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alternative conrod choices
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anthonimartini  



Joined: 17 Feb 2020
Posts: 240
Location: portland oregon

PostPosted: Sat Mar 27, 2021 2:44 am    Post subject: alternative conrod choices Reply with quote

i know this has been covered previously but im wondering if anybody actually followed through.

i have decided to build the 931 head/vw flat top combo but i would like to find a suitable rod that is slightly longer for a bit more compression. i believe the vw flat top route will net low 8's. what are our stock rod lengths? i may take a rod up to the machine shop and see if they can help me find something easily sourced and lighter. i will let you know if i find anything.
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anthonimartini  



Joined: 17 Feb 2020
Posts: 240
Location: portland oregon

PostPosted: Sat Mar 27, 2021 3:00 am    Post subject: Reply with quote

well hell check these out!

https://www.ebay.com/itm/CXRacing-H-Beam-Connecting-Rod-bolts-for-BMW-M10-4Cyln-Engine-146mm-Rod-Length/184480795789?epid=668804799&hash=item2af3e98c8d:g:Ok8AAOxyTjNSev4F

CXRacing H-Beam Connecting Rods (4 PCS) + Bolts
For BMW M10 4 Cyln Engine. 146mm or 5.748'' Rod Length

Rod Length: 146mm (5.748'')

B/E Bore: 52mm

S/E Bore: 22mm

B/E W: 24mm

S/E W: 24mm

Bolts: 3/8''

This Auction is for Rod x 4 pcs + Bolt x 8 pcs


they have the in 148mm as well. i just need to figure out what 146mm will yeild in compression and decide what length i need to get me in the high 9's. then i have to figure out which rod bearing i need but it seems i will just need to get my crank cut to a bmw m10 undersize and use those bearing. simple and very cheap. this combo using lightweight vw flattops will drop hundreds of grams off the rotating mass of our engines and using 40mm dcoes and a euro points dizzy will probably net around 130bhp possibly more. this will absolutely transform my car and for very cheap.
_________________
ive saved alot of 924's
current:
79 MochaSchwartz 931
83 944
87 924S
Instagram @anthoni.jpeg
formerly known as turbellion, lost password.
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Juho  



Joined: 03 Oct 2018
Posts: 377
Location: Finland

PostPosted: Sat Mar 27, 2021 5:17 am    Post subject: Reply with quote

check b230 rods also
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Juho  



Joined: 03 Oct 2018
Posts: 377
Location: Finland

PostPosted: Sat Mar 27, 2021 5:24 am    Post subject: Reply with quote

some discussion on topic.
http://www.924board.org/viewtopic.php?p=401390&highlight=#401390
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Juho  



Joined: 03 Oct 2018
Posts: 377
Location: Finland

PostPosted: Sat Mar 27, 2021 4:51 pm    Post subject: Reply with quote

Why stop high 9s? I want to do similar build but i want like 11:1 compression
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Juho  



Joined: 03 Oct 2018
Posts: 377
Location: Finland

PostPosted: Sat Mar 27, 2021 5:14 pm    Post subject: Reply with quote

STUUUPID idea but listen

Volvo rods, custom bmw flat tops cheap cheap for forged, deck height 227
Compression would be 10.5:1
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Juho  



Joined: 03 Oct 2018
Posts: 377
Location: Finland

PostPosted: Sat Mar 27, 2021 5:18 pm    Post subject: Reply with quote

http://www.924board.org/viewtopic.php?p=279976&sid=0f512332912cee072c89d6184599b0cb
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Juho  



Joined: 03 Oct 2018
Posts: 377
Location: Finland

PostPosted: Sat Mar 27, 2021 8:03 pm    Post subject: Reply with quote

anthonimartini wrote:
well hell check these out!

https://www.ebay.com/itm/CXRacing-H-Beam-Connecting-Rod-bolts-for-BMW-M10-4Cyln-Engine-146mm-Rod-Length/184480795789?epid=668804799&hash=item2af3e98c8d:g:Ok8AAOxyTjNSev4F

CXRacing H-Beam Connecting Rods (4 PCS) + Bolts
For BMW M10 4 Cyln Engine. 146mm or 5.748'' Rod Length

Rod Length: 146mm (5.748'')

B/E Bore: 52mm

S/E Bore: 22mm

B/E W: 24mm

S/E W: 24mm

Bolts: 3/8''

This Auction is for Rod x 4 pcs + Bolt x 8 pcs


they have the in 148mm as well. i just need to figure out what 146mm will yeild in compression and decide what length i need to get me in the high 9's. then i have to figure out which rod bearing i need but it seems i will just need to get my crank cut to a bmw m10 undersize and use those bearing. simple and very cheap. this combo using lightweight vw flattops will drop hundreds of grams off the rotating mass of our engines and using 40mm dcoes and a euro points dizzy will probably net around 130bhp possibly more. this will absolutely transform my car and for very cheap.


with these rods and vw pistons your compression ratio is going to be 10.6:1

with 148 rods 13.4:1 that seems a fun na engine
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anthonimartini  



Joined: 17 Feb 2020
Posts: 240
Location: portland oregon

PostPosted: Sun Mar 28, 2021 1:40 am    Post subject: Reply with quote

now were talkin! seems these rods might be the solution to our build problems! at least us that are building na engines with 931 heads. i wouldnt build a engine with flat pistons and a flat head. so i guess the only real question at this point is at what deck height we need to worry about piston valve interferance with stock cams. i am going to order a set of the m10 rods (havent determined length yet) and use 88mm vw pistons. ill use the bmw rod bearing and cut my crank to match. i still have to cc my recently rebuilt 931 head to know exactly what rod length i want. this will be my daily using 40mm dcoe carbs so i dont think i want to exceed 10;1.

as i side note, i have built about 100 aircooled vw and 914 engines over the years and usually i use the cheap hypereutectic AA branded pistons. i even used them on a 914 engine i built to a 2276, it had around 9.5:1 compression and dynoed at 137 rwhp. i put it in a 1978 porsche 911 chassis with a 915 gearbox. i drove the shit out of that car and those cheap little pistons never complained. my point is that i have faith in these cheap pistons. if they can take abuse in aircooled applications with higher temps and worse thermal control i think they will be wonderfully reliable for our stable watercooled engines. i honestly dont even think we need the forged units unless were boosting them. the only downside i can see is since we run such big deck heights we really dont have a good quench area. the vw engines i built had a rather large combustion chamber all in the head so i would run at low at .037" deck. depending on how much head volume i measure ill be running between .249 and .275". not really the most efficient way to build a engine. im hoping to at least get 120hp at the wheels. if i can achieve this with decent mpg ill be happy.
_________________
ive saved alot of 924's
current:
79 MochaSchwartz 931
83 944
87 924S
Instagram @anthoni.jpeg
formerly known as turbellion, lost password.
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Juho  



Joined: 03 Oct 2018
Posts: 377
Location: Finland

PostPosted: Sun Mar 28, 2021 1:56 am    Post subject: Reply with quote

you need some kind of washers in big end
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Juho  



Joined: 03 Oct 2018
Posts: 377
Location: Finland

PostPosted: Sun Mar 28, 2021 1:57 am    Post subject: Reply with quote

if you want decent mpg dont get webers i had them in my na and swapped them for bike carbs
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Juho  



Joined: 03 Oct 2018
Posts: 377
Location: Finland

PostPosted: Sun Mar 28, 2021 6:12 pm    Post subject: Reply with quote

where you saw the 148mm rod, i cant find it
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anthonimartini  



Joined: 17 Feb 2020
Posts: 240
Location: portland oregon

PostPosted: Mon Mar 29, 2021 9:31 am    Post subject: Reply with quote

i also found this.

https://aapistons.com/collections/type-4-914-rods/products/5-325-h-beam-rod-type-4-2-0-journal-22mm-pin
.......................................................................


2.0 porsche /vw type iv rod
available in 144.7mm length

22mm pin

Big end bore-52.8mm
-we could machine the rod journal diameter to a 914 undersize bearing. easy

big end width-25.68 (1.0110")

this gives us .0865" big end side clearance. still pretty large but much better than the 0.153" we get with the bmw M10/s14 rods.
_________________
ive saved alot of 924's
current:
79 MochaSchwartz 931
83 944
87 924S
Instagram @anthoni.jpeg
formerly known as turbellion, lost password.
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View user's profile Send private message
anthonimartini  



Joined: 17 Feb 2020
Posts: 240
Location: portland oregon

PostPosted: Mon Mar 29, 2021 9:34 am    Post subject: Reply with quote

these are the pistons im going to use.

https://aapistons.com/collections/piston-sets/products/vw-88mm-type-1-piston-kit
_________________
ive saved alot of 924's
current:
79 MochaSchwartz 931
83 944
87 924S
Instagram @anthoni.jpeg
formerly known as turbellion, lost password.
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madislandguy  



Joined: 25 Nov 2009
Posts: 26
Location: Madeline Island, WI and Elburn IL

PostPosted: Mon Mar 29, 2021 12:12 pm    Post subject: Reply with quote

anthonimartini wrote:
i also found this.

https://aapistons.com/collections/type-4-914-rods/products/5-325-h-beam-rod-type-4-2-0-journal-22mm-pin
.......................................................................


2.0 porsche /vw type iv rod
available in 144.7mm length

22mm pin

Big end bore-52.8mm
-we could machine the rod journal diameter to a 914 undersize bearing. easy

big end width-25.68 (1.0110")

this gives us .0865" big end side clearance. still pretty large but much better than the 0.153" we get with the bmw M10/s14 rods.


How would you tighten the bolts on these rods in a 924 engine? They would be pointing inward towards the pistons, wouldn't they?
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