Molnar Rods?
 

Hello all,

I'm comparison shopping new rods for my 3.4 build, compared to having my orignals resized, rebushed, and using ARP bolts.

New rods are going to be more expensive but there are some weight savings to be realized.

Other than the usual $1,700 options from Carrillo and Pauter one option is Molnar:

https://molnarrods.com/porsche-964-connecting-rods

They are considerably cheaper than the other alternatives and are competitive in terms of weight.

Not sure the typical weight of a stock 3.2 rod w/bolts but pretty sure it's +100g or more than these so that's a pretty nice weight savings.

Anyone have experience with them? A search comes up completely empty.

Thanks in advance!!!

They're well known (well, mostly, anyway) in American V8 circles. AFAIK, they're lumped with Scat and Eagle, being overseas 4340 steel. Tom Molnar used to own K1. Some of their stuff WAS US made, but I'd call them and ask.

Carrillos are 4330 vacuum melt steel, US made. Pauters are also US made.

A builder of Porsche and American engines who I trust told me he likes Molnar. Granted it was a few years ago, no idea whether anything about their situation changed since then. I didn't realize they're making Porsche rods now.

My stock 1986 930 rods weight 694 g each. This is a big savings in weight. I had not planned on rods for my rebuild......... But this does have me thinking.

They are made in China, and "finish machined in the USA" to what they claim is a decent tolerance.

with 3/8" ARP bolts they should be actually a bit stronger than the 9mm replacement bolts for the 3.2 Carrera.

I'm guessing if they made an SC rod (they don't) they'd have gained a bit more attention on the forum. As it happens the 3.2 and 964 share rod dimensions.

I would stand to shave a good 100g per rod off the motor. Not a bad thing.

Carrillos for 3.2/3.3/964 weigh 585 gms, for reference.

I'd venture a guess that any quality aftermarket rod is a substantial upgrade over stock, even with better rod bolts.

The lightweight version of the Pauter rods are 545g.

The Molnar rods are 565g so right in there. Seem like a lot of positive reviews from around the net of the products, and the man himself (Tom Molnar) has experience and reputation to back up the product.

Not trying to crap on the idea of better rods. Just thought it worth mentioning that I have always found it odd what a wide spread in weight groups there are for the factory rod groups. So I figured i'd ask if anybody knows why Porsche did that.


Took this snapshot from the '84 to '87 tech spec booklet. On the low side of the weight groups, 615g to 687g is a BIG difference!


http://forums.pelicanparts.com/uploa...1643996952.jpg

There’s also a Swedish brand called Verdi. Good quality

Not sure who sells them in the US

Autoverdi makes some of the best stuff money can buy. We used their dry sump oil pumps in nascar. They also make an oil pump for 911's.

Wow, so my engine has the heaviest rods? Weight group 11. They are nicely matched, one is 693 grams, 5 are 694. Was this normal on the turbo cars??

I have wondered that exact thing. I guess there can be a wide range of rough forging weights, maybe as tooling wears or ?? so easier to sort them than manufacture them to a finer tolerance??

Getting closer to pulling the trigger on these but notice that the small end width matches that of the 993 rod which is 5mm smaller than the 3.2/964 rod.

Seems like this would impart more bending force on the pin than necessary. Of course custom pistons would be the fix for this but I'm trying to avoid that since pistons are on order.....

Thoughts anyone?

Porsche apparently rethought the rod design in the 993 engine. Take a look at this 964 vs 993 engine document from Porsche. The mention of the 993 rod vs 964 rod is on pdf page 57, which is published page 53

https://drive.google.com/file/d/0B7Bo-8bAu9G9NUY3YUJQcjFJTW8/view?usp=drivesdk&resourcekey=0-wMe-9TYRISmq7Hi3nqVJpw

Apparently the “footprint” of the earlier rods (they all look the same shape, despite different dimensions) is needlessly fat? I’m not saying the 993 rods are spectacular. Just saying the earlier rods are fatties

Edit: I would contact Molnar and ask them if it’s a typo in their specs for the 964. Like you said, the pin end dimension is narrower and the description in the grouping of specs on the 964 page says “993 and 996”

The 993 rods were thinner do to crank webbing being made thicker. The 964 rod is thicker and design goes all the way back to 78 3.3 turbo (probably earlier). 993 rod design was able to take advantage of CAD analysis and looks very different from earlier rods.

993 Rod

http://forums.pelicanparts.com/uploa...1645126012.jpg

964 Rod

http://forums.pelicanparts.com/uploa...1645126104.jpg

I believe that for racing and high RPM use (GT3) Porsche went back to wider journal. GT3 rods and same dimension as 3.0 SC and 996 Turbo uses same rod as 993.


john

I think Porsche kept playing with the trade offs between crank stiffness and bearing area.

Larger journals have more overlap with the mains making a stronger stiffer crank, the 993 crank should be the stiffest of the stock cranks with the narrow journals and large diameter but at the cost of bearing area and higher bearing speed.

GT3 they went back to the SC size journal for more oil pump clearance/bigger rod bolts and evidently deemed the crank stiffness to be acceptable. GT3 rods have the same shape as a 964 and earlier style rod but are titanium for the weight savings.

That factory tech info is fascinating, thanks!

I'm thinking of a plan "C", that being scouring ebay for the lightest set of matched 3.2/964 rods I can find and sending them off as cores to be resized with the ARP bolts and new bushes.

Yeah the Molnar website leaves a bit to be desired. Didn't get around to calling them yet but it's on the to-do list.

Why not get some Carrillo rods and be done with it?

The money you save with other options just ended up costing you more down the road....

Great Information

Well, the plot thickens, the specs are not a typo..... Molnar doesn't make a 964 Rod at all, it's a 964 RSR rod. Which is a 964 rod with a 993 width small end. From what I can tell this is useful if you wanted to use 993 pistons and a 964 crank.

They also make a 993 Rod which also fits 996 Turbos.

What they don't make is a 3.2/3.3/964 rod.

Unless someone knowledgeable can advise me that the narrow pin end of the 964 RSR spec rod is OK to use with the off the shelf 3.2/3.4 JE pistons I've ordered, I'm going back to plan "A", to use rebuilt and balanced stock rods with ARP bolts.

Short answer - money. Longer answer below:

The bigger picture on this motor is it's a street/auto-x motor and I've chosen to give it a nice meaty torque rich powerband, which means it's never going to be an 8KRPM screamer.

$1700 Aftermarket rods are most definitely overkill. But if I can install something for a few hundred bucks more than stock rebuilts and lose 100g+ per hole of reciprocating mass, that would be money well invested.

As far as connecting rods costing money down the road? Not sure how that is, as we are discussing viable options.

Your choice should always be based on your engines performance. Then budget.

I assume the engine's performance is going to be around the mid 300 BHP? That's around 55- 60HP per cylinder/rod/pin/pin boss. Not a lot. The stock rod will work if rebuilt correctly. The weight will be the only consideration.

Many of the current options are still heavy for this type of engine. You stated a "meaty" torque band. I'm not sure what that means. You will be limited to the amount of torque produced by air intake limitations and displacement. The engine's VE is going to be at the lower end of the scale. So what you need to consider is the time it takes for the engine to reach the usable torque. The quicker you can rev the engine into the Torque that will accelerate the car forward the better. This is where the component weight makes a difference.

You need to consider trying to get all cylinders making the same torque. Heavier rods won't do anything to help get more air into the cylinders.

Lighter rods will cost $$$. another factor to consider now is, delivery time. You may be a shock on how long it will take.

Hi Neil, thanks for the thoughtful reply.

The below quote is a better overview of the plans from this thread: http://forums.pelicanparts.com/911-engine-rebuilding-forum/1109850-hot-3-4-build-max-safe-rpm-stock-rods-w-9mm-arp-bolts.html

I'd be happy if the engine made 300Hp and had good wide range of torque (or at least isn't super peaky). I always prioritize light weight where I can, car currently has a Patrick Motorsports DME flywheel, and aluminum pressure plate but planning to switch to an RSR style flywheel which takes a few pounds off current configuration.

The ECU, 46mm ITBs and 12 COP units are in hand and I'm working on the heads right now as parts trickle in from around the world..... yes the supply chain issues are real. And yes I'm already over budget!!!!

I'm aware that the phrase "building a 911 on a budget" makes as much sense as "Fuc%#@ for viginity".......

The car also has close ratio gears in 2nd and 3rd and weighs 2335 pounds empty so getting to that torque is a little easier than some cars.

Hi Jonny,

What rods did you end up with now for your hot 3.4 build, are you going to stick with stock ones? I think that Neil is working with an Italian company to produce some that are strong enough for a NA application and lighter than Carillo and Pauter? Not sure about the price however..

Cheers,
Lukas