333pg333

Yes there are other threads in here but I want to know some more specifics re this process. Note that this is for a racecar that would be spinning in the upper rpms (4000-7500) for much of it's operational hours. Hence balance and strength are crucial.

What are the upsides and downsides of offset cranks? Is there any possibility of upsetting the balance of the crank or weakening it? What about hardening via nitride or other process...is there any possibility that it can make the crank 'brittle'?

Assume that you can spend a little less $ on a crank that isn't in great condition and have it offset however the costs will be 'offset' by the process which looks to range from about $700 - $1000.

I'm potentially looking for a crank either in very good condition to use without offsetting or a suitable candidate for the offset process. This would be for an internal US sale & ship, not to me down here.

333pg333

I believe we had one shipped but the description was "2 burnt journals and was undersized."
Not sure if this would be 'offsettable' though?

MAGK944

It really depends on what else you are doing to the engine and yes, making the stroke longer will increase friction and put more stress on the crank. Also just lengthening the stroke alone will just aid in keeping the torque low down where you don't want it in your situation.

If you are upping the bore at the same time this will help you get peak torque at a higher speed like you desire. Most (if not all) the hybrid strokers on here have steel sleeves fitted and bored out to accommodate much larger pistons. This allows you to fit less expensive off-the-shelf pistons and also accommodate larger valves, which you will also need to get the power in the high rpm range.

I would look at the big picture of what you want from the engine and build to achieve that aim rather than concentrating on stroking it. Lots of people here have more knowledge than me and willing to help. GL

333pg333

The motor will be built using a 3 ltr block that is sleeved and deckplated, big headstuds, dowelled girdle etc. The head will be a worked 16v head with larger valves etc.

While the emphasis isn't on necessarily making the size larger than 3ltr, my questions on the crank 'revolve' around being able to procure one that is either in good condition to use as is or in the case of not being able to find one, we go down the offset route as this may speed up acquiring one for this motor. I don't have a ton of time to get this motor built and shipped, installed and tuned. So I'm exploring options for exigency plans.

thingo

I would have thought you would not be using the standard bearings so the crank would have to be ground anyway?

333pg333

Well the crank would obviously be looked at and a determination made but assuming it's not brand new and 100% perfect then they'd do whatever they do, however this may not necessarily mean offset grinding.

michaelmount123

I have a good 3L crank if you still need one Patrick.

Highly recommend you use a proper race rod bearing (not Porsche!) and an S2 harmonic balancer.

refresh951

Several potential benefits to the offset grinding as I see it:

1) Much larger selection of quality rod bearings. The ACL Race bearings we use (4G63) are far superior to the OEM rod bearings. They are NASCAR quality.

2) The use of smaller journals can provides better bearing lubrication. No #2 bearing failure has occurred to date on an offset grind motor.

3) Offset grinds can allow components to be used that greatly reduce the weight of the internal components (crank, rods and pistons) thus reducing reciprocating mass.

4) Offset grinds can allow the use of "off the shelf" components that otherwise may not be an option. This can improves build flexibility and can greatly reduce costs.

5) Mildly damaged cranks can be salvaged.

In my mind there is no downside to an offset grind if it is done properly. The 944/968 cranks are unbelievably strong and hard. When have we EVER seen a 951 crank fail? My crank guy, who does a lot of exotic motor work, says our cranks are harder than ANY crank he has ever dealt with. Nitride treatment is a surface treatment to make the surface harder. Nitriding should not cause brittleness if done properly. I personally do not think nitriding is mandatory with our cranks but I understand the reasons to do it. Sid and myself did not nitride our cranks and we have both done inspections and bearings and cranks were perfect. If things go bad at the track (very high oil temps) I am sure nitride treatment will buy you more margin so for a dedicated track car it makes sense. A critical aspect after offset grinding is max runout. The cranks tend to bend during the grind process so they need to be straightened. Michael Mount recommends a max runout of 0.001".

I think with Gustaf's motor (race car) , Sid's motor(s) (drag car), and my motor (street car) the process has been pretty well proven on big power applications.

refresh951

Just for reference wrt proper race rod bearing (without offset grind):

https://rennlist.com/forums/944-turb...aring-fix.html

333pg333

Thanks Michael. PM sent about that crank.

Thanks Shawn. Always value your input and experience. We would definitely be looking to employ a non stock bearing.

MAGK944

You have pretty much answered your own question. Look for a good 3L crank and if you find one buy and fit as is. If you can only find a 3L crank with some rod bearing damage, you can get it offset ground. That's your plan correct?

A machine shop will tell you how much they can offset the crank and this will obviously depend on how much damage there is.

michaelmount123

Great info from Shawn! A little further detail on surface treatment for crankshafts: Nitriding provides additional strength as well as a harder surface, so its benefits are two-fold. The strength is a function of having a harder 'shell' that encapsulates the crank. It's only a few thousands thick (usually .003-.004"), done before final straightening and grinding. Porsche uses, BTW, induction hardening on the 944 cranks. You can see the tell-tale bluing around each journal. The induction hardening process provides a much thicker layer - around .060". So when cutting journals to undersize, or even going to a 2" Chevy size journal (so a good bearing can be used) there's still a hardened layer left. I always use the Nitride surface treatment on a repaired journal or if I'm stroking a crank by welding the journals.

Shawn and Sid have determined that a Nitride treatment is not necessary for the street or for short power applications. For the track and endurance applications though, it's highly recommended.

mikey_audiogeek

Just to expand on the great info above:

Nitriding increases the fatigue strength of the crank.
It has minimal effect on the overall mechanical strength and stiffness because the layer is so thin.

Nitriding increases fatigue strength because the surface layer into which the nitrogen has diffused is now in compression. This inhibits fatigue crack formation in a similar way to shotpeening or rolling the crank fillet radii.

The compressive forces in the nitrided layer are because the chemically altered layer now takes up a greater volume, like water freezing. This is why you expect a slight diameter change on a nitrided crank.

The fact that the layer is harder (which means stronger) also helps inhibit crack formation. There is a direct correlation between material hardness and strength.

Cheers, Mike

333pg333

Thanks for the great advice guys.

I'm following up on that crank Micheal.

mikey_audiogeek

No worries, Patrick!
Lots of holes in my explanation, it's oversimplified. Didn't want to get into a p1ssing match, just wanted to expand on the great info above. MM always has something to bring to the party!

Were you thinking of offset grinding the mains or the throws?

Cheers,
Mike