Rod Failure

I made 57 passes on a set of aluminum rods when #7 or #8 failed on the line. My setup is a 565 BBC with a 14-71 roots blower and a 44 mag in a boat. On the line I rev limit the motor to 8K rpm, take out 12* of timing and ramp it back in in 2.5 sec with a Digital-7. Initial timing is set at 30* (crank trigger) and boost is 20 psi. It takes 2.5 sec before the boat hooks up and the motor lugs down to ~7K rpm through the run. When the rod failed the data logger showed #7 and #8 egt's were around 800*, which is high. From the previous pass #7 and #8 temps dipped ~100* at the launch and then headed up to ~1050* at the end of the pass.

My questions are did I detonate the motor at that launch which in turn caused one of the rods to fail or did I exceed the rod life due too the 57 passes I put on them?

I was told that you should change Al rods after ~30 passes. I thought I should be able to do better than that with my setup? If this is the case why not go too steel rods?

Thanks in advance for any advice for my future motor. Joe

 

Rods

You made about 30 more run than you should of!!!!!!!!!!!!!!!!!!! I just don,t understand people who run blower motors and don,t understand the concept. If you don,t want to do all the maintenance, and expense, don,t go there!!!!! Don

 

Don, I would like your input on why you think 30 passes they should be changed, Critical Mass is only putting 20psi boost, I understand alot of other things come into play.

 

rods

i think what don is saying if he was looking in the oil pan about every 5th pass he would have found his problem before it went BOOM !

 

Joe, Without all the techno speak, everything that roates and is cycled under pressure eventually changes shape and or strength. An atopsy(SP) would help if there is enough to sift thru for answers. Usually lightly loaded and safely run motors can go beyond 30 passes if checked and measured every time out. I cannot get it thru to people I help out with, that rods are cheaper than the carnage they cost.
You can buy 3 sets of rods for one block, Guys claim they can't afford that, but come up with the money to fix the block????, AND buy rods and pistons, and cranks
Set a comfortable number of runs for your parts, maybe it is 30, Change them out!! Finding out the hard way that the number is 31 SUKS.
Steel rods when they break throw parts much further IMHO...
My 2 cents
Hal

 

I believe that rev limiting the motor on the line at 8 grand doesn't help either. I also agree with others that rods need to be replaced every 30 runs maximum on a blown motor, especially one with a 14:71.

 

BAE hemi here GRP rods 1471- 42 lbs 3300 lbs drag truck 60 passes as of today season is over. new set will go in.

 

Rods

There's only one way to find out how many runs you can get out of a set of rods....and it's very expensive. I wouldn't run more than 30 regardless of manufacturer. Stories you hear are just that....stories....give them your bill for the damage when their estimation doesn't work out.

Brandon Booher
NHRA TAD #34

 

For those who test the outer limits in cycle life.. The amount of runs is usually never exactly the same. But the bill for the carnage is preety close to the same in most cases.

 

I'm not sure why but I get way more runs on rods than what some of you guys talk about. Maybe its because my pistons are lighter or its because we are tractor pulling. We routinely peak at 8500-9500 RPM, our runs last 11-12 secs and we put about 75 runs on them before changing.

One thing to look at in the autopsy is if #5 main was going out that junk will get pushed into #7/8 rod bearings, seizing them and causing them to exit.

Also if you have a vibration from something else. I bent a driveshaft on a 9200 RPM pass and it caused rods to exit, bolts to come loose, broke brackets, etc, etc...

 

With that kind of boost and RPM's you should still be in the safe zone for rod life. I would try some other way to control the starting line RPM's besides the rev limiter. What is the rod length and weight? Where did the rod break? Are you taking any fuel out on the two step? Did you have a rocker adjustment are pushrod problem that caused more cylinder pressure and heat in that hole? Some of these things could cause the early exit.

 

I first want to thank you all for responding to my post. I now there's nothing more to do except suck it up, learn from the experience and shell out the cash for your mistake.

I'll try to answer some of your question about my rod failure.
Rods - Venolia aluminum rods #14172, length 6.535 in, 559 grams,
Piston, pin & buttons - Venolia, #2881, 1154 grams,
Bob weight - 2697 grams
As you can see this is a heavy assembly. I wanted to build it stout, but maybe to stout might not be the best way to go? The rod and main bearings all looked good, no detonation or vibrations.

I use a high-speed at 62 psi with a 0.065 pill. The high-speed is connected to the port line, right off the BV. The pressure reaches 100 psi in 0.250 sec; therefore, the high-speed is open while on the limiter and taking fuel away.

According to the MSD spec sheet the Digital-7 won't drop the same cylinder twice in a row. The Digital-7 box is connected to the 8145 box, at the rev limit chip location, which in turn controls the ignition timing ramp and the max rev limit of the 44 mag.

Thanks, Joe

 

That seems like a really light weight rod for that combo. The normal rod weight for a bracket style BB Chevy with low boost and that length is around 800 to 830 grms. Give me a call and I will give you some details. Bill @ 337-543-2028

 

The rods is 100% too lights, my suggestion is put something in there (like Mr. Doucet said) over 800g, I think 850-870g is much better. What is your wrist pin weight?, the bob weight seems off?

Brandon Booher
NHRA TAD #34

 

Are you running .990" wristpins or bigger? Is part of the rod hooked to the piston yet or did the pin break first?

FYI, I just looked and my rods are 731 grams and are longer than yours.

 

Sorry, I got the weight of the rod and piston reversed; rod weight is 1154, and piston weight is 559 grams. Also, I'm running the 1.094 pins.

Thanks, Joe

 

Joe, are you measuring the weights yourself because I don't even think that you could buy a steel rod weighing that much & if that is the weight of your piston assembly then that it miles toooo lite.

 

MKR-588,
Below are the numbers that were used to arrive at the initial bob weight:
Piston, Pin & buttons = 1154
Recip. rod = 269
Rings = 57
Bearings = 96
Rotating oil = 3
Bob weight = 2697

I'm having the crank magnafluxed before I put the motor together again. If it checks out ok I will have it rebalanced again and double check the bob weight. I hope this helps.
Thanks, Joe

 

You may want to send that digital 7 box in and have it checked, msd has had a bad batch of those boxes causing severe problems.

But yes cycle your rods out around 30 passes. the higher the starting line rpm the heavier the piston becomes.

 

Critical Mass... what kind of boat? Hook the thing up. Hell, running on the limiter has never been easy on parts.
Oh, you also may have other issues. But this is not easy on parts.

Wags