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Cheap-a$$ long rod + stroker + high CR pistons?
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ideola  



Joined: 01 Oct 2004
Posts: 15548
Location: Spring Lake MI

PostPosted: Tue Feb 14, 2012 8:04 am    Post subject: Cheap-a$$ long rod + stroker + high CR pistons? Reply with quote

So, I got bored today and decided it's been slow around here, so why not do some muck-raising.

Check this out:
Rod: 150mm length, 45mm big end diameter, 21mm pin diameter, 28.3mm width
Piston: flat-top 87.00mm bore, 31.6mm compression height, 21mm pin diameter

Both above items are available off-the-shelf from relatively modern Japenese cars (two different makes). My initial inquiries indicate the rods can be had for less than $350 per set. The pistons can be had for less than $250 per set with pins & retainers, but I don't believe it includes the rings.

In order to use the rod, the 924 rod journals would have to be ground down from the stock 51.6mm diameter to 45mm. So this would involve some additional $$$ spent to service the crank. Also, while grinding the journals to their new diameter, they would need to be widened from their stock 27.9mm width by .4mm to accommodate the slightly larger width of the Jap rod (which shouldn't be a huge deal, as the 924 crank is quite beefy and has plenty of material there).

If you simply ground the rod journals to maintain the same center location relative to stock location (i.e. maintaining the stock stroke of 84.4mm), using an otherwise stock 931 head and gasket, you end up with ~8.4:1 CR (which could be further reduced if desired by using a thicker gasket).

Now, here's the cool part. If you offset ground the rod journal, you could move the stroke out by as much as 3.3mm, which would allow you to achieve ~10.0:1 CR using a stock 931 head and gasket. You could get ~10.6:1 with a .027mm gasket, and that doesn't take into account additional CR increase possible by decking the block or head.

The other potential benefit is the >4% increase in rod length will increase top-end HP, reduce side loading of the piston (freeing up even more power) and improve combustion efficiency.

I don't know how much the offset grinding would cost, but it should be far less than the stroker cranks I had prepared because it doesn't involve any welding, just downsizing the rod journals. I also have not been able to determine the weights of these components, so I don't know how they might compare to stock.

In any event, I think one could achieve a high compression long rod stroker using the better 931 head, with CR tuned for either forced induction or NA applications. A set of NA headers with elongated holes and a Series 1 931 intake or a stock NA intake with elongated holes would both mate directly to the 931 head. So we could end up with a one size fits all combination of components, if you will.

FWIW, I believe this approach is better than the VW GTI rod approach discussed by Peter Barnett some time ago because those rods are not nearly wide enough for our crank-centered setup. You need a rod that is at least as wide as the stock rod to avoid piston slap and loss of oil pressure. I also looked into flattop VW pistons, but have yet to get the exact compression height, not to mention, I believe their wrist pin diamter is 22mm, rather than the 21mm required in order to use these rods.

So. What do you all think????
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Last edited by ideola on Wed Feb 29, 2012 11:19 am; edited 1 time in total
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ideola  



Joined: 01 Oct 2004
Posts: 15548
Location: Spring Lake MI

PostPosted: Tue Feb 14, 2012 8:16 am    Post subject: Reply with quote

Just found another piston option with a dish, available in 87 or 88mm bores, with dishes in 12cc, 8cc, and 2.5cc versions. This setup would be good for an NA application, as it would yield 9.5:1, 10:1, and 11:1 CR respectively (everything else being stock).

{EDIT}
...but I don't know the prices on those pistons at the moment...
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flosho  



Joined: 01 Jul 2004
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Location: Eau Claire, Wisconsin

PostPosted: Tue Feb 14, 2012 10:12 am    Post subject: Reply with quote

I like what I'm hearing, but doesn't sound like any of these options would make the motor non-interference, correct?
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Last edited by flosho on Tue Feb 14, 2012 1:00 pm; edited 1 time in total
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BionicBalls  



Joined: 05 Jan 2009
Posts: 642
Location: Charlotte, NC

PostPosted: Tue Feb 14, 2012 11:24 am    Post subject: Reply with quote

If you get everything worked out, put me in for a kit. I got a spare 931 engine laying around with plans to do something like this at some point.
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ideola  



Joined: 01 Oct 2004
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Location: Spring Lake MI

PostPosted: Tue Feb 14, 2012 12:37 pm    Post subject: Reply with quote

Actually, I think it might be non-interference. Here are some figures to mull:

stock stroke with flat top piston = ~6.4mm deck clearance
87.7mm stroke with flat top piston = ~4.75mm deck clearance
stock stroke with dished piston = ~3.075mm deck clearance

Bearing in mind the 931 head has a recess to it, i.e. the valves aren't flat like on an NA head, there may be enough deck clearance with all of these options to avoid interference. I don't know offhand how far beneath the head surface the 931 valves are at full lift. The latter two options might be close. Of course, a good machine shop should be able to cut valve reliefs, although I don't know how much material is on these piston crowns...
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flosho  



Joined: 01 Jul 2004
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PostPosted: Wed Feb 15, 2012 1:29 am    Post subject: Reply with quote

Dan are you going to be packaging this stuff or do we only get the info and have to get the parts ourselves? Either way is ok.
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ideola  



Joined: 01 Oct 2004
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Location: Spring Lake MI

PostPosted: Wed Feb 15, 2012 1:56 am    Post subject: Reply with quote

I'd like to offer a complete package, but of course, the only hitch being having an opportunity to try it out myself first.

The components I'm looking at all appear to be readily available from a couple of suppliers with whom I already have reseller status. Once I know whether I have a package that might work, I'll release the detailed specs, and people can choose to source the components on their own, or buy from me if they feel my prices are competitive and that my research is a value-add .

What I'm discovering is that there are bunch of alternatives on the pistons, which opens up a whole world of possibilities. For example, I've found 5 different manufacturers, and some of them have different dish volumes which would allow us to create different CR combinations. One of the difficulties I'm having is finding conclusive documentation on the wrist pin diameters for the pistons. There are two rod configs that will work, one with 21mm and one with 22mm wrist pin, so I have to figure out which piston goes with what. Oddly, very few of the piston spec sheets include the wrist pin diameter.

The other really cool thing is that the rods are available in a variety of configurations: H-beam are the cheapest, then I-beam, then X-beam; and there are at least 4 manufacturers that have them as a standard catalog item, all available with standard ARP fasteners. Point being, a package could be put together for bargain basement all the way to "ultimate".

I'm also going to reach out to a local crank shop to get a guestimate on what offset grinding might run.

Early indication looks like the least expensive package would be right around $750 (rods, rod bolts, pistons, pins, retainers, and ring pack); if the crank could be serviced for $250 or less, you could have a turn-key solution for $1K, maybe less depending on what discounts I might be able to negotiate on the parts.
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morghen  



Joined: 21 Jan 2005
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PostPosted: Wed Feb 15, 2012 2:52 am    Post subject: Reply with quote

thats cheap allright...but what about the "cement" layer on the journals?

if you reduce the diameter by that much....not only you make the crank weaker but the friction surfaces on the original journals are(should be) treated surfaces and the material does not have the same properties deeper.

i'll have to consult with my coleague who is an expert...but thats my initial thought.
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ideola  



Joined: 01 Oct 2004
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PostPosted: Wed Feb 15, 2012 3:08 am    Post subject: Reply with quote

This was not an issue when welding and offset grinding my stroker cranks. We inquired about nitriding the crank at the time, and he advised against any additional heat treating. But, I will definitely inquire about this when I speak to my crank guys, see what he says.
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Fifty50Plus  



Joined: 28 Feb 2008
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PostPosted: Wed Feb 15, 2012 6:01 am    Post subject: Reply with quote

Dan, I'm really curious about what the crank guys say.
From what I remember, the nitriding done on 356 and 911 cranks was about 10 thou thick (.25mm). Turning a worn crank first undersize .25mm usually removed the nitirided metal. Doing it again for second under or even third undersize meant that we wouldn't race that crank. That's a total of less than 1mm. What you are proposing is that the 924 crank be turned 6.6 mm undersize or more than 12% of it's original rod journal diameter.
Hopefully that's not a concern with the Audi crank because the idea is very tantalizing. See what they say and whether they would suggest imposed RPM limits.
Chuck
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ideola  



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PostPosted: Wed Feb 15, 2012 6:30 am    Post subject: Reply with quote

Will do...but I'm not even sure if the 924 crank was nitrided to begin with...
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tyfighter123  



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PostPosted: Wed Feb 15, 2012 7:37 am    Post subject: Reply with quote

This would be so much cheaper!
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morghen  



Joined: 21 Jan 2005
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PostPosted: Wed Feb 15, 2012 5:46 pm    Post subject: Reply with quote

ideola wrote:
Will do...but I'm not even sure if the 924 crank was nitrided to begin with...


AFAIK its a standard to treat such surfaces since the dinosaurs went missing...

i'm quite sure the surfaces there are hardened(just like japanese swords...hard shell and softer inside)..thats why the factory says you have a certain amount of material to shave off (RI RII RIII) and then you have to replace the crank....so unless you re-harden the surfaces i dont see how it could hold.

if the crank holds with -6.6mm and you find a propper way to harden the surfaces it is a go IMO.
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Carrera RSR  



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PostPosted: Wed Feb 15, 2012 8:19 pm    Post subject: Reply with quote

Dan, awaiting updates on this. As per my engine build thread in the 931 section I either have to source a new block with good 86.5mm bores or rebore the current block and source new 87mm pistons.

This could be a solution for me?!
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Cedric  



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PostPosted: Wed Feb 15, 2012 9:28 pm    Post subject: Reply with quote

Nitriding isnt so deep since its a chemical process. thickness of the hardened layer depends on were you want to measure and how, but at 100µm i would expect the hardness to already be lot lower than at the surface (Do you want more specific numbers?). "normal" heat treatment is a lot deeper.

If you could find a small company there should be possible to nitrid harden it for a fair price. But be aware, if you are not aware in the process you could ruin dimensional tolerances.
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