to pipe in with 505 telling me around 425Chp as well.

I believe turbo tom was around 400 hp with his stock 4.0 when that motor let go. Stock motor, mileaged just addeda turbo & dynoed it. Vid's are on youtube.

[SilverXJ]

I believe his head gasket failed though.

[casual]

I believe his head gasket failed though.

Yes he pushed the headgasket out around 13psi? Correct me if I'm wrong, but also snapped 2 pistons or rods.

[Cheromaniac]

505 has a 600+hp turbo but I don't know what rods.

Eagle forged 6.150" length rods.

Any one know a rough hp figure that the 4.0L can safely handle?

The stock 4.0L rods can easily handle 400+hp and 6000+rpm. Other components in a stock 4.0L engine are likely to fail (e.g. snapped timing chain, cracked crankshaft rod journals) before the stock rods or rod bolts, and the stock composite head gasket will fail if subjected to more than 8psi of boost.
Shotpeening the rods, using ARP rod bolts, nitriding the crank, using a MLS head gasket, and accelerating through the ~5500-5700rpm cam harmonic should take care of the most likely points of failure.

[SIXPAK]

Tom cracked the pistons in the wrist pin area with 13lbs of boost. I had a set of factory rods last me 17yrs, my engine guy threatened me and I replaced them. Many years of standard transmission abuse followed by many years of 5300 stall converter action too.

[casual]

Tom cracked the pistons in the wrist pin area with 13lbs of boost. I had a set of factory rods last me 17yrs, my engine guy threatened me and I replaced them. Many years of standard transmission abuse followed by many years of 5300 stall converter action too.

Aww ok. I thought it was the rods that went.

Curious how much could you lighten the stock rods? I know mine were all off balance..a few grams variance between them. Would it be worth it to try and lighten them up? Could you drop 50 grams per rod easily? That'd be a lose of 300 grams which is over half a pound. Is this possible?

[Cheromaniac]

Curious how much could you lighten the stock rods? I know mine were all off balance..a few grams variance between them. Would it be worth it to try and lighten them up? Could you drop 50 grams per rod easily? That'd be a lose of 300 grams which is over half a pound. Is this possible?

You're asking a lot to drop 50g from each rod without compromising strength. What you could do is polish the rod beams to remove stress risers, grind away the balance pads on the small ends, balance them to equal the lightest rod, and shotpeen to add strength.
The stock 4.0 rods, at ~660g each, are not exactly heavy in the first place considering that they're long at 6.125". The 4.2 rods are ~30g heavier yet 0.25" shorter.

[casual]

Curious how much could you lighten the stock rods? I know mine were all off balance..a few grams variance between them. Would it be worth it to try and lighten them up? Could you drop 50 grams per rod easily? That'd be a lose of 300 grams which is over half a pound. Is this possible?

You're asking a lot to drop 50g from each rod without compromising strength. What you could do is polish the rod beams to remove stress risers, grind away the balance pads on the small ends, balance them to equal the lightest rod, and shotpeen to add strength.
The stock 4.0 rods, at ~660g each, are not exactly heavy in the first place considering that they're long at 6.125". The 4.2 rods are ~30g heavier yet 0.25" shorter.

Ya I understand. I guess I'm trying to make any gains where ever possble..even if it takes extra time! My lightest rod currently untouched weighs 663g, heaviest being 676g's. These are out of an 87 motor. I just think thats far too much of a difference to be a smooth running motor..I guess getting them all to 660g would be much better than having them 13g apart. Maybe I should focus more to get them all together.

[Cheromaniac]

My lightest rod currently untouched weighs 663g, heaviest being 676g's. These are out of an 87 motor. I just think thats far too much of a difference to be a smooth running motor..I guess getting them all to 660g would be much better than having them 13g apart.

Definitely. I worked on the 4.2 rods to get them all within 1g of each other before putting them in my stroker and yes, it runs smoothly right up to the 5200rpm rev-limiter.

[casual]

Dino is that normal for the rods to be that far out? I've seen maybe 5g or so...but these are almost tripple that!

Would it be a good idea to get a second set of rods and try to find a set thats more evenly matched? As in combine 2 sets and get them all much closer? Or do you think its possible to get the heaviest even or at least within a gram of the lightest?

[Cheromaniac]

Dino is that normal for the rods to be that far out? I've seen maybe 5g or so...but these are almost tripple that!

Would it be a good idea to get a second set of rods and try to find a set thats more evenly matched? As in combine 2 sets and get them all much closer? Or do you think its possible to get the heaviest even or at least within a gram of the lightest?

That's too much of a variance in rod weights so I agree it'll be a good idea to buy another set of rods, choose six that are most closely matched in weight, and go from there.