Show full size 924Board.org
Discussion Forum of 924.org
 
 FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   RegisterRegister 
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 
 Technical FAQ924 FAQ (Technical)   Technical924 Technical Section   Jump to 924.org924.org   Jump to PCA 924 Registry924 Registry

Cheap-a$$ long rod + stroker + high CR pistons?
Goto page 1, 2, 3, 4, 5, 6, 7, 8, 9  Next
 
Post new topic   Reply to topic    924Board.org Forum Index -> Performance Upgrades
View previous topic :: View next topic  
Author Message
ideola  



Joined: 01 Oct 2004
Posts: 15501
Location: Woodstock IL

PostPosted: Tue Feb 14, 2012 9:04 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????
_________________
15 981 GTS | 88 924S SE | 82 931 Holbert | 82 931 Rallye | 81 937 | 81 Euro 931 | 81 Weissach | 80 US 928 | 80 US 931 '941' | 80 US 931 | 80 931 GTR | 79 Sebring | 78 D-Prod Replica | 78 w/D-Prod kit | 78 Poli-Form | 78 Limited Edition | 77 Martini


Last edited by ideola on Wed Feb 29, 2012 12:19 pm; edited 1 time in total
Back to top
View user's profile Send private message
ideola  



Joined: 01 Oct 2004
Posts: 15501
Location: Woodstock IL

PostPosted: Tue Feb 14, 2012 9:16 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...
_________________
15 981 GTS | 88 924S SE | 82 931 Holbert | 82 931 Rallye | 81 937 | 81 Euro 931 | 81 Weissach | 80 US 928 | 80 US 931 '941' | 80 US 931 | 80 931 GTR | 79 Sebring | 78 D-Prod Replica | 78 w/D-Prod kit | 78 Poli-Form | 78 Limited Edition | 77 Martini
Back to top
View user's profile Send private message
flosho  



Joined: 01 Jul 2004
Posts: 3158
Location: Eau Claire, Wisconsin

PostPosted: Tue Feb 14, 2012 11:12 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?
_________________
[This Space For Rent]


Last edited by flosho on Tue Feb 14, 2012 2:00 pm; edited 1 time in total
Back to top
View user's profile Send private message Send e-mail Visit poster's website AIM Address Yahoo Messenger MSN Messenger
BionicBalls  



Joined: 05 Jan 2009
Posts: 643
Location: Charlotte, NC

PostPosted: Tue Feb 14, 2012 12:24 pm    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.
_________________
1980 924 NA
1982 931
Back to top
View user's profile Send private message
ideola  



Joined: 01 Oct 2004
Posts: 15501
Location: Woodstock IL

PostPosted: Tue Feb 14, 2012 1:37 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...
_________________
15 981 GTS | 88 924S SE | 82 931 Holbert | 82 931 Rallye | 81 937 | 81 Euro 931 | 81 Weissach | 80 US 928 | 80 US 931 '941' | 80 US 931 | 80 931 GTR | 79 Sebring | 78 D-Prod Replica | 78 w/D-Prod kit | 78 Poli-Form | 78 Limited Edition | 77 Martini
Back to top
View user's profile Send private message
flosho  



Joined: 01 Jul 2004
Posts: 3158
Location: Eau Claire, Wisconsin

PostPosted: Wed Feb 15, 2012 2:29 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.
_________________
[This Space For Rent]
Back to top
View user's profile Send private message Send e-mail Visit poster's website AIM Address Yahoo Messenger MSN Messenger
ideola  



Joined: 01 Oct 2004
Posts: 15501
Location: Woodstock IL

PostPosted: Wed Feb 15, 2012 2:56 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.
_________________
15 981 GTS | 88 924S SE | 82 931 Holbert | 82 931 Rallye | 81 937 | 81 Euro 931 | 81 Weissach | 80 US 928 | 80 US 931 '941' | 80 US 931 | 80 931 GTR | 79 Sebring | 78 D-Prod Replica | 78 w/D-Prod kit | 78 Poli-Form | 78 Limited Edition | 77 Martini
Back to top
View user's profile Send private message
morghen  



Joined: 21 Jan 2005
Posts: 7858
Location: Romania

PostPosted: Wed Feb 15, 2012 3:52 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.
_________________
red 924 and not red 924
Back to top
View user's profile Send private message Visit poster's website Yahoo Messenger MSN Messenger
ideola  



Joined: 01 Oct 2004
Posts: 15501
Location: Woodstock IL

PostPosted: Wed Feb 15, 2012 4:08 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.
_________________
15 981 GTS | 88 924S SE | 82 931 Holbert | 82 931 Rallye | 81 937 | 81 Euro 931 | 81 Weissach | 80 US 928 | 80 US 931 '941' | 80 US 931 | 80 931 GTR | 79 Sebring | 78 D-Prod Replica | 78 w/D-Prod kit | 78 Poli-Form | 78 Limited Edition | 77 Martini
Back to top
View user's profile Send private message
Fifty50Plus  



Joined: 28 Feb 2008
Posts: 1002
Location: Washington DC area

PostPosted: Wed Feb 15, 2012 7:01 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
_________________
1979 924 NA race car
1982 924 NA race car - Sold
1982 924 Turbo almost a PoS
1981 924 Turbo a real PoS, new engine
1982 924 Turbo nice body, blown engine
1972 911 E race car - going to Vintage
Various 944s to become IT-S race car
Back to top
View user's profile Send private message
ideola  



Joined: 01 Oct 2004
Posts: 15501
Location: Woodstock IL

PostPosted: Wed Feb 15, 2012 7:30 am    Post subject: Reply with quote

Will do...but I'm not even sure if the 924 crank was nitrided to begin with...
_________________
15 981 GTS | 88 924S SE | 82 931 Holbert | 82 931 Rallye | 81 937 | 81 Euro 931 | 81 Weissach | 80 US 928 | 80 US 931 '941' | 80 US 931 | 80 931 GTR | 79 Sebring | 78 D-Prod Replica | 78 w/D-Prod kit | 78 Poli-Form | 78 Limited Edition | 77 Martini
Back to top
View user's profile Send private message
tyfighter123  



Joined: 19 Jan 2010
Posts: 540
Location: Colorado

PostPosted: Wed Feb 15, 2012 8:37 am    Post subject: Reply with quote

This would be so much cheaper!
_________________
Porsche 924 1977 N/A
Mustang GT/CS 2007
Porsche 924S 1987 (parts car)(cut up and recycled)
Porsche 911S 1976
Porsche 931 1980
Porsche 931 1980 (parts car)
Back to top
View user's profile Send private message MSN Messenger
morghen  



Joined: 21 Jan 2005
Posts: 7858
Location: Romania

PostPosted: Wed Feb 15, 2012 6:46 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.
_________________
red 924 and not red 924
Back to top
View user's profile Send private message Visit poster's website Yahoo Messenger MSN Messenger
Carrera RSR  



Joined: 08 Jan 2010
Posts: 1565
Location: Somerset, UK

PostPosted: Wed Feb 15, 2012 9:19 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?!
_________________
1980 931 - forged pistons, Piper cam, CGT turbo, 951 ducted FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife LSD
Now www.924board.org/viewtopic.php?t=34690
Then www.924board.org/viewtopic.php?t=31252
Back to top
View user's profile Send private message
CÚdric  



Joined: 27 Aug 2004
Posts: 1732
Location: Sweden

PostPosted: Wed Feb 15, 2012 10:28 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 100Ám 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.
_________________
┤79 924
http://www.garaget.org/show.php?car=211
┤80 924 Turbo
http://www.garaget.org/?car=160275
Back to top
View user's profile Send private message MSN Messenger
Display posts from previous:   
Post new topic   Reply to topic    924Board.org Forum Index -> Performance Upgrades All times are GMT + 11 Hours
Goto page 1, 2, 3, 4, 5, 6, 7, 8, 9  Next
Page 1 of 9

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Powered by phpBB © 2001, 2005 phpBB Group