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Cheap-a$$ long rod + stroker + high CR pistons?
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tyfighter123  



Joined: 19 Jan 2010
Posts: 549
Location: Colorado

PostPosted: Fri Mar 02, 2012 7:23 am    Post subject: Reply with quote

Quote:
Just to clarify, there is NO WAY to increase stroke without modifying the crank. The long rod does not make it a stroker (but has it's own benefits). The increase in stroke can only come by modifying the crank, i.e., specifically by moving the rod journal further out.


That makes sense! I am still trying to learn all of this stuff so its great to have someone explain it.

When do you plan on getting dynos? I think it will be nice to see what the actual numbers are.
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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)
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ideola  



Joined: 01 Oct 2004
Posts: 15506
Location: Woodstock IL

PostPosted: Fri Mar 02, 2012 7:37 am    Post subject: Reply with quote

tyfighter123 wrote:
When do you plan on getting dynos? I think it will be nice to see what the actual numbers are.


As soon as I can this spring, once I've gotten the Club Sport running. I am aiming for end of March to get the motor in. I have a couple of minor nits to sort out with the suspension, and I have to fab a mounting arrangement for the radiator, but otherwise, it should be very doable, especially since we're getting warming than usual weather already this March.

I have four cars I want to take in so I get a good baseline, so I'm going to try to find a place where I can get a good deal on a whole-day session:
-- GEBLASN: stock US-spec S2 931
-- 941: long rod, high CR, ported head, integral cam, scraper, S1 turbo
-- 937: stock ROW-spec 937
-- Club Sport: stroker, big valve head, integral cam, windage, S1/937 turbo

Should be VERY interesting!!! All running stock CIS, of course, but the 941 and Club Sport both have flow matched CIS injectors...
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Martijnus  



Joined: 29 Dec 2006
Posts: 2019
Location: Netherlands

PostPosted: Fri Mar 02, 2012 9:25 am    Post subject: Reply with quote

Woops. You're totally right about the centerline when grinding the crank.

And I didn't mention custom pistons... because that's a way that's not appealing to me. Like I said before, the off-the-shelf part makes it interesting. But that doesn't mean you're restricted to a particular piston/rod combo. You could use mazda rods with mercedes pistons... if it fits...

How did you have access to rod catalogs? Are there specialized suppliers in the US for rods providing catalogs?

The reason I hate the stock rods are because they're heavy. Almost a kg a piece. Include the piston and you've got about 1300 gr per cylinder. That's more than 5kg of mass going up and down. Also, the pin bushings are NLA so I made them myself, which is no problem, but for most it's nice to be able to buy this stuff (however, most bushings are semi finished so you need machine shops anyway).

edit:
there's one thing I can't understand since I've first read it on this board years ago.
Flattop pistons are afaik in no way performing better than dished pistons in a blown application. The dish is always present in turbo pistons and could easily be 'flattened' during manufacturing. Still, all manufacturers create the dish in blown applications... Some guy working for BMW at the time told me that on blown engines the dish increases the whole VE and compared it to filling a long-drink glass and soup dish or something. Can't really remember the whole point, but he convinced me

The advantage is that the stock N/A pistons are imho very suited for a turbo application
Doesn't really matter, and I guess there are some alternatives with a dish.

Dan, can you tell something about the brand of the pistons? I'm fond of Mahles because they're just beauties and the manufacturing precision is so bizarre. While I often find other (even custom) pistons do differ in specifications, weight etc.
So are the pistons of a certain quality? Price doesn't really say anything. I've seen jaguar pistons from a top brand for very little money while the same brand manufactures pistons for french cars for the triple price.
_________________
"Rule: Turbo's make torque, and torque makes fun." (C. Bell)

924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment


Last edited by Martijnus on Fri Mar 02, 2012 9:41 am; edited 1 time in total
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ideola  



Joined: 01 Oct 2004
Posts: 15506
Location: Woodstock IL

PostPosted: Fri Mar 02, 2012 9:33 am    Post subject: Reply with quote

Martijnus wrote:
... if it fits...

Well, you have to get the right bore, the right wrist pin to match the rod, and a compression height that works with our deck height. I also eliminated all possibilities that had strange valve pockets because those create unnecessary hot spots and weaknesses that add no benefit to our application (since they won't line up with our valve size or spacing), and further compound proper calculation of CR. I also eliminated all possibilities with domes. So now all you have are those with flat top or slight dish...trust me, it *really* narrows down the possibilities to maybe 5 or 6 different engines.

Martijnus wrote:
How did you have access to rod catalogs? Are there specialized suppliers in the US for rods providing catalogs?

All aftermarket suppliers, Google is your friend

Martijnus wrote:
The reason I hate the stock rods are because they're heavy. Almost a kg a piece. Include the piston and you've got about 1300 gr per cylinder. That's more than 5kg of mass going up and down. Also, the pin bushings are NLA so I made them myself, which is no problem, but for most it's nice to be able to buy this stuff (however, most bushings are semi finished so you need machine shops anyway).

Agreed, plus all of these rods have integral bushings for a floating pin, which is what we want.
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Martijnus  



Joined: 29 Dec 2006
Posts: 2019
Location: Netherlands

PostPosted: Fri Mar 02, 2012 9:54 am    Post subject: Reply with quote

Don't get me wrong I'm not saying there's a heap of pistons and rods out there for our application, just that it does not have to be a brand-combo. But I read the starting post again, and you've already got two different engine types for rods and pistons, so that's exactly what I meant

edit: with fitting, I obviously meant the wrist pin. Every 21mm pinned piston could fit the rod. I wasn't talking about the other specs, just the pin... but I do take your word that it all narrows down to just a few.

I just googled for connecting rod catalogs and then I already understood that you based on aftermarket that makes sense. I sincerely hoped for a catalog covering all sorts of models and OEM specs, not only the models for which aftermarket is available... but those don't exist I guess (that's why I never searched for rods).
If there are aftermarket rods, that's a nice advantage too even more choice.

edit2: last remark
Quote:
In order to use the rod, the 924 rod journals would have to be ground down from the stock 51.6mm diameter to 45mm


51.6mm seemed very unusual for european stuff, so I checked haynes and it stated:
connecting rod bearing journal diameter, new: 47.95 - 47.97mm

Where did you get the 51.6 from?

edit: I'm pretty sure 47.95 is the correct diameter....which has two consequences. Less material needs to be removed for the 45mm rods, but less stroking.
This in turn influences the piston choice.

Please do correct me if I'm way off, I'll take measurements of my crank when I get to it some day but 99% of the metric engines are close to round numbers (48.00 including the average 0.05mm play in this case). That's why I ask. Best to be sure.
_________________
"Rule: Turbo's make torque, and torque makes fun." (C. Bell)

924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment
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ideola  



Joined: 01 Oct 2004
Posts: 15506
Location: Woodstock IL

PostPosted: Fri Mar 02, 2012 10:16 am    Post subject: Reply with quote

The rod I have in my hands at this moment and my digital calipers beg to differ The bore in the rod itself is 51.6mm, vs. 48mm and 45mm bores for the two rods I'm looking at. The figure you are looking at is for the crank journal. So there would be a corresponding difference in size for the journal itself, after you take into account the bearings. It still amounts to the same difference in material removed.

{EDIT}
To clarify the above, all rod specs are listed based on the bore of the big end, not the actual size of the crank journal. So if you look at the specs on the Crower rods, you will see that the big end bore is listed at 2.0316", which is 51.6mm. Consequently, the rod specs in the catalogs I perused were listed at 48mm and 45mm respectively. As noted above, the crank journals for the respective cranks will be proportionally smaller to take into account the bearing size.

{EDIT}
Also, the tech specs booklet list the stock rods at 815...927 grams (quite a spread!), while the Crowers (I-beam construction) weigh in at 620 grams. The H-beam rods I'm looking at are less than 600 grams, so more weight savings!
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Martijnus  



Joined: 29 Dec 2006
Posts: 2019
Location: Netherlands

PostPosted: Fri Mar 02, 2012 10:40 am    Post subject: Reply with quote

The weight of the rods is spread indeed, but I only saw moments ago that the weight difference is max. 8gram, so in the piston/rod combo, stock maximum of 14+8 = 22 gram is allowed. I read it as more than 100 which is completely not logical.

Anyway, I understand it. Apparently my english is so bad that I can't understand the difference between bore and journal diameter We call it 'ground bore' litteraly translated... the bore of the rod in which the bearing goes.

On the other hand... to quote you again:
Quote:
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 the stock diameter still does not go from 51.6 to 45, but from 48 to 45.

I think I know which rods you're looking at and I think they're a very good option. This thread has been worth much already.
_________________
"Rule: Turbo's make torque, and torque makes fun." (C. Bell)

924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment
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ideola  



Joined: 01 Oct 2004
Posts: 15506
Location: Woodstock IL

PostPosted: Fri Mar 02, 2012 10:52 am    Post subject: Reply with quote

Martijnus wrote:
On the other hand... to quote you again:
Quote:
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 the stock diameter still does not go from 51.6 to 45, but from 48 to 45.


Yeah, I misspoke there...I should have written:

"the 924 rod journals would have to be ground down from the stock 48mm to XXmm"

where XX = the journal diameter associated with the 45mm big end bore rods, which is probably something like a 42mm journal size by the time you account for bearings. Accordingly, 48mm to 42mm is still a 6mm difference, and still achieves an increase of 6mm in stroke. Follow?
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Martijnus  



Joined: 29 Dec 2006
Posts: 2019
Location: Netherlands

PostPosted: Fri Mar 02, 2012 11:01 am    Post subject: Reply with quote

No, if we are talking about the same rods, the bigend bore is 48.00mm and the pin (journal) diameter is 45.00.
All the other specs are exactly the same as you mentioned.

edit: still, I have to thank you a lot, because I also found out that acl race bearings are freaking cheap for these rods and come in 3 oversizes. Rebuildable for life.
_________________
"Rule: Turbo's make torque, and torque makes fun." (C. Bell)

924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment
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ideola  



Joined: 01 Oct 2004
Posts: 15506
Location: Woodstock IL

PostPosted: Fri Mar 02, 2012 11:12 am    Post subject: Reply with quote

Martijnus wrote:
No, if we are talking about the same rods, the bigend bore is 48.00mm and the pin (journal) diameter is 45.00.
All the other specs are exactly the same as you mentioned.

edit: still, I have to thank you a lot, because I also found out that acl race bearings are freaking cheap for these rods and come in 3 oversizes. Rebuildable for life.


If we are talking about the same rod, there is one manufacturer who makes a set that is identical to all others except with a 45mm big end bore
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Martijnus  



Joined: 29 Dec 2006
Posts: 2019
Location: Netherlands

PostPosted: Sun Mar 04, 2012 4:46 am    Post subject: Reply with quote

Ok haven't found that specific one I guess... but I see 45mm housing bore as a downside, while you probably see it as an advantage for the stroking, am I right?
_________________
"Rule: Turbo's make torque, and torque makes fun." (C. Bell)

924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment
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ideola  



Joined: 01 Oct 2004
Posts: 15506
Location: Woodstock IL

PostPosted: Sun Mar 04, 2012 7:22 am    Post subject: Reply with quote

Well, if the goal is to get as much stroke as possible (and therefore, as much displacement as possible) without having to weld-and-regrind, then yes, the smaller journal size is advantageous.

The beauty is, rods are available in BOTH journal sizes, so we can come up with a package that is fine-tuned to exactly what the individual wants
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ideola  



Joined: 01 Oct 2004
Posts: 15506
Location: Woodstock IL

PostPosted: Fri Mar 09, 2012 2:04 am    Post subject: Reply with quote

Update: crank guy still hasn't gotten around to doing his critical measurements. Need his insight to determine whether we can use the slightly wider or slightly narrower rod.

In the meantime, I took a closer look at the rod specs, and the one manufacturer that I thought had a 45mm big end bore does NOT, it is a 48mm big end bore like everyone else's. Their online catalog lists the CRANK PIN diameter instead of the BIG END BORE, unlike every other manufacturer out there. Unfortunately, some of the aftermarket resellers erroneously report the big end bore to be 45mm, which is incorrect.

So the actual dimensions we're working with is taking our stock 48mm crank pin down to 45mm, using the off-the-shelf rods with 48mm big end bore, and using off the shelf bearings.

In summary, this means the maximum additional stroke with simple offset grinding is +3mm.

More to follow...
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Euro924S2  



Joined: 06 Mar 2008
Posts: 183
Location: UK

PostPosted: Wed Mar 14, 2012 9:15 am    Post subject: Reply with quote

Nice work dan.

I'm probably not in the market for the stroker kit at the moment as i've spent on new OEM rings and bearings etc for the 931 engine but the manifold discussion intersts me greatly.

I can 100% confirm that a NA cast iron mani can be ground to fit a 931 head, but as said, it's a long process - took me about 12 hours for the 4 ports IIRC. I then made up a custom gasket from 3 layer tin covered universal gasket material.

My blend in between the 38mm head and 32mm manifold is only over a short distance (maybe <1cm) since reaching any reasonable distance down the minifold is pretty difficult.

I am very keen on doing something to improve my gas flow but unfortunately, despite my steering wheel being on the right side (in both senses of the word!! ) my steering shaft is on the wrong side to allow for a straight forward header bolt on.
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ideola  



Joined: 01 Oct 2004
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PostPosted: Thu Mar 22, 2012 2:01 pm    Post subject: Reply with quote

I have confirmed with the crank specialist that the rod to use is the wider width, plenty of room on the rod journals to grind and accommodate the .4mm additional width. I am hoping he will have my stroker crank completed by the weekend. Will be finalizing selection of rods and pistons this week as well, and as soon as I have all the components will have a block bored to match, build the reciprocating assembly, and confirm clearance. Hope to have complete confirmation within 2-3 weeks. More to follow...
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