Rod bolts.........

Gumba11 · 09-22-2005, 10:57 AM

Hey guys,

I need your thoughts on rod bolts. I have a 3.2 litre turbo engine that I had originally planned on going close to 550 rwhp with or a little over 600 at the fw.

There is a lot of talk about ARP being the only rod bolts to go with. My engine was built by a very experienced tech who also now resides at Rothsport road and race. When my engine was built he told me the stock bolts were fine. He has used them in my case and says he does not use the stretch method for torque. He sets them without stretching them. Bottom line I would trust Gordon with my life but I am now being told that my factory rod bolts will only live up to 400 fwhp and I am already beyond that number.

My question is this. Am I in deep ***** and asking for trouble if I build my motor too hot when I do my new turbo and EFI setup in the spring? I dont want to end up with a rod hanging out of the side of my case.

Thanks in advance!

.................................................. C

Zeke · 09-22-2005, 11:10 AM

Although this forum is a lot more active, you might post something like this on the engine forum. Cross posts are not encouraged, however.

My thoughts are why take any chances? If you are asking, you either have doubts yourself, or are looking for some to comfort you with your decision. Or both. You either go with the mechanic and his experience or you get the best you can buy and worry about the next thing.

I'm afraid I'm not the only one that will put it this frankly.

Thomas Owen · 02:49 AM

No better time than now to put your mind at ease. The stock bolts are, as the name implies, designed and installed by porsche in a 'stock' engine and also those of higher performance. Though there are documented cases of many high performance engines being run with them, why take the risk? In short, you have a lot to lose, if nothing, peace of mind.
After all, who will pay to fix the (severe) damage if there is a failure?
Good luck -

Gumba11 · 09-22-2005, 02:57 PM

Well the engine is 9K miles new. I guess at this point it is risk it, Have no fun, or tear it down to put the damn things in.

.............................................C

Any other thoughts before I put my new budget together????

sammyg2 · 09-22-2005, 03:21 PM

Required rod bolt strength has absolutely nothing to do with the amount of horsepower you are making, it is all about the rotational mass and the maximum rpm with the emphasis on rpm because the force=mass times the speed squared.
If you plan to increase the rotational mass, or you intentionally or accidentally increase the maximum rpm, you may need stronger rod bolts. If you are not going to increase the amount of rotational mass or increase the maximum rpm you will take the engine to, there will be no extra force applied on the bolts.
If they work now they will work then.

BTW, unless your mechanic forgot to tighten the bolts, he stretched them.
It sounds like he didn't measure the amount of stretch and used a torque value instead which is fine, but the bolts still got stretched.

Gumba11 · 09-22-2005, 03:58 PM

Sammy you ROCK! Thanks for the reassuring words. It makes more sense now that you explain it that way. I am sure Gordon will probably tell me the same thing when I here from him. He just hasnt called me back yet.

Thanks again....................................C

Wayne 962 · Wayne 962's Garage

It's well proven that the smaller, stock rod bolts are underdesigned for the stock 3.2 and 3.6 engines. I have a picture of a broken rod bolt in my Engine Rebuild Book for all who don't believe this.

Running stock bolts is like driving your car on bald tires. Sure, it will probably work, but your margin of safety is much, much reduced.

-Wayne

WydRyd · 09-22-2005, 10:25 PM

I went with ARP fasteners all round (rod bolts & head studs) for peace of mind because I planned to run ~1.2bar of boost!

I never want to go inside that engine again, so better to do it right first time

Valve guides and rod bolts have always been a weak point in the 3.2 Carrera engines, so I went with AlSiBr valve guides and ARP fasteners.

randywebb · 09-23-2005, 11:09 AM

"rod bolt strength has absolutely nothing to do with the amount of horsepower you are making"

- well, the problem is that the force on the rod bolt/nut is a F(inertia), which is a F(of the engine speed), which is a F(hp for a given displacement, CR)

So, it actually is well-correlated with hp increases in the engine ....

Bottom line - replace them at any rebuild

the stock ones will do pretty well for a stock engine operated by your granny

smdubovsky · 09-23-2005, 12:17 PM

Randy, thats not really true. The bolt are only under a tensile load from 90 deg BTDC to 90deg ATDC. The other 180deg the rod is in compression and the bolts really aren't doing anything other than bringing the rod cap along for the ride. Increasing boost can actually REDUCE the tensile load on the rod at high rpm when the inertia forces outweigh the compression forces. Corky Bells book has really neat graphs of this (because they sure as heck aren't intuitive - until you see the graphs and say, well, duh - of course!). So, Im in agreement w/ Sammy on this one.

All that said, *I* would have replaced the bolts for the peace of mind (that and what Wayne has said, the 9mm bolts are known to be marginal already.)

SMD

Quicksilver · 09-23-2005, 12:24 PM

Quote:

Originally posted by randywebb
"rod bolt strength has absolutely nothing to do with the amount of horsepower you are making"

- well, the problem is that the force on the rod bolt/nut is a F(inertia), which is a F(of the engine speed), which is a F(hp for a given displacement, CR)

So, it actually is well-correlated with hp increases in the engine ....

Bottom line - replace them at any rebuild

the stock ones will do pretty well for a stock engine operated by your granny

Increasing horsepower in of itself won't increase the load on the rod bolts. The rod bolts are loaded when the rod is in tension at the top of its stroke when it is trying to keep the piston from flying through the head. RPMs and piston/rod mass are what effects this.

Horsepower is generated when the intake charge is pushing the piston down. The rod is in compression at this point at the rod bolts are not subjected to any additional stress.

The one loophole in this is that an engine is an air pump so to increase power you must increase the quantity of air you are pumping. If you pressurize the intake you aren't really effecting the rod bolts. If you are spinning it faster you are effecting the rod bolts.

That being said, the 3.2 factory rod bolts are a joke. If you over rev the motor then you will end up with many $$ of junk. (I like to call the 3.2 rod bolts "Rev Limiters") Rod bolts are amazingly cheap insurance.

If I was building an expensive turbo motor the 5 things inside the motor that I would absolutely change from stock are: Compression ratio, Cam, Oil pump, Valve springs and Rod bolts.

Either ARP or Raceware are good replacements.

Jeff Alton · 09-23-2005, 01:00 PM

How many rod bolts have actually failed? How many were due to an over rev? Wayne, you have a picture in your book, yes, why did that one fail? Are they failing without being abused? The rod bolt debate is kinda like the eshuast header size debate, lots of opinions, little hard data posted. It just seems strange if it is "well proven" that the factory, in 89, starts building 3.6 high compression motors using the same rods and bolts that have been used for years. Wouldn't the weakness have been exposed by then?

That said, I have a set of ARP ones sitting on my shelf for my upcommig 3.4. However this motor will be revved to at least 7500.....

Cheers

Bill Verburg · Bill Verburg's Garage

Quote:

How many rod bolts have actually failed?

All it takes is one in your very own personal car

i used Raceware when I did my 3.8, saving a few bucks here is not a wise decision. nothing wrong w/ ARP either

Jeff Alton · 09-23-2005, 01:25 PM

Bill, I agee. That is why I am putting in ARP bolts. My point was that the someone getting a stock rebuild for his daily driver maybe does not need them. It is just one of those topics that seems to get so much "press" that sometimes it comes across like every last one of them is going to fail.

Jeff

Gumba11 · 09-23-2005, 01:41 PM

Im starting to regret ever starting this thread because the inevitable arguments seem to always flair up when someone states their opinion here. I thank you all for the help. I got the answers I needed. However I am very upset at the fact that someone has e mailed me to give me an ear full of Bull***** about my post, right down to taking my mention of a very great engine builder and the shop I think he still works at personally and twisting it around. He let me know that he thinks I am out of line.

Bottom line I dont give a damn what he thinks and I dont see the problem with mentioning who has worked on your car here. I guess I still have a lot to learn about the things you should and should not say here, being that there are some very sensative people here.

So I will clarify what I said. Gordon Ledbetter built my engine. That is a fact. He did a great job. That is a fact. I dont question his work or his knowledge.....EVER. That is a fact. I mention Rothsport because I thought he worked there. Maybe I am wrong and I will discuss that on the phone with him and let you guys know where he is in the event someone local in the Oregon area might want him to perform some work on their engine. After all that was what my aim was. To try and help friends get more business by giving positive feedback about them.

I will clear all this up with Gordon personally so his dear friend who has a personal problem with me doesnt call him and screw up my relationship with Gordon. However I think Gordon knows me better than the above mentioned party. I most of all apologize to Gordon who is the inocent party here and I will not mention his or Rothsports name again without permission.

Thanks for reading. I am still confused as to how I am out of line here!

.................................................. C

Bill Verburg · Bill Verburg's Garage

Quote:

My point was that the someone getting a stock rebuild for his daily driver maybe does not need them

True, as long as the rev limit is not raised stock bolts are fine

Gumba11 · 09-23-2005, 02:23 PM

Thanks Bill, I am not upset with anyone but the guy that e mailed me and he knows who he is.

.................................................. ...C

Quicksilver · 09-23-2005, 04:40 PM

Quote:

Originally posted by catca
How many rod bolts have actually failed? How many were due to an over rev?
. . .

One of the problems with the 'stretch to torque' rod bolts is identifying a cause of failure or even the time it was damaged. These bolts are designed to be torqued to a load where they will take a permanent stretch. When assembled like this they will have the correct preload to hold the rod together in the designed RPM range.

It you over rev it enough, any movement of the big end's parting line will add additional permanent stretch to the bolt. This reduces the preload on the bearing. If you keep over revving it the reduced preload makes additional damage easier.

Here we get to the diagnostic problem. How and when will the failure become apparent? The reduced preload can allow the rod bearing to spin. When the motor is taken apart it will usually be diagnosed as an oiling problem. Another possibility is that the rod bolt may loosen over the following months (or years) after the over rev started the damage. By the time the grenade goes off you have long since forgotten the missed shift.

The spun bearing in the 3.2 on the stand in my garage is possibly from an over rev. It could be from an oiling problem. I'm betting on the rod bolts though...

Quote:

Originally posted by Gumba11
. . .

Thanks for reading. I am still confused as to how I am out of line here!

.................................................. C

I can't find any possible objection in what you have written here. I would seriously question anyone who objects to you trying to educate yourself. Anyone you are doing business with should welcome a serious educated customer.

- - -
Back to original question. If the engine is together and you trust yourself to never over rev it don't worry. If the engine is apart then spend the little bit extra for some real bolts.

randywebb · 09-23-2005, 04:57 PM

BTW - who said anything about boost? If you increase the revs, you increase inertia & hence stress on the fasteners.

We all agree to replace the bolts for racing, etc. - ARP or Raceware. I've used both.

I agree re Gordon & the Gamroth org. in general. For stock use, do what he said - or spend more if you want or think you might use the engine for racing before the next rebuild...

RLJ · RLJ's Garage

Here I am the one who emailed this guy.

I "felt" like he was saying that Leds gave him bad information!

Gordon has forgot more than Chris and I will ever know about these cars.

I sent him an email using Pelican's source for his email addy telling him that I didn't think slamming Leds was cool. Rather on the side, then in public. Is that wrong?

If I was wrong, don't go postal on me, reply in a civil maner, talk about touchy! I have known these folks for years, you I have never meet and I have been around Porsches in Portland starting in 1969. I am very protective of my friends, and yes I have known them for "years" probably before you bought your first Porsche.

So if I hurt your feelings, I'm sorry!

Randy Jones
1971 911 "Iris"