2.5 long stroke: harmonic vibration/other concerns?

[crater64]

Here's a photo of my recent mild hot rod motor build.



I never posted a dedicated build thread but I raised this question here: 1969 2.0 T to 2.5 S/T Build Apologies for trying to hijack that post. Cliffs Notes version:

• case line bored, squirters added and bypass mod performed
• 87.5mm Nickies
• 70.4mm 2.4 crank
• 2.4 rods
• 10.5:1 JE pistons
• twin plugged heads
• ported
• Mod-S cams
• 3.0 SC oil pump
• Weber 40s
• SSIs
• Scart Sport muffler

I'm super concerned about this:

For 2.5 there is choice between short stroke or long stroke 2.5. If the goal is a peak power under 7K then either will work. Short stroke 2.5 (66 x 90) has the advantage that you can rev to 8K plus with the right cams where as the long stroke (70.4 x 86 or 87.5) you will generally want to keep the redline below 7.5K. The longer 70.4 stroke crank has harmonics at high RPM that causes the flywheel to come loose (and other problems).

I drove it for about six weeks before winterizing. It's a great runner but I'm just a little worried about the long-term robustness of the setup. I honestly wanted to stick with my stock crank and rods but the builder didn't want to use my 2.2T crank. (He described the end product as being similar to the old IMSA 2.5GTU motors of which he'd built a few.) I hate the idea of cracking open a fresh motor to swap in 2.2 rods and a 2.2E/S crank, losing displacement, horsepower and torque as a result. I hate the idea of this engine damaging itself even more.

Short of replacing the crank and rods, what advice can anyone offer beyond staying below 7500rpm? I'm all ears.

[Jeff Higgins]

Where did you get that information? I'm not doubting it, I'm asking because I hear the exact same thing from a very respected local engine builder. We were at a vintage race where my buddy and his client was running his '69 equipped with a 2.7. Monty, the builder, said he loves 2.7's, but we need to keep the revs down because of these crankshaft harmonics. I asked him where he got that information, but he couldn't remember.

All of that said, I don't think Mod S cams will continue to make power in a 2.5 above 7,500 rpm anyway. That, and even ignoring the harmonics, would stock 2.4 rods, even nitrided "S" rods, survive revving that high? I don't think they would last long. Pretty much every motor I've seen that is intended to rev that high has Pauter, Carrillo, or similar rods.

If it were my motor, I would just keep revs to 7,100, 7,200 maybe, and live happily ever after. Looks like a street car, so I don't think you are racing it, so there really is no need to spin it up like that. That, and like I said, Mod S cams will fall off by then anyway in a motor of that displacement. 7,000 or so is still pretty high, these motors make glorious sounds up there, and plenty of power. Plus they live much longer. I see no downside to just leaving it be and enjoying it. Sounds like a very fun build.

[Henry Schmidt]

The post about crank harmonics looked like something I've posted for years.
Even the wording sounded oddly familiar. It was not a quote from me.
The issue with the 2.4/2.7 crankshaft is pretty simple.
In an attempt to build a more stable bottom end (not sure why, the 2.0/2.2 crank was very robust) , Porsche engineers designed a crankshaft with very wide rod journals. (maybe the widest of any 911 air-cooled engine)
The issue is that the crank length is determined by the case. Beginning a very small company, Porsche tried their best to "improve" the engine by changing as few parts as possible. To make the engine larger, they just increased the stroke. Other dimensions, like total engine width, created another problem, shorter rods, (a topic for another thread).
So by increasing the rod journal width without reducing main bearing width they were left with less room for flyweights. Result was a "very" thin flyweight.
Under load, all crankshafts twist. The amount of twist in the 2.4/2.7 is excessive.
The flyweights flex (lack of torsional rigidity) and the crank develops a rocking couple.
This creates an imbalance and that into translates in a serious vibration. That vibration creates harmonics that would often cause the flywheel to come loose even searing the bolts at times.
911 engine vibrate so a lower RPMs the condition is less noticeable.
In the 2.8 and 3.0 RSR engines, Porsche made a special crankshaft to address this problem. It has narrower rods (like the SC), larger journal radii and a super light flywheel to reduce twist.
As far as I can tell, all factory race engine incorporated this narrower rod design.
Even the water cooled GT3 of the 2000s had a connecting rod that can be exchanged with a production SC rod (all be it, rod lengths vary).
Simply put, the 2.4/2.7 has a limitation as do the magnesium cased they come in.
Learning to live with these (incurable) limitations is just part of being the owner of an engine from a by-gone era.
To the OP. Understanding your limitations should ease your concerns. Go forth and rev, just keep in mind that RPM can be expensive.

On a side note: higher RPMs can create frantic situations so take your own counsel on how "experienced" your driving skills are.

[crater64]

Originally Posted by Jeff Higgins

Where did you get that information? I'm not doubting it, I'm asking because I hear the exact same thing from a very respected local engine builder. We were at a vintage race where my buddy and his client was running his '69 equipped with a 2.7. Monty, the builder, said he loves 2.7's, but we need to keep the revs down because of these crankshaft harmonics. I asked him where he got that information, but he couldn't remember.

It does seem like an urban legend. I'll try to track down online sources and update this post when I do. I do see where there are people running 2.5LS motors with success today, both on the street and on the track. I don't intend to track this car, although I would like to try my hand someday at one of those multi-day events like the Targa California or Mountain Mille.

Here's a set of 2.5LS P&Cs being sold because of a flywheel coming loose.

[crater64]

Originally Posted by Henry Schmidt

Simply put, the 2.4/2.7 has a limitation as do the magnesium cased they come in.
Learning to live with these (incurable) limitations is just part of being the owner of an engine from a by-gone era.
To the OP. Understanding your limitations should ease your concerns. Go forth and rev, just keep in mind that RPM can be expensive.

On a side note: higher RPMs can create frantic situations so take your own counsel on how "experienced" your driving skills are.

There is always that matter of ensuring that the nut behind the steering wheel is well secured.

Originally Posted by Jeff Higgins

If it were my motor, I would just keep revs to 7,100, 7,200 maybe, and live happily ever after. Looks like a street car, so I don't think you are racing it, so there really is no need to spin it up like that. That, and like I said, Mod S cams will fall off by then anyway in a motor of that displacement. 7,000 or so is still pretty high, these motors make glorious sounds up there, and plenty of power. Plus they live much longer. I see no downside to just leaving it be and enjoying it. Sounds like a very fun build.

I'm planning on replacing my stock tach with a 2.2E tach to keep myself honest.

Thank you both for your input and your reassurances. I've valued your opinions over the years and having them applied to my personal situation just makes me value them that much more.

[Neil Harvey]

Something that should be known, the damage caused by these harmonic vibrations are not because of high engine speeds. Lowering the engine speed may help the engine life but not the damage caused by these vibrations.

Testing has shown the damage is at lower engine speeds but the results of these can be seen all the way through the RPM ranges. Cam timing goes out the window once these nasties start, the engine never recovers even above the RPM number.

The bearing shells and housing bores have shown this for years in abundance. It seems it has been one of many things that was seen but never really addressed.

I remember my time at Andial building the race engines for Porsche. We used to see the cranks break between the #1 and #2 main journals, but only after running at certain tracks. It was at these tracks that the engine ran the most in the RPM that the noise was the at it's highest.

This is an area of development we have been involved in for many years now. We developed a Damper with ATI for the later GT3 and Turbo engines some 8/9 years ago.

The 911 air cooled engine Damper has also been developed along with the Alternator Pulley and Tensioner. We have done our own Tensioner or the Tensioner from Richard (Clewett) can be used.

Unfortunately, the only way to fit this on a stock crankshaft is when the engine is rebuilt. The Damper requires an overlap onto the end of the crankshaft more than the stock FMS will allow.
The front #8 Bearing sleeve has to be modified and a thinner 7.00mm seal used. Use of the longer 996/997 GT3 crankshaft length allows the Damper to be fitted without engine disassembly.

Here is a simple but effective way of finding out the RPM your engine creates these harmful vibrations. Attach something inside the car that can rattle against something. I have been told by many that race the later GT3 cars that the hook they have to hold their helmet when not racing seem not to rattle as much when the Damper is installed. The street engines have sensors on the Camshafts and the deviation between cams can be logged. This gets worse as the engine runs in these harmful RPM's.

Try to establish the RPM your engine makes the most "noise" and try to drive around those RPM's.

The Damper weighs in at 6 lbs and is added to the end of the crank that moves the most. It's best described as a wide band Damper, so that it will aid engines with different strokes and masses. But it has it limits.

[Jeff Higgins]

Thank you Henry and Neil for your explanations. It's invaluable for us mere duffers to hear from guys like you with real experience in all of this. Experience earned over decades of working on and playing with more examples of these cars than the rest of us will ever get a chance to do. Much appreciated.

[Geneulm]

Expert in laboratory of life and science above!

2.8/3.0 Rsr crank has a standard size journal, but with larger radii. I have one in my new build - w carillos. Bearings had to be champfered. I thought a better/lighter option for the crank thank uber rare Rsr rods of unknown history and their impossible to find double tang bearings. Also using a sandcast case -- to avoid problems w mag case.

History books say racers back in the day dowell pinned mag cases and held max rpm to 7800 to yield a workable reliable package. 8k is def no go zone. They also say harmonics peak extreme at 8k and throw flywheel. Same history books say always new bolts, loctite and over torque.

Per Henry's comment about the SC journal size I have in my stash -- can't remember where I got it a T 2.0 crank that has been stroked to long stroke size but to use an SC rod. Some racers enterprising way to do what Porsche did for race motors -- use an SC size journal to change harmonics.

[Henry Schmidt]

Originally Posted by Geneulm

Expert in laboratory of life and science above!

2.8/3.0 Rsr crank has a standard size journal, but with larger radii. I have one in my new build - w carillos. Bearings had to be champfered. I thought a better/lighter option for the crank thank uber rare Rsr rods of unknown history and their impossible to find double tang bearings. Also using a sandcast case -- to avoid problems w mag case.

History books say racers back in the day dowell pinned mag cases and held max rpm to 7800 to yield a workable reliable package. 8k is def no go zone. They also say harmonics peak extreme at 8k and throw flywheel. Same history books say always new bolts, loctite and over torque.

Per Henry's comment about the SC journal size I have in my stash -- can't remember where I got it a T 2.0 crank that has been stroked to long stroke size but to use an SC rod. Some racers enterprising way to do what Porsche did for race motors -- use an SC size journal to change harmonics.

What we been doing to 2.0/2.2 is to regrind the rod journal to fit the GT3 Gen II titanium rods. After years of testing this modification have proven to be stellar.
The 130mm OAL and larger radii is capable of well over 8200 RPM when driven in anger. The down side, when we started using these rods the price was around $ 2.5K. Now they are around $6.5K.......yikes!

[Old H2S]

And the local dealer has those rods for 14k..