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

931 engine N/A k-jet tuning????
Goto page 1, 2  Next
 
Post new topic   Reply to topic    924Board.org Forum Index -> Performance Upgrades
View previous topic :: View next topic  
Author Message
anthonimartini  



Joined: 17 Feb 2020
Posts: 232
Location: portland oregon

PostPosted: Mon Nov 29, 2021 2:17 pm    Post subject: 931 engine N/A k-jet tuning???? Reply with quote

hey guys so i just finished my weird 931 NA build and am attempting to tune it with the stock 79 k-jet using the 1980 WUR, the 931 intake manifold and currently the N/A throttle body.

so the engine is 9.5:1 using 88mm flat top VW pistons.
stock 1979 cam with adjustable cam pulley.
the head is a 1980 931 that has been lightly ported using stock size stainless valves. for ignition im using a euro points distributor with pertronix and a bosch red coil.
exhaust is a MSDS header with the stock 2.25" rear section from a 84 944,a resonator and no cat. i spent hours modifying the msds header to fit the 931 ports and i have the pretty much perfectly port matched without the 931 fire sleeves.
clean fuel tank, new bosch pump, new filters, new accumulator, new injectors and i rebuilt the cis fuel distributor.

im currently running stock euro spec timing 10 degrees btdc at idle which brings it to around 32-33 all in, the vacuum can brings in about another 10 degrees. it actually runs pretty good but i can tell the fueling is not perfect and i know theres plenty of power hidden in this combo.

i have yet to hook up my pressure gauges and check out whats going on but its kind of meaningless until i get a wide band and see what adjustments i need to make to the warm up regulator and possibly system pressure.

i guess my real question is to anybody that had built anything that required mods to the stock k jet pressures, larger displacement etc or to anybody that has built a similar combo.
_________________
ive saved alot of 924's
current:
79 MochaSchwartz 931
83 944
87 924S
Instagram @anthoni.jpeg
formerly known as turbellion, lost password.
Back to top
View user's profile Send private message
Cedric  



Joined: 27 Aug 2004
Posts: 2269
Location: Sweden

PostPosted: Mon Nov 29, 2021 6:21 pm    Post subject: Reply with quote

Cool build! I would expect it would need some more timing with the flat tops dude to the lack of squish, the fuelling shouldn't be so hard to adjust when you get that wideband.

A shorter intake manifold and more agressive cam would make it much more fun at the top end. Whats the plan and use of this peculiar build?
_________________
1980 924 Turbo
Back to top
View user's profile Send private message MSN Messenger
gegge  



Joined: 27 Jul 2007
Posts: 1108
Location: Sweden

PostPosted: Tue Nov 30, 2021 7:14 am    Post subject: Reply with quote

IIRC running 87mm flat tops on a 931 increase CR to 8,7:1 and to 8,8:1 with 88mm. Have you decked the block or skimmed the head 1mm?

I think you will get ROW 924 power with that setup, maybe slightly more. The headers wont do any good with stock cam I afraid. You need higher CR and a hotter cam.

I run stock CIS but relocated the WUR from the inlet manifold to keep it cooler in order to lower the controlpressure and run richer. Stock timing is tricky, I need a lot more advance mid and up.

87mm Mahles, 10,5:1 CR, a good flowing 924 head, wild cam, solid lifters, cam pulley, Audi TB, headers and some more work to the bottom end:

https://youtu.be/ZEepQkhJKV4
_________________
Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs
Back to top
View user's profile Send private message
anthonimartini  



Joined: 17 Feb 2020
Posts: 232
Location: portland oregon

PostPosted: Tue Nov 30, 2021 8:06 am    Post subject: Reply with quote

I ended up calculating the compression after cc’ing the head measuring deck height and adding the thickness of a used crushed head gasket. My chambers were around 19cc. I’ve built at least 100 weird old engines and used my same methodology on this one and I was surprised I got it do high. Oddly enough I built this as a daily driver. Just wanted to make a bit more efficient engine and I had pretty much all the parts already. I may have to recurve the dizzy to get more timing earlier without getting too much up top. I drove it about 20 miles to work today and all went great although I need. To adjust the valves.

One thing I am a bit confused with is the head bolt torque. I used the ft lb method over the angle torque but I did have the new serrated bolts which I later realized may be stretch. Should I just loosen them and re torque to the angle method one by one, or should I just get new ones and do the same?
_________________
ive saved alot of 924's
current:
79 MochaSchwartz 931
83 944
87 924S
Instagram @anthoni.jpeg
formerly known as turbellion, lost password.
Back to top
View user's profile Send private message
safe  



Joined: 18 Mar 2017
Posts: 352
Location: Sweden

PostPosted: Tue Nov 30, 2021 10:16 pm    Post subject: Reply with quote

What compression height do you have on the flat tops?

When I calculated this swap using flat tops with 41mm CH on a stock NA I would loose compression from the 9.3:1. Adding a turbo head to that would make it much worse.
_________________
/Magnus, Stockholm Sweden
=======================
Porsche 924 -79 NA, under EFI and turbo conversion
Back to top
View user's profile Send private message
anthonimartini  



Joined: 17 Feb 2020
Posts: 232
Location: portland oregon

PostPosted: Wed Dec 01, 2021 3:45 pm    Post subject: Reply with quote

Oh boy I made some progress tonight, I recurved my dizzy to have more total so I can run less initial but pull all the way to 36 without vacuum and I adjusted my warm control pressure lower, raised the system pressure. this thing is coming to life. Feels very similar to my 83 944 now just more buzzy. I’m just going by plug chops now and they are still showing pretty lean maybe ever so slightly tannish gray. Tomorrow I’m going to try adding one more shim to the pressure regulator and see if I can get my plugs to color nicely. After I get my mixture in a safer zone I’ll try adding more timing. I feel like a I’m probably in the 130hp zone now which is what I was aiming for now I’m wondering if I can get hit 140?
_________________
ive saved alot of 924's
current:
79 MochaSchwartz 931
83 944
87 924S
Instagram @anthoni.jpeg
formerly known as turbellion, lost password.
Back to top
View user's profile Send private message
anthonimartini  



Joined: 17 Feb 2020
Posts: 232
Location: portland oregon

PostPosted: Thu Dec 02, 2021 12:58 am    Post subject: Reply with quote

Drove the 30 miles to work today and aside from some hot start issues this thing rips! Pretty weak low end but pulls hard past 3k with nice power band all the way to my 6500 rev limiter. I’m going to try and get on a dyno soon to have a baseline for this combo as it is easy to replicate. I may look into some more aggressive cams after I put some miles on it. As for the compression height it is just a standard mahle 88mm type 1 vw piston. I’ll find my measurements and post them. I’ll also take some driving videos soon and link to my YouTube.
_________________
ive saved alot of 924's
current:
79 MochaSchwartz 931
83 944
87 924S
Instagram @anthoni.jpeg
formerly known as turbellion, lost password.
Back to top
View user's profile Send private message
Fasteddie313  



Joined: 29 Sep 2013
Posts: 2421
Location: MI

PostPosted: Thu Dec 02, 2021 4:07 am    Post subject: Reply with quote

From what I have tested raising system pressure increases the fuel flow a surprisingly small amount..
_________________
80 Turbo - Slightly Modified
Back to top
View user's profile Send private message Yahoo Messenger
Mike9311  



Joined: 14 Dec 2004
Posts: 1216
Location: Chicago-ish

PostPosted: Thu Dec 02, 2021 4:26 am    Post subject: Reply with quote

Fasteddie313 wrote:
From what I have tested raising system pressure increases the fuel flow a surprisingly small amount..


Funny I just read your EDIT in RED on this very topic in another thread yesterday. You postulated an increase but later found it to be minimal

Back to main topic: Congrats on the build! I like projects like this
_________________
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
Back to top
View user's profile Send private message
anthonimartini  



Joined: 17 Feb 2020
Posts: 232
Location: portland oregon

PostPosted: Thu Dec 02, 2021 5:02 am    Post subject: Reply with quote

I actually added one shim and it seemed slower so I removed it. I may actually just be at the limit of my current setup. Stock 79 Na cis with 88 mm pistons 931 head,intake and msds header 2.25” exhaust. Not sure what else I can do but as it is I’m happy with it. I kept up with a 924s this morning and I was hoping to at least reach that baseline. I haven’t heard any detonation yet running 91 octane. I also switched to the stock 79 warm up regulator and changed the position of the warm circuit as I don’t want to modify the rare 931 unit. Now I just got to remedy my starting issues. Likely just add a toggle to ground the cold start injector.
_________________
ive saved alot of 924's
current:
79 MochaSchwartz 931
83 944
87 924S
Instagram @anthoni.jpeg
formerly known as turbellion, lost password.
Back to top
View user's profile Send private message
anthonimartini  



Joined: 17 Feb 2020
Posts: 232
Location: portland oregon

PostPosted: Thu Dec 02, 2021 5:19 am    Post subject: Reply with quote

Also what’s your experience with larger than stock throttlebodies. Im currently using the stock Na one to try again some low end but I do have the s1 turbo with the larger primary. I may swap it and see what happens?
_________________
ive saved alot of 924's
current:
79 MochaSchwartz 931
83 944
87 924S
Instagram @anthoni.jpeg
formerly known as turbellion, lost password.
Back to top
View user's profile Send private message
gegge  



Joined: 27 Jul 2007
Posts: 1108
Location: Sweden

PostPosted: Fri Dec 03, 2021 8:33 pm    Post subject: Reply with quote

Try the Audi 5000 TB from the 5-cyl. Same bolt pattern and the rubber J-boot fits. The secondary is much larger.
_________________
Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs
Back to top
View user's profile Send private message
anthonimartini  



Joined: 17 Feb 2020
Posts: 232
Location: portland oregon

PostPosted: Fri Dec 10, 2021 2:21 pm    Post subject: Reply with quote

well i finally worked all the kinks out and my car has been driving really well for the past week. playing with the timing i found that i actually got some piston slap when cold when i had my timing further advanced around 15 degrees at idle. which cured when warmed up. i backed off the timing a bit and i dont really get that anymore. ive heard these pistons can be noisy when cold and im running pretty tight clearances 0.015" thou. it was not predetonation it was very clearly piston slap, im thinking the poor quench of this combustion chamber probably tends to push the piston down favoring the spark plug side of the chamber since there isnt a controlled flame front. oh well im happy with the engine. i may look into installing a supercharger to pick up some more low end grunt since the 944 gearbox is pretty tall geared.

heres a you tube video, enjoy the sound!

https://youtu.be/DySDcQS5Jbk
_________________
ive saved alot of 924's
current:
79 MochaSchwartz 931
83 944
87 924S
Instagram @anthoni.jpeg
formerly known as turbellion, lost password.
Back to top
View user's profile Send private message
Fifty50Plus  



Joined: 28 Feb 2008
Posts: 1223
Location: Washington DC area

PostPosted: Sat Dec 11, 2021 3:02 am    Post subject: Reply with quote

Piston to wall clearance on my race engines in the '80s was .004 - .006 in. Newer engines from the factory are now in the .0015 - .002 in with today's metallurgy. Fifteen thou seems excessive and explains the piston slap.
_________________
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
safe  



Joined: 18 Mar 2017
Posts: 352
Location: Sweden

PostPosted: Sat Dec 11, 2021 3:27 am    Post subject: Reply with quote

Fifty50Plus wrote:
Piston to wall clearance on my race engines in the '80s was .004 - .006 in. Newer engines from the factory are now in the .0015 - .002 in with today's metallurgy. Fifteen thou seems excessive and explains the piston slap.


Yeah, 0.015 sounds like a lot!

924 spec for new is 0.0011, were limit at 0.0031.
I think the flat tops are hypereutectic so you can run them really tight, more so than with a forged piston.

But it looks like it pulls nicely!
_________________
/Magnus, Stockholm Sweden
=======================
Porsche 924 -79 NA, under EFI and turbo conversion
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topic   Reply to topic    924Board.org Forum Index -> Performance Upgrades All times are GMT + 10 Hours
Goto page 1, 2  Next
Page 1 of 2

 
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