924Board.org

brit2514 · Joined: 21 Feb 2007 Posts: 18

i have now nearly completed my project, to fit a 924 turbo head on my 924 n/a car. i am having a few drama's and would like some advice

i have fiteed the following to my car:

1. 924 turbo head with 75 tho skimmed off, and inlet manifold
2. 924 turbo series 2 euro pistons
3. 924 turbo dizzy
4. pipercross adjustable cam sprocket
5. i have also advanced the timing 12 degrees
6. 924 turo injectors

the car starts ok but only revs up to 5250rpm and there is a lack of power when driving. it also sound a bit rough at idle but that soon go's away. i haven't fitted a 924 turbo coil could this cause it?

i would welcome any help i can get as i am so close to having it finished.

also is there anyone who has complete this mod that have bhp details?

NEMESIS · Posted: Sun Apr 29, 2007 7:15 am Post subject:

Let me start by saying I have no experience whatsoever in running the 931 engine without the turbo. I understand what you are trying to do and here are my observations given the information you posted.

A couple of thoughts.

1. Sounds like your compression ratio may be quite a bit lower than you calculated.

2. You are adding a lot of fueling capacity by swapping in 931 bits, why? I suspect it's becasue of the increased flow of air you were expecting with the 931 head and such, but without a turbo I would think the fueling would be way off.

What do the plugs look like after it's been run?

These are mearly guesses on my part but I hope in some way they help you figure this out.

Andrew NZ · Joined: 22 Jun 2004 Posts: 744 Location: New Zealand

If you aren't running a turbo why are you using the turbo dizzy? As far as I know the advance curves are very different and you'd be much better off with an N/A distributor.
_________________
Andrew
1977 RX924 race car
12a bridgeport supercharged
www.race4-dcup.co.nz

brit2514 · Joined: 21 Feb 2007 Posts: 18

i have fitted the 931 injectors because the 924 one don't fit the head.

not checked the plugs yet. will cheak in the morning

the compression ratio should be approx 10:1 as the stock for a 931 euro spec series 2 is 8.5:1 add in the 75 tho head skim which raise's it up again

stock compression on a euro 924 n/a is 9.3:1 so i should have at least that if not heigher

brit2514 · Joined: 21 Feb 2007 Posts: 18

iam using the turbo dizzy because i got it with the car thought it would be the same. if you think differntly i might go buy a n/a dizzy.

also any idea why there are 2 vacuum advance pipe on a turbo dizzy?
i just blocked one off and it has no effect on running

RC · Joined: 25 Mar 2007 Posts: 2637 Location: Australia

Mate, Andrew NZ wrote

leadfoot · Posted: Sun Apr 29, 2007 8:46 am Post subject:

one tube is for vaccum retard, one is for vaccum advance, on the n/a distributor there is only vaccum advance.
the ignition curves are very different though.
I would suggest once you replace the dissy taking your car to a dyno to tune it, it may cost you a couple of hours time but it will give you a visual representation of your timing and air/fuel ratios... this is very important as you only have a small amount of adjustment in respect to timing. To make sure your not killing your engine running it too rich/lean or to far advanced will cause excessive combustion temps and damage your valves/pistons.
It would be a shame to ruin all your hard work.
Leadfoot
_________________
1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...

NEMESIS · Posted: Sun Apr 29, 2007 9:10 am Post subject:

Sorry I assumed you meant fuel dizzy not ignition. So not adding 'that much' extra fuel.

Sounds like these guys are on to something with the curve though. That sounds like a simple enough thing to try first.

Curious to know what the cc volume of the head was once you skimmed it. Don't suppose you measured it?

brit2514 · Joined: 21 Feb 2007 Posts: 18

no sorry didn't mesure the cc volume of the head. i was in a bit of a rush to get it back together as it was taking up a lot of space.

i have a n/a dizzy i will test in the morning.

if it does not work i will buy one soon

daveo · Joined: 02 Sep 2005 Posts: 183 Location: GB

hi sory to jump in

leadfoot said (one tube is for vaccum retard, one is for vaccum advance) i have just posted a question on the bike carb post i need to know haw to conect the vac retard, is it to the same as vac advance after the buterfly?.

hope you can help

Dave

leadfoot · Posted: Sun Apr 29, 2007 9:36 am Post subject:

probably only a few cc difference, the stock head is 21.5cc, so 2mm of that is really going to be little difference, 2mm stroke difference on the other hand...
What's your piston dish volume though?? and how far do they sit down in the bore??
IIRC they almost come to the top of the bore as they were the higher comp pistons,
as a simple calc with a new head pocket of 19cc and a dish of 28cc your compression should be around 9.76:1
if you drop the pocket to 15cc you get around 10.76:1
Leadfoot
_________________
1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...

Smoothie · Posted: Sun Apr 29, 2007 10:30 am Post subject:

On the vacuum advance and retard -
The inner one is retard and the outer is advance.
For advance, you want a source that provides higher vacuum when the throttle is open. You get that from the port near the throttle valve - its' internal opening is actually just upstream of the throttle valve when the throttle is closed, so it sees very low vacuum at idle and higher vacuum when the throttle is opened.
The distributors' retard side vacuum is sourced downstream of the throttle valve (intake manifold vacuum).
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

endwrench · Joined: 07 Dec 2002 Posts: 1631 Location: Victor, Montana

Just my 2 cents but I would definetly get rid of the 931 ignition distributor. The centrifical advance actually retards ignition up to 3000 rpm then only slight advances above that. It relies on a specific vacum signal for advance. I would think your best bet would be the Euro 125HP model NA distributor. Much more aggressive curve.

Todd
_________________
'79 924NA. Rebuilt 9.5:1, MSDS header, Mega Squirt Injection, MJLJ-EDIS Ignition, 1.6L Whipple Charger and Intercooler, 10lbs Boost, 944 Trans, Custom HD Clutch.
"simsport" said....superchargers are better than turbos its official!....

brit2514 · Joined: 21 Feb 2007 Posts: 18

i have just bought a new 924 euro dizzy off ebay

the spare i had was knackered

should be here tuesday or wednesday will keep you informed of progress

gohim · Joined: 02 Nov 2002 Posts: 4459 Location: Rialto, CA

With that much shaved off the head, I would be worried about the pistons hitting the valves, AND the ignition timing will be hard to set, because when the head was shaved, you changed the relationship between the crankshaft and the camshaft turning, because the timing belt is not going to be effectively shorter.

While I have not had to deal with this on my 924, or on a 931 before, Porsche took this into consideration. Porsche spec'cd a limitation on the amount that the head can be shave (and a cast in mark on the cylinder head of the 924S and 944) to mark exactly how far the head can be shaved. There are also special instructions for dealing with heads that have been shaved when installing the head and timing belt.

Did you calculate your valve clearance, and followup with a check, after shaving the head, before assembly the engine? Maybe this has something to do with the poor running.