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'S 'Rods

In the process of assembling parts to build a 2.6 backup motor for my 914-6
PCA/SCCA race car.
I have a real set of "S" rods and was wondering what the general opinion is regarding taking these to 8,000 rpm.They are fully reconditioned and ready to go.If this would not be a good idea they would then be for sale,but thats for another forum.
Tom B.
Old 10-13-2010, 11:08 AM
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JMHO, but I would not be using Porsche rods to 8K, even with better bolts, due to their weight.

I'd be using Pauter rods.
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Old 10-13-2010, 03:28 PM
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What stroke? 66mm should be ok at 8k.

-Andy
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Old 10-13-2010, 08:19 PM
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All of these rods are old. I prefer new rods when getting 8K angry. 7.5K not at all.

I know Henry uses S rods on some of his 2.8SS motors, so really shouldn't be an issue, but I'd magnaflux them jsut for peace of mind. Or just buy Pauter or R&R rods.
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Old 10-13-2010, 09:48 PM
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I would certainly have them shot peened by a reputable compnay with a good reputation as this will help to re-life them from a fatigue point of view. They will need some plates to protect the side faces and soome reduced head bolts so that this area of the rod is accessible.
Old 10-13-2010, 10:15 PM
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Steve is right, the stock rods are heavy, and do not maintain their big end dimensions if a mistake is made (hard over-rev). I have used stock 2.2 rods with 10mm bolts (not nitrided "S") in a 66x90 twin turbo application years ago running it to 8800 and making 450 plus to the rear wheels with no problems. I refreshed this engine twice at over 50 hours--the rod bearings were pretty used up at that point and the big end dimensions were on the limit. Grady Clay has posted that he used stock 2.0 rods to 8000 ( I was surprised at this because the 2.0 rods had a reputation of breaking early on. He was, however, using a lightweight Mahle piston which reduces the loads considerably) I have also used the stock rod with ARP bolts with the 70.4 stroke with a 8000rpm hard rev limiter. Those engines were going 40-50 hours before refresh as long as no mistakes were made. You are definitely better off with Pauters if the budget allows, but if if not I would use stock rods.(After magnafluxing and reconditioning with ARP bolts)

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Old 10-14-2010, 08:04 AM
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Thanks guys I appreciate the feedback.
As you can tell the reaction is kinda mixed with caveats gelore.
This is gonna be a short stroke 2.6 using 92mm 10.5 pistons and either an "S" cam or modified Elgin S cam.I'd like to see a bit more grunt below 5000rpms.
The "S" rods I've got have been lightened and resized but I'm not sure they have been magnafluxed and I know they have not been shotpeened both of which I know need to be done.

I think I will weigh them then try to get some #'s from Pauter and see exactly what the weight difference is.
I'll keep you posted .
Tom b.
Old 10-18-2010, 03:31 PM
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Tom

Lightened? I suppose one could shave a few grams off, but normally these are only lightened to attain a balanced set, no?

No place to shed an appreciable amount of weight, seems to me. A hot rod shop doing a farm out job balance job from my machinist once ground off quite a bit of the rib under the cap. OK for Chevies, but not at all for a 911 - made the rods into junk as that is what keeps the cap stiff and round.

My 2.3 eventually had a rod let loose, because I used stock bolts. At the time nothing better was generally available, and on another discussion here it was pointed out that machine work is needed on the 66mm rods in order to take the stronger ARP or other aftermarket high strength steel bolts.

Money for that and the bolts might better be applied to Pauters.
Old 10-19-2010, 05:50 PM
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has anyone tried the rods from ireland engineering? they look just like carillos but priced at $160ea.
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Old 10-20-2010, 04:14 PM
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Tom,

I went through the same discussions in the late ‘70s and early ‘80s when I built my 914-6 engines. They are 66x91, the SCCA CP limit at the time. The P&Cs are a Mahle Nikasil and forged race piston, made for SCCA racing with a 2.7 (90+1 mm).

At the time, the choices of rods was much more limited. I opted for modifying 2.0 rods as the 2.2 rods are way too heavy.

While they were eventually somewhat lighter than original, the purpose was to eliminate any ‘stress risers’ by first grinding/sanding and then polishing to a ‘perfect’ finish. They were then carefully shot peened.

Since these are steel rods, I was very careful to never handle them without oily gloved hands – no fingerprints. After peening, they were never oil-free.

I started with about 150 rods and ended up with four matched sets plus a few spares for each. Three sets are still in the engines with one spare matched set.

I X-rayed all of them, some several times. There were about a dozen out of 150 that were ‘suspicious’ and rejected. I had several ‘experts’ review the films. I never got around to trying breaking them so I cannot say for sure that there were flaws.

I then magnafluxed them and inspected them with dye penetrant. I found no flaws.

I repeat the inspection at each ‘maintenance rebuild’.

There were not alternate rod bolts available. I used new stock Porsche bolts and nuts each time. They were installed using torque and stretch method. Even when measuring big-ends, I used previously stretched bolts and nuts. The race hardware was stretched once. I rejected about a dozen on installation, suspecting something strange with the stretch. Porsche bolts are difficult to measure length so I suspect it was more paranoia than an actual flaw.


The power peaks about 8300 but doesn’t stop there. The engines turn 8300-8400 every shift. I intentionally have turned them to about 8700 in competition to save a shift going into a corner. I have unintentionally turned about 9000 a few times (no rev limiter).

Perhaps I was just lucky. Perhaps all the work paid off. Whatever, I have never had a failure.


Fitting these Mahle pistons to modified 2.2T heads, I struggled to get the highest CR. The result was a relatively ‘large’ squish area around the perimeter. Over time, I was able to reduce the clearance so this area remained relatively ‘clean’ of combustion debris but never any contact.

I was very careful with engine warm-up and cool-down so differential expansion would not cause a contact issue. All this care may have helped prevent other failures.


I intend to recycle these three identical engines and possibly build a fourth.

I will make them 70.4x92 using modified stock crankshafts and new Mahle race P&Cs so I need to decide on what rods and bolts.

I intend to go back to 906 cams (from 935 sprint cams) and limit the rpm to about 7800 for longevity (and use a ‘soft’ rev limiter).


I would like to hear everyone’s observations and experience.


Experience dictates building everything ‘best possible’. Engine failure is not in the plan – ever.

This will be a much less ‘serious’ race effort with the car speced for compliance with SCCA GT-3, PCA Club Racing, RMVR, etc. and ‘play days’ for my son. I may have to do some ‘testing’.

I’m going to build the engines where they comply with everyone’s rules – not necessarily try and push the letter of any group’s rules or mechanical limits for maximum performance.

Best,
Grady
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Old 10-21-2010, 05:38 AM
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grady,
Wow!Sounds like you've run the gammet with these.
In speaking with Walt @ CE,he says I should have no problem going to 8,000rpm while still being able to use the OEM Verbus nuts and bolts.I don't think ARP makes a ready to fit bolt/nut combo for the 'S" rods without some machining.
I will however try to be as meticulous with their preparation as you have been.I only have six rods to work with so i'll keep my fingers crossed they will all pass muster.
I hope to use an "s" cam or some variable or modified "s" cam profile that D. Elgin has come up with.Of course twin plugging will be necessary and i've decided on a 12 plug distributor by JB racing.Its been about eight years since i last built a six ,so Wayne's book will be by my side the entire time.my last eight years have been spent building Type IV's for use in my F prod SCCA car during which time I went to the Runoffs numerous times and was even able to compete for a while until the Miata's and Acuras' just simply rendered the 914-4 a mid pack car.
Thanks for your input as I find it to be very informative and useful.
Tom B.
Old 10-21-2010, 03:47 PM
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If memory serves, weren't both the 2.0 and 2.2 S rods selected for light weight group then Nitride treated ("Tenifer" treated in manual)?

Is Nitriding the rod worth anything anyway?
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Old 10-21-2010, 03:59 PM
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Tom

I had a stock rod bolt let go, ending my 2.3 liter motor (5 heads, 5 Cosworth Ps and Cs, 5 rods, one cam of two, one cam carrier, both chain boxes and covers, 3 of 4 valve covers, fan and alternator left over, case and other innards junk). Mind you, I wasn't as careful as Grady. But he had a shop, tools, knowledge, skills, etc. to be that meticulous. At the time there was no aftermarket bolt made for those rods, at least not one whose existance filtered down to the likes of me.

I wasn't spinning the motor over 8,000, though I was running it up there.

I wouldn't use stock bolts on a race motor ever again. Yes, it appears there is a machining step (Mike Bruns explained that in a different discussion here recently) to get the ARP bolts to fit. Even calls for torquing the bolts and resizing the big end to compensate for the tighter squeeze they give. Pretty advanced stuff for an engine assembler like me. But worth it.

Or perhaps what is really worth it is a stronger aftermarket rod, like the Pauter.

I'm pretty much satisfied that the stock springs can do the job up to 8,000. Helps if you use titanium retainers to lighten things a little.

But not those bolts. Not for me, anyway. I see it as one of those things I can't afford not to improve upon. Maybe if you only run 25 hours between replacements? But the 911 takes, I'd guess, about twice as long to pull apart, refresh, and reassemble as those 4 bangers. At least for me, moving from VWs to 911s with their heads and rocker shafts and chain boxes and cam timing was a mildly rude awakening.
Old 10-21-2010, 09:54 PM
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Here is the end game question. Stock Porsche rods with good rod bolts are good to 7500 RPM on a 70.4 stroke, this with stock weight pistons and pins are considered by many to be "safe". If you run light weight pistons and pins you will have a higher RPM potential. If the pistons are lighter than stock, and pins are lighter than stock we will see more RPM potential. So what I calculated is if the stock rod and pistons are good to 7500 with good bolts what will a new "safe" RPM be with light weight pistons and pins only?

The parts evaluated:

Stock piston 457 grams 95 mm
Stock Pin 137 grams
Total 594 grams

New Piston 433 grams 97 mm
New Pin 100 grams
Total 533 grams

New Piston 433 grams 97 mm
TI Pin 71
Total 504

Here is what I found out, the large numbers are effective load in grams. The piston G Loads are calculated with Engine Analyzer Pro for a 70.4 mm stroke.

RPM Piston G Load @ RP 594 grams Stock CP and Steel Pi CP and Ti Pin
7500 2790G 1657260 1487070 1406160
7750 2980G 1770120 1588340 1501920
8000 3180G 1888920 1694940 1602720

Now in pounds because this is how I think.

RPM Piston G Load @ RPM 594 grams Stock CP and Steel Pin CP and Ti Pin
7500 2790G 3653 lbs 3278 lbs 3100 lbs
7750 2980G 3902 lbs 3501 lbs 3311 lbs
8000 3180G 4164 lbs 3736 lbs 3533 lbs


So the rod weight is important but so is the weight of the total package. The Ti pins are lighter than the CP pins by a small 29 grams but at RPM it does make a difference. Working in pounds the stock piston and pin at 7500 RPM have a load on the rod of 3653 pounds, considered by many to be "safe" with good rod bolts.With light weight pistons and a Ti pin we have a load of 3533 pounds at 8000 RPM, or less load then the "safe" load of the stock parts at 7500 RPM.

This is not including any concerns over oiling and such just information on rod load / weight and RPM.

Stay Tuned,

PFM
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Old 10-23-2010, 10:47 AM
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i'm not sure this means anything but in a running engine the piston is what is pushing and pulling on the crankshaft. the big end weight and rod ratio i would think has more effect on high rpm capability than the piston weight. i'm thinking that a lighter piston only helps when compression and combustion are not occuring and the lighter rod big end and/or shorter stroke benefits 360 degrees of crank rotation. just a thought.
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Old 10-23-2010, 03:52 PM
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Does anyone make a piston pin that is tapered on the inside?
We used to get race pins that had a taper from each side (thin wall) extending inwards to the center (thicker wall).
These pins were usually about 2/3 the weight of regular pins.
Bob
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Old 10-23-2010, 05:08 PM
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Quote:
Originally Posted by gestalt1 View Post
i'm not sure this means anything but in a running engine the piston is what is pushing and pulling on the crankshaft. the big end weight and rod ratio i would think has more effect on high rpm capability than the piston weight. i'm thinking that a lighter piston only helps when compression and combustion are not occuring and the lighter rod big end and/or shorter stroke benefits 360 degrees of crank rotation. just a thought.
I believe your thoughts are on the right track, but the whole crank/rod/piston/pin assembly must be able to swing around TDC at top revs with no benefitial gas pressure. This is because one cylinder fires, but another is on TDC exhaust stroke, so it is actually pulling a vacuum on the other side, adding to the tension on the rod and the moment/shear in the pin. Rods are strongest in tension but they also have to cope with the compression on power stroke.

So, you still must be able to turn the revs in a vacuum, just looking at force and inertia, because there are not always benefitial gas pressures.
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Old 10-23-2010, 05:35 PM
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gestalt1,

If we are discussing Porsche rods, and we are, the highest compressive load the rod will see is at peak torque, if the motor knocks that will likely be the highest compressive load. This will likely happen well below peak horsepower and well below max RPM. Skipping to the 3.0 ltr rods these rods were used in the Porsche Turbo and made over 400 ft/lb of TQ, this is a significant compressive load on the rod. So the Porsche rod with a factory rod bolt can handle health amounts of compressive load. Compressive load is combined force load tied to the weight of the parts and the power per cylinder, but again this peaks at well below peak RPM.

Now when we look at tensile load, the load trying to pull the cap of the rod as the piston gets to top dead center, the weight of the rod, wrist pin and piston assembly all factor into the rod failure. The tensile load increases with RPM, this load is not a linear increase. We already know the factory rod bolt is a weak link, in tensile load, so we upgrade the bolts. Since the rod bolt is a problem we can be quite sure the rod has a problem in tensile strength, so a light weight piston and wrist pin with a stock rod and good bolts will increase the max RPM potential. We could also attempt to reduce the weight of the rod as well but if the strength of the rod is compromised in reducing the weight we have failed. If the big end of the rod cannot retain the round shape the rod fails.

HawgRyder,

I am sure tapered wrist pins are still being made, Probe sells a good selection, a quick check shows the tapered pin is lighter than a non tapered pin (6 grams less) but still over 20 grams more than a Titanium pin.

Stay tuned,

PFM
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Old 10-23-2010, 09:02 PM
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I once saw the result of lightening a wrist pin too much. Ugly.
Old 10-23-2010, 11:06 PM
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Walt,

Yes a wrist pin failure is ugly, failed rod bolt is worse when the crank throw drives the rod through the bottom of the motor. Touching a wrist pin on a lathe is not a good plan, the companies in the business are their for a reason.

The Titanium wrist pins have come a long way, casidium coated and steel caps to resist ware due to the wire lock. If they live in a NASCAR engine past 9000 RPM I trust them in a Porsche too.

Stay tuned,

PFM
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Old 10-24-2010, 03:22 PM
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