924Board.org

[anthonimartini]

what have you guys done to get a NA exhaust manifold onto the 931 head? buidling the engine is simple, VW flattops, resized small end bushing i also have a really nice ported and polished 931 head to use. im just unsure what id do to get an exhaust on there. i was also considering using weber dcoe but i dont think there is a manifold available for the 931 head. im either going to build this or sell the head and use the NA head with euro spec pistons. which will definitely be simpler.
_________________
ive saved alot of 924's
current:
79 MochaSchwartz 931
83 944
87 924S
Instagram @anthoni.jpeg
formerly known as turbellion, lost password.

[Juho]

what compression ratio youre going for and can you link me what pistons?

[TJC]

I'm considering a similar build for my '80 924 Turbo and will be looking further into the old D-Production setup as the Turbo head was used in an NA application, (albeit fuel injection), in that case to produce around 180 horsepower.
After following the constant issues that always seem to be present with the turbocharged engines in the 931's for the last couple of years it seems like a more trouble free way to go to enjoy one's car instead of always working on it or trying to source parts, hee, hee!!!
Okay, flame suit donned...ready to get thrown out of the group!!!
_________________
'95 BMW 318i/5 ..."Pearl"
'87 Porsche 944 NA... "Liebchen"
'02 Porsche Boxster..."Sunbeam"
'04 BMW X3..."Xander"
Still on the Prowl!

www.ttrs1.com

[Juho]

I want to do similar build but i want to run off the shelf pistons....

[anthonimartini]

you can use off the shelf 87mm aircooled vw pistons. they are flat and will give you just shy of 9:1 with the stock deck. you can of course deck the block and head for increased compression and run a adjustable cam pulley to get back your cam timing, or run one tooth advanced for a bit more low end power. when running these pistons you do have to resize your small end rod bush to 22mm. these are them

https://www.ebay.com/itm/PISTON-KIT-87-MM/163356414314?fits=Make%3AVolkswagen&hash=item2608cd116a:g:EjcAAOSwJZ5f71Mr

ive personally built many air cooled vw engines with similar pistons all the way up to 100mm for 914's. even though they are cheap as hell they are pretty decent quality and i would not hesitate to run them hard. they usually come out of the box within 1 gram of each other and in the air cooled world deal with much hotter temps then would be present in our water cooled world. you can even opt for much better wiseco or mahle forged units. the 87-90mm sizes can save alot of our worn out std bore blocks!
_________________
ive saved alot of 924's
current:
79 MochaSchwartz 931
83 944
87 924S
Instagram @anthoni.jpeg
formerly known as turbellion, lost password.

[Juho]

how those will give9:1 comp ratio, they are lower comp height than stock 931 pistons?

[anthonimartini]

the 931 pistons have a huge bowl in piston. the vw are totally flat.
_________________
ive saved alot of 924's
current:
79 MochaSchwartz 931
83 944
87 924S
Instagram @anthoni.jpeg
formerly known as turbellion, lost password.

[Fasteddie313]

Just under 9.0:1?
Dang, I could probably still even run boost on those..
Why not?

Do they cause the engine to become non-interference?
_________________
80 Turbo - Slightly Modified

[anthonimartini]

you could totally run boost on those especially if you opted for the forged units! im not totally sure if it becomes interference since i have yet to assemble and check one of these combos. it does have a few mm lower compression height so it may have clearance.
_________________
ive saved alot of 924's
current:
79 MochaSchwartz 931
83 944
87 924S
Instagram @anthoni.jpeg
formerly known as turbellion, lost password.

[Juho]

what about taking some material off block and machining valve pockets on those pistons? to achieve something like 11:1 compression

[MikeJinCO]

Long and a bit rambly.

A standard NA Weber intake manifold can easily be adapted to the NA head by enlarging some of the bolt holes in the manifold. Some not too drastic port matching is required. Since the head is already ported I would only work on the manifold only. I used some Splashzone epoxy and have plenty left over. There is some oil field equipment repair epoxy which is probably the best, but it is hard to find I think maybe some place in Caper WY. I think Moroso may have some also. The intake fits both 40 and 45 DCOE's. For street use I would use the 40mm webers. The 45's really don't shine at low rpm running. For 40's probably the standard Redline/Pierce setup is best. I'm at altitude so all standard bets are off. My car even with an expert set up idles a little unhappily at about 1100. I have about 10 CR with 931 S2 pistons and 040 MLS head gasket and a 264 deg. Elgin cam on NA head and 45's. About 135 corrected HP with the timing at 34 vs 41.

On the exhaust the port centerlines are the same as the NA. I made a dxf design file for the 931 exhaust if you want to cut one. The difference besides port size is the bolt spacing- wider. Ideola went thru this with one of the header outfits and ended up just taking a standard set of headers and cutting the hole slots out to edge of the flange. The MSDS headers except for the bolt spacing fit the 931 head perfectly. I'd be happy to send you a sketch, I can't confirm that Dan's mod worked. MDS headers are the correct port size for the 931, a bit too big for the NA head.

I talked to EBS a couple of weeks ago and a set of JE pistons with rings and pins is about $750. Besides being probably 450gm vs 700-800, you get any compression ratio you want and lower drag ie modern piston rings. JE made many/most of the old race pistons.

When I tested my 931 head The flow was really climbed well(much better than NA head) up to the point where the valve leaves the opening of the combustion chamber ,then the shrouding of the valve against the cylinder wall holds it back and the flow struggles the rest of the way open. I my opinion that is why many of the cams are of a longer duration lower lift. My 264 deg Elgin reground and hardened cam works really when you figure out how to get the valve clearance right.

If you are looking for an intake manifold, contact Jeff Winter at Rallye Sport in Thornton CO, he had one a while back.
_________________
Mike


'67 MG Midget Dp
'71 Ocelot Dsr Kawasaki 1000(under rebuild)

[Mike9311]

Dan's D-Prod reproduction now sold. I was also looking at it (dreaming) but can only absorb so much. Just wish I could have heard this baby before it sold. This think ripp'in with lightweight everything back to a 931 LSD... oh man

Think the MSD header mod worked just fine




For fun...this view
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_________________
1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA ohne 650 mit 471
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car

[anthonimartini]

did he use the intake manifold for a na and modify the holes? doesnt seem to bad. im not really worried about the exhaust. i can easily modify a header. the cast aluminum is a bit more tricky.
_________________
ive saved alot of 924's
current:
79 MochaSchwartz 931
83 944
87 924S
Instagram @anthoni.jpeg
formerly known as turbellion, lost password.

[MikeJinCO]

I have no idea what Dan did. What i did was slightly enlarge the upper holes to fit. It was no big deal, not even noticeable after it is bolted on. If there ever was a manifold made for the 931 head I have never found a reference for it. The way I did this was first get the manifold to fit the head, then see where the head did not match the existing port. Then I filled the places where they didn't match with epoxy and then reworked the manifold and port entrance to match. Looking at the port I typically finished with some epoxy from about 12 to 2 in the head and 6 to 8 in the manifold. From there I went on porting the head. When I did my NA head(different manifold) it did not match those ports all that well either, generally larger as I recall the NA ports are smaller about 33mm dia and the 931 ports about 36mm(estimated). They are both designs from the dark ages. Jeff Winter who builds Datsun vintage race motors( won the COTA fall race in his 260Z) and cars told me that he remembered when folks were racing 924's and were always short on power as nobody could get the air around that turn in the head.
_________________
Mike


'67 MG Midget Dp
'71 Ocelot Dsr Kawasaki 1000(under rebuild)

[anthonimartini]

hmm you have some great info there! so let me ask your opinion. im just trying to build a fun reliable daily driver, not trying to build a racer but am tired of my 95hp 924's. do you think it would be worth it to build this hodge podge NA engine with the 931 head or should i build a really nice balanced euro spec NA using a massaged NA head. i already have all of these parts on hand including a nice set of euro spec pistons and euro distributor. im starting to think a nicely built euro spec engine with a header and a set of 40mm dcoe's will be the most fun and problem free car for me. i already acquired a 84 944 for the 5 lug brakes and 14" 14x5.5 fuchs for it. this is the look im going for, im even swapping my hatch for a spare 76 hatch with the greenish glass and chrome trim. i got hooked on the brown with chrome when i worked on a brown 911e targa. it was beautiful!

https://www.beverlyhillscarclub.com/1969-porsche-911e-coupe-c-9657.htm
_________________
ive saved alot of 924's
current:
79 MochaSchwartz 931
83 944
87 924S
Instagram @anthoni.jpeg
formerly known as turbellion, lost password.